Build: An LED Enlarger Head

Purpose

To give the enlarger a light source you can describe in numbers: a diffusion head running at extra-low voltage from a certified supply, whose flux, warm-up behaviour, uniformity and repeatability are measured before it prints anything.

The design is settled before a tool is picked up, and it is settled by the optics page: a mixing chamber with a diffuse interior and an opal exit port, sized to the donor enlarger’s lamphouse mount and to the largest negative you will print, with room in it for a second colour channel that Part XVIII will want and this page does not fit.

And there is a second purpose, which is a boundary. The donor is a mains appliance. This build does not make it into a different mains appliance; it takes the mains out of the light path entirely and replaces it with 12 or 24 V from a supply that arrived certified and sealed. That is not a caution bolted onto a build — it is the reason this build is a reasonable thing for a reader to do at all, and it is set out before the Parts list rather than after it.

Learning objectives

By the end of this session you should be able to:

• decide, from the donor enlarger in front of you, which of the three mains routes this page names applies, and say why the other two do not;
• state where the course’s electrical work stops and a competent person’s begins, in the words the guidance uses rather than in your own;
• size a mixing chamber from an exposure target, using the illuminance relation rather than by guessing at a wattage;
• read a white, blue and green LED datasheet for the five numbers that decide a channel, and compute a junction temperature from the thermal resistance;
• explain why the head is driven at a fixed current and timed, rather than dimmed, and cite the evidence;
• build a light-tight joint between a home-made head and a bought casting, and a cable exit that does not leak;
• run the five commissioning tests in order, and say what each one licenses you to do next;
• write a head record that a stranger could use to reproduce your exposures;
• name the point at which buying a commercial head is the better answer.

Prerequisites

Enlarger optics and design, in full. The conjugate relation, the (1 + m) factor, the condenser-against-diffusion argument, the falloff arithmetic and the alignment tolerance are all settled there and are used here without being re-derived.

Low-voltage electronics for the darkroom, in full and without exception. Constant-current driving, MOSFET low-side switching, the current budget, why a heatsink is a photometric component and the five datasheet numbers are established there. Nothing electrical is re-taught on this page.

The LED light source and exposure timer, whose lamp is the same architecture at a twentieth of the power, and whose warm-up and repeatability tests are the ones repeated here at the easel.

Making a room dark and the safelight fog test, both passed. A head’s light leak cannot be found in a room that leaks, and it cannot be found by somebody working under an untested lamp.

Safety classification

Level B, on two of the rubric’s criteria at once: a high-brightness LED source that is an eye hazard at close range, and a low-voltage electronics build where a certified power supply provides the isolation from the mains. Neither of those is negotiable by care; both are handled by construction.

What decides the letter and what does not. The donor enlarger is a mains appliance, and the rubric puts mains-voltage construction at Level C — “switching enlarger lamps or heaters from a home-built controller” is the example the rubric itself gives. This page performs no such work, which is why it is at B and not at C: the three routes below are constructed precisely so that nothing at mains potential is opened, rewired, re-terminated or re-energised at any point.

One step is declared at a higher level of care within a Level B session, under the mixed-level rule. Removing the donor’s lamp assembly, where route B applies, is done with the appliance unplugged at the socket, by undoing fixings only, with nothing cut, stripped or unscrewed from a terminal — and it stops the moment a conductor would have to be disturbed. The controls for that step are written out in stage 1 rather than left to be inferred.

What is not a hazard here. No chemistry is handled during the build: the Chemicals section a lab page would carry is replaced by Parts, and the only substances present are workshop adhesives and paint. Nothing is heated deliberately, and the ventilation in the head is not there for a gas — a white LED emits no ozone and produces no fume, so the vents are a thermal control and nothing else. And there is no shock hazard from anything the reader builds, because every conductor in the finished head is at 12 or 24 V from a certified, enclosed supply with a moulded lead, which is HSE’s own first remedy: limit the supply voltage to the lowest that will do the job.

The chemistry arrives later, in Testing and calibration, and it is the ordinary print sequence at Level A — the same trays, tongs and dilutions as the fog test.

Hazards

The emitter at close range. A white module at 700 mA is bright enough to leave an afterimage and there is no aversion response fast enough to protect you at 100 mm. The rule is Part XIV’s, unchanged: the emitter is never looked into, the current goes up only with the diffuser in place, and alignment work is done with the array dark and a torch.

The heatsink. It is doing its job when it is too hot to hold comfortably. Route the cable so it cannot lie across the fins, and give the head thirty minutes after a session before it goes back in its box.

The donor enlarger’s mass and its column. A head at the top of a column is a weight at the end of a lever, and a baseboard with a heavy head above it will tip if the head is not locked. Fit and remove the head with the carriage locked and the column at its lowest, with a second pair of hands for anything above shoulder height. Glass condensers, if you are removing them, are heavy, thick and slippery, and they are handled over a padded surface and stored on edge in a box, not stacked.

Soldering, on the discrete-emitter route: Part XIV’s controls unchanged — iron in its stand, extraction at the joint, lead-free solder, nothing touched for thirty seconds.

Mains, in one sentence and one only. The appliance is unplugged at the socket before it is touched, and stays unplugged for the whole session. HSE’s instruction is that equipment is switched off and unplugged before cleaning or making adjustments, and this is an adjustment.

Required PPE

Safety spectacles with side protection, for the whole build: swarf from drilling aluminium, and glass if condensers are being handled.

Gloves are a judgement here rather than a rule. Nitrile for the paint and the adhesive; bare hands for anything near a hot heatsink, because a glove hides the warning you need. Never gloves near a rotating drill bit.

Nothing optical. There is no UV in this build and no filter to specify. The eye protection that matters is procedural — the diffuser goes on before the current goes up — and it is written into the build order rather than worn.

For the commissioning prints at the end, the Level A print-processing controls apply: nitrile gloves, eye protection, tongs, one pair per dish.

Ventilation

For the build: ordinary workshop ventilation, and local extraction at the soldering iron on the discrete route, exactly as Part XIV requires. Matt paint inside the chamber is applied and cured with the window open and the head out of the darkroom.

For the finished head: ventilation is a thermal control and is not there for a gas. The head’s vents exist so that convection can carry heat away from the heatsink, and every one of them is a potential light leak, which is why they are baffled rather than open — the same construction the UVA unit uses for the same geometric reason.

For the darkroom the head runs in: unchanged, and owned by Part II and by the room page.

Parts

Nothing here is a recommendation to buy a named product. As on the safelight and the contact printer, the course specifies geometry, datasheet figures and acceptance tests, and leaves the part number to you.

The optical and thermal core

Part	Quantity	What it does	Notes
High-output white LED module or emitter array, on a star or metal-core board	1 to 6	The source, single-channel head	Choose on flux, forward voltage, forward current, thermal resistance and derating. Cree’s XLamp XP-E2 is the worked datasheet on this page because it is already in the course’s bibliography and it publishes all five; it is an example to read, not a part to buy
Blue and green emitters, on their own boards	optional, in pairs	The two channels of a variable-contrast head	Only if you are building the dual-channel head now. See the deferral note below before you do
Heatsink, extruded, with fins	1	Keeps the junction cool, and therefore keeps the output constant	Sized in stage 3 from the dissipation, mounted so its fins are outside the chamber
Thermal interface pads or paste	1 per emitter	The link the datasheet’s thermal resistance stops at	The datasheet gives junction to solder point; everything after that is yours
Constant-current driver, one per channel	1 to 3	Sets the current, which sets the flux	Adjustable, then set once and left. Not a constant-voltage supply
Certified, enclosed low-voltage supply, 12 or 24 V, moulded lead	1	Power, and the isolation from the mains	Rated at least 1.5 times the total draw. Its plug and fuse are checked as the safelight build sets out
Fuse holder and fuse, low-voltage side	1	Fault protection at the head end	Sized to the head, not to the supply
Opal acrylic sheet, 3 mm, for the exit port	1 or 2	The diffuser that makes the head a diffusion head	Opal, not clear and not frosted-one-side. Two thin sheets spaced apart diffuse better than one thick one and cost more light
Matt white paint for the chamber interior	~250 mL	Makes the chamber integrate rather than image	The highest-reflectance matt white you can get. See the note on finishes in stage 2

The mechanical and light-tight parts

Part	Quantity	What it does	Notes
Chamber body: aluminium sheet, or 6 mm ply lined with foil and painted	1	The integrating volume	Metal conducts heat away and is easier to seal; ply is easier to cut. Either works if the joints are light-tight
Mount adapter, to the donor’s lamphouse register	1	Makes the head fit the enlarger	Traced from the donor. This is the single part that cannot be bought
Dense felt or closed-cell foam strip, 3 to 5 mm	1 m	The light-tight seal at the joint	Compressed, not merely touching
Matt black paint or flocking, for everything outside the diffuser	1 tin	Stops the chamber’s light finding its way out sideways	White inside the diffuser, black outside it — the same rule as the safelight housing and the densitometer head
Cable gland or a baffled cable exit	1	Brings the low-voltage cable in without bringing light out	A straight hole is a light leak with a wire in it
Strain relief, internal	1	Stops a tug on the cable reaching a solder joint
Connector, low-voltage, polarised	1 pair	Head to supply, and head to timer	Polarised so it cannot be reversed in the dark
A lifting handle, and a positive location	1 each	So the head can be taken off without dropping onto the negative stage	The stage is directly under the head. Design for the drop you will one day make

🛠Four things this page will not decide for you

A part number for the emitter. No high-output white module available in the United Kingdom was verified against its own datasheet while this page was written, so none is named. The Cree XP-E2 figures below are used because that datasheet is in the course’s bibliography and publishes every number the design needs; they show you what to look for on whatever part you buy.

The number of emitters. That is an output of stage 0’s arithmetic and of your donor’s geometry, not an input. Build the chamber so you can add emitters later and set the driver below its maximum so you can raise the current: a head that can be brightened is a head that can pass its test on the second attempt.

Whether to build the dual-channel head now. The case for waiting is that Part XVIII owns variable-contrast theory and has not yet established what the control has to do — how finely the ratio must be settable, whether the two channels must be speed-matched, whether a third channel earns its place. The case for building now is that the chamber geometry is the expensive decision and retrofitting a second channel means opening the head again. The course’s position is: build the chamber for two channels and fit one. Leave the second emitter position drilled, tapped and wired to a blanked connector. That costs an hour now and saves the whole head later, and it commits you to nothing Part XVIII might overturn.

An illuminance target in lux. The temptation is to publish a number for the illuminance a mid-grey exposure needs at the easel. ILFORD’s statement that Multigrade RC papers are roughly equivalent to a film ISO of 3 to 6 is a comparison rather than a speed point, and no source this course has read licenses the step from it to a lux-second figure for a mid-tone. So this page gives the relation, the ratio method and the measurement, and publishes no lux figure of its own.

Tools

Tool	For	Notes
Multimeter	Channel currents, and the continuity checks	Read the current once, at the fuse, and write it on the label
Drill, step drill, files, deburring tool	The chamber, the mount, the heatsink	A step drill makes a clean round hole in thin aluminium where a twist drill grabs
Soldering iron, stand, extraction	The discrete-emitter route only
Tape measure, square, vernier or digital calipers	The mount adapter, the chamber depth, the exit port	The mount adapter is the one part where 0.5 mm matters
A torch, small and bright	The leak hunt, from inside and outside
The Part XV photodiode head or a lux meter	Illuminance at the negative stage and at the easel, and the warm-up curve	Relative readings are enough for everything on this page except comparison with somebody else’s head
A densitometer with a reflection head, or the visual fallback	Reading the uniformity map	The contact printer’s T3 sets out both
Thermocouple or infrared thermometer	Heatsink and chamber temperature over a session	An infrared thermometer reads a painted surface far better than a bare one

Estimated cost

Cost band £££, the highest in this part alongside the UVA unit, and the money is in three places: the donor enlarger, the emitters and drivers, and the heatsink.

The planner prices almost none of it, and says so. Its enlarger record carries one listing — a new LPL 7700 VCCE at £1,549, dated 5 September 2026 — and its own note calls that the least representative number in the file, because nothing in the first eight parts requires an enlarger and because almost every darkroom enlarger comes from the second-hand market, which is a transaction rather than a listing. There is no priced line for a white LED module, a constant-current driver, a heatsink, opal acrylic, aluminium sheet or a low-voltage supply. Every cost statement here is a band, not a figure.

Two dated numbers do bear on the decision and both are worth knowing before you start.

A replacement enlarger lamp is £11.40 to £11.80. The planner’s record names the two families a donor turns out to carry: an A1/231, 12 V 100 W, fed from the enlarger’s own transformer and named by the retailer as the usual fitting in colour and variable-contrast heads, and a P3/4, 240 V 150 W screw lamp for condenser heads. Which of those two you find inside the donor decides which of the three mains routes below applies, and it costs nothing to look before you buy.

A commercial enlarger timer with its own switched mains outlet is £112.20 to £429. That is the plug-in route, and it is priced here because it is the alternative to this whole build: leave the donor’s lamphouse alone and buy a certified appliance that owns the mains. The switching capacities come from the makers — 500 W for one, up to 750 W for another with the note that most enlargers draw 100 to 250 W, and “any enlarger up to 5 amps” for the third.

The honest comparison. If your donor is free or nearly free, the LED head is much cheaper than a new enlarger and roughly comparable with a bought timer plus a tungsten lamp. If your donor costs real money, the build is not the economy — the measurement is, because a bought head arrives with no certificate either.

Estimated consumables cost

One run of this page means building the head and commissioning it, and the consumption is entirely in the commissioning. The chamber, the emitters, the drivers, the supply, the heatsink and the tools are capital.

Consumed	This session	Sourced price	Cost this session
Photographic paper, variable-contrast RC, 8 × 10 in — leak check, illuminance checks, warm-up series, two uniformity maps and the first print	14 sheets	£33.50 for 25 (£1.34 a sheet) to £96.18 for 100 (£0.96), dated 5 September 2026	£13.44 to £18.76
Paper developer, working strength	1 L of 1+9, that is 100 mL of concentrate	£10.52–£20.03 per 500 mL to 1 L of concentrate	£1.05 to £4.01
Stop bath, working strength	1 L of 1+19, that is 50 mL of concentrate	£10.66–£12.18 per 500 mL of concentrate	£1.07 to £1.22
Fixer, working strength	1 L of 1+4, that is 200 mL of concentrate	£21.05–£25.98 per 1 L of concentrate	£4.21 to £5.20
Matt white paint, chamber interior	about 150 mL	None. The planner carries no line for it	—
Matt black paint or flocking, exterior surfaces	about 100 mL	None	—
Thermal interface pads	1 to 6	None. The planner carries no line for them	—
Solder and flux, discrete route	under 2 m	None	—
Felt or foam seal, adhesive, cable tie	as needed	None	—
Wash water	about 15 L	Not priced by the course	—

The priced rows give £19.77 to £29.19 for one run, and that is a floor rather than a total. Six of the ten rows carry no dated price and none of them is free. The paper figure also assumes the commissioning works first time, which it will not: budget twenty-five sheets, and cut the uniformity sheets from 8 × 10 rather than buying 10 × 12, because the map wants position rather than area.

Equipment is excluded on purpose. The enlarger, the multimeter, the densitometer, the photodiode head, the trays and the tongs are not consumed by this session, and a page that quietly counted them would corrupt the only figure the consumables calculator has.

Waste streams

The build produces workshop waste: aluminium and ply offcuts, swarf, packaging, and a little solder and flux residue on the discrete route. None of it is photographic.

The removed lamp assembly is electrical waste, and it is the one stream this page creates that is not ordinary. WM3’s list of waste puts household and household-type electrical equipment in chapter 20: 20 01 35* where the equipment contains hazardous components and 20 01 36 where it does not, with chapter 20 taking precedence over chapter 16 for domestic-type items. A tungsten lamp holder and its cord contain no battery and no mercury and fall in the second. HSE’s instruction for equipment taken out of use is that it is disposed of to prevent its further use — which is the same route the course’s disposal policy sends every other stream down, and local regulation governs it.

Commissioning produces the ordinary print streams: spent developer and stop to general chemical waste, spent fixer and the first wash to silver-bearing waste for recovery, as Part XII teaches. Local regulation governs disposal and this page gives no jurisdiction-specific instruction.

Alternative route

This is the most expensive and least accessible build in the course, and there are four honest ways out of it.

Buy a commercial LED head, or keep the tungsten one. Both are legitimate and neither exempts you from a single test in Testing and calibration. A bought head arrives with a manufacturer’s name on it and no measurement of your uniformity at your magnifications. If you keep the donor’s own certified lamphouse, you keep a working enlarger, you give up the drift-free and dust-tolerant behaviour this build is for, and you switch it by the plug-in route below.

Switch a mains lamphouse with a bought, certified enlarger timer. This is the plug-in route the course specifies wherever a mains appliance has to be switched: a self-contained certified appliance with its own switched outlet, into which the enlarger’s plug goes. No fixed wiring, no enclosure opened, nothing built. Dated at £112.20 to £429 on 5 September 2026. The fixed-wiring alternative — a switched outlet installed in the darkroom wall — is work for a competent person and this course does not describe it; Kodak’s own darkroom guide says the same thing in the same words, which are to have a licensed electrician install or inspect the wiring. Whether the course’s own timer could ever switch mains was settled by product survey on 5 September 2026, and the answer is no: eleven candidates were tested against five criteria and the sealed, certified, low-voltage-triggered appliance switch the design wanted is not sold to a hobbyist in Britain. Part XVII therefore builds a single low-voltage output and reports that survey rather than recommending a class of product. Which is why this route is a bought timer and not a built one — and why a low-voltage head, switched on the low-voltage side, asks the question of nobody.

Print by contact only. The complete alternative, and the one this part is built to make real: the contact printing frame needs no enlarger, no column, no alignment and no head. What you give up is enlargement, which is a real loss and not a small one. What you keep is every other thing this part teaches, and a great deal of photography’s best work is contact-printed.

No workshop, no bench, no space for a donor enlarger. A full-size enlarger is a metre of column and a baseboard that has to live somewhere, and in a shared bathroom it cannot. Take the contact route and revisit this page if the space changes. If you have access to a shared or hired darkroom with an enlarger in it, the commissioning page is the one to take with you: it works on any enlarger, and a machine nobody has measured is the normal state of a shared darkroom.

Build

Stage 0 — Write the specification, and do the one calculation that decides the head (30 minutes)

Four numbers, on paper, before anything is bought.

1. The largest negative the head must illuminate evenly. This sets the exit port.
2. The largest magnification you will print at. This sets the exposure through (1 + m)².
3. The working aperture. Two stops down from your lens’s maximum, from the optics page — Rodenstock’s own recommendation for a six-element lens.
4. The exposure time you want at that worst case. ILFORD’s answer is the right one: about ten seconds, because shorter times are hard to time accurately and longer ones are tedious.

Then the relation. Combining the illuminance formula from the optics page with the definition of a Lambertian diffuser’s luminance — exitance divided by π — gives a result in which π cancels and only measurable quantities are left:

E = Φ T ÷ [4 A N² (1 + m)²]

Illuminance at the easel, from the head's own numbers

E is the illuminance on the paper in lux, Φ the luminous flux actually leaving the exit port in lumens, T the transmittance of the negative at the tone you are printing for, A the area of the exit port in square metres, N the marked f-number and m the magnification.

Three things fall straight out of it, and they are the whole design.

• The exit port should be as small as the largest negative allows. A is in the denominator: halving the port’s area doubles the illuminance for the same flux, because what the lens sees is the diffuser’s luminance, not its total output. A generously oversized port is light thrown away.
• The head must cover a range in Φ that you can compute now. Work out N²(1 + m)² for your easiest case and your worst one. At f/5.6 and 2× it is 31.4 × 9 = 283; at f/8 and 4× it is 64 × 25 = 1600. That is a factor of 5.7 between them — and it is handled by time, not by current, for the reason in stage 4.
• The chamber’s efficiency is the term you cannot look up. Φ is not the emitter’s rated flux: it is what survives several reflections off the chamber walls, and a matt white chamber loses a large fraction of it. Assume something between a third and a half, note the assumption in the notebook, and measure it at T2.

🧮Worked, so the shape of the answer is visible

Take a 6 × 6 head: exit port 70 × 70 mm, so A = 4.9 × 10⁻³ m². Take one emitter in a middling flux group — Cree publish their XP-E2 in groups measured at 350 mA, and group R3 spans 122 to 130 lm — and assume 40 per cent of it leaves the port, so Φ ≈ 50 lm. Print a mid-tone of the negative, T ≈ 0.2, at f/8 and 4×.

E = 50 × 0.2 ÷ (4 × 4.9 × 10⁻³ × 1600) = 10 ÷ 31.4 = 0.32 lux.

Is that enough? This page does not know, and neither does any source it has read, for the reason in the Parts callout. What the arithmetic is for is ratios: run it again with four emitters and it reads 1.3 lux, and if your first test exposure at one emitter comes out at 40 seconds instead of 10, four emitters is exactly the answer. The number that settles it is the one you measure at T5, and everything before that is sizing.

Stage 1 — Survey the donor, and settle the mains boundary before anything else (30 minutes)

Unplug the enlarger at the socket. Leave it unplugged. Then look at how its lamp is held and its cord is routed, and place yourself on one of three routes.

Which mains route the donor puts you on, and what each one permits

1. 1Route A — the lamphouse lifts off completeMany enlargers are built so the whole lamphouse, with its lamp, holder and cord, unclips or unbolts from the negative stage as one assembly. Nothing is opened; the mains leaves the machine as a unit and goes to WEEE disposal. The new head occupies the mount. THIS IS THE DEFAULT AND THE ONLY ROUTE THIS PAGE DESCRIBES IN DETAIL.
2. 2Route B — the lamp assembly is separate but fixed insideThe lamp holder is on its own bracket and the cord passes through a grommet to a connector or a terminal block. Permitted only where the whole assembly, cord included, can be freed by undoing fixings and unplugging an inline connector, with nothing cut, stripped or unscrewed from a terminal. It stops the instant a conductor would have to be disturbed.
3. 3Route C — the mains is integral to the machineA transformer bolted into the chassis, a cord anchored and terminated inside, a switch let into the casting, a lamp holder wired into a block. This is where the course stops. Either a competent person removes the mains parts, or the lamphouse stays exactly as it is and you take the plug-in route: keep the certified lamphouse, switch it with a bought certified timer, and do not build this head.
4. 4Whichever route: nothing is re-energisedNo mains conductor is rewired, re-terminated, extended, insulated, taped or switched on again at any point in this build, on any route. The finished head runs from a separate certified low-voltage supply that has never been opened.

The question is never 'am I careful enough'. It is 'can the mains parts leave this machine complete, without a conductor being disturbed'. If the answer is no, the answer is no.

⚠What the guidance actually says, and what this course reads into it

The research plan for this page asked a specific question: does any authority say whether a non-electrician may remove the lamp assembly and cut off the flexible cord of an unplugged appliance? The answer is that none of the sources this course has read addresses it, and the course will not pretend otherwise. What HSE do publish, and what the routes above are built out of, is this.

• Competent means “having suitable training, skill, and knowledge for the task to prevent injury to themselves and others”. It is a description of a person, not a level of care.
• Equipment taken out of use is “either repaired by someone competent or disposed of to prevent its further use”. Those are HSE’s own words, and the second half of the sentence is the one this build relies on.
• “More complicated tasks, such as equipment repairs or alterations to an electrical installation, should only be carried out by people with knowledge of the risks and the precautions needed.”
• Equipment is switched off and unplugged before cleaning or making adjustments, and “even simple tasks such as wiring a plug can lead to danger”.

The course’s reading, stated as a reading. Undoing the fixings of an unplugged appliance and lifting out a complete assembly that is then discarded is disposal to prevent further use, not repair and not alteration: nothing is re-terminated and nothing is put back into service. That is route A, and it is what the course teaches. Cutting a cord is not part of it — a cut cord is a conductor that has been disturbed, and there is no reason to cut one, because a complete assembly can go to WEEE with its plug still on the end of it. Where the mains cannot come out complete, the reading runs out and route C applies.

If that boundary feels conservative, notice where it puts the burden: on the donor you choose, not on the care you take. Choosing a route-A enlarger is a decision made in somebody’s garage with a torch, for free, and it removes the question entirely.

Then measure the mount. Calipers on the lamphouse register: the diameter or the rectangle it seats on, the depth of the spigot, the position of any locating pin or screw, and the clearance above the negative stage. Draw it. This is the one part of the head that cannot be bought, redesigned or substituted, and everything else is built around it.

Stage 2 — The mixing chamber (50 minutes)

The chamber’s job is to arrive at the diffuser with the same luminance everywhere. Three variables do it, and they trade against each other.

Depth against exit-port size. The rule the contact printer’s light box established holds here: the emitter-to-diffuser distance should be at least as large as the emitter spacing and preferably twice it, so that the pattern of individual sources is washed out before the light arrives. In a chamber with one emitter the equivalent rule is that the emitter must not be visible as a bright patch through the diffuser from any angle the lens can see. The cheap test is your eye, at the negative stage, with the chamber running at low current: if you can see where the emitter is, the lens can too.

Interior finish. The chamber integrates by reflecting light many times, so its wall reflectance appears raised to a high power in the efficiency: a chamber whose walls reflect 0.85 loses far more than half again as much as one at 0.95 after five bounces. Three finishes are used in practice, and the course can rank them by what it can and cannot source:

Finish	What the course can say	What it cannot
Matt white emulsion or acrylic paint	The obvious and cheapest option, and what the contact printer’s box uses	No reflectance figure for any specific paint was verified while this page was written. Buy on the maker’s stated reflectance if one is given, and compare candidates by measurement rather than by the word “brilliant” on the tin
Barium sulfate coating	Used as a high-reflectance diffuse standard in optical work, and the reason the course’s densitometer work refers to a white reference tile at all	No coating product, price or reflectance figure was verified for this build, and the course names none
PTFE sheet	Diffusely reflecting and machinable	Same: nothing verified, no price, no figure

The honest instruction is therefore comparative rather than absolute. Paint the chamber matt white, measure the illuminance at the negative stage at T2, and if you later re-line it, measure again. A 20 per cent improvement in efficiency is a 20 per cent shorter exposure, and it is measurable in an evening.

What to do with the donor’s condensers. Three choices, with different consequences:

• Remove them. The default, and the simplest. The chamber’s opal exit becomes the source and the head is a clean diffusion head. Store the condensers; they are part of the machine’s value.
• Keep them as a mechanical element only. Some enlargers use the lower condenser’s mount as the negative stage’s upper reference or as a dust shield. Keeping the glass in place beneath a diffuse source does not turn the head back into a condenser head — a condenser works by gathering light from a compact source, and there is no compact source any more — but it does add two glass surfaces to reflect between, and it absorbs a little. No optics reference or manufacturer statement covering this specific case was found in this course’s corpus, so the claim above is stated as reasoning from what a condenser does, and the measurement that settles it is one you can make: read the illuminance at the negative stage with the glass in and out.
• Replace them with the diffuser. Where the condenser mount is the right size and position for the exit port, this is neat and saves fabricating a bracket.

The head in section: emitter, heatsink outside, chamber, diffuser and the sealed joint to the donor

1. 1Heatsink, fins outside the head — the waste heat leaves the enclosure rather than warming the negative by convection
2. 2Emitter on a thermal pad, on the chamber roof — the datasheet stops at the solder point; everything below that is your build
3. 3Chamber, matt white, depth d against exit width w — if you can see where the emitter is through the diffuser, so can the lens
4. 4Opal diffuser on a ledge, with a second ledge above it — two thin sheets spaced apart beat one thick one, and cost more light
5. 5Baffled vent, black-lined — thermal, not for a gas; no straight line from inside to outside
6. 6Cable gland, internal loop, strain relief — a straight hole is a light leak with a wire in it
7. 7Felt seal at the lamphouse joint, compressed — the commonest leak on a converted head
8. 8Matt black everywhere below the diffuser and outside the head — white inside the diffuser, black outside it
9. 9Handle and positive location — the negative stage is directly underneath

Drawn readable rather than to scale. The two proportions worth settling on paper are d against w, and the spigot against the donor's register; everything else is checked by measurement in Testing and calibration.

Stage 3 — Thermal design, and the calculation you can actually do (40 minutes)

An LED’s flux falls as its junction warms. That is not a reliability question here, it is a photometric one: a head that warms through a session prints a different exposure at the end of it than at the beginning. Part XIV settled the principle — a heatsink is a photometric component — and this stage applies it at twenty times the power.

The calculation the datasheet lets you do, and only this one:

ΔT = P × R_{θ, junction to solder point}

Junction rise above the solder point

P is the electrical power the emitter dissipates, which for design purposes is all of it: current times forward voltage. Cree’s XP-E2 publishes what you need. White: thermal resistance junction to solder point 5.8 °C/W, forward voltage 2.99 V typical at 700 mA and 85 °C, maximum DC forward current 1500 mA, maximum junction temperature 150 °C. Blue is 5.7 °C/W and green 9.0 °C/W — green is much the worst of the three, which is a real constraint on the dual-channel head and a reason to run the green channel at a lower current and compensate with time.

Worked, for one white emitter at 700 mA: P = 0.700 × 2.99 = 2.09 W, and ΔT = 2.09 × 5.8 = 12 °C above the solder point. Six of them is 12.6 W into the heatsink.

✕What that calculation does not cover, which is most of the thermal path

It covers one link: junction to solder point, inside a part somebody else made and measured. Everything after the solder point is your build and none of it is on any datasheet — the board to the pad, the pad to the heatsink, the heatsink to the air, and the air to the room. If the solder point sits at 70 °C because your heatsink is undersized, the junction is at 82 °C and the flux has fallen accordingly.

So the calculation sizes the design and the thermometer settles it. T3 measures the drift at the easel, which is the quantity that actually matters, and it does not care which link was at fault.

Three construction rules follow, and they are the ones that make the difference:

1. The fins go outside the chamber. Heat dumped into a sealed white box comes out through the exit port, warms the negative by convection and undoes the one advantage an LED head has over a tungsten one.
2. The thermal path is mechanical. A pad squeezed by two screws is a thermal path; a pad resting under its own weight is not. Flat, clean, and tight.
3. Vents are baffled, not open. Convection needs a path in at the bottom and out at the top; light must not have one. Two sets of offset slots with a black-lined plenum between them pass air and stop light, which is the same construction the UVA unit uses.

Stage 4 — The electrical side, entirely at extra-low voltage (40 minutes)

Every conductor in the head is at 12 or 24 V from a certified, enclosed supply with a moulded lead, on HSE’s own reasoning: limit the supply voltage to the lowest that will do the job.

Wiring: certified supply, per-channel constant current, low-side switch, focus mode

1. 1Certified enclosed supply, moulded lead, never opened — the only mains in the build, and it arrived finished
2. 2Fuse on the low-voltage side, sized to the head — protects the head, not the supply
3. 3Constant-current driver, one per channel, set once — brightness is a current you set and then measure, not a knob you turn
4. 4Logic-level MOSFET low-side switch, with a gate pull-down — off while the controller is unpowered or resetting
5. 5Second channel, wired and blanked — the hour that saves opening the head for Part XVIII
6. 6Focus mode: a separate low fixed current — not a dimmed printing current
7. 7Polarised connector to the timer, low voltage only — it cannot be plugged in backwards in the dark

Fixed current, switched cleanly on and off. The exposure variable is time, and stage 4 says why that is not a preference.

Constant current per channel, set once with a trimmer and then measured with the meter at the fuse, because the flux is a function of the current and nothing else you can control. Low-side switching through a logic-level MOSFET with a gate pull-down, so the channel is off while the controller is unpowered or resetting — the exact circuit Part XIV builds and this page does not repeat.

The focus mode is a second fixed current, not a dimmed printing current. Focusing wants perhaps a twentieth of the printing output and it wants it continuously; give it its own low-current driver and its own switch, and the printing channel never runs at anything but its calibrated current.

!Why the head is timed rather than dimmed, and why this is not a preference

Pulse-width dimming is the obvious way to control an LED and it is forbidden here for a photographic reason, established with measurements a century old. Part XIV sets out the evidence: Sheppard and Mees compared a sector wheel at 1520 revolutions per minute with one at 9.5 and found the fast one worse, concluding that a wheel below 100 revolutions per minute makes the intermittency effect negligible. A PWM signal at hundreds of hertz to tens of kilohertz is thousands of times faster than the regime they were prepared to call safe.

The critical frequency above which a modern paper emulsion stops resolving flashes is not published in any source this course has read, and the course refuses to assume one. So the rule is Part XIV’s, applied unchanged: fixed direct current for the whole exposure, switched cleanly on and cleanly off, and the time is the variable.

That rule is also why stage 0’s factor of 5.7 between the easiest and hardest exposure is handled by time. A head that has to cover 283 to 1600 in N²(1 + m)² covers it with a 2.8-second exposure and a 16-second one, and its current never moves.

Stage 5 — Mechanical and light-tight fit (40 minutes)

The joint to the donor is the commonest leak on a converted head and it is worth over-building. A compressed felt or closed-cell strip on both mating faces, a spigot that enters rather than merely rests, and a locating feature so the head goes back in the same rotation every time. If the donor’s own lamphouse had a light trap moulded into the casting, copy it.

The cable exit is the second commonest. A cable through a plain hole is a light leak with a wire in it: either a proper gland, or a hole in a small black-lined plenum with the cable entering one side and leaving the other with a bend in between. Inside, a loop of slack and a cable tie anchored to the wall, so a tug on the lead reaches the tie and not a solder joint.

Every surface below the diffuser is matt black, and so is the outside of the head. White inside the diffuser and black outside it — the same division the densitometer’s optical head uses, for the same reason: you want light that has come through the diffuser and nothing that has come off a wall.

And design for the drop. The negative stage sits directly under the head. Fit a handle on top, make the location positive enough that a partly seated head is obviously wrong, and get into the habit of removing the head with the carriage locked and the stage empty.

Stage 6 — The label and the log (15 minutes)

On the head, in permanent marker or on a bonded label: the channel currents, the supply voltage, the warm-up rule from T3, the date, and the exposure that produced a mid-grey at a stated aperture, magnification, paper and developer. An exposure without those five qualifiers is not a number, it is an anecdote.

Open an hours log at the same time. LEDs age; the only way you will ever know by how much is a dated count of hours beside a dated exposure.

Testing and calibration

Five tests, in this order, and no negative goes in the carrier until T1 has passed. Each one assumes the one before it.

Commissioning order, and what each test lets you do next

1. 1T1 — Light-leak check, lens cappedEstablishes that the head keeps its light inside. Until it passes, every other test is measuring the head plus an unknown, and any fog on any sheet has two possible causes.
2. 2T2 — Illuminance at the negative stage and at the easelEstablishes what the head actually delivers, and lets you measure the chamber efficiency you assumed at stage 0. It is also the reading everything later is compared against.
3. 3T3 — Warm-up drift, measured at the easelEstablishes the warm-up rule. Until it passes you do not know whether the exposure you find at the start of a session is the exposure you have at the end.
4. 4T4 — Uniformity at two magnificationsEstablishes the field. It needs T3, because a map made during warm-up has a gradient in time as well as in space.
5. 5T5 — Repeatability over ten switch cycles, then the first printEstablishes that the same command gives the same exposure. Only then is a number worth writing on the label.

Each is a precondition for the next, which is why a head half-commissioned is a head that is not commissioned.

T1 — The light-leak check, with the lens capped

The head is supposed to put light through the negative and nowhere else.

1. Put the lens cap on, or a piece of black card under the lens board. No negative in the carrier.
2. Put a sheet of paper on the easel, emulsion up, with an opaque card covering half of it.
3. Dark-adapt for at least five minutes with everything off — Kodak’s own room test uses the same five minutes for the same reason.
4. Switch the head on at printing current and look at the head, not at the paper: at the joint to the donor, the vents, the cable exit, every fastening, and the bellows.
5. Leave the head running for four times your longest printing exposure, then switch off, remove the card, and process the sheet with the session’s prints.

Pass: no visible line between the covered and uncovered halves, and no glow visible at any seam to a dark-adapted eye. A visible line means light reached the paper without going through the lens — from the head, from the bellows, or from the room, and the room’s leak test tells you which of those you have already excluded.

If you find a leak: black tape from the inside, more felt on the seam, a deeper plenum at the vent or the cable exit, a bend in the cable’s path. Then re-test. A head that leaks at one seam usually leaks at three.

T2 — Illuminance at the negative stage and at the easel

With the photodiode head or a lux meter, and relative readings are enough.

• At the negative stage, with the carrier out: read the centre and the four corners of the exit port. This is the chamber’s own uniformity, before the lens has touched it, and it is the number to improve if T4 later shows a symmetric falloff.
• At the easel, at your working aperture and magnification, with no negative: this is the working illuminance, and it is what you compare against stage 0’s estimate.

Then compute the chamber efficiency you actually got. Rearranging stage 0’s relation, Φ = 4 A N²(1 + m)² E with T = 1 for an empty carrier. Divide by the emitters’ rated flux and you have the fraction that survives the chamber, measured rather than assumed. Write it down: it is the number that tells you whether a better interior finish is worth an evening.

T3 — Warm-up drift, measured at the easel

Switch the head on cold, and read the easel illuminance every minute for thirty minutes without switching off. Plot it.

The shape a warm-up curve takes, and what the rule is read off

• Output falling as the junction warms

Show the numbers behind this plot

A plot of relative illuminance at the easel against time from switch-on in minutes, running from zero to thirty minutes on the horizontal axis and from about 0.90 to 1.01 on the vertical. A single curve starts at 1.00 at switch-on, falls steeply over the first four minutes to about 0.955, continues falling more gently to about 0.935 at ten minutes, and then flattens, reaching about 0.928 at fifteen minutes and holding essentially level at 0.926 out to thirty minutes. A horizontal guide is drawn at 0.936, one per cent above the settled value, and a vertical guide is drawn at nine minutes where the curve crosses it, marked as the warm-up rule. A note records that the total fall from cold to settled is about seven per cent, which is a tenth of a stop, and that the first exposure of an unwarmed session would therefore be about a tenth of a stop lighter than the rest. The shape is exponential decay towards a thermal equilibrium and it is drawn to teach that shape rather than measured from any head.

Series	Minutes from switch-on	Relative illuminance at the easel
Output falling as the junction warms	0.00	1.00
Output falling as the junction warms	1.00	0.98
Output falling as the junction warms	2.00	0.97
Output falling as the junction warms	3.00	0.96
Output falling as the junction warms	4.00	0.95
Output falling as the junction warms	6.00	0.94
Output falling as the junction warms	8.00	0.94
Output falling as the junction warms	10.00	0.94
Output falling as the junction warms	12.00	0.93
Output falling as the junction warms	15.00	0.93
Output falling as the junction warms	20.00	0.93
Output falling as the junction warms	25.00	0.93
Output falling as the junction warms	30.00	0.93

Drawn to show the shape, not measured from a head. Your curve's depth and time constant are properties of your heatsink and your room, and the number that goes on the label is yours. The curve is drawn to show the shape, not measured from a real material. Your own materials will differ, and measuring them is what the sensitometry part of the course is for.

The warm-up rule is the time at which the curve comes within one per cent of its settled value, which is the same criterion the sensitometer’s lamp uses. Write it on the label. Then work to it: switch the head on when you switch the safelight on, and leave it on between prints rather than switching it off, because the drift is thermal and a switched-off head starts cooling immediately.

Record the depth of the drift as well as the time, in stops: log₂ of the ratio between cold and settled. A seven per cent fall is 0.10 of a stop, which is at the edge of visible on a print and squarely inside the range that will make a test strip disagree with the print that follows it.

T4 — Uniformity at the easel, at two magnifications

The full treatment belongs to the commissioning experiment, which measures the enlarger as a system. What the head owes at this stage is narrower, and it is answered by two sheets.

Expose a whole sheet to a mid-grey, with no negative, at your working aperture, at the smallest magnification you print at and again at the largest. Process both with the session, dry them, and read the centre and eight surrounding positions.

The two sheets separate the head from the lens, and this is the point of doing it at two magnifications:

• A pattern that is the same at both magnifications, expressed as a fraction of the sheet, is the lens and the geometry. It is what the optics page’s cos⁴ reference describes and it is not the head’s fault.
• A pattern that changes shape — a stripe, a bright patch, an off-centre bias that moves — is the chamber. Go back to stage 2: more depth, a second diffuser, or an emitter that is not where you thought it was.

T5 — Repeatability over ten switch cycles

Ten identical exposures, measured at the easel with the photodiode head, each preceded by a full switch-off and switch-on so that the test includes everything the operating cycle includes. Take the mean and the standard deviation, and then convert the standard deviation into stops, because that is the form that says whether it matters:

stops = log₂(1 + s)

Repeatability in the units that decide

where s is the standard deviation as a fraction of the mean. Two per cent is 0.029 of a stop, which is well under the smallest adjustment any printer makes; five per cent is 0.07 of a stop and will show as inconsistency between a test strip and the print.

Then, and only then, the first print. Establish the exposure that gives a mid-grey at your stated aperture and magnification, write it on the label with all five qualifiers, and file the head’s record beside the enlarger certificate the next page produces.

Troubleshooting

Symptom	Likely cause	Test that distinguishes it
Exposures far longer than stage 0 predicted	Chamber efficiency below the assumed third to a half, or an exit port larger than it needs to be	T2, and compute the efficiency you actually got
Exposure creeping shorter through the first ten minutes of a session	No warm-up rule, or the rule not being kept	T3, then switch the head on with the safelight and leave it on
Exposure creeping longer over months	Emitter ageing, or a chamber interior that has yellowed	The hours log, then re-run T2 and compare with the commissioning figure
A stripe or a bright patch on the uniformity sheet, same at both magnifications	Not the head. Lens falloff or a mechanical vignette	Stop down two stops and re-map; optics improve, metal does not
A stripe or a bright patch that changes with magnification	The chamber. Too shallow, or the emitter visible through the diffuser	Look at the exit port from the negative stage at low current
Fog on the first commissioning sheet with the lens capped	A leak at the joint, a vent, the cable exit or the bellows	T1 again, seam by seam, watching the head rather than the paper
The head is hot and the drift will not settle	Fins inside the chamber, a thermal pad not clamped, or an undersized heatsink	Infrared thermometer on the heatsink base and on the chamber roof; a large difference means the pad
The negative goes soft partway through long exposures	Film popping. Heat is reaching the negative by convection	Where do the fins vent? Then a glass carrier, and pre-warming
Two channels, and the green one dims faster than the blue	Green’s thermal resistance is 9.0 °C/W against blue’s 5.7	Run green at a lower current and give it more time
The head will not seat the same way twice	No positive location	Fit one; the whole calibration assumes the head goes back where it was

Questions

1. Your exit port is 60 × 60 mm and you are printing 6 × 6 negatives. A friend suggests enlarging the port to 90 × 90 mm “so the corners are definitely covered”. Using the illuminance relation, say what that costs you in exposure time, and what it buys.
2. A single white emitter at 700 mA has a junction 12 °C above its solder point. Your infrared thermometer reads the heatsink base at 58 °C in a 22 °C room. State the junction temperature you can defend, state the one you cannot, and say what measurement would close the gap.
3. Explain, to somebody who thinks the low-voltage rule is a general caution, why converting the head to extra-low voltage does not solve the mains-switching problem so much as abolish it — and why that matters more than usual, given that Part XVII’s survey found no certified product with which to solve it.
4. You have a donor whose lamp holder is on a bracket, whose cord runs through a grommet to a terminal block screwed to the casting. Which route are you on, what may you do, and what are the two acceptable ways forward?
5. Your warm-up curve falls 12 per cent from cold to settled and takes 14 minutes. Express the drift in stops, and say what would happen to a printing session in which you made a test strip on a cold head and the final print twenty minutes later.
6. Ten exposures give a mean easel reading of 4.10 arbitrary units with a standard deviation of 0.21. Convert that to stops and say whether you would print with this head.
7. A uniformity sheet at 2× shows the corners 0.05 in density lighter than the centre; the same sheet at 5× shows 0.05 as well. Is that the chamber or the lens, and what single further test would confirm it?
8. Your head passes every test and your prints are still a grade flatter than they were on the donor’s original condenser lamphouse. Nothing is broken. Explain, and say what you would change and where.

Further experiments

Measure the chamber efficiency against its finish. Read the easel illuminance, re-line the chamber with a different white, and read it again at the same current. The ratio is the only reflectance comparison this course can honestly offer, because it publishes no reflectance figure for any paint — and it is the one that applies to your chamber rather than to a coupon in a laboratory.

Find your own PWM threshold. Part XIV forbids dimming on evidence from 1907 and admits the critical frequency for a modern emulsion is unpublished. You now own a head, a timer and a paper. Expose a series of identical mid-grey sheets at the same average power, one at fixed current, the others at 100 Hz, 1 kHz and 10 kHz PWM, and read them. If they match, you have measured something the literature this course reads does not tell you. If they do not, you have confirmed a rule you were following anyway.

Put a number on the condenser question. If your donor’s condensers can be refitted, print the same negative to the same shadow density with them in and out, and read the highlight densities. The difference is your own answer to the condenser-against-diffusion question, on your own film, and Part XV explicitly asks for it because the course could not source the size of the effect for a named film and a named enlarger.

Log the ageing. Re-run T2 every fifty hours on the counter and plot the illuminance against accumulated hours. In two years you will have a decay curve for your own emitter that no manufacturer in this course’s corpus publishes for a photographic duty cycle.

Build the second channel and characterise it before Part XVIII asks. With blue and green fitted, expose a step wedge at several blue-to-green ratios and read the results. You will arrive at the variable-contrast part with your own data rather than with somebody’s filter numbers, which is exactly the position the course tries to put you in everywhere else.

Recap

The head is a mixing chamber, and it is sized by one relation: E = ΦT ÷ [4AN²(1 + m)²]. That formula says the exit port should be no bigger than the largest negative, that the range of exposures the head must cover can be computed before anything is bought, and that the chamber efficiency is the one term you cannot look up and must measure. The emitter is chosen on five datasheet figures, driven at a fixed current, and heatsinked with its fins outside the chamber, because a head that warms the negative by convection has given back the advantage an LED had over a tungsten lamp.

Dimming is forbidden and the reason is photographic rather than electrical: Sheppard and Mees found faster chopping worse, and the critical frequency for a modern paper is unpublished, so the head runs at a fixed current and the time is the variable. Focus mode is a second fixed current, not a dimmed printing current.

The mains boundary is the reason this build is reasonable at all, and it is settled by the donor rather than by care. Route A — a lamphouse that lifts off complete — is the default and is disposal to prevent further use in HSE’s own words. Route B stops the moment a conductor would have to be disturbed. Route C hands the job to a competent person, or keeps the certified lamphouse and switches it with a bought certified timer, which is the plug-in route this course specifies wherever a mains appliance has to be switched. Nothing on any route is rewired, re-terminated or re-energised, and the finished head asks the mains-switching question of nobody, because it is switched on the low-voltage side.

And nothing is believed until it is measured: leak, illuminance, warm-up, uniformity, repeatability, in that order, with the first print at the end rather than the beginning.

Check your understanding

Question 1. Your head has one emitter and its exit port is 80 x 80 mm for a 6 x 6 negative. Reducing the port to 60 x 60 mm, still covering the negative, does what to the exposure time?

• Nothing, because the emitter still produces the same flux
• Cuts it to about 56 per cent of what it was, because the lens sees the diffuser luminance, which is flux divided by the emitting area, and the area has fallen by that factor
• Doubles it, because a smaller port passes less light
• It cannot be predicted without knowing the chamber depth

Show the answer and why

Answer: Cuts it to about 56 per cent of what it was, because the lens sees the diffuser luminance, which is flux divided by the emitting area, and the area has fallen by that factor

E = Phi x T / (4 A N squared (1 + m) squared), and A is the exit-port area. Going from 6400 to 3600 square millimetres is a factor of 0.5625 in area, so the illuminance rises by 1/0.5625 = 1.78 and the exposure falls to 0.56 of what it was, which is five-sixths of a stop for nothing. The reason is that an imaging system responds to the luminance of what it is imaging rather than to total flux, so concentrating the same output into a smaller emitting area is free brightness. What you must not do is make the port smaller than the negative, because then you are vignetting the negative itself.

Question 2. Why does this build put the heatsink fins outside the chamber rather than inside it, where there is room?

• Because aluminium reflects badly and would spoil the chamber uniformity
• Because heat dumped into a sealed chamber leaves through the exit port and warms the negative by convection, which is the fault an LED head was supposed to avoid, and because a hot chamber means a hot junction and a drifting output
• Because the fins would collect dust
• Because the datasheet requires it

Show the answer and why

Answer: Because heat dumped into a sealed chamber leaves through the exit port and warms the negative by convection, which is the fault an LED head was supposed to avoid, and because a hot chamber means a hot junction and a drifting output

Two failures for the price of one. Convected heat through the exit port is the mechanism of film popping arriving by a different route than the infrared it was supposed to replace; and a chamber warmer than the room raises the heatsink temperature, which raises the junction temperature, which lowers the flux and lengthens the warm-up. Reflectance and dust are real considerations but neither decides this. And no datasheet says where to put a heatsink: the datasheet stops at the solder point, which is exactly the point this page makes about what the thermal calculation does and does not cover.

Question 3. A donor enlarger has its mains cord anchored through the chassis and terminated on a block screwed to the casting, with the lamp holder wired to that block. What does this page permit?

• Cutting the cord close to the block with the appliance unplugged, since an unplugged appliance is dead
• Nothing at mains potential: this is route C, and the two acceptable ways forward are a competent person removing the mains parts, or keeping the certified lamphouse and switching it with a bought certified timer
• Unscrewing the terminal block and taping the ends, provided the appliance stays unplugged
• Removing the lamp holder only, leaving the cord and block in place

Show the answer and why

Answer: Nothing at mains potential: this is route C, and the two acceptable ways forward are a competent person removing the mains parts, or keeping the certified lamphouse and switching it with a bought certified timer

The test is not whether the appliance is dead at the moment you work on it; it is whether a conductor has to be disturbed. Undoing a terminal, cutting a cord and taping ends are all alterations to equipment, and HSE put those with people who have knowledge of the risks and the precautions needed. The distinction the page draws is between removing a complete assembly for disposal, where nothing is re-terminated and nothing goes back into service, and rework, where something does. Option four fails on the same test the moment you reach the holder terminals, and it also leaves live parts in a machine you intend to keep using.

Question 4. You want to halve the exposure for a large print by running the emitter at twice the current for the same time, rather than doubling the time. What is wrong with that as a routine method?

• Nothing; current and time are interchangeable
• The flux is not exactly proportional to current, the junction runs hotter so the warm-up rule and the drift both change, and every calibrated figure on the label was established at one current — so the exposure becomes a two-variable problem for no gain
• The MOSFET cannot switch twice the current
• It would trigger the intermittency effect

Show the answer and why

Answer: The flux is not exactly proportional to current, the junction runs hotter so the warm-up rule and the drift both change, and every calibrated figure on the label was established at one current — so the exposure becomes a two-variable problem for no gain

The rule is one current, measured, and time as the variable, and this is why. LED flux rises sub-linearly with current, and it rises less at a hot junction than a cold one, so twice the current is not twice the light and how much less depends on how long you have been running. Every number on the label - the mid-grey exposure, the warm-up rule, the drift, the repeatability - was established at one current and none of them transfers. The intermittency effect is the reason PWM is forbidden, which is a different objection to a different practice: a steady higher current is not a train of flashes.

Question 5. The uniformity sheet at 2x shows a bright band across the middle of the sheet; the sheet at 5x shows the same band, in the same place relative to the paper, and about the same width relative to the paper. Chamber or lens?

• The chamber, because a band is an emitter pattern printing through the diffuser
• The lens or the geometry, because a chamber pattern is fixed in the negative plane and would keep the same size relative to the NEGATIVE, so it would grow with the paper only if it scaled - a band holding the same relative position and width at both magnifications is behaving like a property of the projection rather than of the source
• Neither: it is uneven development
• It cannot be told apart without a densitometer

Show the answer and why

Answer: The lens or the geometry, because a chamber pattern is fixed in the negative plane and would keep the same size relative to the NEGATIVE, so it would grow with the paper only if it scaled - a band holding the same relative position and width at both magnifications is behaving like a property of the projection rather than of the source

The two magnifications are the discriminator and the reasoning is about what a pattern is fixed to. A feature belonging to the source is fixed in the negative plane and is projected, so its size relative to the sheet is the same at both magnifications - which is exactly what is described here, and is why this particular result is ambiguous and needs the third test: stop down two stops and re-map. Optical falloff improves and a mechanical vignette does not. Uneven development is a genuine confounder and it has its own test, which is to rotate the paper in the tray and see whether the pattern follows the paper or the easel: that is the break/fix page at the end of this part.

Question 6. The page tells you to build the chamber for two channels and fit one. What is the argument, and what would change it?

• Two channels are always better, so fit both now
• The chamber geometry is the expensive and irreversible decision while the second emitter is an hour of work, and Part XVIII has not yet established what variable-contrast control needs - so drilling, tapping and wiring the second position commits you to nothing while retrofitting the chamber would mean rebuilding it
• Green LEDs are unreliable and should be avoided
• The timer cannot drive two channels

Show the answer and why

Answer: The chamber geometry is the expensive and irreversible decision while the second emitter is an hour of work, and Part XVIII has not yet established what variable-contrast control needs - so drilling, tapping and wiring the second position commits you to nothing while retrofitting the chamber would mean rebuilding it

It is a decision about which mistakes are cheap. Chamber depth, exit-port size and mounting geometry are set once and are painful to revisit; an emitter on a blanked connector costs an hour. What would change the argument is Part XVIII establishing a requirement the chamber cannot meet - a third channel, a much larger mixing volume, speed-matched channels demanding a different emitter family - which is precisely why the course does not commit to the electronics now. Green is not unreliable, though its thermal resistance of 9.0 degrees C per watt against blue at 5.7 is a real design constraint and is why it is run cooler and given more time.

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