Build: Making a Room Dark and Proving It

Purpose

To turn a room you already have into one that can be made dark on demand, ventilated while it is dark, worked in without groping, and — the part that makes it a build rather than a tidy-up — proved dark against the material you are actually going to handle.

Two things separate this session from hanging a curtain. The first is that darkness is not a property of a room; it is a relation between a room and a material. Photographic paper is slow and nearly blind to red. Panchromatic film sees everything. A room that passes for one fails for the other, and the only way to know which you have is to test the room with the material in it. The second is that a blacked-out room stops exchanging air at exactly the moment you start pouring chemistry into it, so the ventilation is designed before the blackout, not bolted on afterwards.

You finish holding four things: a room that goes dark, a vent that passes air and not light, a plan of where wet and dry work happen, and a processed test strip beside its control that says in tone rather than in opinion whether the room is fit for the paper in your cupboard.

Learning objectives

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

• say what “dark enough” means for a named material, and why the answer changes between a chlorobromide paper, an orthochromatic film and a panchromatic one;
• describe the two-phase course of dark adaptation well enough to explain why a leak invisible at one minute is obvious at twenty, and why a phone screen undoes it;
• carry out a systematic leak hunt rather than a glance round, and name the six places a leak is usually found;
• black out a window, a door and a vent with materials that are removable and leave no mark;
• design a ventilation path that admits air and refuses light, and state why the room may not simply be sealed;
• lay out a wet side and a dry side for printing, and say where the safelight and the paper stock go relative to both;
• decide, with a manufacturer’s reasoning behind you, which surfaces in the room should be black and which should not;
• run a leak test with a control, read the result, and write the record that makes it repeatable;
• state honestly what your room is not cleared for.

Prerequisites

Laboratory layout and workflow, which owns the wet-and-dry division, the ventilation arithmetic, secondary containment, surfaces, water and waste. Every one of those decisions is made there and is not remade here. This page adds the one thing Part II deliberately left out: the light.

Spectral sensitivity, because the room is tested against a material and you have to know what the material can see.

The latent image made visible, whose Station 4 ran a compressed safelight test under interim conditions. This part runs it properly, on a lamp you build, three pages from now.

Safety classification

Level A. The classification rubric puts a procedure at Level A when the substances handled are at most irritant or harmful in home quantities, nothing is heated, no mains construction or modification takes place, and the waste can be collected ordinarily. All four hold. The work is measuring, cutting, taping, screwing, and one tray sequence of ordinary print chemistry at the dilutions the printing part uses.

What is not a hazard here, and why. No chemical on this page is concentrated: the developer, stop and fixer are working-strength baths at the dilutions their sheets specify, handled in trays with tongs, and the reason they earn no higher letter is the operation rather than the substance — nothing is weighed, nothing is heated, and nothing is decanted from a concentrate. There is no dust, because nothing is a powder. There is no vapour worth extraction beyond what Part II already sizes the room for. The same fixer becomes a different proposition when it is the concentrate in the bottle, and the silver-bearing waste procedure treats the spent bath as what it is.

One step is declared higher. Fitting a window panel or a ceiling-mounted lamp means working at height, and under the mixed-level rule that step carries controls that the rest of the session does not: a stable step ladder rather than a chair, both feet on it, nothing carried up that needs two hands at the top, and somebody else in the building who knows you are up there. If that cannot be arranged, the panel route in stage 2 has a floor-level alternative and you take it.

Hazards

Cutting. Foam board, corrugated plastic and blackout fabric are all cut with a blade, and the blade is the most likely injury on this page. Fresh blades — a dull one needs force and force is what slips — a steel rule, a cutting mat, and every stroke away from the hand that is holding the work. Used blades go into a rigid container, not into a bin bag.

Working at height. Declared above. It is a Level B step inside a Level A session and it is the only injury here that could put you in hospital.

Loss of air exchange. A blacked-out room is a room whose window no longer opens and whose door is now sealed. That is a hazard created by this build, not one it inherits, and it is why ventilation is stage 1 rather than stage 5. The rule the course enforces: nothing is poured in a room that has been sealed shut.

Electricity and water in one room. HSE’s guide is plain that 230 V AC can kill and that the risk is greatest in wet surroundings. Nothing on this page wires anything. What it asks is that every powered item stands outside the splash zone, that leads run at high level or behind the bench rather than across a wet floor, and that the socket feeding the room is protected by a residual-current device — a plug-in RCD adaptor needs no wiring and costs less than a box of paper. A darkroom is not a bathroom in the regulations’ sense, and this page does not claim it is; the reason to take the precaution is the water, not a special classification.

Adhesives and paints. Use tape and mechanical fixings in preference. Where a paint is used, use a water-based matt emulsion and paint with the room ventilated or the panel taken outdoors.

Print chemistry. Level A, in trays, with the controls Part II sets: nitrile gloves, eye protection, dedicated utensils, one pair of tongs per bath.

Required PPE

Nitrile gloves for the tray sequence, and for handling spent fixer. Eye protection for the tray sequence and, separately, for cutting rigid board, where the hazard is a chip rather than a splash. Closed shoes, because the floor gets wet and because a dropped blade lands point down. Nothing respiratory, and that is a statement about this page rather than a general reassurance: no powder is opened, no concentrate is decanted, and no solution is heated, so the controls here are the glove, the goggle and the tray rather than a mask.

For the leak hunt itself the protective equipment is different in kind: the lights stay off and the door stays shut for twenty minutes, so know where the door handle is by touch before you switch off, and clear the floor of anything you could trip on.

Ventilation

Part II owns this and gives the arithmetic. The figures it establishes are the ones this page builds to: Kodak’s five air changes per hour for an amateur darkroom, Kodak’s ten room volumes an hour for open-tray processing, HSE’s more-than-five with a through draught, and CIBSE’s 6 to 8 for photographic darkrooms as ILFORD quote it. Treat 5 to 10 changes an hour as the band.

What belongs here is the one sentence in that CIBSE passage that decides the shape of this build: small darkrooms for occasional use may often be ventilated naturally through a suitable light trap. That is a manufacturer-quoted building-services source saying, in terms, that the baffled opening in stage 1 is a legitimate ventilation strategy for a room like yours — not a compromise. For longer sessions ILFORD’s own advice is a light-trapped extract fan, which is the same idea with a motor in it.

Two rules follow and neither is negotiable. Air must be able to get in as well as out, or the fan strains against a sealed box and moves nothing. And the room is opened up between processes, which costs you nothing because the room only needs to be dark while paper is out of its packet.

Parts

The list assumes the common case: one window, one door, one vent or extractor, in a room you may not drill into. Every line has a low-cost substitute because most readers of this page are working in somebody else’s building.

Part	Quantity	What it does	Substitutes and notes
Blackout fabric, or heavy blackout-lined curtain	window area plus 150 mm all round	The window	Sold by the metre for curtain making. The overlap is the specification: fabric that ends at the reveal leaks all the way round. A camping tarpaulin or a heavy woollen blanket both work; the blanket is what Kodak’s own advice reaches for when it says to close large leaks with heavy cloth
Foam board 5 mm, or twin-wall corrugated polypropylene	one panel per window, cut 3 to 5 mm oversize on every edge	The rigid window panel — the better answer where the window is a regular shape	Corrugated plastic survives damp and foam board does not; in a bathroom, use the plastic. Cut it oversize so it wedges into the reveal under slight compression
Neodymium magnets, 10 mm, or hook-and-loop tape with self-adhesive backing	6 to 10 per panel	Holds the panel without a fixing in the wall	Magnets need a steel frame or a steel washer stuck to the reveal. Hook-and-loop is the universal answer and its adhesive comes off plasterboard paint with heat from a hairdryer
Self-adhesive brush-pile draught strip, 6 to 12 mm pile	door perimeter, about 5 m	The gap around the door leaf	Brush pile compresses and springs back, which is what a door that opens and closes needs
Self-adhesive closed-cell foam or E-profile rubber strip	door perimeter, about 5 m	The compression seal where the door meets the stop	Foam takes a set and needs replacing yearly; rubber profile does not
Brush or drop-down threshold seal	door width	The gap under the door, which is the biggest single leak in most rooms	A rolled towel is the zero-cost version and works; it is also the version you will forget
Matt black paint, water-based emulsion	250 mL	The wall immediately behind the enlarger, and the inside of the light trap	Kodak specify flat black in both those places and a light colour elsewhere. Buy matt: a gloss black surface is a mirror and is worse than white
Timber or plastic angle, 20 × 20 mm	2 m	The baffles inside the vent trap	Offcuts of the foam board work if the trap is not in a wet position
Ducting or a plain rectangular box, 100 to 150 mm	one trap	The vent light trap body	A shoebox lined with black paper is a legitimate prototype and will tell you whether the geometry works before you build it properly
Black photographic or gaffer tape, 50 mm	1 roll	Sealing panel edges, temporary fixes, blacking a hinge line	The structural fastener of this whole page
Aluminium foil tape	1 roll	Sealing round a duct where tape has to survive damp	Also the answer for a leak through a material that turns out to be translucent
Photographic paper, the one you print on	about 6 sheets, 5 × 7 in or larger	The leak test and its control	It must be the paper you actually use, from one box, opened for the first time in the dark
Working-strength developer, stop and fixer	one tray each	Processing the test	The printing part’s standard sequence; the dilutions are on the manufacturer’s sheet
A dark, opaque card larger than the test sheet	1	The mask that makes half a sheet a control	Any offcut of the foam board, painted matt black

Tools

Tool	For	Notes
Craft knife with fresh blades, steel rule, cutting mat	Every panel, every seal	Three light passes beat one heavy one, on board and on fabric alike
Tape measure and a pencil	Window reveals, door gaps, bench heights	Measure the reveal in three places: window openings are rarely square
Step ladder	The window panel and any ceiling fitting	Not a chair. See the declared step in Safety classification
A small torch	Leak hunting from the outside	The most useful tool on the page — see stage 3
A watch or timer you can read without lighting the room	The twenty-minute sit, and the leak test	A phone is not this tool. Its screen resets your eyes
A sheet of white paper or card, A4	Kodak’s own light-tightness test	Held against a dark background; if you can see it, the room is not dark
Trays, tongs, thermometer, graduate	The leak test’s processing	Part II’s dedicated set, not the kitchen’s

Estimated cost

Cost band ££, and it is dominated by whichever blackout route the window forces on you. Fabric for one domestic window, seals for one door, a vent trap made from ducting and a tin of matt black emulsion sit at the lower end; a purpose-cut rigid panel for each of three windows sits at the upper.

This page quotes a price only where the planner carries a dated one, and for the blackout materials it carries none. Blackout fabric, foam board, corrugated polypropylene, magnets, hook-and-loop tape, draught seal, threshold seal, ducting, foil tape and matt black emulsion are all absent from src/data/prices.json, and the planner’s gap register does not yet name them — this page is the first to ask for them. What the page can give instead of a price is the specification that outlives one: an overlap of 150 mm on a fabric, a panel cut oversize rather than to size, a compressible seal on the door stop and a brush seal on the threshold, and a trap whose light path turns at least twice.

The one line the planner does price is the paper, and it is the line that recurs.

Estimated consumables cost

What a single run of this page uses up is paper, chemistry and tape. The panels, seals and the trap are capital: they are fitted once and taken down when you move.

Consumed	This session	Sourced price	Cost this session
Photographic paper, variable-contrast RC, 5 × 7 in	4 sheets — two test, two control	£16.06–£44.71 per 25 to 100 sheets, dated 5 September 2026, which is £0.45–£0.64 a sheet	£1.80–£2.56
Paper developer, working strength	about 500 mL of 1+9 working solution, that is 50 mL of concentrate	£10.52–£20.03 per 500 mL to 1 L of concentrate	£0.53–£2.00
Stop bath, working strength	about 500 mL of 1+19, that is 25 mL of concentrate	£10.66–£12.18 per 500 mL of concentrate	£0.53–£0.61
Fixer, working strength	about 500 mL of 1+4, that is 100 mL of concentrate	£21.05–£25.98 per 1 L of concentrate	£2.11–£2.60
Black photographic tape	about 4 m	None. The planner carries no line for it	—
Blackout fabric, board, seals, magnets, foil tape, matt black emulsion	one room’s worth	None. The planner carries no line for any of them	—

The priced rows give £4.97 to £7.77 for one run. That is a floor and not a total: two of the six rows carry no dated price at all, and the second of those two is the whole blackout, which is most of what you will actually spend. The concentrate figures also assume you mix the smallest useful volume and pour it away after one session, which is what a test demands and not what a printing session does.

Equipment stays out of the table on purpose. Trays, tongs, the thermometer, the step ladder, the knife and the torch are not consumed, and neither is the room.

Alternative route

This is the page that will be read by the most people who cannot do it, so the alternatives are listed in the order of how much they give up.

A room that is dark only at night. The cheapest blackout in the world is the Earth’s shadow. If your window faces a garden rather than a street lamp, a heavy curtain plus a towel at the door is a working darkroom between dusk and dawn, and the leak test in Testing and calibration measures exactly that room at exactly that hour. Record the hour on the certificate; a room that passes at midnight has said nothing about noon.

A bathroom conversion, put up and taken down each session. The most common real answer. Stage 5’s plan is drawn for it. A bathroom brings a water supply, a wipeable floor and usually an extractor fan; it brings back a fixed extractor that leaks light, a door that opens inward onto the only floor space, and no bench. The panel-and-magnet route exists for this room, and so does the rule that the whole installation must come down in under five minutes for the next person who needs the room.

A changing bag or a changing tent instead of a dark room at all. A bag makes any lit room into a loading and unloading space, which is enough for film into a daylight tank and enough for putting paper into a printing frame. It is not enough for tray processing under inspection, because you cannot watch a print come up inside a bag. A tent is a bag you can get your head into and is the route that gets closest to a darkroom without one.

A daylight tank and no darkroom for film at all. Load in a bag, process in the light. This is what Part VI already assumes, and it means a reader with no dark space can still do the whole negative half of the course.

Contact printing by inspection under a bought lamp in a windowless cupboard, with the door open for air between sheets and the room lights off. It is not a good darkroom. It is a real one, and the test below will give it a real number.

What none of these substitutes is the test. Every route above still gets a leak test and a written verdict, because the point of the certificate is not the quality of the room but knowing what the room is.

Waste streams

Three, and Part II and the SOP library own all three. Spent developer and stop bath go to their own labelled containers under the general chemical waste procedure. Spent fixer and the first wash are silver-bearing and go to that stream for recovery, because that is where the silver is. Offcuts of board, fabric, foam and tape are ordinary solid waste or their recycling streams; used blades go into a rigid sharps container and never loose into a bag. Read the disposal caveat before anything leaves the house: local regulation governs all of it and it varies more than it has any right to.

Build

Stage 0 — Decide what the room is for, and write it down (10 minutes)

Before anything is cut, answer one question in writing: what is the fastest material this room will ever handle?

• Photographic paper only. The easiest case. ILFORD describe their variable-contrast RC papers as roughly equivalent to a film of ISO 3 to 6, which is between a hundred and a thousand times slower than the film in your camera, and the papers are blind enough to red that a deep-red lamp is permitted at all.
• Orthochromatic film as well. ILFORD’s ORTHO Plus sheet allows either total darkness or a 906 deep-red lamp with a 15 W bulb at not less than 1.2 m. Faster than paper, and its red blindness is what buys the lamp.
• Panchromatic film. Kodak’s list of materials to be handled in total darkness begins with panchromatic black-and-white films and plates. There is no lamp. The room has to be dark rather than safelit, and the standard it must meet is higher than any of the above.

Write the answer on the certificate you will fill in at the end. Every number this page produces is attached to it.

Stage 1 — The vent, before the blackout (25 minutes)

Build the air path first, because a room you have already sealed is a room you will be tempted to leave sealed.

A light trap is a duct that air can go round and light cannot, and it works for one reason: light travels in straight lines and does not turn a corner unless something reflects it there. Give the path two turns and blacken every interior surface, and each turn removes what the geometry does not already remove.

A baffled vent light trap, in section

1. 1Outside opening, with insect mesh — mesh outside, so the trap can be cleaned from the room side
2. 2Baffle A, standing up from the floor of the duct — reaches four fifths of the depth; the first turn
3. 3Baffle B, hanging down from the ceiling of the duct — overlaps baffle A by at least a third of the duct depth — the overlap is what kills the straight path
4. 4Baffle C, standing up again — the second turn; two turns is the specification, three is better and costs only board
5. 5Every interior surface matt black — Kodak specify flat black around light locks for exactly this reason: a grey duct reflects round the corner you just built
6. 6Room opening — free area between baffles at least equal to the opening, or you have built a restriction

Drawn readable rather than to scale. Build it from the key: two turns, one third overlap, matt black inside, free area preserved. Test it by putting a torch at the outside end in a dark room and looking from inside.

Three ways to get this into a real room. Through an existing extractor: fit a shallow black hood over the inside grille, open at the bottom, so the fan still draws but nothing sees through the blades. Through an openable window: build the trap into a panel that replaces the open sash, so the window can be open and the room dark at once. Through a doorway: an L-shaped cardboard baffle standing on the floor across the gap under the door, black inside, which passes make-up air and stops the corridor light.

Then check it the only way that works: a torch at one end, your eye at the other, in a dark room. If any light gets through, add a baffle rather than blacking more paint onto the same one.

Stage 2 — The window (30 minutes)

Two routes, and the choice is not about quality but about whether the window is a regular shape and whether you may fix anything to the wall.

Fabric on a tension rod, hung inside the reveal, is the tenant’s answer. The specification is overlap: the fabric must be at least 150 mm wider and longer than the opening on every side, and it must be held against the wall along its edges rather than hanging free, because a hanging curtain leaks along its whole perimeter. Hook-and-loop tape down each side, or a second tension rod at the bottom, turns a curtain into a seal.

A rigid panel is better and is the answer where the reveal is square. Cut foam board or twin-wall plastic 3 to 5 mm oversize on every edge and push it into the reveal, where it holds by compression alone. Where the reveal is not deep enough for that, magnets onto steel washers stuck to the frame, or hook-and-loop, hold it. Run a strip of black tape round the outside of the panel where it meets the reveal, on the first fitting, and then leave the tape on the panel rather than the wall so the panel is still removable.

Both routes fail the same way: at the top corners, where a rod’s brackets hold the fabric off the wall, and along the sill, where a panel that was cut to size rather than oversize has settled. Stage 3 will find both.

Door seal, window panel and where each one leaks

1. 1Compression seal on the door stop, latch side and head — foam or E-profile rubber; the leaf presses onto it when closed
2. 2Brush-pile strip on the hinge side — a compression seal cannot work on a gap that opens as the door swings; bristles can
3. 3Threshold seal, brush or drop-down — the single biggest leak in most rooms, and the one people notice last
4. 4Keyhole and letterplate — holes, not gaps: a taped black flap each, hinged so the door still works
5. 5Rigid panel cut 3 to 5 mm oversize, held by compression — oversize is the specification; a panel cut to size is a panel that will drop
6. 6Foam edging on the panel — takes up the difference and lets the panel come out without a fight
7. 7The sill, and the top corners — the two places a window blackout actually fails; look here first in stage 3

Both drawings are sections, not elevations, and the gaps are drawn far wider than they are. Every seal here is removable and leaves no fixing in the wall, which is the constraint most readers of this page are working under.

Stage 3 — The leak hunt (30 minutes, and 20 of them are sitting still)

This is the stage people skip and it is the one that finds everything.

Switch the room lights off. If any of them are fluorescent, wait five minutes before you conclude anything: Kodak’s test procedure says so explicitly, because a tube keeps glowing after it is switched. Shut the door. Then sit down and do nothing for twenty minutes.

The twenty minutes is not patience for its own sake. Vision science describes dark adaptation as two mechanisms in sequence: threshold falls rapidly over the first minutes as the cones adapt, then a second mechanism enters after about five to eight minutes and threshold falls again, more slowly, reaching its minimum — around 10⁻⁵ candela per square metre — only after about forty minutes in the dark. The two manufacturers put working numbers on the same curve from opposite ends: Kodak says it takes at least ten minutes for the eyes to become fully adjusted, ILFORD says the only satisfactory way to check for leaks is to wait until the eyes have adapted, which can take about fifteen. Twenty minutes is comfortably past both and short of the plateau.

!The phone is the problem, not the clock

A screen at arm’s length puts you back at the start of that curve, and the recovery is not instantaneous — it is the same several minutes over again. This is why the tool list asks for a watch you can read without lighting the room, and why the discipline in a working darkroom is that phones stay in another room rather than face-down on the bench. It is also, incidentally, the reason a leak you could not see at one minute is unmissable at twenty: nothing about the leak changed.

Then sweep, in this order. Door frame — all four edges, slowly, from 300 mm away. Under the door. The keyhole and the letterplate. The window: the sill first, then the top corners. The extractor or vent. Every powered device in the room, for indicator LEDs — a standby light, a timer display, a charger, a smoke alarm, a router. The face of any clock that glows. And the ceiling, for a downlighter that is not as blanked as it looks.

Two checks close the stage. Kodak’s white-paper test: hold a sheet of white A4 against a dark background at arm’s length. If you can see it, the room is not dark, whatever the sweep found. And the torch reversal: have somebody walk round the outside of the door and window with a bright torch while you watch from inside. A leak that is invisible against daylight becomes obvious against a moving beam, and this is how you find a pinhole in a panel.

Close what you find. Kodak’s instruction is the simplest one on the subject: small leaks with black tape, large ones with heavy cloth.

Stage 4 — Surfaces, and the myth of the black room (15 minutes)

The instinct is to paint everything black. Both manufacturers say otherwise, and their reasoning is worth having because it is not aesthetic.

Kodak’s darkroom-design guidance is that ceilings are painted flat white where indirect safelighting is used, and that walls should be a light colour — preferably one similar to the colour the safelight filter transmits, with a neutral light tan or buff suitable in most cases. The logic is that indirect safelighting works by reflection: a bounced lamp in a black room throws almost nothing back, so you turn it up, and a brighter lamp is a worse lamp. ILFORD list the colour of the walls and ceiling among the nine factors that determine whether safelighting is effective and safe, for the same reason and in both directions.

Two places are black, and they are specific. The wall immediately behind the enlarger, so that white light from the lamphouse does not bounce off it onto the paper. And the interior of any light lock or trap, which is stage 1’s requirement restated. Kodak add one exception worth knowing: an all-black darkroom is best where the work is copying and duplicating, where the material is fast and no safelight is being bounced off anything.

Everywhere else, leave the walls alone — which, for a reader who may not paint at all, is the cheapest good news on this page.

Stage 5 — Laying it out for printing (25 minutes)

Part II divided the room into wet and dry and gave the one-way workflow. Printing adds three constraints that chemistry alone did not.

The paper stock is a light-sensitive object that sits out for the whole session. It is the thing in the room with the longest exposure to the safelight, and it wants to be in the dimmest place, ideally in a light-tight box or drawer under the dry bench, and never in a pool of direct lamp light.

The safelight has to light the tray line without shining on the easel. Kodak’s placement guidance is explicit on both halves: no direct illumination closer than 1.2 m from the working surface, and do not put a direct safelight where it will shine on an enlarging easel, because it interferes with dodging, with cropping and with judging exposure — and because the paper on the easel is exposed paper, which is the state in which paper fogs most easily. They add the ergonomic point too: avoid pools of relatively bright light against a dark, unilluminated background, which is fatiguing to work in.

There has to be somewhere to put a wet print down that is not the dry side. A washing tray, a sloping draining board, or at minimum a tray of water at the wet end. Without it, wet prints migrate to the enlarger bench, and a drop of fixer on the dry bench ends up on the next sheet of paper.

Two plans: a small dedicated room, and a bathroom put up for the evening

1. 1Paper store: light-tight box or drawer, on the dry side, out of direct lamp light — the longest-exposed sensitive material in the room
2. 2Enlarger, with the wall behind it matt black — the only wall Kodak asks you to blacken in an otherwise light-walled room
3. 3Tray line in working order: developer, stop, fixer, wash — left to right, so the workflow never doubles back over the dry bench
4. 4Safelight above the wet side, at least 1.2 m from the trays, aimed away from the easel — lights the trays; must not shine on the enlarging easel or on the paper store
5. 5Seated reach envelope, about 700 mm radius — if the timer and the tongs are outside it, the room is not usable seated
6. 6Vent light trap at the wet end, make-up air at the door end — air crosses the room away from you, not past your face
7. 7Cables at bench-back height, never across the floor — the floor is the wet part of the room
8. 8Bathroom version: board across the bath is the wet side, board across basin and lavatory is the dry side — everything comes down in five minutes

Plans, not elevations. The distances that matter are the 1.2 m from safelight to tray and the reach envelope; everything else adapts to the room you have.

Heights and reach. A wet bench that is too high makes you lift a dripping tray at shoulder level; one that is too low makes you stoop over a bath of fixer for an hour. Work standing at about elbow height minus 100 mm for the wet bench, and set the dry bench so that the enlarger’s focusing control falls to hand. If the room will be worked seated, the test is simpler and harsher: sit in the chair and see whether the timer, the tongs, the paper drawer and all four trays fall inside about 700 mm of reach. If they do not, move them, because a room you have to stand up in every ninety seconds is a room you will not use.

Stage 6 — The certificate (5 minutes)

Write, on a card that lives in the room: the date; the material class the room is cleared for; the hour, if the blackout depends on it; what the blackout consists of; where the vent is; and a line for the leak-test result you are about to obtain. The date matters more than any of the rest, because the result belongs to a configuration and every configuration eventually changes.

Testing and calibration

The build is not finished when the room looks dark. It is finished when a sheet of paper says so.

T1 — The white-paper check

The go/no-go, done at the end of every leak hunt. Twenty minutes adapted, a white A4 sheet held against a dark background, arm’s length. Visible means not dark. This is Kodak’s own test and it takes ten seconds; its virtue is that it uses the most sensitive detector in the room, which is you, and its limit is that you are not the detector that matters.

T2 — The leak test on paper, with a control

The measurement. It answers one question: does this room, over the longest time paper is out of its packet in a real session, put any density on that paper?

The leak test, in the order it happens

1. 1Measure your real handling timeWatch in hand, during an ordinary session: from opening the paper box to the sheet entering the fixer.
2. 2Two sheets from one box, in the darkOne goes straight back into its light-tight packet and is marked CONTROL.
3. 3Test sheet emulsion up, half covered by an opaque cardAt the place in the room where paper actually sits during a session.
4. 4Dark and undisturbed for twice the handling timeEverything in the room switched off, the safelight included, and the door shut.
5. 5Process both sheets togetherSame trays, same times, same temperature, same agitation, in total darkness.
6. 6Dry both, then compare in even lightCovered half against uncovered half, and the whole test sheet against the control.
7. 7Write the verdict on the room certificateWith the duration used, the material class, the date and, where the blackout depends on the sun, the hour.

Seven steps, and the two that people leave out are the first and the second. Without a measured handling time the duration is a guess; without a control sheet from the same box, a grey result cannot be told from a grey box of paper.

Why twice the handling time, and where that number comes from. The often-repeated instruction to test for ten times the handling time is not in either manufacturer’s literature and this course could not source it, so it is not used here. What is sourced is Kodak’s definition of a safe time: an exposure less than or equal to half the time required to produce a detectable change. Turn that definition round and it says what a test must do — if you want a session of length t to sit inside the safe time, the material must show no detectable change at 2t. So the room is tested at twice the longest handling time, and that is the course’s own derivation from a published rule rather than a figure of folklore. If your handling time is fifteen minutes, the sheet sits for thirty.

Why the control is not optional. The covered half of the test sheet controls for the room. The separate sheet that never left the packet controls for everything else: the paper’s age, its storage, the developer’s temperature and freshness, and your processing. If the control comes up grey, the room is not what you have measured — you have measured a fogged box of paper, and the base fog entry in the troubleshooting atlas separates the causes.

Reading it. Visually, in even light, dry — paper looks different wet. The pass is that the line between the covered and uncovered halves is invisible, and that the test sheet matches the control. Where you have built Part XV’s densitometer with its reflection head, read the two halves instead of looking at them, and record the difference in density units.

📓The threshold this page does not invent

The course has no sourced density criterion for the leak test on paper. ILFORD publish a figure of about 0.04 in density as the change that condemns a safelight after one minute, and that figure belongs to that test rather than this one. So the leak test’s verdict is visual, and the densitometric reading is recorded beside it as a number you own and can compare with your own future tests, not as a pass mark taken from anybody. Where the course has no threshold it says so rather than making one up.

T3 — The same test on film, if film is handled here

Paper is a forgiving detector. If orthochromatic or panchromatic film will ever be handled in this room, repeat T2 with a sheet or a strip of that film, in the dark, for twice the film’s handling time, and process it with a strip from the same batch that never left the box. Kodak’s parallel test for sheet film uses longer intervals than the paper test and reads the result against the film’s own gross fog, which is the transmission density of a completely unexposed, normally processed sample — another reason the control strip is the whole method.

Expect to fail this where you passed on paper. That is not a defeat; it is the room telling you the truth about which of the two materials it is cleared for, and the certificate carries both verdicts.

T4 — Repeat conditions

The verdict expires whenever anything in the list changes: a different or faster paper, a hand-coated emulsion, an orthochromatic film, a new lamp, a moved bench, a raised dimmer setting, a new tenant in the flat opposite with a security light, or a season. Re-run T2 and date it again. ILFORD’s advice to record the installation date of a safelight filter is the same discipline applied to a different component: the room, like the filter, degrades without announcing it.

Troubleshooting

The whole test sheet is grey, control included. Not a room problem. Suspect the paper’s age or storage, a fogged box, or developer fog from a warm or over-worked bath. Settle it with a fresh box and fresh developer before you touch the blackout.

The test sheet is grey and the control is clean. A real leak, or the safelight was left on. Check the switch first, then repeat the leak hunt with the torch reversal, then repeat T2 with the test sheet moved to a different position in the room — a difference between two positions tells you the direction the light is coming from.

One edge or one corner of the sheet is darker. A directional leak, which is much easier to find than a general one: the gradient points at its source. Put the sheet back in the same orientation, and the darker edge is the side the light came from.

A lighter band along one edge of every print, which survives changing paper and developer. This is the assessment question and the answer is not fog. A band that is lighter is a band that got less exposure — so it is the enlarger, the easel or the negative carrier, not the room. The single test that separates them: expose a sheet with no negative in the carrier and the easel blades pulled right back. If the band persists, it is the lamphouse or the lens; if it disappears, it was the easel.

The room passes at midnight and fails at noon. The blackout depends on the sun, which is a real answer and not a fault. Either improve the window blackout or write the working hours on the certificate and keep them.

The vent trap passes the torch test and the room still fogs. Look for the path you did not build: a keyhole, a letterplate, a downlighter, a cable grommet, a loft hatch, or a translucent material you assumed was opaque. Kodak name plywood specifically as a material that can look opaque and still admit infrared, and their list of overlooked sources — an enlarger head, a lighted dial, a pinhole between the darkroom and a lit space — is worth reading as a checklist rather than as prose.

Condensation on the window panel. Twin-wall plastic instead of foam board, and take the panel down between sessions. A damp foam-board panel delaminates and then leaks along the delamination.

Questions

1. Your handling time, measured with a watch, is eleven minutes. State the duration of your leak test and justify it from Kodak’s definition of a safe time, in one sentence, without using the word “ten”.
2. A room passes T2 on variable-contrast RC paper and fails T3 on ORTHO Plus film. Explain the result in terms of the two materials’ speed and spectral sensitivity, and say precisely what the room’s certificate should now say.
3. You are told to paint the whole darkroom matt black. Give the manufacturer’s reasoning against it, name the two places that should be black, and give the one application in which an all-black room is the better choice.
4. Your leak test’s control sheet reads 0.09 above the paper’s white, and the covered half of the test sheet reads the same. What have you learnt about the room, and what have you learnt about the box of paper?
5. A friend hunts leaks for two minutes, declares the room dark, and fogs a box of paper. Using the dark-adaptation time course, explain what they measured and what they failed to measure. Then say what they should have been holding.
6. Sketch a light trap for a 100 mm circular duct — the section is circular rather than rectangular — and state the two things about your baffles that decide whether it works.
7. You may not fix anything to the walls, the window is a bay with three angled panes, and the door opens inwards over the only floor space. Write the blackout specification for this room in five lines, and say which of the alternative routes you would take if it fails.

Further experiments

Measure the leak instead of looking at it. Put the densitometer’s reflection head on the two halves of a failed test sheet and get a number. Then double the test duration and read it again. Two points is enough to say whether the fog is growing linearly with time, which tells you it is a steady leak, or faster than linearly, which tells you something else is going on.

Map the room. Run T2 with four sheets in four positions at once — over the trays, on the easel, at the paper store, and in the darkest corner — and you have a fog map instead of a verdict. It is the cheapest way to find out that the enlarger bench is the worst place in the room to leave paper.

Test the same room at four hours of the day. Dawn, noon, dusk and midnight, same duration, same box. If your blackout depends on the sun, this is the experiment that tells you by how much.

Time your own dark adaptation. In the blacked-out room, with a single sheet of white card at a fixed distance, note the minute at which you can first see it and the minute at which it stops getting easier. Compare your two times with the published account: a rapid early phase, a second mechanism entering at five to eight minutes, and a plateau that takes far longer than anybody’s patience.

Recap

A room is not dark; it is dark for a material. Paper is slow and red-blind, orthochromatic film is faster and still red-blind, and panchromatic film has no safelight at all — so the standard the room must meet is set before any tape is cut.

Ventilation is designed first, because the blackout is what removes it, and a baffled light trap is a published answer rather than a compromise. The blackout itself is a set of removable parts: a fabric with 150 mm of overlap or a panel cut oversize, a compression seal on the door stop, a brush seal on the hinge side and another at the threshold, and a flap over every hole a perimeter seal cannot close.

The leak hunt takes twenty minutes because your eyes take that long, and the phone that tells you the time is the thing that undoes them. Kodak’s white sheet against a dark background is the ten-second check; the leak test with a control is the measurement. It runs for twice your real handling time — derived from the published definition of a safe time, not from folklore — and its verdict goes on a dated certificate that expires the moment anything changes.

Most of the room stays a light colour, on the manufacturers’ own reasoning, because indirect safelighting works by reflection. Two surfaces are black: behind the enlarger, and inside the trap.

Check your understanding

Question 1. Your printing session, timed with a watch, has paper out of its packet for twelve minutes. How long does the leak test run, and what published rule fixes that duration?

• Two minutes, because that is how long the eye takes to adapt
• Twenty-four minutes, from Kodak's definition of a safe time as an exposure no longer than half the time at which a detectable change appears
• One hundred and twenty minutes, from the widely repeated rule of ten times the handling time
• Four minutes, from ILFORD's general safelight recommendation

Show the answer and why

Answer: Twenty-four minutes, from Kodak's definition of a safe time as an exposure no longer than half the time at which a detectable change appears

Kodak define a safe time as any exposure less than or equal to half the time required to produce a detectable change. Read that backwards and it tells you what a test has to demonstrate: for a twelve-minute session to be inside the safe time, the material must show nothing at twenty-four minutes. The ten-times rule is repeated everywhere and appears in neither manufacturer's literature, so this course does not use it; the four-minute figure is ILFORD's recommendation for direct safelight exposure of their papers, which is a different quantity measured under a different lamp.

Question 2. A reader hunts for leaks for two minutes, sees nothing, and later fogs a whole box of paper. What went wrong, physiologically?

• Nothing; the fog must have come from the developer
• Two minutes is inside the cone phase of dark adaptation, before the second mechanism enters at five to eight minutes and threshold falls again, so the detector was thousands of times less sensitive than it would have been at twenty minutes
• The leak only appeared later, when the sun moved
• Two minutes is enough; the error was not holding a white card

Show the answer and why

Answer: Two minutes is inside the cone phase of dark adaptation, before the second mechanism enters at five to eight minutes and threshold falls again, so the detector was thousands of times less sensitive than it would have been at twenty minutes

The published account of dark adaptation is biphasic: a rapid fall in threshold over the first minutes as the cones adapt, then a second mechanism entering after five to eight minutes and a much slower fall to a minimum around 10^-5 candela per square metre after about forty minutes. At two minutes you are on the first plateau and the eye is a poor photometer. Option three is possible in a particular room but it is not the general answer, and option four gets the tool right and the cause wrong: the card helps, and it helps most once the eye behind it has adapted.

Question 3. A room passes the leak test on variable-contrast RC paper. What may you conclude about handling panchromatic film in it?

• It is cleared for panchromatic film, because a pass is a pass
• Nothing, and the presumption runs the other way: paper is roughly equivalent to a film of ISO 3 to 6 and is nearly blind to red, whereas panchromatic film is fast and sees the whole visible spectrum, so it is on the manufacturer's total-darkness list
• It is cleared, provided the safelight is switched off
• It is cleared for panchromatic film but not for orthochromatic film

Show the answer and why

Answer: Nothing, and the presumption runs the other way: paper is roughly equivalent to a film of ISO 3 to 6 and is nearly blind to red, whereas panchromatic film is fast and sees the whole visible spectrum, so it is on the manufacturer's total-darkness list

ILFORD describe their variable-contrast RC papers as roughly equivalent to a film ISO of 3 to 6 — between one hundred and one thousand times slower than ordinary camera film — and papers are red-blind, which is the whole reason a deep-red lamp exists at all. Kodak's list of materials that must be handled in total darkness begins with panchromatic black-and-white films and plates. A leak that a slow, red-blind material never noticed can be plenty for a fast panchromatic one, which is why the certificate names the material class and why T3 exists. Option four inverts the speed order: orthochromatic material sits between the two.

Question 4. Why do Kodak tell you to paint the darkroom ceiling flat white and the walls a light colour, when the room is supposed to be dark?

• Because dark paint is more expensive
• Because indirect safelighting works by reflection, so a black room forces you to run the lamp brighter, and a brighter lamp is a worse lamp
• Because white walls make leaks easier to see during the hunt
• Because light-coloured walls reflect less of the enlarger's light

Show the answer and why

Answer: Because indirect safelighting works by reflection, so a black room forces you to run the lamp brighter, and a brighter lamp is a worse lamp

The room is dark when the lights are off; the paint has nothing to do with that. What the paint decides is what happens to the safelight. A bounced lamp in a black room returns almost nothing, so the printer turns it up — and the safe working time falls with the illuminance at the paper. Kodak even suggest matching the wall colour to what the filter transmits, and ILFORD list wall and ceiling colour among the nine factors that determine whether safelighting is effective. Option four has the physics backwards, which is exactly why the wall immediately behind the enlarger is the one surface Kodak do ask you to paint flat black.

Question 5. You seal the room completely — door, window, vent — and it passes every light test. What have you done wrong?

• Nothing, if the tests pass
• Removed the room's air exchange at the moment chemistry enters it; the ventilation path is designed before the blackout, and CIBSE's own guidance, as ILFORD quote it, is that a small occasional-use darkroom may be ventilated naturally through a suitable light trap
• Made the room too dark for the eye to adapt
• Nothing, provided the session is under an hour

Show the answer and why

Answer: Removed the room's air exchange at the moment chemistry enters it; the ventilation path is designed before the blackout, and CIBSE's own guidance, as ILFORD quote it, is that a small occasional-use darkroom may be ventilated naturally through a suitable light trap

A sealed room is a hazard this build creates rather than one it inherits, and it is why the vent is stage 1 and the blackout is stage 2. The published position is not that ventilation and darkness are in tension but that a light trap resolves them: CIBSE give photographic darkrooms 6 to 8 air changes an hour and say small darkrooms for occasional use may often be ventilated naturally through a suitable light trap. Option four is the reasoning that leads people to work in cupboards; the vapour concentration rises for as long as you are in there, and an hour is not a control.

Question 6. The leak test comes back with the covered half and the uncovered half indistinguishable — but the control sheet, which never left its packet, is visibly grey. What do you conclude?

• The room leaks, and the leak reached inside the packet
• The room passed, and the paper or the processing is the problem — the control has caught a fogged box, poor storage or a developer fault, and the room verdict stands only because the two halves agree
• The test is void and tells you nothing at all
• The developer was too cold

Show the answer and why

Answer: The room passed, and the paper or the processing is the problem — the control has caught a fogged box, poor storage or a developer fault, and the room verdict stands only because the two halves agree

The two controls answer different questions. The covered half controls for the room: same sheet, same processing, no light. The separate packet sheet controls for everything that is not the room. When the packet sheet is grey and the two halves agree with each other, the room did not add anything — the base level was already high before the test began. The next moves are a fresh box and fresh developer, and the troubleshooting atlas separates base fog from developer fog. Option three is the counsel of despair: a test with a control that fails still tells you which of two things failed, which is what a control is for.

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