Experiment: The Safelight Fog Test

Purpose

To convert the lamp built on the previous page into a number: the longest time a sheet of your paper may spend at your working position, at a stated distance and dimmer setting, before that lamp begins to show on the print.

The hypothesis. At its tested distance and dimmer setting, the lamp adds no detectable density to paper that has already received a mid-grey printing exposure, for any safelight exposure up to the longest step in the series.

The control. A piece of the same sheet, from the same box, given the identical mid-grey printing exposure and kept in complete darkness while the others are exposed to the lamp, then developed alongside them in the same trays at the same temperature for the same time. Everything except the safelight has happened to it.

The one variable that changes. The duration of safelight exposure, and nothing else. Same paper, same box, same printing exposure, same lamp, same distance, same setting, same developer, same dilution, same temperature, same agitation, same session. If any of those moves, the experiment has two variables in it and its answer means nothing.

This is a measurement the course has run once before, in compressed form, at Station 4 of Part IV, under whatever interim lamp a reader had. It is run properly here, on a lamp whose current, spectrum and setting are known, in a room whose leak test has passed — and then extended to every other light in the room, because the safelight is rarely the worst of them.

Learning objectives

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

• design a fog test whose result cannot be explained by anything but the safelight;
• say why the paper must be pre-exposed, and what a test on fresh paper systematically over-reports;
• run a stepped exposure with a moving mask and record the times so the steps are recoverable afterwards;
• judge a threshold visually against a control under a stated viewing condition, and read the same threshold as a density where an instrument is available;
• apply the published definition of a safe time and explain what the margin is for;
• extend the same method to an enlarger lamphouse, a timer display and a room full of indicator lamps;
• write a lamp label and a calibration record that a stranger could act on;
• state what a failed test does and does not license you to do.

Prerequisites

Build: an LED safelight, finished, with its drive current measured, its dimmer detented and its illuminance recorded at each setting.

Making a room dark, passed. A room that leaks cannot be used to test a lamp, because you cannot tell which light did it.

Paper sensitometry, for the paper characteristic curve this test’s result lands on, and for what reflection density means.

The latent image made visible, whose Station 4 is the compressed version of this procedure.

The procedure is also written as a bench-side standard operating procedure. Read this page to understand it; take the SOP to the darkroom.

Safety classification

Level A. Ordinary print processing in trays at working-strength dilutions, in a ventilated room, with dedicated utensils. The rubric’s Level A criteria all hold: no substance here is worse than irritant in these quantities, nothing is heated, nothing at mains potential is built or modified, and every waste stream is one the household routes already handle.

What is not a hazard here, and why. No powder is opened and no concentrate is decanted: developer, stop and fixer are made up to working strength before the session and handled with tongs. The developing agents in a paper developer — metol and hydroquinone — are the substances on this page that deserve respect, and the reason they earn no higher letter is the operation rather than the substance. Weighing them out of a jar to mix D-72 from scratch is a different session with different controls, described on its own page; pouring a bought concentrate at 1+9 into a dish is this one. The same distinction is the ruling of 5 September 2026: the operation sets the level, not the substance.

One step is worth naming even though it does not raise the level. If a tungsten enlarger head is used for the pre-exposure, its lamphouse gets hot enough to burn, and it is switched off and left to cool before anything is adjusted inside it.

Hazards

Developer on the skin: metol is a recognised skin sensitiser and the control is that the hands do not go in the dish. Tongs, one pair per bath, and gloves.

Stop bath and fixer: irritant to eyes at working strength. Eye protection, and no splashing when sheets go in.

Cross-contamination, which is a hazard to the experiment rather than to you but wrecks it completely. ILFORD’s instruction is one dish per solution, each dish marked, chemistry kept in matching coloured containers, and a separate pair of tongs per dish — because a trace of fixer or stop in the developer gives inconsistent results or, at worst, completely blank prints. On this page that failure mode is indistinguishable from a control sheet that will not develop.

Working in the dark, for two hours, with liquids. The floor is clear before the lights go off, the trays are in a fixed order that does not change during the session, and everything electrical is outside the splash zone on a residual-current-protected socket.

A hot lamphouse, if you use one. Named above.

Spent fixer is silver-bearing and is not a drain stream. See Disposal considerations.

Required PPE

Nitrile gloves whenever a hand may enter a dish or handle a wet sheet, and for pouring spent baths. Eye protection for the whole tray sequence, because the risk is a splash at eye level when a sheet is dropped in. Closed shoes, because the floor will be wet and dark at the same time.

No respiratory protection, and that is a statement about this session specifically: nothing is weighed, nothing is heated and nothing here produces a vapour that the room’s ventilation does not handle. The control against the substances present is the glove, the tongs and the eye protection.

Ventilation

The room’s normal print-processing ventilation, sized in Part II and provided through the light trap built on the room page. Three trays of working-strength chemistry for two hours is an ordinary printing session and is ventilated like one: air in at the door end, out at the wet end, and the room opened up between the exposure phase and the processing phase.

The one thing this session must not do is close the trap to improve the test. A sealed room is a worse hazard than a marginal leak, and a leak that only appears with the vent open is a result about the vent, which is information rather than a nuisance.

Materials

Material	Quantity	Notes
Photographic paper, the grade and surface you print on	6 to 8 sheets, 5 × 7 in or 8 × 10 in, one box	The whole test is a comparison, so a second box is a second variable. Variable-contrast RC is the course’s default
An opaque masking card, larger than a sheet	1	Rigid, matt black, with one straight edge. A painted offcut of the room page’s foam board
A second opaque card or a light-tight envelope	1	To hold the control in darkness inside the room while the others are exposed
Adhesive labels or a chinagraph pencil	a few	Sheets are identified in the dark; a notch cut in one corner is the classic dark-readable mark
Blotting paper or a squeegee and a clean surface	1	Sheets are read dry

Chemicals

All three baths are made up to working strength before the session, so that nothing is measured in the dark.

Chemical	Quantity	Form
Paper developer — a bought concentrate at its stated dilution, or D-72 made up separately	about 500 mL working solution	Working-strength dish solution at 20 °C
Acetic acid or citric acid stop bath	about 500 mL working solution	Working-strength dish solution
Fixer — sodium thiosulfate or an ammonium thiosulfate rapid fixer at its stated dilution	about 500 mL working solution	Working-strength dish solution; non-hardening
Water for the wash	as the paper’s sheet requires	RC paper washes in minutes; fibre base does not

The developing agents in the developer — metol, hydroquinone — and its sulfite, carbonate and bromide are listed because the page depends on them, not because they are weighed here. A bought concentrate contains the same chemistry in a bottle.

Equipment

The safelight, on its bracket, at the distance recorded on its label. A tape measure. A timer that can be read in the dark without emitting light. Three dishes and three pairs of tongs, marked. A thermometer. A graduate. The enlarger, or the contact printing frame and its light source built later in this part, for the pre-exposure. And, if you have built it, Part XV’s reflection head.

Estimated cost

Cost band £. Two or three sheets of paper, a few hundred millilitres of three working solutions, and two hours. Everything else is equipment you already have because the previous three pages asked for it.

The only line that can surprise you is paper, and only if you run the test on 8 × 10 in when 5 × 7 in would do. It would do: the test needs four areas on a sheet and a control, not a big print.

Estimated consumables cost

Consumed	This session	Sourced price	Cost this session
Photographic paper, variable-contrast RC, 5 × 7 in	4 sheets — one range-finding, one test, one control, one spare	£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	500 mL of 1+9, 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	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	500 mL of 1+4, that is 100 mL of concentrate	£21.05–£25.98 per 1 L of concentrate	£2.11–£2.60
Wash water	a few litres	Not priced by the planner	—
Labels, chinagraph	a few	None	—

The priced rows give £4.97 to £7.77 for one run. That is a floor and not a total: two rows carry no dated price, and the concentrate figures assume the smallest useful volume is mixed and discarded after one session, which is what a controlled test requires and is more wasteful than a printing session would be. Running the extended room test in Procedure stage 6 costs another sheet or two of paper and the same three baths, which are already in the dishes.

Equipment is excluded on purpose: the safelight, the enlarger, the dishes, the tongs, the thermometer and the densitometer are not consumed by this session and would distort the figure the consumables calculator needs.

Alternative route

Without a densitometer. This is the ordinary case and the published procedures assume it. Both manufacturers’ tests are read by eye against a control, and Kodak offer a printed reflection density guide as the comparison aid. The visual method gives a threshold that is conservative — the eye finds a difference later than an instrument does — which means your safe working time comes out longer than a densitometric one would. The margin in the analysis exists partly for that reason.

Without an enlarger. The pre-exposure is a controlled, even, mid-grey exposure and an enlarger is only one way to make one. A bare lamp at a measured distance with a card mask, or the light box of the contact printing frame built later in this part, will do it, provided the illumination is even over the test area and the exposure is repeatable. Use the same source for the control.

Without a darkroom of your own — a borrowed or shared room. The test is a property of a geometry, so a result from a hired darkroom is a result about that darkroom, valid only while your lamp is in the position you tested it in. If you work in a shared darkroom whose lamps you did not install, run the test anyway: you will be measuring somebody else’s installation, which is exactly the situation in which nobody has measured it.

Without a dark room at all. There is no route to this measurement without a space that can be made dark, because the quantity being measured is the light in that space. What a reader without one should do instead is honest and small: keep paper in a changing bag until the moment of exposure, use a daylight tank for film, and accept that contact printing under a bought lamp in a temporarily darkened room is untested until the room exists. The room page lists the temporary blackouts that turn “no dark room” into “a dark room for two hours”.

Waste streams

Three, unchanged from every printing session and owned elsewhere. Spent developer and stop bath go to their labelled containers under the general chemical waste procedure. Spent fixer and the first wash carry dissolved silver and go to silver-bearing waste for recovery, as Part XII teaches. Processed test sheets are ordinary solid waste once dry — they are the record, so keep them.

Preparation

Do all of this in the light, the day before if you can. Nothing here should be happening while you are trying to time a one-minute step in the dark.

Prepare the lamp. Confirm the distance from the lamp to the working plane with a tape measure and write it down. Set the dimmer to the detent you intend to work at, and write that down too. Switch the lamp on and leave it for its warm-up time, from the label — the reading you take from a cold lamp is not the lamp you print under.

Prepare the room. Kodak’s preparation list is the right one and is not optional. Fit a new bulb if your lamp has one. Turn the white lights off; if any are fluorescent, wait five minutes for the residual glow. Confirm there is no white light entering. Confirm no white light is escaping from the enlarger or from the safelight housing. Correct anything you find before spending paper.

Prepare the baths. Three dishes, marked, in a fixed order you will not change: developer, stop, fix. Working-strength dilutions from the manufacturer’s own sheet, at 20 °C, with the thermometer left in the developer. One pair of tongs per dish.

Prepare the sheets. In the dark, take four sheets from one box. Notch one corner of each in the same place, so that in the dark you know which way up and which way round a sheet is. Put one sheet straight back into its light-tight packet: that is the control and it does not come out again until processing.

Prepare yourself. Sit in the dark for the fifteen minutes ILFORD ask for before checking a room, or Kodak’s ten for full adaptation. You are about to make a visual judgement, and you will make it badly for the first ten minutes of the session.

Procedure

Stage 1 — Find the exposure that gives a mid-grey (20 minutes)

You need a printing exposure that lands the paper in the middle of its scale, because that is where the paper actually is when it is most vulnerable and where a small addition is most visible.

With every light off, including the safelight, give one sheet a doubling series of exposures under the enlarger with no negative in the carrier — lens well stopped down, short times. Process it fully and read it in the light.

The target. Kodak specify a light grey of reflection density 0.25 to 0.50, which is 0.15 to 0.40 above the paper’s own density. ILFORD specify a pale grey of approximately 0.2 to 0.3 in density. The two overlap and either is acceptable; pick one, record which, and use it for every sheet in the test including the control.

📓Why the pre-exposure is the experiment, not a preliminary

It is tempting to skip this and expose a blank sheet to the safelight. Kodak name that shortcut and reject it: a test that partially covers a piece of paper with a coin and exposes it to the safelight tests only for fog, and not for the added effects of safelight and enlarger exposure together.

The reason is on the previous lesson’s account of latensification. A grain carrying a latent image is partway to developable and needs few more photons to finish; a fresh grain has to be started from nothing. So a coin test on fresh paper reports a working time longer than the one you will actually have — and reports it confidently.

Stage 2 — Give the “after” exposure (10 minutes)

With every light off, give one end of a fresh sheet the mid-grey exposure from stage 1, using the notch to know which end. Label that end “after”, because the safelight exposure is about to happen after it.

ILFORD are explicit that this is the more critical half of the test: paper is more sensitive to safelight fogging after it has been exposed in the enlarger than before.

Stage 3 — The stepped safelight exposure (15 minutes)

Still in the dark, carry that sheet to the place in the room where the safelight illumination is brightest. Both procedures say to determine this rather than assume it; it is usually over the developer dish, which is also where paper spends longest.

Lay the sheet emulsion up on a card. Then, with the masking card, uncover successive areas so that the four regions receive a geometric series of times.

The test sheet: two enlarger exposures, four safelight times, one control

1. 1Notch in one corner, cut in the light — how you know which way up and which way round a sheet is, in the dark
2. 2AFTER patch: mid-grey enlarger exposure given BEFORE the safelight steps — the critical half — paper fogs more readily once it carries a latent image
3. 3BEFORE patch: identical enlarger exposure given AFTER the safelight steps — checks hypersensitisation; expect it to survive longer than the AFTER end
4. 4Four bands, cumulative safelight times 0, 1, 3 and 7 minutes — ILFORD's alternative series is 0, 1, 2 and 4 minutes; use one, and record which
5. 5Masking card moved stepwise to the right — moved after 1 minute, then 2 more, then 4 more — the increments are not the totals
6. 6CONTROL sheet: same grey patch, no safelight at all — kept in a light-tight envelope inside the same room and processed in the same dish at the same time
7. 7Everything else in the room switched off — enlarger, timer, indicators — those get their own test in stage 6

A plan of the sheet, drawn to the proportions of a 5 by 7 inch piece. The geometry is what makes the result readable: two grey patches at opposite ends of one sheet mean the after and before cases are compared on the same paper, in the same developer, in the same minute.

Kodak’s series is 0, 1, 3 and 7 minutes, made by moving the card after 1 minute, then after 2 more, then after 4 more. ILFORD’s is 0, 1, 2 and 4 minutes. Either is a published series; use one, write down which, and never mix them, because the whole point of a geometric series is that the ratio between adjacent steps is constant and a threshold can be bracketed.

Everything else in the room is off. The enlarger, the timer display, the phone that is not in the room anyway. They get their own test in stage 6 and they are not allowed into this one.

Stage 4 — Give the “before” exposure (5 minutes)

Still in the dark, give the other end of the same sheet an identical mid-grey exposure, using the notch to place it so that it does not overlap the first. Label it “before”.

You now hold one sheet carrying two grey patches and four safelight durations, and one control sheet in an envelope carrying one grey patch and no safelight at all.

Stage 5 — Process everything together (25 minutes)

Take the control out of its envelope in the dark, and process both sheets together in the same dish: full development to the manufacturer’s time and temperature, then stop, then fix, then wash. Kodak’s instruction is to process in total darkness, and it is not a formality — developing under the lamp you are testing puts the lamp back into the experiment at the one moment the paper is wettest and most sensitive.

Full development matters. Pulling a print early because it looks right is a technique for making prints and a disaster for making measurements: it makes development time a second variable and it suppresses exactly the low-density difference you are looking for.

Dry both sheets before reading them. Wet paper is darker and the difference you are hunting is small.

Stage 6 — Now test everything else in the room

The lamp you built is rarely the worst light in the room. Repeat stages 2 to 5, one source at a time, with the safelight off and the source under test on:

• The enlarger lamphouse, lamp on and the lens capped. This measures what leaks from the head, the bellows and the negative carrier rather than what comes through the lens.
• The timer’s display, at the distance and orientation it actually faces the bench from.
• Every indicator LED: the power supply, a charger, a router, the smoke alarm, the extractor.
• The door edge and the vent, with the corridor light on — which is the leak test from the room page run again with a pre-exposed sheet, and it is a harsher test than that page’s.
• The whole room, everything on at once, which is the only test that measures what a printing session actually exposes paper to.

The last of those is the number that governs your working practice. The individual ones tell you what to fix first.

Expected observations

A room and lamp that pass show a test sheet on which the four bands are indistinguishable at both ends, and a test sheet indistinguishable from the control. Kodak’s Test A is this: no added density on either side, and a safe time of at least the longest step.

The typical result is not that. Kodak describe their Test C as more typical: the paper is safe for up to 7 minutes of safelight exposure before the enlarger exposure, and only up to 3 minutes after it, so the conditions are safe if total safelight exposure is limited to 3 minutes. Expect the “after” end to fail first. If it does not — if your “before” end fogs first — something is wrong with the experiment rather than interesting about your paper, and the first thing to check is that the two patches really did receive identical exposures.

A room that fails badly shows a visible step edge on the “after” end at the first or second band, and the highlights of the whole test sheet look milky beside the control.

The control should look like nothing happened to it, because nothing did. If it does not, read the troubleshooting section before you conclude anything about your lamp.

What is happening chemically

Two things, and only the second is chemistry in the ordinary sense.

The exposures add. Kodak’s term for the combination of a printing exposure and a low-level overall exposure is a super-additive exposure, and their statement is that virtually all exposures are cumulative and can cause both density and contrast changes. The paper does not know that one exposure came from an enlarger and the other from a lamp on the ceiling. It integrates.

Where the addition lands is what makes it visible. On the paper’s characteristic curve, a uniform safelight exposure shifts every point to the right by the same amount in log exposure — but the density consequence of that shift depends on the local slope. In the toe, where the highlights live and the curve is shallow, a shift lifts density out of the toe and destroys the separation between the lightest tones. On the straight line it moves everything together and is barely noticed. Near the shoulder, where the paper is already close to its maximum black, it does almost nothing at all.

Where safelight fog lands on a paper's curve

• Clean paper
• The same paper with a uniform safelight fog added

Show the numbers behind this plot

Two S-shaped curves on axes of relative log exposure against reflection density, running from a paper-white base of about 0.06 up to a maximum black of about 2.1. The lower curve, drawn solid and labelled clean paper, leaves the base flat, rises through a toe, straightens steeply, and shoulders off at the top. The upper curve, drawn dashed and labelled the same paper with a uniform safelight fog added, is the same curve displaced to the left along the exposure axis by a small constant amount. The two curves are indistinguishable near the top, where both are approaching maximum black, and diverge markedly at the bottom: at the lowest exposures the fogged curve has already left the base while the clean one has not, so the paper's white is no longer white. Two shaded regions mark the consequence. A region low on the exposure axis is labelled highlights, and within it a vertical gap between the two curves is annotated as the separation that has been lost, since a range of exposures that produced distinguishable pale tones on the clean paper now produces nearly the same density on the fogged one. A second region high on the exposure axis is labelled shadows, and there the two curves lie almost on top of one another, annotated as the reason the print does not look darker overall. A horizontal guide at density 0.06 marks base plus fog for the clean paper; the fogged curve leaves the left edge of the plot at about 0.10, above that guide, which is the raised base the safelight has contributed. The whole drawing says that a small uniform exposure costs highlight separation and maximum black and does not simply darken the print.

Series	Relative log exposure	Reflection density
Clean paper	-2.40	0.06
Clean paper	-2.00	0.06
Clean paper	-1.70	0.07
Clean paper	-1.50	0.10
Clean paper	-1.30	0.18
Clean paper	-1.10	0.34
Clean paper	-0.90	0.60
Clean paper	-0.70	0.94
Clean paper	-0.50	1.32
Clean paper	-0.30	1.66
Clean paper	-0.10	1.91
Clean paper	0.10	2.04
Clean paper	0.30	2.09
Clean paper	0.60	2.11
The same paper with a uniform safelight fog added	-2.40	0.10
The same paper with a uniform safelight fog added	-2.00	0.12
The same paper with a uniform safelight fog added	-1.70	0.16
The same paper with a uniform safelight fog added	-1.50	0.22
The same paper with a uniform safelight fog added	-1.30	0.33
The same paper with a uniform safelight fog added	-1.10	0.52
The same paper with a uniform safelight fog added	-0.90	0.80
The same paper with a uniform safelight fog added	-0.70	1.14
The same paper with a uniform safelight fog added	-0.50	1.50
The same paper with a uniform safelight fog added	-0.30	1.80
The same paper with a uniform safelight fog added	-0.10	1.99
The same paper with a uniform safelight fog added	0.10	2.07
The same paper with a uniform safelight fog added	0.30	2.10
The same paper with a uniform safelight fog added	0.60	2.11

Shapes drawn to teach, not measured from a paper. What is taken from published sources is the direction of the effect and its location: Kodak state that excessive safelight exposure degrades highlights and lowers print contrast before fogging is visible in the borders, and ILFORD describe the symptom as reduced contrast and a lack of clear highlights. Your own paper's curve comes from Part XIII. 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 chemistry underneath is the latent image: the safelight’s photons produce silver specks on grains that already carry sub-developable specks from the printing exposure, tipping them over the threshold at which the developer will reduce the whole grain. That is why the effect is disproportionate, why it is worse after the printing exposure than before, and why the developer amplifies a difference in photons into a difference in density you can see.

Data to record

Everything in this list, on one sheet, in the calibration records worksheet:

• The lamp: its emitter, drive current, dimmer setting and distance to the working plane, copied from its label.
• The geometry: where in the room the test was run, and why that position was chosen.
• The paper: make, grade, surface, size and batch, and the date the box was opened.
• The pre-exposure: source, aperture, time, and the reflection density it produced, with the target you were aiming at.
• The step series used, 0/1/3/7 or 0/1/2/4, and the clock times of each card move.
• The processing: developer and dilution, temperature, development time, agitation, stop and fix times.
• The result: the shortest time at which a difference is visible on the “after” end and on the “before” end, judged how, under what light.
• The derived safe working time, and the margin applied to it.
• The date, and the room certificate’s date, because a lamp result is only valid inside a room result.

Analysis

1. Find the threshold

On the “after” end, find the shortest step at which you can see a difference from the 0-minute band. That is your first detectable change. Do the same on the “before” end; the two will usually differ and the “after” one governs.

Judge it under a stated light. The course could not source a print-viewing standard in its corpus, so it does not cite one. What it does instead is fix its own condition and ask you to record it: both sheets dry, side by side, touching along the edge being compared, under an even diffuse white light bright enough to read small print, with the safelight off and the room lit normally. Write down what that light was. A threshold judged under a desk lamp and one judged at a north window are not the same threshold, and only one of them is repeatable.

Where you have a reflection densitometer, read the 0-minute band and each other band and record the differences in density units. That converts an opinion into a number you can compare with your own future tests. It does not, on its own, give you a pass mark — see the next paragraph.

2. Apply the published criteria, and be clear which is which

Three published statements bear on the verdict and they are not the same statement.

Kodak’s definition of a safe time: an exposure time less than or equal to one half of the time required to produce a detectable change. This is the rule this course uses, because it is a definition rather than a threshold and it applies whatever your detector is.

ILFORD’s pass criterion: no density change between the 0-minute and 4-minute areas means the conditions are safe; a change of about 0.04 in density after just one minute means they are inadequate.

And an inconsistency the course reports rather than hides. The same ILFORD passage also says that “typically good results” will leave a density change of 0.2 to 0.4 on the “after” strip after four minutes of safelight. A change of 0.2 is five times the 0.04 that the previous sentence calls inadequate. The course cannot reconcile the two and does not try: it uses the 0.04-at-one-minute figure as a published indication of what “detectable” means on paper, and it uses Kodak’s half-the-time definition to turn a detected change into a working time. No density pass mark is invented here, because none is available.

🧮Working the rule

Suppose the “after” end first shows a difference at the 3-minute band and nothing at 1 minute.

Your first detectable change lies between 1 and 3 minutes, and a geometric series only brackets it — that is what a series is for. Take the largest time that showed nothing, which is 1 minute, and take the smallest that showed something, 3 minutes.

Kodak’s rule halves the time at which a change appears: 3 ÷ 2 = 1.5 minutes. That is your safe working time, and it is conservative in two ways at once, because 3 minutes was an upper bracket and because a visual threshold is later than an instrumental one.

Now measure your real handling time with a watch — box to fixer, in an ordinary session. If it is under 1.5 minutes, you are inside the number. If it is 4 minutes, you are not, and stage 12 of the SOP lists the remedies in order of cost.

3. Write the label and the record

Fill in the four blanks left on the lamp’s label: the paper, the distance, the dimmer setting, the safe working time — and the date. Put the same block in the calibration record.

The label is written on the lamp rather than in a notebook for one reason: the next person to wonder whether the room is safe, including you in eighteen months, will look at the lamp.

4. Read a failure honestly

A failed test tells you exactly one thing: at this distance, at this setting, on this paper, the working time is shorter than you need. It licenses four responses, all of which change something measurable, and ILFORD and Kodak between them name all of them: move the lamp further away, fit a lower output, use only indirect light, or develop part of the time with the lamp switched off.

It does not license the sentence “the safelight is nearly safe”. There is no such state. There is a working time, and either your session fits inside it or it does not.

Troubleshooting

The control sheet is grey. Not a safelight result. Three causes and a test for each. Fogged paper — open a fresh box and repeat one sheet. Developer fog — check temperature and age; a warm, over-worked or under-restrained developer raises base fog, which Part VIII explains and the troubleshooting atlas separates from the alternatives. A white-light leak during processing — repeat the leak test from the room page.

Nothing develops at all, on any sheet. Contamination. ILFORD’s warning is the diagnosis: a trace of fixer or stop bath in the developer gives inconsistent results or completely blank prints. Fresh developer, clean dish, one pair of tongs per bath, and repeat.

The “before” end fogs before the “after” end. The expected order is the other way round, so suspect the experiment. Most likely the two grey patches did not receive the same exposure — check that the lens aperture and the timer were untouched between stages 2 and 4 — or the sheet was turned over between them.

A difference appears at one minute. ILFORD’s published indication is that a change of about 0.04 in density after a single minute means the conditions are inadequate. Do not compute a safe time from it. Change the geometry and test again.

Every band looks the same but prints still show milky highlights. The safelight passed and something else did not. Run stage 6: the enlarger lamphouse with the lens capped is the usual culprit, and it is the one source in the room that is on during the exposure.

The result is different from last month’s, with nothing changed. Something changed. The most common candidates in order: the lamp has been knocked and the distance is not what the label says; the dimmer was moved; the paper is a new box or a new emulsion; a new device in the room has an indicator on it; the corridor beyond the door is lit differently. Re-measure the distance first, because it is the one with a quadratic effect.

Clean-up

Sheets out of the wash and onto a drying rack or blotter. Baths poured into their own labelled containers — never into each other. Dishes rinsed in the order developer, stop, fix and left to dry face down; tongs rinsed and hung with their own dish. Thermometer rinsed and put away dry. The safelight is left exactly where it is, because moving it voids what you have just spent two hours measuring; if it must come down, mark its bracket position first.

Wipe the bench, dry the floor, and put the lights on before you carry anything.

Storage

The test sheets are the record. Dry them flat, write the date, the lamp and the step series on the back in pencil, and keep them with the calibration record. A sheet from a year ago beside today’s is the fastest way to see that a lamp has drifted.

Working-strength baths do not keep well and paper developer keeps worst of all; discard them rather than carrying them forward into a printing session, since a partly exhausted developer would make the next test incomparable with this one. Concentrates go back to their own bottles, capped, labelled and dated, as Part II sets out.

The paper box goes back into the dark, with the date it was opened written on it, because the box’s own age is one of the things that can make the next control sheet grey.

Disposal considerations

Spent developer is alkaline and carries oxidised developing agents; spent stop bath is a dilute acid. They are neutralised by each other only if you do it deliberately and with care, and the course’s practice is to keep them in separate labelled containers rather than mixing streams in a bucket.

Spent fixer carries dissolved silver as thiosulfate complexes, and so does the first wash. That is not a drain stream: it is the stream Part XII exists for, and the chemistry of why is on its pages. Collect it.

Local regulation governs all of this, and it differs between authorities and between countries. Read the disposal caveat, which is the course’s standing statement on what it can and cannot tell you, and check your own authority’s rules before anything leaves the house.

Questions

1. Your “after” end shows nothing at 1 minute and a clear difference at 3. State your safe working time, show the arithmetic, and say in one sentence why the answer is a bracket rather than a measurement.
2. Give three causes of a raised density on the control sheet that are not the safelight, and the single test that distinguishes each.
3. A printer proposes testing their safelight by putting a coin on a sheet of unexposed paper for ten minutes. Give the published objection, and then explain the mechanism that makes the objection correct.
4. Design the test that would establish a safe working time for a hand-coated orthochromatic emulsion from Part V. Say which parts of this procedure transfer unchanged, which need a number you would have to find, and what you expect to change in the result.
5. Your densitometer says the 3-minute band is 0.03 above the 0-minute band; your eye says they are identical. What do you record, what do you do, and which of the two published criteria bears on the answer?
6. You test the safelight alone and it passes at 7 minutes. You then test the room with everything on and it fails at 1 minute. What have you learnt, and what is the first thing you would switch off?
7. Explain why full development matters more in this experiment than in ordinary printing, and what a pulled print would do to the threshold you measured.

Further experiments

Test the same lamp at two distances and check the inverse-square prediction. Run the whole procedure at 1.0 m and again at 1.7 m. The previous lesson predicts the working time should go up by (1.7 ÷ 1.0)² ≈ 2.9. Whether it does is a real measurement of how far your diffused lamp is from being a point source, and the course has not made it.

Test direct against indirect at the same working illuminance. This is the question the build page explicitly deferred to measurement: set the direct head and the indirect head to give the same illuminance at the tray, then fog-test both. The answer decides how a second head should be used and the course does not know it in advance.

Test the same room on two papers. A variable-contrast paper and a graded one, or an RC and a fibre base. The safe working time belongs to the paper as much as to the lamp, and two numbers on one lamp label is a more honest label than one.

Extend the procedure to sheet film. Kodak’s parallel test for blue-sensitive and orthochromatic sheet film uses steps of 0, 4, 7 and 13 minutes and reads the result against the film’s own gross fog — the transmission density of a completely unexposed, normally processed sample. It substitutes a camera exposure of a grey card on a black ground for the enlarger’s grey patch. If you handle ORTHO Plus in this room, this is the test that clears it.

Repeat the whole test after a year and change nothing. The most useful experiment on this page is the one that takes twelve months and five minutes of work: same lamp, same setting, same distance, same paper if you can still buy it. An LED lamp has no filter to fade, so if the number moves, the answer is in the room rather than in the lamp.

Recap

The test measures one thing: the longest safelight exposure that adds nothing detectable to paper that has already been printed on. Pre-exposure is the whole design, because paper carrying a latent image fogs far more readily than fresh paper, and a coin on a blank sheet reports a working time you will never have.

Four cumulative safelight times in a geometric series, two identical grey patches at opposite ends of one sheet to separate latensification from hypersensitisation, and a control from the same box that never saw the lamp. Everything is developed together, to full development, in the dark, and read dry under a light you wrote down.

Kodak’s definition converts the threshold into a rule — the safe time is half the time at which a change first appears — and ILFORD’s 0.04 density at one minute says what “detectable” has meant to somebody who measured it. Where the two sources disagree, this page says so and invents nothing.

Then the same method goes round the room: the lamphouse with the lens capped, the timer, the indicator lamps, the door, and finally everything at once, which is the only number that describes a real session. And a failure is not a lamp that is nearly safe. It is a working time, and a decision about distance, output, or how long paper is out of its box.

Check your understanding

Question 1. Why does this procedure give every test piece a mid-grey printing exposure before the safelight steps?

• So the fog is easier to see against grey than against white
• Because paper carrying a latent image fogs far more readily than fresh paper, so a test on unexposed paper reports a working time longer than the printer will actually have — and Kodak state directly that a coin-on-blank-paper test checks only for fog and not for the added effects of the two exposures together
• To use up the paper that would otherwise be wasted
• Because the developer needs a developed image to work against

Show the answer and why

Answer: Because paper carrying a latent image fogs far more readily than fresh paper, so a test on unexposed paper reports a working time longer than the printer will actually have — and Kodak state directly that a coin-on-blank-paper test checks only for fog and not for the added effects of the two exposures together

The pre-exposure is the experiment, not a preliminary. A grain that already carries a sub-developable speck needs only a few more photons to tip past the threshold at which the developer will reduce the whole grain; a fresh grain has to be started from nothing and its sub-threshold events decay. ILFORD state that paper is more sensitive to safelight fogging after the enlarger exposure than before, and treat the "after" strip as the critical one for that reason. Option one is a real but secondary benefit — a mid-grey does make a small change easier to judge — and it is not why the step exists.

Question 2. Your "after" end shows no difference at 1 minute and a clear difference at 3 minutes, using the 0, 1, 3, 7 series. What is your safe working time?

• 3 minutes, the time at which the change appeared
• 1.5 minutes: Kodak define a safe time as an exposure less than or equal to half the time required to produce a detectable change, and 3 minutes is the shortest step at which one appeared
• 1 minute, the last step that showed nothing
• 7 minutes, because the series ran that far

Show the answer and why

Answer: 1.5 minutes: Kodak define a safe time as an exposure less than or equal to half the time required to produce a detectable change, and 3 minutes is the shortest step at which one appeared

Kodak's definition is the one this course uses because it is a definition rather than a threshold: the safe time is half the time at which a detectable change appears, so 3 ÷ 2 = 1.5 minutes. Option three is not wrong so much as unjustified — the last clean step is a bracket, not a limit, and it carries no margin. The answer is conservative twice over: the true threshold lies somewhere between 1 and 3 minutes, and a visual judgement finds a difference later than an instrument would, so the real margin is larger than the arithmetic shows.

Question 3. The control sheet — same box, same grey exposure, no safelight at all — comes out visibly grey. What have you measured?

• A safelight that leaks even through a light-tight envelope
• Nothing about the safelight. The control isolates everything that is not the lamp, so a grey control points at the paper, its storage, the developer, or a white-light leak during processing, and each has its own test
• That the developer was too dilute
• That the test is void and must be repeated from the beginning with a different paper

Show the answer and why

Answer: Nothing about the safelight. The control isolates everything that is not the lamp, so a grey control points at the paper, its storage, the developer, or a white-light leak during processing, and each has its own test

The control exists to answer exactly this question. It received the printing exposure and the same processing and no safelight, so anything it shows arrived from somewhere else: an old or badly stored box, a developer that is warm, exhausted or short of restrainer, or white light reaching the paper during processing. Each has a test — a fresh box, fresh developer at a measured temperature, a repeat of the room leak test. Option four is the counsel of despair: a control that fails still tells you which half of the experiment failed, which is what a control is for.

Question 4. ILFORD say a change of about 0.04 in density after one minute means the conditions are inadequate, and in the same passage that "typically good results" leave a change of 0.2 to 0.4 after four minutes. How does this course handle that?

• It picks the more permissive figure, since the source is a manufacturer
• It reports the inconsistency, uses the 0.04-at-one-minute figure as a published indication of what "detectable" means on paper, uses Kodak's half-the-time definition to turn a detected change into a working time, and invents no density pass mark of its own
• It picks the stricter figure and states it as the pass mark
• It ignores ILFORD and uses only Kodak

Show the answer and why

Answer: It reports the inconsistency, uses the 0.04-at-one-minute figure as a published indication of what "detectable" means on paper, uses Kodak's half-the-time definition to turn a detected change into a working time, and invents no density pass mark of its own

Two numbers five times apart in one passage cannot both be a pass criterion, and quietly choosing one would hide a real problem in the source from the reader. The course's rule is that where it has no sourced threshold it says so rather than making one up. What survives from ILFORD is a useful published sense of what magnitude of density change a person has called detectable on paper; what supplies the rule is Kodak's definition, which does not depend on a threshold value at all because it is stated in terms of whatever your own detection is.

Question 5. You test the safelight alone and it passes out to 7 minutes; you then test the room with everything switched on and it fails at 1 minute. What follows?

• The first test was wrong and should be repeated
• Both results are correct and they measure different things. Your working practice is governed by the whole-room figure, and the individual source tests in stage 6 tell you which source to fix first — the enlarger lamphouse being the usual culprit, since it is on during the exposure
• The room fails and there is nothing to be done
• The safelight must be moved further away

Show the answer and why

Answer: Both results are correct and they measure different things. Your working practice is governed by the whole-room figure, and the individual source tests in stage 6 tell you which source to fix first — the enlarger lamphouse being the usual culprit, since it is on during the exposure

The lamp you built is one of the light sources in the room and rarely the worst. Kodak's own list of what fogs paper in a room with a correct safelight includes light escaping from an enlarger head and lighted dials on equipment controls; a modern darkroom adds timer displays, chargers, routers and smoke alarms. The whole-room test is the one that describes a real printing session, and the per-source tests are the diagnosis. Option four fixes the one thing the first test showed was already fine, which is the most common wasted afternoon in this part of the course.

Question 6. Why does the procedure insist on full development rather than pulling the sheets when they look right?

• Because full development gives a darker print and darker prints are easier to compare
• Because pulling a print early makes development time a second variable and suppresses exactly the low-density difference the test is looking for; the experiment has one variable, and it is safelight time
• Because the fixer will not clear an under-developed print
• Because the manufacturer's time is a legal requirement

Show the answer and why

Answer: Because pulling a print early makes development time a second variable and suppresses exactly the low-density difference the test is looking for; the experiment has one variable, and it is safelight time

Inspection development is a printing technique and a measurement error. The fog you are hunting is a small increment of density at the low end of the scale, which is precisely the part of the image that comes up last; stopping early removes it from the sheet before you can see it, and does so by an amount that differs between sheets. Since the whole design holds everything constant except safelight time, a development time that varies from sheet to sheet puts a second variable into the experiment and the result no longer belongs to the lamp.

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