Testing a safelight

Purpose

To turn the lamp you have been calling a safelight into a number: the longest time a sheet of your paper may spend under it before the light it is giving off starts to show on the print.

When to use it

Before the first printing session on a new paper, after any change to the lamp, the bulb, the filter or its position, and once a year for the filter’s age alone. It is a property of a geometry, not of a lamp: move the lamp and the verdict is void.

It does not clear the room. The test measures the one brightest point at the one distance you tested, so a second lamp, a reflective ceiling or a glowing timer display is outside it.

Before you start

You should already have run lab opening, read the safelight recommendation on your paper’s own sheet, and know that paper is more easily fogged after it has been exposed than before, which is the latensification effect and the whole reason for the two exposures below.

On the bench: about six strips of the paper you actually print on, cut under the safelight and notched at one corner; an opaque masking card; the enlarger or a lamp clamped at a measured height that will not move; a timer; developer, stop, fixer and wash at their published dilutions and 20 °C; the record sheet; the labelled waste containers, silver-bearing among them.

Procedure

1. Fix the geometry and write it down: the filter and its age, the bulb wattage, the distance from lamp to working surface, and where the lamp points. Nothing moves again, during the test or after it.
2. Range-find the white-light exposure. Give one strip a doubling series under the enlarger or lamp, process it fully, and find the step where a just-perceptible tone first appears. If tone appears on the first step, move the lamp back or stop down and repeat; if nothing shows, come closer.
3. Set the test exposure at a light grey tone — a reflection density of about 0.25 to 0.50, which is roughly two stops above the threshold from step 2.
4. With every light off, give one end of a fresh strip that grey-tone exposure, and label that end “after”.
5. Carry it, still in the dark, to the point in the room where the safelight illumination is brightest — usually over the developer tray.
6. Lay it emulsion-up on a card and, masking with the opaque card, give successive areas 0, 1, 2 and 4 minutes of safelight. The alternative published series is 0, 1, 3 and 7 minutes; use one or the other and write down which.
7. In the dark again, give the other end of the same strip an identical grey-tone exposure and label it “before”.
8. Process the strip in total darkness alongside an unexposed strip that has had no safelight exposure at all, to the same times and temperatures you print at.
9. Dry both and read them in even light against the 0-minute area of the same strip.
10. Find the shortest safelight time at which any difference is visible on the “after” end. Halve it. That is your safe time.
11. Measure your real working time: from taking a sheet out of the box to putting it in the fixer, in a normal session, with a watch.
12. Compare the two. If the safe time is shorter than the working time, the room fails, and the remedies in order of cost are: move the lamp further away, fit a lower-wattage bulb, replace an aged filter, use only indirect light, or develop with the safelight switched off.
13. Route the spent baths — developer and stop to their own containers by the general waste procedure, fixer and the first wash to silver-bearing waste — and read the disposal caveat before any of it leaves the house.

How you know it worked

• A room that passes shows no difference between the 0- and 4-minute areas at either end.
• A room that fails shows it on the “after” end first, and only at longer times on the “before” end. That order is the evidence that you tested the right thing.
• The control strip, which saw no safelight at all, is as clean as the 0-minute area.
• The record carries a time in minutes, the geometry it belongs to, the paper it was measured on, and the date.

If a step fails

Step 2, every step of the series is black. A white-light leak, or paper already fogged. Sit in the room with the door shut and your eyes adapting for a full fifteen minutes and look again — for the door edge, the enlarger head and any indicator lamp — before spending more paper.

Step 2, nothing appears at any step. Contaminated developer, or the strip went in emulsion side down. A trace of fixer or stop in the developer gives blank prints. Mix fresh developer into a clean tray and repeat one strip.

Step 6, you have no way to time four minutes in the dark. Use a clock that does not light the room. A phone screen face-up at the tray is a second light source, and it is in the frame of the test.

Step 9, the 0-minute area is itself fogged. That is not a safelight result: the fog arrived before the test did. Suspect the paper’s age or storage, a white-light leak, or chemical fog from the developer, and settle that first — the entries in the troubleshooting atlas separate the four.

Step 10, a difference appears at one minute. A density change of the order of 0.04 after a single minute is the published criterion for conditions that are inadequate. Do not compute a safe time from it; go straight to step 12 and change something.

Step 12, the safe time passes but prints still show veiling. Suspect the geometry you did not test. Repeat at a second position, and remember the published limit on direct illumination for graded paper is a few minutes at the recommended distance whatever your own figure says.

Somebody suggests putting a coin on a sheet instead. A coin test checks only for fog, not for the added effect of safelight and enlarger exposure together, so it can pass a lamp that is quietly flattening your highlights. It is not a shortcut to this procedure.

What to record

One block on the record sheet: the date; the paper, its grade and batch; the filter, its age, the bulb wattage, the lamp distance and where it points; which step series you used; the shortest time at which a change was visible on each end; the safe time; your measured working time; the verdict; and what you changed if it failed. Put the same date on the lamp itself, because the next person to wonder whether the room is safe will look there first.