Timer display fog

Owned by Break/Fix — the timer that lies, which names this entry as the compressed version of the instrument treated as a source of fog, and by Part XVI’s break/fix page, whose fog differential gives the signature in one line: a local patch shaped like whatever faced the paper, rather than a veil or a gradient across the sheet. Both were written after this entry, and the timer page states the rule for the pair — where a page and an atlas cell disagree, the page is right and the cell is a bug. This entry was built from Kodak’s K-4 and ILFORD’s beginner’s guide.

What you see

A soft patch of fog, roughly the shape and position of something on the bench. It is usually faint, often noticed first as highlights that will not stay white on one side of a print, and it is in the same place on sheet after sheet because the bench does not move.

Against a room leak: a room leak has a hard direction from a small aperture, usually at the edge of the sheet. A display is close, broad and low, so its fog is a soft pool rather than a gradient across the whole sheet.

Likely causes

1. A lighted instrument facing the working surface. A timer, a thermometer, an exposure monitor, a process controller.
2. An indicator lamp on a power strip, a charger or a heater — the ones nobody counts as darkroom equipment.
3. A display whose colour the paper still sees. A red display is not automatically safe; Kodak’s own physics note is that emulsion sensitivity does not end abruptly at a wavelength and most materials retain some sensitivity to the colours a recommended filter transmits.
4. Time. A source too weak to fog in ten seconds fogs in ten minutes, and a printing session is not ten seconds.

What is happening: the chemistry and physics

The chemistry is ordinary fogging: photons arriving where no image was intended, moving grains towards developability. What makes this entry worth separating from the general safelight fog entry is the exposure arithmetic.

Photographic exposure is intensity multiplied by time, and both factors here work against you. Intensity falls off with the square of distance, and a display on the bench beside the developing dish is a great deal closer than a safelight at ILFORD’s recommended 1.2 m. Time is not the four minutes ILFORD specify for direct safelight illumination on MULTIGRADE RC; it is the whole session, because the display is never turned off.

A source at a tenth the distance delivers a hundred times the intensity for the same emitted power, and if it is on for ten times as long the product is a thousand. That is why a display too dim to read across the room can fog paper on the bench beside it.

!The confirming test: mask the display, change nothing else

Tape black card over every lighted display in the room and repeat the print exactly.

If the fog goes, you have the cause and you also have the cure. If it does not, the display was innocent and the room leak and enlarger head entries are the next two tests.

The stronger version of the test, when several sources are candidates, is to put an unexposed sheet on the bench with a coin on it and leave it for the length of a session with everything switched on as usual, then repeat with the displays masked. Two sheets, one variable, and the answer is on the second one.

Note Kodak’s caution about this whole family of tests: a coin test checks for fog alone and not for the combined effect of room light and enlarger exposure, and fogging may show up only in the image area. For paper, run the sheet through the enlarger first.

Diagnostic questions

• What is lit, and which way does it face? Walk the room in the dark, after five minutes of adaptation, and count.
• How long is the paper out of its box? A dark drawer shortens that to seconds; an open packet on the bench does not.
• Is the fog in the same place every time? Fixed geometry means a fixed source, and a fixed source is on the bench.
• Does the fault survive with the safelight off? If it does, the safelight is not the cause.
• Is the display dimmable, or can it be turned away? The cheapest fixes first.

Corrective action

Mask, turn away, or move. A strip of black card, a display turned to face the wall, or an instrument moved off the wet bench are the three fixes and they cost nothing.

Do not accept “it is red, so it is safe.” Test the specific instrument against the specific paper by the method above, because the claim is about a filter and the fault is about a spectrum.

Re-run the fog test after the fix, so that the improvement is measured rather than assumed.

Prevention

Keep the paper in a lighttight container until the moment it is used, which is Kodak’s own advice and the reason for the dark drawer in the room-leak entry.

Handle paper emulsion side down, also Kodak’s, which halves the exposure of the sensitive surface to everything in the room.

Audit the room’s light sources whenever anything is added to it. A new charger is a new safelight nobody tested.