Safelight-fogged control strip

Lifted from paper sensitometry, whose closing section argues that safelight fog is a sensitometric shift rather than a general greyness, and from process control with control strips.

What you see

D-min up, highlights closed, everything else where it was. On a control chart the fog row moves and the speed and contrast rows barely do. On the strip itself the clear end is no longer clear and the last few steps before it have lost their separation.

That combination is the diagnosis, and it is different from the general safelight fog entry in one important way: this entry is the fault seen sensitometrically, on a strip, by measurement, which is what makes it separable from age, heat and chemical fog rather than merely nameable.

Likely causes

1. The filter. Wrong for the material, faded, or cracked. Kodak list all three first.
2. The lamp and its geometry. Wattage too high, distance too short, or simply too many safelights in the room.
3. Time. ILFORD’s recommendation for MULTIGRADE RC is no more than four minutes of direct illumination at not less than 1.2 m; a printing session runs longer than four minutes.
4. Something that is not the safelight. Kodak name light escaping from an enlarger head, lighted dials on equipment controls, and non-opaque construction materials in the same sentence as the filter itself.

What is happening: the chemistry and physics

A safelight is not a neutral part of the room. ILFORD define one as illumination that does not cause a significant visible change to the material during use, and note in the same sentence that “safe” is relative. Kodak put the physics behind it: the colour sensitivity of most emulsions does not end abruptly at a wavelength, most materials retain some sensitivity to the colours a recommended filter transmits, and safelight exposure should always be minimised.

What that residual sensitivity does to a curve is specific. A small uniform exposure added everywhere lifts the bottom of the curve, where the slope is nearly zero. It raises D-min and it compresses the highlights, which are the tones sitting on that flat foot. Mid-tones and shadows barely move, because there the curve is steep and a small added exposure is negligible beside what they already received.

So the symptom is not a grey print. It is a print whose whites are not white and whose highlight separation has quietly gone, with everything else looking normal — which is the failure that is hardest to see, because there is nothing in the room to compare against.

!The confirming test: two strips, one of which never met the room

Process a strip that was loaded in total darkness alongside one handled normally, in the same tank, in the same run.

• Both fogged: the material or the developer. Go to base fog and chemical fog.
• Only the handled strip fogged: light in the room, and the safelight test SOP is the next step.

Then heed Kodak’s warning about the obvious test. A coin laid on paper under the safelight tests only for fog, not for the added effects of safelight and enlarger exposure together — and excessive safelight exposure may show up only in the image area, because that area received the enlarger’s light as well. A coin test can pass while the working material fogs. Kodak’s own test exposes the paper through the enlarger first and then to the safelight, and that order is the point of it.

Diagnostic questions

• Which material, and for how long? Safe time is a property of the pair. A four-minute figure for RC paper says nothing about a panchromatic film, which has no safe safelight at all.
• How far away, and how many? Distance and count both multiply. Kodak’s guidance spaces direct lamps at least 2.5 m apart for a reason.
• How old is the filter? A faded filter transmits more, and it fades slowly enough that nobody notices.
• What else in the room is lit? Timers, thermometers, exposure monitors and enlarger heads. A digital display is a light source pointed at the bench.
• Did D-min move without speed or contrast moving? That asymmetry is the fingerprint. Fog with contrast up is over-development; fog with speed down is bromide restraint; fog alone is light or material.

Corrective action

Nothing recovers the strip, for the reason elevated fog gives: density added uniformly cannot be subtracted selectively.

Find the source before changing the filter, because the commonest answer on a home bench is not the safelight. Work in complete darkness for one session and see whether the fault goes; if it does, put the sources back one at a time.

Then re-measure D-min and re-establish the aim, because a control chart whose aim was set under a fogging safelight has the fog built into it.

Prevention

Test for the material and the time you actually use, by Kodak’s method rather than the coin, and repeat the test when the filter, the lamp or the paper changes.

Keep paper in a light-tight box and handle it emulsion side down, away from the lamps, which is Kodak’s own advice and costs nothing.

Put the safelight behind you and above, not over the developing dish where the paper sits longest, and never where it shines on the easel.