Safelight fog

Lifted from Break/Fix — emulsion coating defects, entry 11, and from the six-signature comparison in commissioning the pinhole camera. The register names Part XXVIII as this row’s owner and records that the test procedure is owned by Part XVI; neither part is written yet, so the entry is lifted from the written pages that established both, and the test below is the one those pages use.

What you see

A veil that is uniform right to the cut edge of the sheet, because the sheet was out of the camera when it happened, and it grows with handling time.

Its mild form is the important one. ILFORD warn that low-level fogging may show not as visible safelight fog but as a general loss of contrast, so a print that will not snap and a negative whose shadows are grey are both symptoms of it before anything looks foggy.

Likely causes

1. A filter of the wrong class for the material. An orthochromatic emulsion needs a different filter class from a chloride paper.
2. Too long under the light, or too close to it.
3. A material whose sensitivity extends further into the filter’s pass band than the filter’s recommendation assumed — which is normal rather than exceptional, see below.
4. A light leaking around the filter, or a cracked filter.

What is happening: the chemistry and physics

A safelight is not safe; it is slow. Kodak’s darkroom illumination guidance states the position plainly — the colour sensitivity of most emulsions does not end abruptly at a wavelength, and most papers and films retain some sensitivity to the colours that a recommended safelight filter transmits.

So the filter buys time rather than immunity, and safelight exposure is therefore minimised rather than treated as free. Every minute under the lamp adds exposure to the whole sheet, and the addition is uniform, which is why it reaches the cut edges and why it costs contrast at the bottom of the scale first.

Diagnostic questions

• The four-step test, run on your own material. Lay a pre-exposed sheet under the safelight at the working distance, masked so that four areas receive 0, 1, 2 and 4 minutes. Process it. The pass criterion is no density change out to 4 minutes. A pre-exposed sheet is used because a material that already carries a latent image is more easily fogged than a fresh one, which is the condition your paper is in on the easel.
• Does the veil reach the cut edges of the sheet? It must. Flare stops at the image circle; a leak has a direction; this does not.
• Is it on the unexposed control sheet? Safelight fog and a bad developer are; a leak and flare are not.
• Does it grow with handling time? Sort the sheets by how long each was out, and the ranking is the test.

!Test on the material you actually use

A safelight that passes on a commercial bromide paper has not been shown to pass on a hand-coated orthochromatic emulsion, or on a direct positive paper, or on film. Re-run the test whenever the material changes, and keep the result with the material rather than with the lamp.

Corrective action

Increase the distance, reduce the wattage, or change the filter class. Distance is the cheapest lever and it works on the inverse square.

Reduce the time the material spends out. A sheet that is under the lamp for two minutes rather than five has had less than half the dose.

Nothing corrects the fogged sheet.

Prevention

Run the four-step test on every new material and record the result with a date, the lamp, the filter, the wattage and the working distance. All five belong in the record, because changing any one of them invalidates the test.

Keep unexposed material in its box and out of the light until the moment it is used, and put it back.

Watch for the contrast symptom rather than only for the veil. A print that will not reach a black on a grade that used to give one, with a clean developer and a correct exposure, is worth a safelight test before it is worth a new packet of paper.