Sensitometer exposure session

Purpose

To expose a set of strips whose log exposure axis can be defended afterwards, and to leave the bench with a log line that lets somebody else — including you, in March — say what the numbers are worth.

When to use it

Any session that exposes film or paper in the sensitometer for a measurement: a characteristic curve, a developer comparison, a batch of control strips, a repeatability arm.

It does not cover the calibration itself. The warm-up rule, the uniformity map, the repeatability and reproducibility figures and the error budget are the deliverables of the calibration experiment, and this procedure consumes them rather than producing them.

Before you start

You should already have the instrument certificate: the warm-up rule in a sentence, the design exposure interval and drive current, the firmware version, the uniformity figure, and the two log H uncertainties — the relative one, which a developer comparison uses, and the absolute one, which a speed claim uses. Without a certificate you have an instrument that makes marks on film and no scale to report them on.

Cut and label the strips the day before, all of them, from one batch. Fourteen identical strips in the dark is how a session goes wrong; fourteen labelled strips is how it does not.

An hour before, bring every solution to 20.0 °C in the water bath and leave it there. Kodak’s process-control literature is blunt about the tolerance: variations greater than ±0.3 °C in the developer affect process control and image quality, and on this bench a temperature error would be counted as instrument noise.

On the bench, before the lights go off: the strips in order; the wedge, with its emulsion side marked on the edge; the cover glass, both faces wiped; the notebook and a pen you can find in the dark. Put the module’s heatsink where a hand reaching for the lid will not find it.

Procedure

1. Switch the instrument on and start the warm-up your own certificate specifies. Open the serial log at the same moment.
2. Write the session’s predictions in the notebook, dated, before the first exposure: what spread you expect, in density and in log H, and why. A prediction written afterwards is not a prediction.
3. Wipe the platform and both faces of the cover glass. Grit is what scratches a strip you cannot re-expose.
4. In darkness, lay the strip emulsion up on the platform, pushed back against the long fence and left against the short one.
5. Lay the wedge on it emulsion down, against the same two fences. Both parts register against the fences, never against each other. Turned over, the wedge’s base spreads light between the two emulsions and every step boundary softens.
6. Lower the cover glass onto the pair — never slide it — and close the lid. Contact is by weight; nothing is clamped.
7. Fire the design exposure. Record the firmware’s reported interval and the monitor reading with it.
8. Keep a fixed cadence between exposures, the one your bench tests showed the lamp is stable at, and do not vary it because a session is running late.
9. Repeat from step 4 for each strip, keeping the strips in a known order and the tail of each masked so that every one carries its own base-plus-fog patch.
10. Process the set together, with one unexposed control from the same batch, by film processing. Do it promptly: ILFORD’s instruction for their own film is to process exposed film as soon as practical, and a strip left a fortnight tests the box and your latent image together with no way to tell them apart.
11. Take the log exposure of each step as log H(k) = log H₀ − D_wedge(k), and write down which you used: a calibrated wedge’s certificate values, or the nominal 0.05 + 0.15(k − 1), which is a manufacturing specification rather than a measurement of your piece.
12. Quote every log H figure with the uncertainty from the certificate that belongs to it — the relative figure for a comparison, the absolute figure for anything that leaves your own bench.

How you know it worked

• The warm-up that actually ran is the one on the certificate, and the log says so.
• Every strip has a label, an exposure log line and a monitor reading, and the order they were exposed in is recoverable.
• The step boundaries on the developed strip are sharp, which is the evidence the wedge went in emulsion down and the glass made contact.
• The tone is even along each step: one end lighter is your uniformity map appearing on film.
• The control strip is clean, and the developer held 20.0 °C throughout.
• Every number leaving the session carries an uncertainty, and the relative and absolute figures are reported as two numbers rather than one.

If a step fails

Step 1, you have no certificate and therefore no warm-up rule. Then the session is not a measurement. Run the calibration experiment first, or expose the strips and label the whole set no warm-up rule; instrument uncharacterised, and use them only for a comparison made inside that one session.

Step 5, you cannot tell which face of the wedge is the emulsion. Stop. In the dark you will not be able to tell, which is why the mark goes on the edge in the light. Turn the lights on, mark it, and start the session again.

Step 6, the lid will not close, or the lamp lights with the lid open and not closed. A wire is being pinched. Look before you force it; the taped loop of slack inside the box exists for this.

Step 7, the monitor reading drifts through the set. Finish the set, then read the log. Drift shows on the monitor and a light leak does not, so a monitor that is flat while the film is not points at something between the emitter and the film plane — the diffuser, its ledge, or the module on its spacers.

Step 9, two strips get confused. Both are unusable, and so is any conclusion drawn from either. Discard the pair rather than guessing, and note it: this is the failure that most often ends a session with two hours of unusable data.

Step 10, the developer is not at 20.0 °C when you get to it. Bring it back and wait, or record the temperature you actually processed at and treat the set as a within-session comparison only. Do not correct a density arithmetically for a temperature you did not intend.

Step 12, you are about to quote a speed. Do not write “ISO 400” and do not write “accurate to 0.02 log H”. The first is a standard’s number, and the course cites standards by number and never quotes them; the second confuses accuracy with repeatability — a repeatable instrument with a wrong wedge is precisely wrong every time.

What to record

One line per exposure, in the serial log and again in the notebook: the date; the strip’s label; the film batch; the reported exposure interval; the monitor reading; the warm-up elapsed at that moment; and the cadence. Then, once, per session: which wedge was used and whether it was calibrated; the processing conditions; and the two uncertainties copied from the certificate that every figure from this session is to be quoted with.