Operating the UVA exposure unit

Purpose

To give one sheet a dose you can repeat, with the array proved dark whenever the lid opens, and a log line that says afterwards what the exposure was worth.

When to use it

Every time a UVA unit is energised: a print, a test strip, a dose series. It runs from cold to cool-down, and applies to a bought unit or a nail lamp in a home-made box exactly as to the unit the course builds.

Not commissioning: the leak check, interlock test, temperature log, uniformity map and dose series are the build page’s deliverables, and this spends them. Not sunlight, which has no lid and no counter: use a daylight exposure record.

Before you start

You have read the lesson: this source is invisible and painless while it does harm, and nothing warns you. Every step follows from that.

The unit is commissioned and the label says so — peak wavelength, array current, printing area, array-to-paper height, warm-up, highest heatsink temperature, uniformity, and the date the interlock was last tested. A blank field is an uncommissioned unit.

On the bench: the timer, the hours log, the frame and its glass, the dry coated sheet, the negative, a felt or porous-plastic pad for behind the paper, UV-blocking eyewear in reach. If the unit has a viewing window whose material you cannot establish, cover it opaquely and leave it covered: ICNIRP permit ports only of suitably tested absorbing material.

⚠One instruction that is not general advice

ICNIRP state that people with an intraocular lens not designed to absorb ultraviolet, and people with no implant at all, should be fitted with UVA protective eyewear when working with UVA sources. If that is you, it is a primary control, worn as long as the unit runs.

The airflow is a thermal control: the temperature log assumes the ambient it was written at.

Procedure

1. Read the label. A blank or undated field stops the session: run the test that fills it.
2. Clear the vents and the top of the box, and move anything put down against the fan.
3. Read the hours counter and write it down, before the array runs.
4. Test the interlock. Lid shut, array on the manual switch, indicator lit; lift the lid, and the indicator must go out before the lid opens far enough to see inside. Watch the indicator, not the interior. Close it, confirm it relights. Five seconds, every session.
5. If you are in the group ICNIRP name above, put the eyewear on now and keep it on.
6. Shut the lid, switch the array on, and run the warm-up the label gives. Time it.
7. While it warms, build the frame in the light you coat in: glass wiped both faces, negative and paper emulsion to emulsion with the negative facing the glass, the pad behind the paper, the split back closed and sprung.
8. At the end of the warm-up, open the lid — the interlock takes the array dark — and set the frame against all three battens, in the orientation the dose series was made in.
9. Shut the lid and confirm the indicator is lit.
10. Expose for the time the dose series gives for this process, paper and height. Time it. Do not touch the current and do not dim: exposure is by time at a fixed current. Stay in earshot — the unit is not left running unattended.
11. To inspect a printing-out image, stop the timer, open the lid, confirm the indicator is out, then release one leaf spring and fold half the back away without lifting the frame off the battens. Look, close it, shut the lid, restart. For cyanotype, first look at four minutes.
12. Printing to appearance rather than to the series, take it to the endpoint: high values light green, mid-tones firm blue, deepest shadows reversed to a pale blue-grey.
13. Switch the array off, read the hours counter, enter the difference in the log.
14. Cool down with the lid shut, and the fan running if it switches separately, for as long as the temperature log says the heatsink needs to return near ambient. Do not reach into the box, cover it, or put it away hot.
15. Take the frame out, process the print by its own route, and write the log line at the bench.
16. Switch off at the socket; leave the lid shut.

How you know it worked

• The indicator went out before the lid opened far enough to see in, and relit when it shut.
• The exposure ran at the label’s current, on the same three battens, for a time from a dose series still valid for this paper, process and height.
• The hours counter has a start reading, an end reading and a difference in the log.
• The heatsink is back near ambient, the lid shut, nothing standing on the box.

If a step fails

Step 1, a field is blank. Without a dose series you have durations, not doses; without a uniformity date, no evidence the corners match the centre; without a leak check, nothing.

Step 4, the indicator stays lit as the lid opens. Stop; unplug at the socket before investigating. The usual causes are a switch wired normally-open, an indicator taken from the supply side rather than the array side, and a bent lever. Nothing is printed on a failed interlock.

Step 4, the indicator never lights. Do not infer the array is dark — that inference is the accident this unit is built to prevent. Treat it as live, unplug, repair it, test again.

Step 6, the session’s first print differs from the rest. The warm-up was cut short.

Step 8, the frame will not sit where it sat. The battens or the array height have moved, and the dose series is void until you run a new one.

Step 10, the exposure runs far longer than the series says. Check the glazing first — most acrylic stops transmitting below about 370 nm — then the hours log, then the fan.

Step 11, you see fluorescence at a seam. Switch off; re-run the leak check and re-tape from inside before the next session.

Step 14, the heatsink is hotter than the log’s highest steady reading. A blocked vent, a stalled fan or a warmer room. Array off, lid shut, hands out, and repeat the temperature log with the ambient recorded before the unit runs again.

Any step, the array must run with the lid open. That is servicing, not printing, and it is done on the build page’s terms: eyewear on, skin covered, lowest current, shortest time, nobody unbriefed present, never mid-session.

What to record

One line per exposure: date; process; paper and its coating date; negative; array-to-paper height; the time the series called for and the time given; when and how often you inspected; the counter at start and finish; the endpoint you judged.

Once per session: the interlock test and its date, copied on to the label, and the ambient.

Once per dose series: the series, with its process, paper, height and date, on the label and in the log. It is void when the height changes, the paper changes, a year passes, or exposures have crept far enough that accumulated hours are the likelier explanation — which is what the counter is for, where a commercial unit has an integrator accumulating dose rather than time.