[pure-silver] Determining lamp delay / ramp-up

There is an application note on the subject of determining
an enlarger's lamp turn-on time on the Darkroom Automation
web site:

http://www.darkroomautomation.com/support/AppNotePH212LampDelay.pdf

Although aimed at users of the Darkroom Automation timer, the technique
can be used with other timers.  Delay is only important when making
multiple additive exposures - such as test strip prints where the paper
is progressively covered.

When making test strip prints the lamp delay time should be subtracted
to the determined exposure.  The correction is:
Corrected time = determined time - (number of the correct strip - 1) * delay time 

If the lamp delay is 0.16 seconds and strip #5, 17 seconds, is the
correct exposure then the corrected time is:

17 seconds - (5 - 1) * 0.16 seconds ~= 16.5 seconds

Incandescent lamps have a short delay time, a PH212 has a 0.05
second delay time.  Cold light heads can have significant delay
times that vary with the temperature of the lamp and the time
between exposures so test prints should be made in a very consistent
manner in order to be able to compensate a cold light head.

Some timers have their own parasitic delay time that adds to the
lamp's delay and makes additive exposures quite inaccurate.  The
technique described also compensates for the timer delay.

--
Nicholas O. Lindan
Darkroom Automation
A Unit of Cleveland Engineering Design, LLC
Cleveland, Ohio 44121

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