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# Thread: Why doesn't 2+2 = 4 in print exposure?

1. Has anybody actually tried it? I did. Not telling what I found. You'll learn a lot in two, or maybe even a single sheet of paper.

2. Originally Posted by BBarlow690
Has anybody actually tried it? I did. Not telling what I found. You'll learn a lot in two, or maybe even a single sheet of paper.
Yes, I have.

3. 2 + 2 does equal 4; heck, 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 = 20 -- if the timer is accurate and you compensate for any lamp warm-up time. However, if you have a cold light head without a shutter, or the least an integrating/compensating timer, it probably isn't possible.

The following application note on the Darkroom Automation web site gives a method how to determine lamp warm up time:
http://www.darkroomautomation.com/su...2LampDelay.pdf

4. Originally Posted by Nicholas Lindan
2 + 2 does equal 4; heck, 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 = 20 -- if the timer is accurate and you compensate for any lamp warm-up time. However, if you have a cold light head without a shutter, or the least an integrating/compensating timer, it probably isn't possible.

The following application note on the Darkroom Automation web site gives a method how to determine lamp warm up time:
http://www.darkroomautomation.com/su...2LampDelay.pdf
Nicholas

Are you saying: the intermittency effect does not exist?

5. No he's saying you can have an "inaccurate timer" that takes it into account

i.e. has a additional time added to compensate. So 2 secs is 2 + enough milliseconds to compensate for the ramp up/down time, but it would need to be calibrated for different light sources.

Ian

6. Originally Posted by RalphLambrecht
Are you saying: the intermittency effect does not exist?
The 'intermittency effect' is an effect of the quantum response of a silver salt crystal to a photon of light. In conventional emulsions a crystal has to be hit at least twice with a photon within a certain period of time. If too much time passes between photon hits then the crystal 'relaxes' and is not activated (exposed) when the second photon hits. This is the principle behind reciprocity failure, where the photon flux is so low that too much time passes between photon hits and therefore extra exposure time is required to statistically assure a double-hit. It is also the principle behind intermittency where an exposure is paused and the pause time is long enough to allow the crystal to relax and become 'unexposed' again. The critical photon inter-arrival period varies between emulsions - as evidenced by the different reciprocity and intermittency factors for different films and papers.

The lack of interimittency is the reason for the excitement about 'single photon' emulsions - it's not so much that they are twice as sensitive as that there is no reciprocity failure.

Intermittency has little to no effect in the range of exposure times encountered in printing with printing paper emulsions. See the experimental results in the application note. Or make your own instrumented or shuttered experiments and draw your own conclusions. Or, See Meese, et. al., again....

Intermittency effect has nothing to do with lamp warm-up and stabilization time.

7. Originally Posted by Nicholas Lindan
... Intermittency effect has nothing to do with lamp warm-up and stabilization time.
Nicholas

I agree with your statement above, but I don't think anybody claimed that. In my experiments, the intermittency effect is strong enough to through a regular test strip off by up to 1/12 stop.

8. Originally Posted by RalphLambrecht
but I don't think anybody claimed that
I think it was in one of the references you gave, last two paragraphs:

http://jbhphoto.com/articles/intermit/intermit1.htm

Originally Posted by RalphLambrecht
In my experiments, the intermittency effect is strong enough to through a regular test strip off by up to 1/12 stop.

In my experiments a 1 second error accumulated from 20 x 1 second exposures - that is 5% or 0.07 stops or 1/14th of a stop. Adding a 50mSec correction to each exposure removed the error. It may have corrected intermittency but as with the correction 30 x 1 = 1 x 30 and 10 x 1 = 1 x 10 the effect seems to be entirely lamp warm-up time, as confirmed by the measurement of the turn-on and turn-off light output measurements shown in the application note. Additionally, the required correction time changes if a ferroresonant stabilizing transformer is added to the system.

9. All things being equal (if they ever are!?), it is reciprocity failure.

10. Originally Posted by Nicholas Lindan
I think it was in one of the references you gave, last two paragraphs:

http://jbhphoto.com/articles/intermit/intermit1.htm...
Nicholas

Cut me some slack here. I referenced that article for the OP so he could read up on the intermittency effect, but I never claimed that this effect has anything to do with lamp warm-up. They are totally unrelated.

Originally Posted by Nicholas Lindan
... What was your test protocol?

In my experiments a 1 second error accumulated from 20 x 1 second exposures - that is 5% or 0.07 stops or 1/14th of a stop. Adding a 50mSec correction to each exposure removed the error. It may have corrected intermittency but as with the correction 30 x 1 = 1 x 30 and 10 x 1 = 1 x 10 the effect seems to be entirely lamp warm-up time, as confirmed by the measurement of the turn-on and turn-off light output measurements shown in the application note. Additionally, the required correction time changes if a ferroresonant stabilizing transformer is added to the system.
Fair enough. You are right. There are several effect at work here: intermittency, lamp ramp-up/down and stabilized-transformer delay.

I can adjust my timer to compensate for it. My transformer delay was measured at 400 ms, and I'm not fully aware of my lamp ramp-up/down. I will measure it and ask Phillips for the data sheet to compare. Unless that's done, I can't really measure the effect of intermittency.

I have modified my enlarger to mount a large-format lens and shutter, but LF shutters are not accurate enough (+/- 1/6 stop) to reliably measure intermittency either.

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