Taking that first jump, and wanting some reasoning behind this video

[Peter Lewin]

John, I in turn feel like a fool explaining anything to you, sort of like explaining something to Ansel! Picker maintained that even with lamp warmers and timers, the amount of light being emitted from the enlarger was not exact, I.e. a few milliseconds of turn on might be lost due to lamp warm-up or turn on. If this was true, 5 3-second exposures would be slightly different from a single 15-second exposure. So he eliminated a possible variable by replicating the test strip procedure, and those (possible) milliseconds are what he called intermittency effects.

[nolindan]

Paper exhibits no intrinsic intermittency effects -

http://www.darkroomautomation.com/su...ermittency.pdf

Compensating for lamp warm-up time (I'm sure there is another timer that does this, I just don't know of one, in any case the procedure will be the same) -

http://www.darkroomautomation.com/su...2LampDelay.pdf

[Michael R]

...extremely embarrassed here - having taught literally thousands of students, ages 5 to 81, elementary school through graduate (MA) level...what the heck is the "intermittency effect?" Perhaps (probably...hopefully) it is something with which I am already familiar but with another label? Hmmm...

(Darth Vader chiming in..."you are a Fraud...an IMPOSTER - HA HA HA HA...!)

It’s not an intermittency effect having anything to do with the emulsion. It’s more like shutter efficiency. When the enlarger goes on and off the bulb isn’t necessarily “instantaneously” on/off. So the idea is simply that say two 3-second exposures is a little less than one 6-second exposure.

Whether or not it makes any meaningful difference depends on the light source and the ramp-up/ramp down time relative to the measured time.

[jnantz]

the original omega E enlarger, I think it was the E3 had a light source that remained ON the entire time and there was a shutter at the lens stage. not so hard to do something like that today if someone just wants to leave the light on and put a wollensak studio shutter or a Packard or whatever between the lens and camera. probably LEDs these days would easily mime a traditional enlarger bulb and cost pennies compared to the E3's fluorescent (cold cathode ) light bank. either way, I've never really noticed that much of a light change, or exposure change when just setting the timer ( and traditional enlarger bulb or aristo cold head ) and clicking the button, besides there's always burning and dodging or whatever so it really doesn't matter too much anyways ...

[John Layton]

Peter you're setting way too high a bar...but thank you just the same!

And yes indeed, this (intermittency) effect...is exactly why (even with my amazing Heiland LED VC light source) I never depend on split tests on which to base a final print - but will do an undivided "final strip" (or strips) first, and dry them and check with a mix of 3500 and 2800 K lights.

Thing is, my 69 year old eyes aren't what they once were. They're still sharp enough, for which I count myself lucky...but assessing relative brightness gets trickier as the years pass. Anyone else notice this?

[ic-racer]

Shorter times are better if one's workflow involves mostly burning. I actually do very little dodging. Some, but not a lot. I'm going through a phase where I like bleach better than dodging.

Some of my images might need 10 burns or more. This is where it is nice to have a 2000W head for 15 second base times.

[ic-racer]

Cold lights are notorious for changing intensity based on temperature or duration. And this one has a correctly functioning thermostatic heater (notice baseline temp 40C, per thermostat, before the lamp is turned on).

[nolindan]

Cold lights are notorious for changing intensity based on temperature

Interesting result. Normally light output increases with lamp temperature - viz the time it takes a fluorescent or cfl lamp to come up to full light output as it warms up.

Having had an Aristo cold light head, though, I would not put any sort of strange behavior past the beast.

You really need an light integrator to get consistent output with a gas-discharge/fluorescent light source. Additionally, the integrator's light sensor has to have appropriate filtering that matches the material being exposed. With VC paper that can be a real problem as by rights the integrator sensor filtration needs to change to match the contrast filtration used for exposing the paper.

Most systems don't get that elaborate. The simple light integration / timer compensation done by some timers seems to be "good enough." As the saying goes - "Good enough is best."