@bernard_L, it's a valid concern. I briefly went into it in
#63 above.
Problematic when the thermal time constant of the LED-heatsink system is comparable with the exposure time.
Indeed. So the design goal for a cooling system is not so much to keep within the maximum ratings of the device, but to keep heating during exposure to a minimum. You'll notice that as the light source heats up, the difference in downrating between the channels becomes greater, so keeping the temperature rise limited, also limits this effect. It's one of the reasons why I like short exposures, especially for color. The head doesn't heat up much in 4 seconds or so.
Worse when time-additive test strips are made, and the result converted into a single continuous exposure.
Sort of; there's two things at play:
1: The light source may heat up during one partial exposure, stay warm, then heat up further during the second exposure, etc. So there would be a compound effect.
2: The heating up during each exposure creates a non-linear effect in increased exposures; in other words, exposing twice as long will not give twice the density. It 'feels' a bit like reciprocity failure, but of course the mechanism is totally different.
Finally, there is of course the question of how important absolute accuracy is. Is it essential that for instance longer exposures have a nicely linear relationship with higher density, or is it acceptable that there is some non-linearity here
as long as print-to-print consistency does not suffer? Hence, there can be some room for compromise depending on how strict you are in your specifications. I myself am not that strict as long as I can prevent gross non-linearities that make the system behave in a non-intuitive way, or (worse) that would create visible differences between for instance a test strip and a final print.
In a previous version of my system, I actually had an issue like that with the red channel. It turned out that for some reason, there was an effect of using red (along with blue and green) for focusing on the color balance (especially cyan) in the final print. My workaround for this was to disable red when focusing for color prints; i.e. I focused with just blue & green. While it worked, it was not optimal. The current version of my print head does not need this workaround; although the LEDs are without a doubt also prone to these kinds of effects, I gave more attention to thermal management (with some degree of success, although there's much room for improvement) as well as to much better current limiting/LED driver circuitry. I suspect it was especially the latter that was the problem in the previous version, as it had *very* crude current limiting circuitry.
All of which goes to show that your concern is very much justified, and it's even more complex and broad in scope than you might have realized, as it also includes ancillary electronics and not just the LED source itself.