Durst Pictochrome Light Loop ?

[adamlugi]

What exactly is meant by "light loop" and what are the benefits of it in Durst Pictochrome?

 

[koraks]

It refers to a system where a photosensor (usually a photodiode) is used to measure the light emitted by the light source. The system can then adjust the exposure to compensate for fluctuations in the light source; e.g. if the bulb dims for some reason, the exposure can be extended slightly to compensate for this.

The advantage is that you should be able to make very consistent exposures this way; e.g. when going from 0.3 to 0.6 seconds, you will get precisely one stop more exposure, instead of a little more still because of the bulb warm-up time. By definition, it can compensate for things like bulb warm-up time, bulb aging (depending on how the system is designed) and power supply fluctuations.

The disadvantage is greatly added system complexity. For virtually all darkroom printing, it's more of a gimmick than a necessity. Frankly, in my opinion, it's a solution in search of its problem. While it was useful in technical applications where extreme control over exposure is necessary, other approaches have superseded this for the most part (more consistent LED lighting being an obvious one) if the applications still exist anymore in the first place. In artistic printing, the net benefit is near zero since fine arts printers (which includes, by extension, the majority of amateur darkroom enthusiasts) adjust their prints using test strips and visual evaluation.

An argument could be set up that a closed-loop system is beneficial if you want to change e.g. enlargement (going from a small test print to a larger one), which will involve a change in exposure, and the closed-loop control will compensate for any non-linearities that accompany such a change. How practical this is, depends on your own perception and preferences. I've done quite a bit of work building my own enlarger light sources and in all honesty, I've never seriously considered including a closed-loop control along the Pictochrome-lines because I just didn't see the point of having it.

 

[Nicholas Lindan]

Sounds like this may be integrating timer where the counting rate is proportional to the light intensity. This is useful for cold-light heads and heads that use arc lamps where light output varies wildly as the lamp warms up. This would only apply to B&W printing, though.

If the device is meant to 'trim' the exposure to compensate for incandescent warm-up and aging, then I agree with Koraks that the device is largely useless. It likely had a use in critical color work. However, an aging lamp has a definite shift in color temperature, as anyone with a chandelier with lamps of different ages can attest, and this will not be compensated for.

* * *​

LEDs do have a shift in output as they warm up. For photographic purposes it is negligible and the effect will never be seen (oh dear, I fear I have committed a "famous last words" infraction). This temperature effect must be accounted for when using LEDs as a light source in laboratory instrumentation, where a shift of 0.1% would be significant.

 

[adamlugi]

Thank you for the clarification.
I became an owner of a Pictochrom and could not find the information and everyone mentions it.
So far I don't have a probe so I don't know if I will make full use of this enlarger.
I am currently doing colors on an L1200 but I have a Durst measuring device for it and it works great. Previously I did Ciba on an AC800 and that worked great too. I also have an Ac800 but need to get it up and running eventually. The advantage of the L1200 is the simplicity of operation with the measuring device. I also have the AC800 almost embraced ;-). I care to do colors on 2 enlargers because the developing and drying process in the machine is so long that I can prepare the second photo in that time.

And I understand that in PictoChrom having a probe I can work after calibration as with an external device calibrated for a specific paper and negative similar to Heiland devices?

 

[koraks]

This is useful for cold-light heads and heads that use arc lamps where light output varies wildly as the lamp warms up. This would only apply to B&W printing, though.

It's used here (Pictochrom) for an incandescent bulb and also for color printing since the Pictochrom is in essence a dichroic enlarger like any other. It just has lots of bells & whistles. So yes, it's intended for bulb variations due to warm-up, power supply fluctuations (although I assume the Pictochrom used a stabilized PSU) and aging/replacement. This is what the manual claims:

Neat, but as we both agree, of little practical utility.

The system is surely very nice and dandy as long as it works. If it doesn't, repair (either DIY or outsourced) will offset any benefit of the system by such a huge margin that it's just not funny anymore.

LEDs do have a shift in output as they warm up.

Yes, I should have been more careful. I guess we'll both have to eat our hats...

So far I don't have a probe so I don't know if I will make full use of this enlarger.

Note that the baseboard probe is unrelated to the closed-loop system. The closed-loop sensor is situated in the head somewhere; I think it's in the light mixing chamber.

I care to do colors on 2 enlargers because the developing and drying process in the machine is so long that I can prepare the second photo in that time.

Yes, many people print color that way. Makes perfect sense, too. I stick to one enlarger because I don't like to clutter my workspace too much, but from a functional viewpoint, having two or three enlargers when doing color is very sensible.

And I understand that in PictoChrom having a probe I can work after calibration as with an external device calibrated for a specific paper and negative similar to Heiland devices?

Yes. It's unrelated to the closed loop thing. But you're right, you could run a calibrated workflow in either color or B&W. Don't ask me about the specifics; I've only done this with the ColorStar analyzers and regular enlargers. I long ago sold off the analyzers because I didn't see the point in having them and they didn't play nice with PWM-ed LED light sources.

 

[Nicholas Lindan]

It's used here (Pictochrom) for an incandescent bulb and also for color printing since the Pictochrom is in essence a dichroic enlarger like any other.

Ah, I should have thought a bit more (story of my life, really) - quartz halogen bulbs don't shift their spectral output to any great extent as they age. "Incandescent" puts me in mind of PH211's and plain-ole light bulbs.