Question relating to UV enlargers

Steve Goldstein · Nov 13, 2021

The ongoing UV enlarger thread is quite interesting to me. I don't want to sidetrack that conversation with this question, but it's something I really wonder about.

A 20-minute UV exposure to contact print an 8x10 negative exposes that negative to a certain UV intensity for that time.

Printing a 6x7 negative (120 film) onto 8x10 paper requires a linear enlargement of around 3.5x, assuming no cropping. If a UV enlarger is capable of doing this with the same 20 minute exposure, that means the UV intensity at the negative must be at least 3.5^2 = 12.25 times higher. It'll actually need to be more to make up for losses in the enlarging lens. Since the best seem to have UV transmission around 50%, we're looking at 20-25x the UV intensity at the negative for that 20-minute exposure when compared with contact printing.

Are there any studies of the effect of extended UV exposure on negatives? Does it affect the gelatine in any way? Does it embrittle the base? I think this is actually an important question that needs to be answered.

Ethan Brossard · Nov 16, 2021

Interesting question, and one I haven't thought about. I'm fairly certain exposure to intense UV light will age the negative faster, but is a 20 minute exposure, say 30 or 40 times, enough to make an impact?

Here's some math I'm thinking could find a theoretical answer:
It looks like a 40 watt lamp for a 6x7cm is what as worked best for people, but I read somewhere that UV LEDs are at most 30% efficient, so thats 12 watts of light over 42sq cm, or 2857 watts per square meter. A typical UV index for the northeast US where I live during the summer is 5. UV index is the energy of UV light in watts/m^2, multiplied by 40, so a UV index of 5 is 0.125 watts/m^2. Watts are energy/time though, and we are only exposing the negative for 20 minutes, so now to convert the watts into joules, which is done by multiplying the watts by the exposure time in seconds. 2857watts*1200 seconds is 3428400 joules for the 20 minute exposure. 0.125 watts by 3600 seconds (1 hour) is 450 Joules. 3428400/450 is 7619, so a 20 minutes exposure to a 40 watt UV lamp is at most equal to 7619 hours of exposure to the sun.

That seems like a lot to me, but there could be a mistake in my math (can any of you verify if that is correct?). Does anyone know of research on the stability of film in ultraviolet light? I found one study from the 60s which seemed to indicate 25 hours of exposure to sunlight would halve the density of film, but if that is true our exposures would destroy film very quickly. That could have also been talking about early color film, which I know was not very archival at all. I also found a forum thread saying kodak had research on their website, but the thread was from 2004 and I couldn't find any of the research it discussed.

nmp · Nov 16, 2021

Ethan Brossard said:
Interesting question, and one I haven't thought about. I'm fairly certain exposure to intense UV light will age the negative faster, but is a 20 minute exposure, say 30 or 40 times, enough to make an impact?

Here's some math I'm thinking could find a theoretical answer:
It looks like a 40 watt lamp for a 6x7cm is what as worked best for people, but I read somewhere that UV LEDs are at most 30% efficient, so thats 12 watts of light over 42sq cm, or 2857 watts per square meter. A typical UV index for the northeast US where I live during the summer is 5. UV index is the energy of UV light in watts/m^2, multiplied by 40, so a UV index of 5 is 0.125 watts/m^2. Watts are energy/time though, and we are only exposing the negative for 20 minutes, so now to convert the watts into joules, which is done by multiplying the watts by the exposure time in seconds. 2857watts*1200 seconds is 3428400 joules for the 20 minute exposure. 0.125 watts by 3600 seconds (1 hour) is 450 Joules. 3428400/450 is 7619, so a 20 minutes exposure to a 40 watt UV lamp is at most equal to 7619 hours of exposure to the sun.

That seems like a lot to me, but there could be a mistake in my math (can any of you verify if that is correct?). Does anyone know of research on the stability of film in ultraviolet light? I found one study from the 60s which seemed to indicate 25 hours of exposure to sunlight would halve the density of film, but if that is true our exposures would destroy film very quickly. That could have also been talking about early color film, which I know was not very archival at all. I also found a forum thread saying kodak had research on their website, but the thread was from 2004 and I couldn't find any of the research it discussed.

Sounds right, although I didn't understand why you had to convert to Joules.

UV enlarger, 2857 w/m2
Sun at UV index 5, 0.125 W/m2

UV enlarger exposure 2857/0.125 = 22856 as powerful as the Sun (at UV Index 5.)

So 20 mins in UV enlarger = 22856x0.33 = 7619 hours in the Sun.

Regrading the stability study you cite, it is likely more to do with the UV stability of the dye molecules rather then the UV degradation of the film base material (PET, cellulose acetate etc) and the gelatin layer. I would think the former will do much better than the latter. Also for B&W film, what happens to the silver - photo-oxidation? According to the calculation here a single 20 min exposure is equivalent to continuous 635 days of 12 hour UV5 exposure in the Sun. It is most certainly food of thought as the OP has brought up.

:Niranjan.

koraks · Nov 16, 2021

nmp said:
Sun at UV index 5, 0.125 W/m2

That's a simplification that's way beyond what is reasonable for this purpose. A typical UV light source for alt process exposures is generally within the 360nm ~400nm part of the spectrum (with some excursions into visible blue which we can more or less ignore for now). However, the UV index we use for sunlight is very heavily weighted for up to 325nm. In fact, the part of the spectrum where your typical alt process light source has its peak has virtually no impact at all on the UV index. In other words: the UV index pretty much is *not* about the kind of UV we're dealing with in photography. Hence, the already optimistic damage assessment based on the UV index will in reality be several orders of magnitude more optimistic if you account for this!

Ethan Brossard · Nov 16, 2021

koraks said:
That's a simplification that's way beyond what is reasonable for this purpose. A typical UV light source for alt process exposures is generally within the 360nm ~400nm part of the spectrum (with some excursions into visible blue which we can more or less ignore for now). However, the UV index we use for sunlight is very heavily weighted for up to 325nm. In fact, the part of the spectrum where your typical alt process light source has its peak has virtually no impact at all on the UV index. In other words: the UV index pretty much is *not* about the kind of UV we're dealing with in photography. Hence, the already optimistic damage assessment based on the UV index will in reality be several orders of magnitude more optimistic if you account for this!

awesome, that number seemed too big to me, thanks for realizing why!

Do you know how the UV index is weighted?

koraks · Nov 16, 2021

Fortunately this is made accessible & easy to understand: https://en.wikipedia.org/wiki/Ultraviolet_index
Look at the second chart and the green plot labeled 'erythemal action spectrum', that's the weighting curve for the UV index. As you can tell, even 320nm (far below what we get from a typical alt process light source) only counts for 1%, and 365 (the more expensive UV leds) is down to something like 0.0005%...
So it's really apples and oranges.

When applying UV index reasoning to this question, the implicit assumption is that film will suffer from the same kind of wavelengths (and hence, through the same or similar physical mechanisms) as skin does. If that is justified, can be put up for debate. I don't know really; I mean, it's evident that the lower the wavelength, the higher the energy and hence the higher the potential for damage. But I don't know if there's for instance a mechanism for degradation that the polymers in film would suffer from at longer wavelengths that does not affect human tissue as much. Hence, I would be very cautious trying to apply this kind of logic to the case of alt. process printing from negatives.

nmp · Nov 16, 2021

koraks said:
That's a simplification that's way beyond what is reasonable for this purpose. A typical UV light source for alt process exposures is generally within the 360nm ~400nm part of the spectrum (with some excursions into visible blue which we can more or less ignore for now). However, the UV index we use for sunlight is very heavily weighted for up to 325nm. In fact, the part of the spectrum where your typical alt process light source has its peak has virtually no impact at all on the UV index. In other words: the UV index pretty much is *not* about the kind of UV we're dealing with in photography. Hence, the already optimistic damage assessment based on the UV index will in reality be several orders of magnitude more optimistic if you account for this!

Just to make sure, I was just checking the calculation - not the thesis...

:Niranjan.

DREW WILEY · Nov 16, 2021

UV enlargers have been commercial built and marketed. Few ever sold except for highly expensive pulsed Xenon units once marketed to the graphics trade. The bulb itself cost more than most amateur enlargers of that era. Intense UV can wreak havoc on the equipment itself. Even with ordinary high-output colorheads, sooner or later I need to replace all the light gasketing with pure silicone high-temp strip, for example. Seemingly nobody thinks of that in advance. All kinds of details need to be reengineered. Then you have to figure out how to incorporate another light source to compose and focus the image, and how to shield yourself and your eyes for the primary UV exposure. Cooling issues come into play. Film torture - yep, that's a factor too.

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