Help me understand units of exposure

mark · Feb 6, 2012

In my carbon reading I keep reading about people using a Nuarc machines and they list Units of exposure. Does this translate to time or is it solely a Nuarc measurement and cannot be translated for the non nuarc owners out there? I don't have a Nuarc and don't plan on getting one. (Wallet and Wife say no)

holmburgers · Feb 6, 2012

Check out post #5 from this thread -> (there was a url link here which no longer exists)

Greg (gmikol) says, "NuArc integrators just measure the UV output and slow down or speed up the counter. It does not give a calibrated measurement."

I'd like to understand this a little better myself. Namely, can we reliably convert the NuArc output number into mJ/cm²?

The nutshell of that thread I linked to is about using a UV meter, which outputs in mW/cm². If we multiply that number by seconds, we get millijoules/cm², and this is a measurement of total UV energy absorbed.

If the NuArc integrator gives a reliable number that, coupled with the aid of a UV meter, can lead us to a calibrated measurement, then we'd have a much better idea of the energy requirements for exposing carbon tissue.

Bill Burk · Feb 6, 2012

holmburgers said:
If the NuArc integrator gives a reliable number that, coupled with the aid of a UV meter, can lead us to a calibrated measurement, then we'd have a much better idea of the energy requirements for exposing carbon tissue.

Or you can expose a Stouffer step wedge and develop it to see how much exposure you are getting. The units, then, become an arbitrary number that you double or halve for every two steps on the wedge that you want to move.

holmburgers · Feb 6, 2012

Right, but these would be arbitrary numbers, as opposed to an absolute measure of the energy.

Bill Burk · Feb 6, 2012

holmburgers said:
Right, but these would be arbitrary numbers, as opposed to an absolute measure of the energy.

I think that's what they are.

In a practical sense, you can use them immediately that way.

If you want to know (at least approximately) how much energy a unit represents, you can work backwards from the characteristics of the material you are exposing.

holmburgers · Feb 6, 2012

You're right of course, and maybe that's all mark is asking. I'm hoping to know a bit about these NuArc integrator's for my own, ulterior motives...

mark · Feb 6, 2012

You two are over thinking. I was wondering if a unit was = to a set amount of time.

1 unit= 1 min etc, hour, light year etc.. Looks like that is not the case.

Bill Burk · Feb 6, 2012

A unit is sort of a combination of Time & Intensity.

A photocell monitors how bright the light is. You get more or less real time as the light gets "dimmer" or "brighter".

I spent a lot of time in prepress, where there were many different platemaking and exposure units. I seem to recall them being somewhere near a second, and different machines had different settings, so it wasn't standardized.

But again the time changes with intensity, so you get consistent results.

holmburgers · Feb 6, 2012

Well what good is an amount of time if the energy output is variable? I thought the whole point of integrators was that they counted up the UV energy that it received, specifically because the output of these light sources is highly variable, which makes a time measurement useless.

edit: In other words, what Bill said.

Loris Medici · Feb 7, 2012

Integrators measure the UV energy received and output the measurement in an arbitrary manner. In other words, the numbers cannot be translated to even another NuArc integrator unit; they're sensor / integrator specific. Nevertheless, it's possible to calibrate the integrators to make their measurements match (where, additionally / optionally, 1 unit = 1 sec.), but that would be an exercise in futility because a.) Bulbs change output with age and voltage (each bulb at its own pace, depending on usage etc. ...) therefore you would need to calibrate often and b.) you actually don't need to know the absolute UV energy at all, why would you?

Regards,
Loris.

holmburgers · Feb 7, 2012

Loris Medici said:
you actually don't need to know the absolute UV energy at all, why would you?

Regards,
Loris.

No of course we don't need to know. I would just like to know because I have this meter and well... why not encourage such an investigation if someone's willing to do it?

I think it might be an interesting way to compare different carbon tissues in a far less empirical manner. Afterall... "we have the technology"

Loris Medici · Feb 7, 2012

Working in arbitrary units isn't "empirical" at all; you still do it by the numbers and in an exact manner... Working with a light integrator is much more absolute and exact than using a UV lightmeter (in the context of exposing UV sensitive material that is...); UV meters can measure the UV energy at a specific "point" of time whereas light integrators do it for the "whole time continuum / range" of the exposure (think of exposing in sunlight with a mackerel / buttermilk sky and lots of wind...), and in my book that's more exact. (Again, where the context is exposure...) Besides, let me remind that sometimes (often, in the context of alt-processes...) you can't rely on someone else's "absolute" numbers at all! Don't get lost in minute details, you need to see the whole picture... (For instance; in carbon printing, there are a myriad of details which has to be taken into account / that will affect the end result such as: The type / hide source / bloom / batch / percentage / volume per given area of gelatin, type / make / amnt. of the pigment, type / strength / application method of dichromate, drying time / RH, tissue support / final support ect. ect. Unfortunately, you simply can't fix them all so that they're identical to someone else's. Therefore & again, what's the point?)

Regards,
Loris.

Bill Burk · Feb 7, 2012

Wait. A light integrator is likely to be an expensive piece of equipment attached to an expensive vacuum-frame/UV light source device.

So a UV "light meter," if it is cheap enough, and if used on a fairly clear, sunlit day with a glass frame...

This might be a reasonable system.

I would add a Stouffer step wedge to any system. With a step wedge, you can tie in the results of any try to the next attempt.

holmburgers · Feb 8, 2012

Thank you Bill for seeing some potential in all of this. The light meter I bought is $180 new, but I found it on eBay for $40. Not bad, and there are lesser units out there; specifically those aimed at sunburn/skin-cancer prevention.

Loris, certainly working from an integrator isn't empirical, but few people are using these. Instead they're probably using fluorescent bulbs, mercury lamps, the sun, etc, and to use these you must rely on experience, as bulbs age & the wind blows. There's absolutely nothing wrong with that, and obviously it's been working pretty well for the last 160 years.

But the fact that alt.process printers can't say, on average we need X millijoules/cm² of UV to expose this process, seems like a gap in our understanding (and perhaps nothing more). Dependant on the factors you mentioned, certainly there will be wide variability, but wouldn't you like to know the extremes?

I don't disagree with your points at all, I just don't see the point of being so critical. Scientists the world over study the most minute minutia imaginable, and after a while it all adds up to something meaningful.

Loris Medici · Feb 8, 2012

I didn't mean to be critical in a negative sense, just wanted to point out it's useless for practical matters: See, even if we were able to say "we need X millijoules/cm² of UV to expose this process" that wouldn't mean much, because the needed energy would still depend on a myriad number of additional parameters - which may be impossible / impractical or not wanted to reproduce... That's my point.

Regards,
Loris.

Vaughn · Feb 8, 2012

Loris Medici said:
...See, even if we were able to say "we need X millijoules/cm² of UV to expose this process" that wouldn't mean much, because the needed energy would still depend on a myriad number of additional parameters - which may be impossible / impractical or not wanted to reproduce... That's my point...Loris.

That was my thought, too. It is difficult to talk to other carbon printers about the finer details as our pigments, pigment load, sugar load, sensitizing techniques (and strengths), our negatives, type and age of the UV bulbs, etc. are all over the place.

Bill Burk · Feb 8, 2012

One of the things that makes daylight unattractive as a UV source is its inconsistency.

Would it be useful to read the meter and say 'Yup - light's good and strong today - think I'll try 15 minutes'?

By keeping notes of the meter number and the times, pretty soon you can work up a chart for the materials you are working with.

Then any day... you can check your chart for the basic time that goes with the meter reading.

When I use a meter in the darkroom, I use it for only one purpose: To set the aperture so I get a base time for my test strip around 32 seconds. I think this kind of attitude towards the meter would be appropriate... Use it to get in the ballpark.

holmburgers · Feb 9, 2012

Well I think we're all in agreement in reality. Truth be told, I just think it'll be interesting to know roughly the exposure requirements and compare it to say, holography. (that's what led me to mJ/cm² in the first place)

Also, we can say, well Vaughn's tissue requires 250 mJ/cm², and Loris' only requires 175... and ask why is that? Right now our only option is to say, well, I expose it from for 20 minutes from 3 feet away from 24 black-lights under a contact frame with 1/4" soda-lime glass and the relative humidity is always 60% and behind the bulbs I've sprayed Krylon's metallic luster paint which has a reflectance of about....

aaggh!!!

So, I just want to see what a little regularity might bring to the understanding of carbon and other processes. And if people ever take aims to increase sensitivity, we'll need to have a benchmark.

Ultimately though, like Bill says, being able to step outside and get in the ballpark is probably the most useful thing you could do with one of these meters.

Nicholas Lindan · Feb 9, 2012

nuArc and other graphic arts integrators aren't calibrated in any sense other than they are set to count down at around 1 second/unit with a fully warmed new lamp. A 1KW unit counts down at 1 second/unit and a 6KW unit counts down at 1 second/unit - a 6:1 variation in what a unit is equal to. However, given that lamps are consistent when new it is possible to transfer exposure information between like models.

Sunlight is very constant - 1 KW/square meter is known as 'the solar constant'. Of course it changes with time of day - but between, say, 10:00 am and 4:00 pm [more in summer] exposure times will be very consistent. Be sure to keep the printing frame pointing directly at the sun. Clouds attenuate UV less than they do visible light so exposures will be about the same with hazy clouds. Moving clouds and heavy cloud cover can be problematic. You can, as Bill mentioned, correlate exposure meter readings to UV exposure times for cloudy days even though an exposure meter doesn't read UV.

UV monitors for sunbathing aren't very accurate - they pretty much tell you the UV is high, medium or low, something one can see for oneself. I tried using one for cyanotypes and the results were pretty miserable.

Interestingly the ratio of UV to visible light is highest when the sun is just below the horizon - that isn't to say there is a lot of UV, just as there isn't a lot of visible light. The UV is scattered from the sky overhead and it is also polarized. The combination makes for very saturated colors when photographing flowers.

Loris Medici · Feb 9, 2012

holmburgers said:
...
So, I just want to see what a little regularity might bring to the understanding of carbon and other processes. And if people ever take aims to increase sensitivity, we'll need to have a benchmark.
...

Sorry but understanding carbon has nothing to do with the mJ/cm2 value of your UV light; it's all about knowing the principles of the system (in whole) and working out the optimum parameters / workflow that gets well with you and your materials, plus, which also gives the technical excellence / artistic results you're after... The more you get into minute details the more you move away of understanding the process as a whole. Just print, and print, and print; knowing the key parameters / principles and a couple of real printing trials / good note taking is worth thousands pages of nonsense theory! (Don't get me wrong; I'm a computer programmer and I also do a lot of operations research at work. I like math and models a lot, BUT, as a matter of fact, past a certain level, math and models can't give you what real / hands-on experience can bring; alt-process printing is mostly intuitive, rather than being strict / rigid / theoretical / reasoned / calculated...)

Regards,
Loris.

holmburgers · Feb 9, 2012

You're misconstruing what I mean by "understanding". I'm talking about the science Loris, not the art.

I know full well that this kind of analysis has little bearing on the production of a beautiful print, but I think it's really silly to suggest that by being curious about it, and spending some effort to figure it out, that it's going to be detrimental to the right side of my brain!

*harumph*

Loris Medici · Feb 9, 2012

Ok, just tell me how you're going to *USE* the information you're craving for then... (And please, use both sides of your brain!)

Vaughn · Feb 9, 2012

holmburgers said:
...Also, we can say, well Vaughn's tissue requires 250 mJ/cm², and Loris' only requires 175... and ask why is that? Right now our only option is to say, well, I expose it from for 20 minutes from 3 feet away from 24 black-lights under a contact frame with 1/4" soda-lime glass and the relative humidity is always 60% and behind the bulbs I've sprayed Krylon's metallic luster paint which has a reflectance of about....

aaggh!!! and harumph!!!!

I think Sandy King has done some comparisons of different light sources that might help up get in the ballpark.

But one even has to take into consideration the heat generated by the bulbs and the resulting temperature of the pigmented gelatin during the exposure as well as all the other factors I gave earlier. I just have a feeling that all these variables would make knowing the exact UV output of the light source to be of little practical use. Sort of like wanting to know one's air speed after a wing has fallen off.

But it would be a nice thing to know why my tissue seems to take one to two stops more exposure than just about everyone else's (probably my low pigment concentration, though). But my exposure under a 750W Merc Vapor bulb is about 90 minutes (about 12" from the neg), while Howard's is 6 minutes under a 1000W "multi spectrum" lamp at 21" from the neg! That is some difference!

Loris Medici · Feb 10, 2012

Vaughn said:
...
all these variables would make knowing the exact UV output of the light source to be of little practical use
...

And vice versa, the mJ/cm2 sensitivity / energy requirement figure which has been worked out exactly, will be ONLY meaningful within the context / exact conditions of the tester - NOT someone else... That would be - if you like - bogus science!

JBrunner · Feb 10, 2012

The only baseline the "units" on my NuArc have are to themselves.

Help me understand units of exposure

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