Metering UV Exposures with a Solartech Model 5.0 Solar Meter

[holmburgers]

Well as a christmas gift to myself I picked up a Solartech Model 5.0 solar meter on eBay for $45 (retails for $179.. so not bad eh!) -> http://www.solarmeter.com/model5.html

The idea for metering UV exposures for alt process printing came to me a while back and I posted some original thoughts in (there was a url link here which no longer exists).

This meter is designed mainly for testing the UV output of tanning beds and that kind of thing. But the meter is well suited for processes that use dichromate sensitization and probably others like cyanotype, Pt/Pd, etc. This meter has a peak sensitivity of 370nm and output values in milliwatts/cm².

But now comes the hard part; figuring out how to make good on these readings.

I think the biggest advantage is going to be the ability to know the UV activity of daylight and to use that information to make accurate sunlight exposures. But getting from the readings to a reliable exposure table is going to take some testing and I'd like your input on devising some methods to get to this point.

 

[keithwms]

If you're lucky, the lamp or solar spectrum and the sensitivity spectrum of your emulsion will be fairly flat around the peak sensitivity of your meter. In that case, you'll be able to do test strips and get reproducible results.

I'd begin with a step-wedge kind of approach using a reproducible (lamp) spectrum. It seems that with a little upfront work, you iwll have a very handy device!

I am a bit more skeptical about using it for the sun exposures- the spectrum we see observe here on Earth changes a lot with the weather (UV scatters a lot in the atmosphere) and has a big slope in the UV:

But at least you should be able to get ballpark exposure estimates.

 

[gmikol]

I suspect that, like a regular light meter, with some experience you'll be able to take the reading from the meter, take a look at the actual conditions, and adjust accordingly. I don't know how many prints you'll have to make before you get to that comfort level.

--Greg

 

[holmburgers]

Thanks guys,

It'll be interesting to start taking readings and getting a feel for the variations during a day and throughout the year. One thing I was wondering about was whether reciprocity effect will apply to this kind of metering. That is, what change in mW/cm² will equal a one-stop (or one step on a wedge) change in exposure on the print, and will that translate from say, my UV-lamp exposures to using the sunlight.

Do the light integrators on Nu-Arc units give a reading, and if so, what unit do they give?

Either way, I know that the only way to know for certain is to just start printing and testing and that's what I'll have to do!

Over time I hope to contribute some meaningful information about using this meter.

 

[gmikol]

It's likely that the meter itself will not suffer from reciprocity failure. It will likely be quite linear over its specified range. Based on my experience, carbon is a fairly linear process (minimal toe/shoulder), so reciprocity failure should be minimal there, as well, unless you're using very high concentrations of dichromate. The combined heat and high dichromate concentration could cause fog very quickly, which I guess, is sort-of the opposite of reciprocity failure. Check either Sandy King's carbon book or his article in unblinkingeye, there are some graphs in there. Don't know much about other processes (Pt/Pd, cyano, etc.) in this regard.

If the meter gives a reading in mW/cm2 (linear units, not log), a one-stop difference is a doubling of the reading. e.g. from 5 to 10 is one stop, from 10 to 20 is one stop, from 20 to 40 is one stop, etc. Best thing to do is measure your UV-lamp output with the meter and them compare your sunlight readings.

# of stops difference between 2 readings = log( R1 / R2) / log(2)

For example: R1 = UV lamp reading, R2 = Sunlight reading. Positive numbers means the UV lamp is brighter (so you need to add that many stops to your sunlight exposure). Negative numbers means the sun is brighter, so you need to subtract that many stops from your sun exposure.

NuArc integrators just measure the UV output and slow down or speed up the counter. It does not give a calibrated measurement. One thing you'll need to make sure of is that the UV reading from your meter stays constant over the course of the exposure, which might be 30 mins. or more on less-than-ideal days. You might need to adjust your time if the UV goes up or down due to clouds or whatever.

Doing some back-of-the-envelope math on my Nuarc:
1000W bulb
1% luminous efficiency (pessimistic? optimistic?)
17x22 inch imaging area (assuming uniform illumination) = 24128 cm^2
10W / 24128 cm^2 = 0.4 mW/cm^2

One more thing...I took a look at the specs on the mfr's webpage, and the meter is fairly sensitive down to ~280nm. When taking a reading make sure to place a piece of glass over the sensor that's identical (or at least similar to) the glass in your contact printing frame. Ordinary window glass starts to absorb significantly around 325nm, so you'd want to exclude any energy that might give an erroneous reading, and to tune the meters' response to be a bit closer to what the exposed paper (or carbon tissue) sees, since it's "seeing" the sun through the glass. If you want to get fancy, you can get narrow band-pass filters that mimic the sensitivity of the alt-process you're using.

Good luck with your testing.

--Greg

 

[holmburgers]

Ok, the lightbulb finally turned on.

I think the most ideal way to meter UV-exposures would be in watt-seconds, or joules (just like the holography folks).

So I can say, my carbon tissues needs a total of X watt seconds of energy to be properly exposed. This value then could easily be translated to black-lights, cloudy days, sunny days, etc.; as long as you are keeping track of the UV levels throughout the exposure & keeping a tally of the total energy.

Does that seem reasonable? Any pitfalls I'm missing?

I've been playing around with the meter a bit. Today at noon, with the bright sun veiled in thin clouds I was getting readings as high as 2.0 mW/cm². In the morning, thru my living room windows, about 0.5 mW/cm². In the evening yesterday, when the sun was just below the horizon, I was only able to eek out a reading of 0.1 mW/cm² from the sky.

 

[holmburgers]

The other day it was brilliant and clear here, and I was getting readings of 3.4 mW/cm² in direct sunlight and 0.5 pointed towards the north sky. This reading held from 11 o'clock in the morning until 2 o'clock that afternoon.

7 CFL black-lights (there was a url link here which no longer exists) measure 0.2 mW/cm² from 2 feet.

 

[holmburgers]

Holographers measure their exposures in mJ/cm² (millijoules per square centimeter), which is easily obtained by multiplying mW/cm² by the number of seconds in an exposure.

So, a reading of 3.0 mW/cm² and an exposure of 1 minute would equal 180 mJ/cm². A typical DCG (dichromated gelatin) hologram requires about 100 mJ/cm², though carbon printing will likely require much more due to its different nature.

Unfortunately reciprocity failure is an issue with DCG as well, but I hope that total energy as expressed in joules per unit of area will still help in determining UV exposures.