Transmission density, UV vs visible light

This is my first post since the forum moved to Photrio. I made a do-it-yourself transmission densitometer from two UV dose meters -- call them, m1 and m2. I use m1 to measure incident exposure and m2 to measure transmitted exposure (m2 goes under the film and looks up toward the light). The two meters measure in different units, so I needed to determine a conversion factor that converts meter2 units to meter1 units. Now I can calculate transmission density as

transmission density = log( incident / transmitted) = log[ meter1 / (meter2 * conversion factor) ]

As a check, I measured the transmission densities of several steps of a Stouffer TP4x5-21 step wedge. The Stouffer has known densities of 0.05, 0.20, 0.35, ... 3.05. That is, they go by half stops (0.15) from 0.05 (clear film) to 3.05.

But when I made the measurements, I got a densities that were consistently higher by 1/2 stop.

Question: do I have a systematic error somewhere? Or, do the published Stouffer densities only hold for visible light? And if so, is the 1/2 stop higher density to be expected for UV?

jisner said:

But when I made the measurements, I got a densities that were consistently higher by 1/2 stop.

Question: do I have a systematic error somewhere? Or, do the published Stouffer densities only hold for visible light? And if so, is the 1/2 stop higher density to be expected for UV?

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Absorption of UV by materials is almost certainly different than absorption of visible light. Generally, common materials that are transparent in the visible are less transparent in the UV, often much less. Thus, your result is the expected one.

Also, remember that "UV" is not a particularly precise term... it covers a wide range of wavelengths, which may be absorbed differently by materials. The shorter the wavelength the less transparent most materials are. Thus, unless the sensitivity of your sensor is similar to the sensitivity of your process your device may give misleading result.

Most alt process, which can be exposed through glass are sensitive to wavelengths in the UVA range. Glass is fairly transparent in the UVA but quite opaque to the shorter wavelengths of UVB or UVC.

Interesting...

When I was thinking about getting a Stouffer step wedge, I tried to do some research precisely on the issue of whether the quoted numbers were UV densities or visible light densities. I could not find this being specified anywhere. Stouffer uses the term "light transmission." I tried to look up the NIST standards they supposedly mearsure their numbers against but was not successful finding them. I figured it had to be UV as it is most used by the folks in plate making and silkscreen making industries who all use UV source for their printing. It would be wrong otherwise. So I left it at that - which was probably a mistake.

The first and the only time I used the step wedge, it seemed to come out quite a bit underexposed than what I thought would be expected based on my previously arrived at time using a simple test strip method with just the transparency material on top. I could not explain it away with the potential difference in the publicly available UV transmittance of the Stouffer base and the Fixxons material I was using. I didn't do any further investigation to confirm this single-point data and went back to doing the old-fashioned way.

To answer the question you raise - yes indeed it may well be true that the numbers on Stouffer signify visible light transmission and not UV. Your method seems fair to me, particularly since the shift is same on all steps. One potential source of error would be the if the step wedge is not the more pricey "certified" kind, there may be some fluctuations from one wedge to another - but I can't see that would be a 1/2 step worth.

:Niranjan.

As an aside, I was wondering why you needed 2 meters - could you not simply use one, first measuring the incident light and then with the particular step above and arrive at the transmission that way. I guess as long as the light is not fluctuating too much within the time of measurement, it should be accurate. Or may be I am missing something?

:Niranjan.

fgorga said:

Thus, unless the sensitivity of your sensor is similar to the sensitivity of your process your device may give misleading result.

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Thanks for your reply. I do use a Stouffer for determining exposure, but I don't go by the numbers. I go by what the process tells me ("I've reached maximum black: stop exposing"). Then I calculate the exposure scale of the process as (Emax - Emin). But subtraction cancels any UV bias, so I should get the same ES for visible light as for UV, assuming the bias is constant over the exposure scale. The ES is a valuable number because it dictates the required density range of the "perfect" digital negative. If I then expose a sample of a pure black negative and a sample of clear film using the same methodology described in the OP, I should be able to determine the density range of the negative and know if it matches the exposure scale of the process. Again, the answer should be true regardless of wavelength. Anyhow, this is how I have been thinking, and I may be totally wrong!

nmp said:

I figured it had to be UV as it is most used by the folks in plate making and silkscreen making industries who all use UV source for their printing. It would be wrong otherwise. So I left it at that - which was probably a mistake.

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I was thinking of calling Stouffer and asking this question. But as you say, I suspect their numbers are for UV transmission, which is why I question my results.

My Stouffer is the uncalibrated kind. There's a screenshot of a calibration here. The differences are typically plus or minus 0.01.

When it comes to determining exposure using a Stouffer, the target tells you the result, so I don't see how using a Stouffer should give a different standard exposure than a test strip. With a Stouffer-21, you have the ability to get within a half step by half-covering, a trick I learned from Keith Schreiber, but it is only practical using the 4x5 version.

I believe there is some variation from nominal so unless the step wedge was calibrated it might be hard know.

What I do know is that the step wedge I have(same model#) reads slightly denser than nominal, if I remember correctly the first step is around .09. Using my X-Rite 361T I don’t see any significant density difference between the UV and visible modes. I believe the UV measurement wavelength is around 380nm.

nmp said:

As an aside, I was wondering why you needed 2 meters - could you not simply use one, first measuring the incident light and then with the particular step above and arrive at the transmission that way. I guess as long as the light is not fluctuating too much within the time of measurement, it should be accurate. Or may be I am missing something?

:Niranjan.

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Right, it could be done with one meter but you would then be relying on time as a measure of exposure. Since my goal has to do with determining negative density, some very long exposures will be required with my DIY setup, and I do not trust time.

Re the two meters: One is an LC2 from Ian Leake (highly recommended). The sensor has a fixed location in the exposure box. The other is a UV Integrator 150 which I had left over from when I exposed in sunlight. I use the UV Integrator to measure the transmitted exposure.

I would love to have an X-Rite 361T for this work, but I can't justify the expense and I'm not sure how much use for it I'd have after completing these experiments.

osella said:

Using my X-Rite 361T I don’t see any significant density difference between the UV and visible modes.

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That is useful information, thanks.

I have my eye on a used 361T, but I'm reluctant to take the chance that something burns out or breaks and replacement parts are unavailable. What has your experience been? Are you the original owner? Any suggestions?

NIST paper on the reference densitometer for measuring visual transmission (Stouffer says their calibration is referenced to NIST standards):
https://www.nist.gov/publications/nist-reference-densitometer-visual-diffuse-transmission-density
I didn't look for the actual standard that describes the measurement of visual transmission, but this paper is freely available and has enough information. Click on the "download paper" link and look at Figure 7, flux and spectral response of the densitometer as a function of wavelength; and Figure 8, transmission and reflectance of step tablets as a function of wavelength. The dashed curve is the step tablet for photographic use. It doesn't say it's a Stouffer, but the Stouffer is likely similar. You can see that the transmission of the tablet falls off by 10-15% in the UV.

I would not assume that a step tablet described as "visual transmission density" is referring to the near-UV. Visual, in optics and spectrophotometry speak, typically means either the 400-700 nm range or specifically about 500-550 nm (and that is where the reference light source peaks). Even if the platemaking industry uses UV, the user needs to figure out how to use the step tablet with their light source and process.

reddesert said:

NIST paper on the reference densitometer for measuring visual transmission

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Thank you for the reference. I will try to make sense of it.

osella said:

I believe there is some variation from nominal so unless the step wedge was calibrated it might be hard know.

What I do know is that the step wedge I have(same model#) reads slightly denser than nominal, if I remember correctly the first step is around .09. Using my X-Rite 361T I don’t see any significant density difference between the UV and visible modes. I believe the UV measurement wavelength is around 380nm.

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Here is a bit of new information. I was discussing my results with a friend who has a 361T. He asked me if I measured with emulsion side up or down. When I told him "down" he said it should be "up." Have you seen any discrepancy in density depending on which side is up?

jisner said:

That is useful information, thanks.

I have my eye on a used 361T, but I'm reluctant to take the chance that something burns out or breaks and replacement parts are unavailable. What has your experience been? Are you the original owner? Any suggestions?

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I’m not the original owner, so I really don’t know how old mine is. It came with all the paperwork and calibration strip so it seems to have been somewhat taken care of.

I don’t use it all that frequently, but it seems reliable. I’ve accidentally left it on for extended periods and that hasn’t seem to hurt it. It seems to keep calibration/adjustment and reads the calibration card always reads correctly.

I bought it mostly to help with digital negatives, but I do use it to check my 8x10 negatives as well. We’re I to buy another I probably wouldn’t bother with the UV as I prefer to work with film negatives, however I have no regrets because it doesn’t hurt to have that capability.

You can apparently still get a new lamp for it, but it’s pretty expensive.

EDIT: I always check emulsion side up, I will check tomorrow emulsion side down and see what I get.