Patrick - you sure earned the name "Gadget Gainer" with that last post!
Now I realize that my conclusion doesn't seem right. I meant that spectrum of the light falling on the (at least my) densitometer does not affect the slope of the response. Certainly the transmission spectrum of the density being measured affects the amount of light transmitted. A truly neutral density reads differently through red and blue filters if the response of the photo cell is different, but that apparent difference is due to a change in the amount of current. The meter still reads log of photo current. In the type of meter that I built, the best way to assure that I am reading log of photo current is to calibrate with a current source, not a light source. I am pretty sure that purchasers of commercially available densitometers do not have that luxury. My photocell is on a plug-in probe. I can easilly substitute any othe current source.
Patrick - you are right that the spectrum of the light should not matter with a properly designed densitometer. That's basic physics. No where does the color of the light come into the equations. The ratio of the incident light and the transmitted light (or reflected for reflection density) are the only thing that matter. But note that we are taking about light as an unchanging property.
The problem I was addressing and JMoore was indirectly asking about is what happens when the spectrum of the light is changing between the inital exposure of the material, the reading of the processed material, and the subsequent exposure of yet another material. Then the changes in color spectrum do matter.
QUOTE=Kirk Keyes]Hi Sandy!
I hope this reasoning makes sense to you!
I have a few questions for you about your densitometer calibration - You mention that your densitometers were recently calibrated. You say recently - do you mean that they were serviced recently and adjustments were made by the service technician? Or that you used your calibration standard that you have from someone like X-rite or MacBeth/Gretag and that you went through your densitometer's calibration routine and the was some time ago and you haven't recalibrated since? Or perhaps something else?
I'm suggesting that we all take the effort to have valid (i.e. not too old) calibration standards for our instruments to make sure that the numbers we read off of them are at least of a known quality, i.e. they are within a known precision and accuracy. I'm an analytical chemist, and no number I produce is worth anything if I can't demonstrate that the quality of my data is known.
I can agree with many of the things you are writing but not with the issue of how the step wedge should be measured. But I have made my point several times so there is no reason going there again. Briefly, to simply my answer to the original question here is what I would recommend.
1. The exposures of the test film through the step wedge should be made with the color of light in which one intends to use the film. In my case I expose all of my film in daylight so I test film with a tungsten light but with an 80A filter to convert to daylight. If you shoot indoors mostly you would not need the 80A filter. In practice it won't make a lot of difference whether you adjust the light or not because panchromatic film is sensitive to Red, Green and Blue light, as well as ultraviolet light.
2. Test the step wedge in Visual, Blue and UV mode and take the readings. If they all match, as one would expect they would, no problem. If they do not match you should use the density values that match the Mode that you will use to measure the test strips. This would be Blue for silver graded papers and UV for alternative processes that require UV light.
3. Next, you measure the test strip densities, Blue for graded sliver papers, UV for alternative processes, enter them and plot the curves.
I am sure we agree on #1 and #2, and though we appear to disagree on #2, the difference should not be great and as Phil Davis said, the main thing is to be consistent.
As for the calibration of my densitometers, I have manufacturer calibration step wedges for both the Gretage D-200 and the X-Rite 810, and I calibrate frequently. I calibrated again just a few days ago when the present discussion began because I wanted to make sure that the readings were accurate.
I don't know why the Visual/Blue and UV modes of the Gretag D-200 read what I would presume to be a neutral step wedge differently, but they do. However, my best assumption is that if the densitometer in UV mode reads the step wedge log 0.18 lower than the Visual/Blue mode it will also read real time film tests the same way in UV mode. The matter would be much simpler if the difference in UV reading was overall, but it is not. The decrease in density is proportional to the increase in silver density, about log 0.05 at Step 1, but log 0.18 at Step 21.
I suspect it is due to the fact that the silver density itself of the step wedge is not entirely neutral in the very small bandwith around 373 nm where the Gretag D-200 reads in UV mode.
Sandy, I'm glad that this thread has made you think about calibrating your densitometer more frequently! That's a good thing.
I agree with you on the first part of step #1 - about filtering your enlarger light to be daylight for film testing. I do that as well - I even checked the color temperature of my light source. It is at 5700K. Pretty close to standard daylight for me!
In the second part of #1 - you say "in practice it won't make a lot of difference whether you adjust the light or not because panchromatic film is sensitive to Red, Green and Blue light, as well as ultraviolet light." Well, then why do you do it? I think if you are trying to determine the speed of your film - where you actually need to measure how much light is being exposed to the film, I think you may have problems there as accurately measuring how much red, green, blue or especially UV light you are applying to your panchromatic film - it will make a difference. After all, that's why we have to use filter factors when using colored filters with b&W films. If the color of the light on the film did not make a difference, then we could dispose of filter factors and shoot with no corrections.
In step 2, you say, "Test the step wedge in Visual, Blue and UV mode and take the readings. If they all match, as one would expect they would, no problem."
We'll that is our issue. If they do match, that is great and it is really kind of serendipidous. Step wedges are not truly neutral in color transmission. And as you have found out, your UV measurements do not match your visible densities. There's your proof. So now you have to pick a color.
"If they do not match you should use the density values that match the Mode that you will use to measure the test strips. This would be Blue for silver graded papers and UV for alternative processes that require UV light." (Instead of test strips, you the film that we are testing, right?)
The problem with your reasoning that is that we have a "function" that is being applied to the input information (the light and film combination) which is generating an output information (the film neg of our step wedge). Information can only flow one way into this system. We can not tell by looking at the film neg of our step wedge what the input was, other than there was some input. Our film processing step is the function, it is a black box into which we are dumping light of a certain spectral energy, a piece of film, and a filter (the step wedge) between the film and the light. Out the back of the black box pops out our test neg. Kind of esoteric, I know, but that is how are system could be modelled.
There are literally an infinite combination of colors and intensities of light that we could shine through our step wedge to generate identical negatives. We can only derive information from this system in one direction - frontwards, not backwards.
"I don't know why the Visual/Blue and UV modes of the Gretag D-200 read what I would presume to be a neutral step wedge differently, but they do."
Well, I believe it is because of the spectral reasoning I gave in my earlier threads. Do you have a reason to think otherwise?
"I can agree with many of the things you are writing but not with the issue of how the step wedge should be measured. But I have made my point several times so there is no reason going there again."
I'm glad that we agree on many of these things! You say you disagree with how the initial readings of the step tablet should be made. I've given my logic as to why they should be read as I've stated above, but I've lost track of your reasoning (it's been a long thread). Could you please state it again? I understand that you appear to be tiring of this thread, but I am truly interested in what the flaw you believe is in my reasoning. I wish we could discuss this face-to-face instead of through a forum, but that is our avenue...
Thanks for all your replies!
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If you are going to use the densitometer readings to estimate exposure and contrast grade of a printing material required to make a full scale print of a negative, the only frequency response that matters is that of the photosensor in the final readings. It must match that of the printing material.
Originally Posted by Kirk Keyes
The meter reading is the log of what the photosensor "sees" in a properly designed instrument. For example, if the photosensor does not know the difference between yellow light and white light, it will not see the stain image in a pyro-developed neg. If the printing paper sees the yellow image as gray, there will be a mismatch and both exposure and contrast grade will not be correctly estimated.
Certainly, if you are using the densitometer to measure light falling on film that is to be developed, the photosensor ought to have the same response as the film. We should therefore have a different sensor for each different material that is being exposed by the light we are measuring. If the sensor has a broad enough response spectrum, we can use filters to simulate the response of the material we are exposing. These filters will have no effect on the calibration of the densitometer as far as reading log of illumination is concerned. Look at it this way:
The linear response of the sensor may be represented by the equation
i = k(I)
where i is the current through the sensor, I is the illumination and k is a constant of proportionality.
The meter reads the logarithm of the current i. Thus
log(i) = log(k) + log(I)
but log(k) is just another constant. Then even if k changes with spectrum or from one sensor to another, the meter reading is still the log of the illumination plus a constant. Once the meter has been calibrated to read logarithm of the input current, it should be sufficient when using it to read density to adjust the level of the input illumination to read 0 density without a negative in place. This adjustment will correct for changes in k that might occur due to aging of the photosensor or other causes outside the meter.
I conclude that the spectral resonse of the densitometer should be the same as the spectral response of the material that is being exposed. If then the step wedge is not exactly neutral, when you measure its density with a densitometer that is properly designed to read log of illumination, it will not matter if the density readings are not the same as the nominal densities of the wedge. They will still be the densities that the material sees.
Patrick writes: "If you are going to use the densitometer readings to estimate exposure and contrast grade of a printing material required to make a full scale print of a negative, the only frequency response that matters is that of the photosensor in the final readings. It must match that of the printing material."
Yes - we must match the freqency response of our photosensor by using filtration that matches spectral properties of our material. That's why when we expose our Tmax film, even if we are using it for Platinum printing, we must use the visible channel on our densitometer. And for each step in the reproduction cycle, this must be done.
Here's another example - say we are going to test IR film and then print it onto Platinum paper. Let's assume we have a densitometer that reads from IR through visible to UV. We have measured the step wedge and found that it transmits more light in the UV than it does in visible. And let's say it transmits less light in IR than visible. (So that the density of any particular step one the wedge follows this pattern, UV density < visible density < IR density.) We know that our IR film, because we are going to use a 87 filter (which blocks all visible light and passes IR above about 740 nm), needs to be exposed to IR light only. Do we mesure the step wedge with the densitometer set to UV - no. Do we use it set to visible - no. We must match the filtration of our densitometer to the spectral sensitivity our our film, and the color spectrum of light that we will be exposing that film.
Once we have processed that film, we have to consider the next step. Since we are printing onto platinum paper, will we use the densities of the step wedge that we measured in the visible or the IR - no. Making measurements with non-actinic light makes no sense. We have to match it to our medium, so we have to make our measurements in the UV.
If I understand what Sandy is saying, for the above example, he would recommend measuring our step wedge with UV light since we are making a platinum print, even though we are using IR film that is being exposed only by IR light and we know that our step wedge has different reading in the IR than it does in the UV. Am I correct in this understanding? I hope this last example has shown that that is not a good procedure.
Patrick writes: "I conclude that the spectral resonse of the densitometer should be the same as the spectral response of the material that is being exposed. If then the step wedge is not exactly neutral, when you measure its density with a densitometer that is properly designed to read log of illumination, it will not matter if the density readings are not the same as the nominal densities of the wedge. They will still be the densities that the material sees."
Correct, as long as we keep matching the filtration of the light and the densitometer to the photographic material. and we cannot expect our step wedge to be completely neutral, as Sandy has found the case when he compares measurements of his step wedge made in the visible or blue with measurements in the UV.
I'm just a tad confused. I would think that once the film is developed, the original exposure spectrographic response would have very little to do with anything in printing.
Originally Posted by Kirk Keyes
The strength of the light that will affect the printing material at a given wavelength will be all that is important.... and that will be a product of the enlarging lamp; transmission properties of the enlarging lens (I have found slight lens-to-lens variations); and the processed film, acting as a dichroic filter.
Ed Sukach, FFP.
Originally Posted by Ed Sukach
Your thoughts would be accurate insofar as conventional developers. With proportional staining developers such as ABC and Pyrocat the matter becomes murky. The reason is that the color of the proportional stain has an effect on the transmission of light. This is dependant on the color of light that the print is exposed to.
Some processes (Pt-pd for instance) use a light exposure source that is heavily UVA. In the contact printing of Azo, for instance, we use primarily the blue componant of the light spectrum due to the sensitivity of that paper's emulsion. When we address the effects of stain on enlarging VC materials this becomes a moving target because of the differing filtration that we use to modify contrast.
Not all proportional stain is of the same color (dependent on the formula). Hence some stain is more actinic then others.
I understand that. My comment was directed at the initial in-camera EXPOSURE spectral sensitivity of the film. Whether it is IR, or "Extended red" or a blue filter was used in exposure, it would have minimal - to say the least -effect on the densitometer reading integrity. Developer "staining" WOULD - most certainly.
Originally Posted by Donald Miller
Ed Sukach, FFP.