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  1. #91
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    I do not have a comercially available densitometer, as I said before, so do not know what the various filters do in detail. What is the bandpass characteristic of the visual filter? Does it look like the average human visibility curve, or is it flat over the limits of human vision?
    Gadget Gainer

  2. #92

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    Quote Originally Posted by gainer
    I do not have a comercially available densitometer, as I said before, so do not know what the various filters do in detail. What is the bandpass characteristic of the visual filter? Does it look like the average human visibility curve, or is it flat over the limits of human vision?
    I have tried and contact X rite about the spectra of their Vis and UV metering in the X rite 369, sadly they have not seen fit to answer me. It is not in their web site, nor is there any info for the Xrite 810 color densitometer. So in that respect I guess your guess is as good as mine. Some speculation, at least in the case of the 369 densitometer would suggest that it is a very narrow band response. The densitometer has a blue light and uses a filter for the UV mode. Either way I am sure it is not even close to the human eye visual range.

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    QUOTE=gainer]I do not have a commercially available densitometer, as I said before, so do not know what the various filters do in detail. What is the bandpass characteristic of the visual filter? Does it look like the average human visibility curve, or is it flat over the limits of human vision?[/QUOTE]


    Pat,

    The measuring geometry of the Gretag D-200-II is a specular/diffuse, ANSI-PH2, 19-1976. The color temperature of the measuring lamp is 3000º K.

    I have measuring tubes for Visual, Blue and UV.

    The center of the bandwith in Visual mode is 555 nm, and the bandwith is on average about 130 nm. The measuring range is 0.00 > 6.00D

    The center of the bandwith in Blue is 458 nm, and the average bandwith is about 45 nm on average, with a measuring range of 0.00 > 4.00D.

    The center of the bandwith in UV is 373 nm, and the bandwith is about 45 nm on average, and the measuring range is 0.00D >5.00D

    The instrument will also measure IR, Ortho (500 nm), Green (543 nm) and Red (638 nm).

    In the absence of measuring tubes of the correct type one could also use narrow band filters between the measuring light and the sample to simulate the filtration in the tubes. Using a 47b filter, for example, I get virtually the same readings with the D-200 as with the Blue measuring tube, and in turn these values are virtually identical to measurements with the X-Rite 810 in Blue mode.

    Sandy
    Last edited by sanking; 08-16-2004 at 06:30 PM. Click to view previous post history.

  4. #94

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    Sandy, sorry for the couple of says absence.

    Sandy wrote, "I have been working with densitomers for over 25 years and am very familiar with calibration procedures. So let’s get this out on the table."

    Thanks for the in-depth info one your calibration procedures. I agree, it looks like you are doing all the manufacturer requires of you in that respect.

    As a side note - I do have one issue though, and it's not an issue with you, but with Gretag. I may be wrong, and please correct me if I am, but it sounds like the calibration wedge that Gretag supplies has only one value on it for each step. Not separate values for red, green, blue, vis, UV channels. Is that correct? And the same wedge/value is used for calibration of all those channels?

    If so, then if the calibration point varies in true density in different wavelenghts, as we really should expect it to, then this type of calibration procedure WILL cause differences in the slope of the densitometers response when changing channels. And that could easily account for errors in readings at higher densities when comapring different channels.

    As you said, "This calibration wedge appears to be perfectly neutral." But if we could determine those fine details very well with our eyes, we would not need instruments like densitometers. Looking at your X-Rite 810-68 calibration wedge, you can see from the numbers on it that a wedge that appears to be "perfectly netrual" is not necessarily so. (See my last post at the bottom of page 8 where I list the RGBVis values for my 810-68 "Cal" step.)

    So I am kind of concerned if Gretag is saying that the calibration wedge has exactly 3.05D in R, G, B, Vis, and UV from the same peice of film. Your measurements of your step tablets show that this is probably not a good assumption - none of them had identical measurements. But as I said, that is really an issue with Gretag's prescribed methods and you a fault of yours.

    "The supposition on your part appears to be that if my reading of a Stouffer TP 45 step tablet is different in Visual and UV mode I must be doing something wrong, or there must be something wrong with my densitometer."

    Not at all, but without a clearer understanding of your procedures one can not be certain.

    Sandy wrote, "This suggests to me that you don’t appear to be aware of the fact that there are considerable variations in step wedges and that very few, if any, are perfectly neutral."

    Not aware of the fact - if you go back through this thread I have been making that claim all along. I brought this up early on in this thread. From my second post in this thread on page 4, "It is quite difficult to make a step wedge for a wide range of the light spectrum that is neutral in color. And we have proved this to ourselves by measuring our step wedge with a calibrated densitometer." It is foolish to assume that a step wedge if completely neutral over a wide range of wavelengths.

    Sandy wrote, "See any pattern? I do not. The densitometer reads higher in UV mode than in Visual with the Kodak Step #2 step wedge, lower in UV than Visual with the X-Rite transmission wedge and all three Stouffer TP-45 step wedges, and the same in UV and Visual with the Gretag calibration wedge."

    The differences in the Kodak wedge are probably actually within the precision error of your densitometer so they can probably truly be considered neutral. You are right, your other step wedges to appear to actually have less absorbance in UV than Blue (Vis). Interesting because that is not what one may suspect a priori. But that's OK, because now we have precisely measured them with instruments that are making measurements of a known quality. That's great!

    Sandy wrote, "As you should immediately see from the above the difference in reading is not due to base filtration. If that were the case the difference between Visual and UV values would be a constant. It is not."

    DIfferences are not due to "only" base filtration. So that certainly means that there is some sort of "staining" or other factor that is going on that is contributing to the non-neutrality of our materials. There are many factors other than film base absorbances that can contribute, things such as the size and shape of the developed silver in the film that could cause this.

    Well, great! I'm glad you've gone though that work and laid it out for us. I'm sorry that you feel like you are having to defend your practices when you know you are doing things as per the manufacturers methodologies, but until you fill us in on your procedures, we have every right to be suspicious of your data and methods.

  5. #95

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    In response to Phil's excellent question, "knowing that you use BTZS for alt printing with great accuracy, I assume you have found that you have improved the accuracy of BTZS use by recalibrating the x axis using UV instead of visual readings of the step tablet?"

    Sandy replied, "Using the [UV - right?] step tablet densities of the Stouffer TP 45 #3 test wedge provides better accuracy of the exact printing density of my pyro stained negatives than was the case with the default values of the WinPlotter, which are virtually identical to the Visual reading of this step wedge."

    (Side note here - there is no reason to use the default values in the software, you should always use data derived from your own materials.)

    "Exactly why this is the case is not entirely clear to me..."

    Well, that's what we've been trying to determine here.

    "...but the important thing to note is that calibration of the X axis to values other than default values or even to the Visual reading of the step wedge used to make the exposures results in different values for CI, EFS and SBR."

    Yes, because of the simple math that is involved in the calculations.

    "In my case the new values suggest develoment times for normal ES values with my processes that give more accurate CI values."

    Using the UV values give you higher results for Ave. G than blue readings did. You did this by using the lower results you got by reading the UV spectrum of your step wedge. I would like to point out to you that you could have gotten the same Ave. G if you had gotten higher results for the y-axis data. Remember the math for the Ave. G is just one number devided by a second number. If your UV meaurements of the test neg had been proportionally higher, then you could have gotten the exact CI as your've gotten by using the UV results for the step wedge. (I hope that was clear - if not let me know.)

    You've come to the conclusion that your visible readings of the step wedge are not valid because you've determined empirically that the UV readings work better. That's fine. But there's no scientific principle to support that conclusion, and many principles not to.

    Have you stopped to think that it may be the readings of test neg that are suspect? Perhaps they need to be looked at?

    I would like to suggest that perhaps the UV filter setting on your densitometer is not matching the "ideal" filtration we need to use for these measurements.

    Sandy wrote, "Kirk has made the case that the use of UV values for the step tablet should not make any difference but the matter may be more complicated than he believes. For one thing film and papers have quite a bit of sensitivity to UV and even tungsten lights put out some UV radiation. There are after all filters called UV absorbing filters that are sold for use in enlargers, even with those with tungsten lights, to eliminate this source of light in printing. The Besler 23-CII enlarger that I use for exposing film in these tests originally had one of these filters but it was lost somewhere down the line so there is at this point no filtration of the UV light."

    Interesting idea - my understanding is that modern enlargers are well designed and essentially have built in filtration. Perhaps if you have a cold light head that uses fluorescent bulbs which are rich in UV then this may be an issue, but not with a color enlarger. Also, the glass in our lenses do quite an effective job of filtering out UV. Most glass transmits about 92% of the visible wavelengths and around 380 nm it really starts to become a very efficient absorber of UV light. But combine this with the fact that a 3200K tungsten-halogen bulb produces virtually no UV light - especially when compared to the total output of all the visible wavelengths, I still believe that any UV contribution to the exposure of our panchromatic test film from our enlarger light source is negligible. If you have some data to suggest otherwise, I am certainly interested in seeing it.

    As I have been saying all along, we MUST match the densitometer settings to match the exposure that we will be giving our material. So far, Sandy, other than your position that 1) it causes problems with Phil Davis' software if we have two sets of data for the exposure, and 2) you empirically have determined that the lower step wedge numbers "work" better, and now 3)your suggestion that UV filtration of the enlarger may be a key issue here, I haven't really heard any other reasons for your position.

    After looking at the filter values you've listed for your densitometer, I see you have 373 nm listed as the center of the filter. Perhaps it is this filter that is not match your printing materials as well as you think for Pt/Pd printing. I believe that Jorge mantioned that according to Dick Arentz, Pt/Pd paper has a peak sensitivity at 290 nm. That is a bit off 373 nm. I don't know what type of light source you are using, I assume a Mercury lamp of some sort? Without knowing the emission spectra of the type of lamp you have, I can't make any suggestions there.

    And we have yet to measure the UV filtration properties of your Pyrocat-HD processed films at wavelengths other than the 373 nm that your densitometer does. Perhaps a wavelength of 350, 320, or 290 nm would give you the extra density that it would take to make the simple math work. Looking at your pyrocat graphs on unblinkingeye.com, there is a trend towards greater density when we compare the blue and UV densities of your graphs. This suggests that a densitometer that could read shorter wavelengths may give better numbers, but there is really no way to predict the absorbance at different wavelengths simply by looking at two points of a spectrum. We'll have to try that and see what we get.

    Again, from above, Sandy wrote, "Using the [UV - right?] step tablet densities of the Stouffer TP 45 #3 test wedge provides better accuracy..."

    If your theory that the UV measurements of the step wedges was truely valid, then there should be no difference between any of the step wedges and we should be able to get predictable results from any step wedge as long as we measured them in the UV.

    Have you asked yourself why Stouffer Step Wedge #3 works best? Is it simply because it has slightly higher numbers than Wedge #1 and 2 in the UV and not as high as any of them in the visible? Aren't you simply picking data sets that tweak the numbers to get results you like the best?

    As I've been saying all along, there is no scientific reason to NOT use the visible density measurements of your step wedge for detemining exposure since you are using standard daylight with panchromatic film as this most closely matches. So now I suggest that instead of trying to rationalize the discrepancies between your calculated data and your empirically derived results, you should look at the measurements used to generate the data for the other side of your data set - we've looked at the x-axis data, so now let's look at the y-axis data. Does it best match the conditions that we will be exposing our materials?

  6. #96

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    I'd like to correct a few things I said above:

    I have been reminded that the density values used for the y-axis of our H&D graphs is the log(opacity). Not the log(density) that I beleive I said above.

    Also, that an H&D curve should properly be called a D log-H curve.

    Thanks, Kirk

  7. #97

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    Kirk,

    In response to your remarks.

    "I may be wrong, and please correct me if I am, but it sounds like the calibration wedge that Gretag supplies has only one value on it for each step. Not separate values for red, green, blue, vis, UV channels. Is that correct? And the same wedge/value is used for calibration of all those channels?"

    I believe you are stating this correctly. The calibration itself is done based on only one density, 3.05, and in Visual mode.

    "If so, then if the calibration point varies in true density in different wavelenghts, as we really should expect it to, then this type of calibration procedure WILL cause differences in the slope of the densitometers response when changing channels. And that could easily account for errors in readings at higher densities when comparing different channels."

    That may be so, but there are in fact no errors at all in readings that I can see, and as I indicated, after calibration the calibration wedge reads identically at 0.00, 2.05 with the Visual and UV tubes, and also with the Blue tube that I checked after the last message.

    "As you said, "This calibration wedge appears to be perfectly neutral." But if we could determine those fine details very well with our eyes, we would not need instruments like densitometers."

    There may be a misunderstanding. I said that the calibration wedge appears to be perfectly neutral because I got identical readings of the low, mid and high densities with the D-200 in both Visual and UV mode. I did not mean that it appears to be neutral because I looked at it and it appeared neutral to my eye.


    "So I am kind of concerned if Gretag is saying that the calibration wedge has exactly 3.05D in R, G, B, Vis, and UV from the same piece of film. Your measurements of your step tablets show that this is probably not a good assumption - none of them had identical measurements. But as I said, that is really an issue with Gretag's prescribed methods and you a fault of yours.

    Again, there must be some kind of misunderstanding. The Gretag calibration wedge is different from all of the others because it actually measures identically in Visual, Blue and UV. All of the other step wedges are way off, except the Kodak #2 which measured very close to the same in Visual and UV mode.


    "Not aware of the fact - if you go back through this thread I have been making that claim all along. I brought this up early on in this thread. From my second post in this thread on page 4, "It is quite difficult to make a step wedge for a wide range of the light spectrum that is neutral in color. "

    OK, you did indeed make this point so I stand corrected.

    BTW, I called Stouffer to discuss this situation and talked to one of their technicians. He told me that they neither test for nor attempt to control the UV absorption characteristics of their step wedges. He said that there was a UV densitometer on the premise and that they would pull it out and try figure out the reason for the discrepancy.

    Sandy

  8. #98

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    Patrick writes, "I do not have a comercially available densitometer, as I said before, so do not know what the various filters do in detail. What is the bandpass characteristic of the visual filter? Does it look like the average human visibility curve, or is it flat over the limits of human vision?"

    Patrick, visible light ranges from 380 nm to 720 nm. Not everyone can see out to the full range of the spectrum, but we should all come close.

    I'm sure you know about rods and cones. The reponse of our rods is from just above 400 to just over 600 nm, with peak sensitivity at 507 nm. But that's for our night vision.

    Our cones come in 3 types, S, M, and L. S does blue response with a peak of 445 nm, M does green response with peak at 535nm, and L does red response with a peak at 575 nm. Your fovea contains the cones in the proportions of 64% L, 32% M, and 2% S. Despite this, your eyes are more sensitive to green than red.

    Sandy's Gretag densitometer seems to be pretty "average" if I remember right - "visible" to our densitometers is usually centered in the green, and somewhat wide in bandpass. He lists 555 nm and a bandpass of 130 nm. 555 is at the upper end of green, and extends from just into the upper blue and into orange, but not quite red. This pretty much covers the middle wavelegths of our vision and near the range of greatest color sensitivity.

    Note that his Gretag's green filter is actually pretty close to the center of the M cones' sensitivity, but because the bandpass of the filter is probably around 45 nm, it is not really suitable for replicating density response for human vision. (But fine for green!)

    Kirk

  9. #99

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    Kirk,


    “Interesting idea - my understanding is that modern enlargers are well designed and essentially have built in filtration. Perhaps if you have a cold light head that uses fluorescent bulbs which are rich in UV then this may be an issue, but not with a color enlarger. Also, the glass in our lenses do quite an effective job of filtering out UV. Most glass transmits about 92% of the visible wavelengths and around 380 nm it really starts to become a very efficient absorber of UV light. But combine this with the fact that a 3200K tungsten-halogen bulb produces virtually no UV light - especially when compared to the total output of all the visible wavelengths, I still believe that any UV contribution to the exposure of our panchromatic test film from our enlarger light source is negligible. If you have some data to suggest otherwise, I am certainly interested in seeing it.”

    The enlarger that I use for testing is a Bes 23-C II and it uses a tungsten light source. The enlarger does not have any built in filtration, and as I mentioned, originally was equipped with a UV absorbing filter that was used in the filter drawer between the light and negative. How much UV light actually reaches the film, considering the absorption of the glass in our lenses and the radiation characteristics of a 3200K bulb, I don’t know. But it was apparently a serious enough issue for Beseler to include a UV absorbing filter as part of the original equipment of the 23 C II and I suspect that film has at least as much sensitivity to UV light as does paper. And it would only require a very small amount of effective UV radiation to skew the results in my plotting. In any event I plan in time to test this myself, but don’t have the necessary narrow band filters to do so right now.

    "As I have been saying all along, we MUST match the densitometer settings to match the exposure that we will be giving our material. So far, Sandy, other than your position that 1) it causes problems with Phil Davis' software if we have two sets of data for the exposure, and 2) you empirically have determined that the lower step wedge numbers "work" better, and now 3)your suggestion that UV filtration of the enlarger may be a key issue here, I haven't really heard any other reasons for your position."

    Three reasons gives me cause to think, especially empirical ones that result in better results.

    “After looking at the filter values you've listed for your densitometer, I see you have 373 nm listed as the center of the filter. Perhaps it is this filter that is not match your printing materials as well as you think for Pt/Pd printing. I believe that Jorge mentioned that according to Dick Arentz, Pt/Pd paper has a peak sensitivity at 290 nm. That is a bit off 373 nm. I don't know what type of light source you are using, I assume a Mercury lamp of some sort? Without knowing the emission spectra of the type of lamp you have, I can't make any suggestions there.”

    Well, I certainly have some thought about these issues.

    As to what Jorge said, frankly I have no idea what he meant because it did not make any sense to me? Approximately 90% of the UV radiation below 330 nm is filtered by the ordinary float glass that is used in contact frames and vacuum easel, and when you get down to 290mm the figure is close to 99%. So even if it is true that the peak sensitivity of Pt./Pd processes are around 290 nm this information is essentially irrelevant for all practical purposes unless you find a way to contact print withouit using any glass.

    By the way, I wrote the appendix on UV light sources for the new revision of Dick Arentz’s book on printing with palladium and platinum. Here is what I said, slightly abbreviated:

    “Pt./Pd. printing requires a light source that emits much of its radiation in the UVA (320-400nm) range and in the violet and blue range up to about 420nm. Palladium and platinum are also very sensitive to UVB between 254nm and 313nm but sources of this type should be avoided because of the severe risk of skin cancer and cataracts. It should also be noted that approximately 95% of Ultraviolet B is absorbed by the glass of most contact printing frames and vacuum easels so in practice almost all UV B radiation is effectively useless to palladium and platinum printing. And above about 436nm the sensitivity of both palladium and platinum is extremely small.”

    So, in essence the situation is this. Over 95% of the useful radiation in the Pt/Pd processes is in the 350 – 420 nm range, which happens to match fairly closely to the bandwith of the Gretag D-200 II densitometer.

    “And we have yet to measure the UV filtration properties of your Pyrocat-HD processed films at wavelengths other than the 373 nm that your densitometer does. Perhaps a wavelength of 350, 320, or 290 nm would give you the extra density that it would take to make the simple math work. Looking at your pyrocat graphs on unblinkingeye.com, there is a trend towards greater density when we compare the blue and UV densities of your graphs. This suggests that a densitometer that could read shorter wavelengths may give better numbers, but there is really no way to predict the absorbance at different wavelengths simply by looking at two points of a spectrum. We'll have to try that and see what we get.

    Again, a very valid question. But regardless of the exact absorption characteristics of a negative processed in Pyrocat-HD, shorter wavelengths, say of 290 – 320 nm, are virtually meaningless for all alternative processes.


    “As I've been saying all along, there is no scientific reason to NOT use the visible density measurements of your step wedge for determining exposure since you are using standard daylight with panchromatic film as this most closely matches.”

    I understand what you are saying, but I would modify it to include another possibility, i.e. that there is a scientific reason to not use the visible measurements but it has either not been discovered or fully documented.

    “So now I suggest that instead of trying to rationalize the discrepancies between your calculated data and your empirically derived results, you should look at the measurements used to generate the data for the other side of your data set - we've looked at the x-axis data, so now let's look at the y-axis data. Does it best match the conditions that we will be exposing our materials?”

    I am not trying to rationalize anything. To the contrary, I am trying to figure out how all of this comes together.

    As for the question of the exposing unit, I expose Pt/Pd with a bank of BLB tubes that emits the great majority of its radiation in the 350 – 420 nm range, almost exactly in the center of the range of the most useful radiation to the Pt/Pd process, and very close to the center of the UV bandwith of the Gretag D-200 II. And my exposures are timed with an Olec light integrator with a photo sensor that almost exactly matches the bandwith of the light source. So I have at least tried to consider the importance of the Y axis.


    Sandy
    Last edited by sanking; 08-17-2004 at 10:43 PM. Click to view previous post history.

  10. #100
    Ed Sukach's Avatar
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    Quote Originally Posted by sanking
    The enlarger that I use for testing is a Bes 23-C II and it uses a tungsten light source. The enlarger does not have any built in filtration, and as I mentioned, originally was equipped with a UV absorbing filter that was used in the filter drawer between the light and negative. How much UV light actually reaches the film, considering the absorption of the glass in our lenses and the radiation characteristics of a 3200K bulb, I don’t know. But it was apparently a serious enough issue for Beseler to include a UV absorbing filter as part of the original equipment of the 23 C II...
    I think you'll find that filter wasn't for the absorption of Ultra Violet (very little of a tungsten source spectrum is UV) .. but for the other end; Infra Red ... there is quite a bit of that ... and its' alter ego - heat.
    IR filters are very common in optical system .. they are commonly called "heat glass" or heat filters".
    Carpe erratum!!

    Ed Sukach, FFP.



 

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