Switch to English Language Passer en langue française Omschakelen naar Nederlandse Taal Wechseln Sie zu deutschen Sprache Passa alla lingua italiana
Members: 68,701   Posts: 1,482,637   Online: 748
      
Page 3 of 4 FirstFirst 1234 LastLast
Results 21 to 30 of 33
  1. #21

    Join Date
    Jun 2004
    Location
    Portland, OR
    Shooter
    4x5 Format
    Posts
    3,268
    Quote Originally Posted by gainer
    I don't know, for example, what the visible channel or the UV channel are measuring.?
    That's why we need to check with the manufacturer and see if they list the specifications for our particular instruments. As Sandy shows with his Gretag D-200 II, they are very specific in this information. Xrite says that his model 810 densitometer uses Status A color filters for reflection and Status M color filters for transmission. I have a MacBeth TD-903 Transmission densitometer that has both Status A and M color filters for transmission. My Noritsu DM-201 uses Status M for transmission. As you can see, each manufacturer may take a different apporach.

    So what are Status Filters? The following is taken from a Tobias densitometry technical bullitin:

    Spectral Response
    The reflection densitometer uses similar color filters to those that produce the separations. So you might think that different densitometers would read the same. However, there are some differences in the filters of various types of densitometers causing discrepancies in readings between units. To overcome this, ANSI specifications have defined several system responses for densitometers. Units conforming to these specifications should have a reasonable agreement. Among these spectral responses are Status A and M which are used in photographic applications and Status T, which is generally accepted as the Wide Band Graphic Arts response in North America. European responses are different giving a higher reading on yellow ink; this response is called Status E. Other responses exist such a Status I which is a narrow band response. Because these varying system responses exist, it is important that the Status response of the densitometer that you are using be included in any communication between customer and vendor. (End of Tobias Quote)

    Note that last sentence - that was something I mentioned in the other, long thread while I we were discussing the calibration of densitometers.

    According to the Kodak Filters for Scientific and Technical Uses book I have from 1980, Status A and M filters "are carefully selected and calibrated to conform to close spectral tolerances. Their use allows more direct comparison of measured densities with other similarly equipped electronic densitometers." It also says they must be used with the #301A IR cut-off filter.

    Kodak does not sell Status filters despite making them - they must be purchased from densitometer manufacturers (at least in 1980...).

    From the Wratten book, it looks like the Status filters are based on the Wratten #92 for Red, #93 of Green, #94 for Blue. The book does say the 92/93/94 combo is used for densitometric measurements of color films and papers. However, it points out that like all Wratten filters, the precision of these filters is +/- 5%. Perhaps it is these #92/93/94 filters that are being "selected" for use in the Status filters. The then goes on to say that it recommends using a #301A Infrared rejection filter in combination with these filters as they all pass IR.

    For less precise work, one could use the narrow band color filters could also be used - the #25 for red, #61 for green, and the # 47B for blue. These filters are often used for making color separations with B&W film, but they will not match the characteristics of the Status filters

    Quote Originally Posted by gainer
    I ass-u-me that the RGB channels are measuring something related to color separation positives for dye separation or similar processes. Do the specs for a commercially available densitometer provide spectral distribution plots of what these channels measure?
    See my last paragraph above. The Kodak book I mentioned above has graphs of the spectral response of all the filters it covers including the Wratten #92/93/94 densitometric color filters I discussed above, but unfortunately not the Status color filters . Gretag gives enough info about the filters in the D-200 II that we could graph the spectral response.

    Quote Originally Posted by gainer
    If so, what does the visible channel have to do with photosensitive materials, when panchromatic films are more sensitive to red and blue than the eye?
    Excellent question! I'm not really sure if a filter is used with all densitometers when they say "Visual" but I suspect there is. The Gretag D-200 II does use one for visible readings - the manual says it has a filter that is centered at 555 nm, has a bandwith of Tmax/2 of 100 nm and Tmax/10 of 180 nm. So most of the light it is seeing is from around 505 to 605 nm, and it will not see anything below 465 nm or above 645. So it is missing all of what would be considered the Violet, Indigo, and Red portion of our visual response (depending on who's wavelength ranges you choose for each color).

    Now there may not be anything really wrong with that, especially since Gretag has told us that this is the property of the filter they chose to use. But is may be a concern as you say if your film is more sensitive to red than blue and your densitometer is not measuring any red but is measuring some (albeit only a little) of the blue...

    But we also need to keep in mind, that we need to pick the filter we are using to try and best match the conditions we are making these measurements for. If you have a relatively neutral material for the range we are making an exposure, say a step wedge, then it will not really matter much if our Visual filter does not cover the entire range. You would get comparible results if the material is fairly neutral. It is when we start measuring things that have significant colors in them that this will come into play and then be a big issue.

    If anyone has access to the old ANSI specs for these things, I would love to see them. Or the current specs either...

    Quote Originally Posted by gainer
    How much of the UV that is measured will get through the glass of a printing frame?
    The float glass I've measured starts to drop significantly around 360 nm and is pretty much opaque by the time you hit 300 nm. These wavelengths are lower than I gave in the other thread. I'm in better agreement with Sandy on this issue now than I was in the other thread...

  2. #22

    Join Date
    Mar 2003
    Location
    Greenville, SC
    Shooter
    Large Format
    Posts
    4,813
    Images
    5
    Quote Originally Posted by Kirk Keyes
    Excellent question! I'm not really sure if a filter is used with all densitometers when they say "Visual" but I suspect there is. The Gretag D-200 II does use one for visible readings - the manual says it has a filter that is centered at 555 nm, has a bandwith of Tmax/2 of 100 nm and Tmax/10 of 180 nm. So most of the light it is seeing is from around 505 to 605 nm, and it will not see anything below 465 nm or above 645. So it is missing all of what would be considered the Violet, Indigo, and Red portion of our visual response (depending on who's wavelength ranges you choose for each color).
    Kirk,

    Do you know how Tmax/2 and Tmax/10 are derived? I assumed that the equation was maximum transmission divided by 2 or 10. But if this is so, how does one determine the figure for maximum transmission? Would this by default be 100%?

    Sandy
    Last edited by sanking; 09-01-2004 at 02:50 PM. Click to view previous post history.

  3. #23

    Join Date
    Jun 2004
    Location
    Portland, OR
    Shooter
    4x5 Format
    Posts
    3,268
    Quote Originally Posted by sanking
    Kirk,

    Do you know how Tmax/2 and Tmax/10 are derived? I assumed that the equation was maximum transmission divided by 2 or 10. But if this is so, how does one determine the figure for maximum transmission? Would this by default be 100%?

    Sandy
    Sandy, yes, you are right in those assumptions. A spectral transmission chart is made for the filter comparing wavelength vs. absorbance (or transmission - it doesn't make much difference since transmittance and absorbance are directly related to each other).

    The wavelength at which the maximum tranmittance occurs is noted. This will probably be close to the "dominant wavelength" of the filter, or its "hue" as seen with our eyes. (Note, the dominant wavelength will shift a few nanometers depending on the color temperature of the light source, the maximum transmittance will not change with color temp. The dominant wavelength also relates more to how the eye sees colors on the color wheel than what its maximum transmittance is.)

    The chart can then be "normalized" by setting the tranmittance at that wavelength to be 100%. All the other transmittance values for each wavelength will then be adjusted by the same ratio as was used to set the 100% point and we can make a new chart.

    So now you have a chart that has been "normalized", with the wavelength of maximum tranmittance at 100%. To find the Tmax/2 point, look for where the plot of the filter crosses the 50% tranmittance mark. For a band-pass filter like we are describing here, we hopefully have a filter that "cuts-off" sharply and is fairly symmetrical. Think of a steep, sharply sided Bell-curve used for grading papers or in statistics, the shape should be similar.

    The #93 and #94 Blue Wrattens both do this, but the #92 Red only cuts-off on one side! It has a transmittance of only 0.40% at 620 nm, a dominant wavelength of about 645 nm where it transmits about 60% but it has a peak transmittance of 88% at 700nm. But it does not drop off on the Infrared side. They must be assuming that since all of the filter in this series must be used with a #310A IR cut-off filter, the IR-blocking filter will take care of that side (it starts to filter out at around 680nm.)

  4. #24

    Join Date
    Jun 2004
    Location
    Portland, OR
    Shooter
    4x5 Format
    Posts
    3,268
    "How is the film's spectral sensitivuty linked to the densitometer's?"

    It's not. I don;t have time for a longer answer now....

    Kirk

  5. #25
    gainer's Avatar
    Join Date
    Sep 2002
    Posts
    3,726
    Images
    2
    TMX and some other panchromatic films are used to make color separation intermediates. I would expect that the individual RGB channels would be of use in their production. I cannot see a use for a visual channel off hand. What I find mising in this thread is what is necessary to make the transmission characteristics of the various filters come true. When a certain fiter of known transmission spectrum is used it is either known or assumed that the spectral response of the photocell is uniform across the photographic spectrum. IOW, the transmission spectrum of the filter is not the whole story.

    I know of no photosensitive materials that are uniformly sensitive across the complete range of the films and papers we use. Unless I know that the native response of my densitometer is the same as yours, I cannot guarantee that the result I might get by measuring through a given filter will be the same you would get from your measurement of the same densities.
    Gadget Gainer

  6. #26

    Join Date
    Jun 2004
    Location
    Portland, OR
    Shooter
    4x5 Format
    Posts
    3,268
    Quote Originally Posted by jdef
    How is the film's spectral sensitivuty linked to the densitometer's? [...] The densitometer reads those densities based on the intensity and spectrum of the light passing through the negative. There is a complete disconnect between the two. How does the densitometer know what spectrum of light exposed the film? What am I missing?
    A film's spectral sensitivity is not in any way linked to the spectral sensitivity of any densitometer. You're right. But depending on how you make the exposure onto the film, your choice of densitometer and filtration could make a difference.

    If you make exposures in a camera by varying the amount of light using a shutter and aperature, it doesn't matter. As you say, there is a complete disconnect between the densitometer and the exposure.

    But if you make the exposure through a step wedge, then your measurements of the step wedge can vary based on the filtration used in the densitometer. The actual exposure that is given to the film is not affected by your choice of densitometer/filter, only the numbers that you generate which will be used in your calculations later on will be affected. This variablity could be significant.

    I'm not talking about measuring densities of the film to which the exposure was made when making these measurements, but the step wedge that was used to control the exposure in the first place.

    So what I was trying to point out in that sentence, "But this may be a concern as you say if your film is more sensitive to red than blue and your densitometer is not measuring any red but is measuring some (albeit only a little) of the blue... " is that if you are controlling your exposure on your film (or paper as you pointed out) and the densitometer is not able to make measurements of your step wedge under conditions that simulate the actual conditions of use for your material, then you may have some concerns about the numbers that you are reading off the display of your densitometer.

    Let me restate some more of what a said in the other post with a few changes to the wording that may make it more clear -

    If you have a step wedge that is a relatively neutral material over the range of wavelengths that your film or paper is sensitive to, then it will not really matter much if our Visual filter does not cover the entire range. You would get comparable results if the material is fairly neutral whether the densitometer can see part of the range of wavelengths or the entire range of wavelengths. It is when we start making exposures through materials that have significant colors in them that this will come into play and then be a big issue.

    Did that help?

  7. #27
    gainer's Avatar
    Join Date
    Sep 2002
    Posts
    3,726
    Images
    2
    Not me. I still don't have a way to determine if my step wedge is neutral. If it is not neutral then I have no way to compare, to within the proverbial gnat's behind, what I measure with what you measure. I know that some of my negs are not neutral, but neither of us will know by how much from either my measurements or yours, even if we use the same filter, unless we know the native spectral sensitivities of our densitometers as well as the transmission spectrum of our filters.

    It is true that when we are dealing with negatives that look neutral we can get a mutually agreeable measurement of contrast index, so that if I tell you that a certain film-developer combination developed under certain conditions gives a certain contrast index, you ought to come within experimental error of the same result in your darkroom. It is also true that what difference there is will be most likely in the form of a difference in film speed rather than in slope if it is due to, say, the difference between a CdS photoresistor and a silicon phototransistor.

    I think we stand a better chance of comparing certain results if we stick to narrow band filters. The color separation filters may be narrow enough for most purposes, and a UV bandpass filter for the rest. Determinations of film speed by densitometer readings are subject to validation by practical use.

    I'm not trying to pontificate here, just bouncing thoughts off the backboard.
    Gadget Gainer

  8. #28

    Join Date
    Jun 2004
    Location
    Portland, OR
    Shooter
    4x5 Format
    Posts
    3,268
    Quote Originally Posted by gainer
    Not me. I still don't have a way to determine if my step wedge is neutral.
    I found a few more sources and found mention of "Status V" densitometry - V for Visual. Of course, no one has gone into enough detail to actually describe the spectral characteristics of the Status V filter. But they did say that they were designed to replicate human vision for densitometric purposes. I suspect that the Gretag Visual filter that was discussed above would comform to this specification, as Gretag seems to know what they are doing when it comes to constructing densitmeters. But as we've seen, the filter used in the D-200 II densitometer, doesn't completely cover the range of human vision. But for densitometric purposes, it is probably sufficient. If more detail about the transmission characteristics of a material are needed, then Gretag and others will gladly sell us a spectrophotometer, which will be able to make density measurements on a wavelength by wavelength basis (or at least every 10 nm according to some of Gretag's brochures).

    But along with the type of filters used are requirements on sensor position, light sources...

    Quote Originally Posted by gainer
    If it is not neutral then I have no way to compare, to within the proverbial gnat's behind, what I measure with what you measure. I know that some of my negs are not neutral, but neither of us will know by how much from either my measurements or yours, even if we use the same filter, unless we know the native spectral sensitivities of our densitometers as well as the transmission spectrum of our filters..
    But if we both have densitometers, and the are both deisgned and contructed to conform to ANSI/ISO specifications that I assume are out there, and the filters used in them conform to the specifications as well, then we should get directly comparable results.

    I would suggest that if we had the same filtration, that doing that would go a long way towards getting comparable results. At least we would be looking at the same band of wavelengths. And since most light sources used in densitometers are going to be incandescent, and even with the small range of color temps that those sources may have, they should be close as well. So that leaves detectors - I would be surprised if any modern densitometers use CdS cells (too sluggish and not much linear range) - I bet Si photodiodes are much more likely. Whether they go to the effort of using a Silicon Blue diode or not, I don't know. If they intend to do UV measurements, then it is probably not used as they drop off response faster than a regular Si photodiode.

    I looked around the ISO site, and they do have several specs on densitometry, but at around $50 a standard, I have to pass on getting them. I'll see if I can find a library that has some of them around here...

    Quote Originally Posted by gainer
    It is true that when we are dealing with negatives that look neutral we can get a mutually agreeable measurement of contrast index, so that if I tell you that a certain film-developer combination developed under certain conditions gives a certain contrast index, you ought to come within experimental error of the same result in your darkroom.
    I agree completely. Problems arise with materials that are not fairly neutral.

    Quote Originally Posted by gainer
    I think we stand a better chance of comparing certain results if we stick to narrow band filters.
    I agree with you when you say we should get better agreeable results if we are both using narrow-band filters. In fact, I'll go further and claim that if we used filters that only transmitted very narrow bands of light, say like if we used a monochromator which could give us bands of light as narrow as a couple of wavelengths, then we should get identical results (within experimental error) as the response curve of the detector and the spectral distribution of the light source will not matter. There will essentially be no slope or curve to either one of those things to come into play. As you have pointed out before, we have reduced all the sources for error down to simply the measurements of the incident vs. the transmitted light.

    PS - I like bouncing thoughts too!

  9. #29
    gainer's Avatar
    Join Date
    Sep 2002
    Posts
    3,726
    Images
    2
    Considering that in the long run, the purpose of most of our densitometer measurements is to predict with some reasonable accuracy what the printing qualities will be of some arbitrary negative on some known printing paper, we should consider other approaches. One is to print the negative of the step wedge on the printing material in question and note the number of steps between black and white. A print of the original wedge directly on the printing material will show the effective number of steps in the SBR. Dividing that by the number of steps in the print of the negative will give the effective contrast index when those development conditions are used on any scene to be printed on that medium. Any kind of densitometer can probably be used to produce the H&D curve well enough to show peculiarities in the shape and give an idea about rendition of mid tones.
    Gadget Gainer

  10. #30

    Join Date
    Mar 2003
    Location
    Greenville, SC
    Shooter
    Large Format
    Posts
    4,813
    Images
    5
    Quote Originally Posted by gainer
    One is to print the negative of the step wedge on the printing material in question and note the number of steps between black and white. A print of the original wedge directly on the printing material will show the effective number of steps in the SBR. Dividing that by the number of steps in the print of the negative will give the effective contrast index when those development conditions are used on any scene to be printed on that medium.
    Alternative printers have been testing their materials for decades with step wedges. We were using the wedges long before any of us had any idea that there were actually densitometers that could measure the UV stain. Or could afford them!

    This method of testing is essential because the range of exposure scale that people get in practice with altternative processes is much greater than the range people get printing different brands of silver papers. The pH of the papers, sensitizing mix, method of coating, and type of developer combine to create a much more variable scenario than silver printers see.

    Sandy

Page 3 of 4 FirstFirst 1234 LastLast


 

APUG PARTNERS EQUALLY FUNDING OUR COMMUNITY:



Contact Us  |  Support Us!  |  Advertise  |  Site Terms  |  Archive  —   Search  |  Mobile Device Access  |  RSS  |  Facebook  |  Linkedin