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  1. #21
    keithwms's Avatar
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    Am I reading correctly that the spectral sensitivity that is reported is a convolution of the lamp spectrum and the actual sensitivity of the film? And these spectra are not corrected for the lamp spectrum?

    I am a bit surprised that these spectra aren't lamp-corrected. At least in spectroscopy, the standard practice is to take a spectrum, and then to correct it for the lamp spectrum and other instrumental factors and background. In other words, the spectrum we report corrects for uneven weighting of the illumination source as a function of wavelength- no lamp is perfect. This lamp-correction procedure is easy enough if the illumination spectrum is smooth and broad, as it must be if the colour temp is typically defined.
    "Only dead fish follow the stream"

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  2. #22

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    Quote Originally Posted by keithwms View Post
    Am I reading correctly that the spectral sensitivity that is reported is a convolution of the lamp spectrum and the actual sensitivity of the film? And these spectra are not corrected for the lamp spectrum?
    Where the temp is listed, we have to conclude the test was done at that temp. So, yes, it is a convolution of the lamp and the film.

    Keithh - When one corrects for the lamp, would that be called an "equal energy" spectrum?

  3. #23
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    Wedge Spectrogram

    Here is a wedge spectrogram. The visible spectrum in nanometers is the x axis and the Log E values in 0.3 increments is the y axis with increasing speed moving upwards and increasing exposure downwards.

    This is a green sensitive material. The peak in the red region is a harmonic of UV light, as this was done with a hand coating on a spectrosensitiometer with no UV filter.

    As you can see, the visual slope varies as a function of wavelength in this case. If it does (It does not sometimes and does other times depending on dye and emulsion), the singe line curve in the example above and shown on Kodak's site are not revealing. You have to construct a curve at each wavelength from the entire exposure such as shown here in the data I have included.

    The line drawing of spectral sensitivity is just an approximation unless you know the full curve.

    PE
    Attached Thumbnails Attached Thumbnails Wedge spectrogram - ISO 100 paper.jpg  

  4. #24
    David A. Goldfarb's Avatar
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    PE, when you showed us one of those in your workshop here in New York, I finally understood why a safelight works.
    flickr--http://www.flickr.com/photos/davidagoldfarb/
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  5. #25
    SAShruby's Avatar
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    The best prevention in B&W against blue and UV is to use yellow filter (result of red and green primary color), if you desire is to achieve the best contrast on your film.

    IMO, it's not always the case.
    Peter Hruby
    LF Silver Photography

  6. #26
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    Keith;

    The lamp in Kodak spectrophotometric work is set to a common output value. We derive everything from that very highly stabilzed and calibrated light source; however release speed is tested at the correct degrees Kelvin. Color speed tests are also done at the correct degrees Kelvin and at each segment of the spectrum, R/G/B, so you can be assured that you are getting daylight and tungsten products balanced correctly.

    As for the example above, the best safelight is in the white (clear) area to the right of center, and that is how to pick a safelight for a given product.

    Now, some additional information. Let us suppose that a green sensitizer causes low contrast in the green region but you have high contrast in the blue region. Your image will be mismatched in color/tone scale. This is rare but does happen. Therefore a full working wedge spectrogram used for engineering purposes looks like the one I posted above. A line drawing will not show you contrast as a function of wavelength, just speed. Both are important, but speed vs wavelength is only useful if the photo engineer has done his job and equalized contrasts.

    IMHO, for the true professional photographer, a photo of a MacBeth color checker is sufficient to reveal the good and bad points of a given film product.

    Peter;

    I'm not sure how to reply to your comment. Of course it is correct, but seems to be a non-sequitur in the context of the preceeding posts. Can you elucidate? Thanks.

    PE

  7. #27

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    Quote Originally Posted by Paul Verizzo View Post
    Another project from the depths of the lah-bor-a-tree on the bayou.....
    -----
    ???

    Looks like the data went Pooffff!
    Anyone know were it went?

    Ray

  8. #28

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    Quote Originally Posted by Ray Rogers View Post
    -----
    ???

    Looks like the data went Pooffff!
    Anyone know were it went?

    Ray
    I just checked, it opened fine for me. It would be difficult for words to disappear from a Pdf file, it's a type of vector graphic.

    I would suspect your PDF reader......

  9. #29

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    Quote Originally Posted by Paul Verizzo View Post
    I just checked, it opened fine for me. It would be difficult for words to disappear from a Pdf file, it's a type of vector graphic.

    I would suspect your PDF reader......

    Sorry, my bad?

    Not the pdf... but the links to kodak web pages.

    I did not think to try Ilford etc. but it looks like the kodak links do not go all the way home.

    Yes, Fuji and Ilford are healthy,
    But Kodak seems to be MIA!
    Last edited by Ray Rogers; 08-30-2008 at 02:12 PM. Click to view previous post history.

  10. #30

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    Quote Originally Posted by Ray Rogers View Post
    Sorry, my bad?

    Not the pdf... but the links to kodak web pages.

    I did not think to try Ilford etc. but it looks like the kodak links do not go all the way home.

    Yes, Fuji and Ilford are healthy,
    But Kodak seems to be MIA!
    Oh, it's not "the data" that is missing, it has "dead links."

    Amazing as I wrote that, what, a month ago? Look up the product, then. You won't find movie films at kodak.com, BTW. Google is your friend.

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