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  1. I have the following code for gamma correcting a reflectance value. For example, if the reflectance is 18% (0.18) and the gamma is 0.45, this gives me a value of 40.9. The 40.9 value agrees with everything I've read.

    =(1.099*0.18^0.45-0.099)*100


    The question is, what is the math to convert this to IRE units? I am expecting to see a value of 43.4 according to everything I've read.

    If I change the reflectance to 90% as found on a typical test chart:

    =(1.099*0.9^0.45-0.099)*100

    I get a value of 94.9. I believe this is correct. After conversion to IRE units it should be 100.

    What is the correct way to convert these gamma-corrected numbers based on reflectance to IRE units?

    Thank you.
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  2. To translate from your 0-to-100 scale to IRE (assuming 7.5 IRE setup), you would use
    (100-7.5)/100 * x + 7.5

    to do the reverse,
    100/(100-7.5) * (x - 7.5)
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  3. Originally Posted by raffriff42 View Post
    To translate from your 0-to-100 scale to IRE (assuming 7.5 IRE setup), you would use
    (100-7.5)/100 * x + 7.5

    to do the reverse,
    100/(100-7.5) * (x - 7.5)
    If there's no setup then:

    (100 - 0)/100 * ( x + 0)

    or

    1 * x

    This puts Y' at 90 IRE for 90% reflectance. It should be 100 IRE or 235 digital.

    This may help: https://docs.google.com/spreadsheets/d/1wGZd5_OEPSE08CC_RpP5X_hTxOgapWw7pihrKeFOWls/edit#gid=0

    I realize the IRE value for 18% reflectance is only off by a little (43.1 vs 43.4), but it seems more like simple rounding error and makes me wonder if I'm doing the calculation correctly.
    Last edited by chris319; 14th May 2018 at 20:34.
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  4. 7.5 IRE setup is only applicable to analog video. Digital video has IRE 0 at Y=16, IRE 100 at Y=235.
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  5. Originally Posted by jagabo View Post
    7.5 IRE setup is only applicable to analog video. Digital video has IRE 0 at Y=16, IRE 100 at Y=235.
    I know that. 7.5 setup doesn't even enter the discussion. See my response above.
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  6. (delete)
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  7. the formulas used in that spreadsheet:

    Corrected reflectance =IF(B4>=0.018,1.099*B4^B$25-0.099,B4*B$25)
    or
    (Reflectance>=0.018)
    ? 1.099 * Reflectance^Gamma - 0.099
    : Reflectance * Gamma


    IRE Units =((100*$C4))*(1/$C$4)
    or
    100 * CorrectedReflectance * 1 / (CorrectedReflectance @ 90%)
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  8. Originally Posted by raffriff42 View Post
    the formulas used in that spreadsheet:

    Corrected reflectance =IF(B4>=0.018,1.099*B4^B$25-0.099,B4*B$25)
    or
    (Reflectance>=0.018)
    ? 1.099 * Reflectance^Gamma - 0.099
    : Reflectance * Gamma


    IRE Units =((100*$C4))*(1/$C$4)
    or
    100 * CorrectedReflectance * 1 / (CorrectedReflectance @ 90%)
    What is your point?
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  9. Your question in post #1 was:
    What is the correct way to convert these gamma-corrected numbers based on reflectance to IRE units?
    I tried to answer it, based on your assumed 90% reflectance = 100 IRE.
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  10. Isn't that what I already have in cell D4?

    https://docs.google.com/spreadsheets/d/1wGZd5_OEPSE08CC_RpP5X_hTxOgapWw7pihrKeFOWls/edit#gid=0

    Does cell D12 look right? If it is right, why does every reference I've read say the value should be 43.4 IRE for an 18% gray card instead of 43.1? It seems like more than just rounding error.
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  11. OK the problem was using 90% reflectance as "white" without normalizing it so that "white" == full scale == 1.00. Make that adjustment and the result comes out as expected: 18% reflectance => 43.4 IRE.

    Image
    [Attachment 45658 - Click to enlarge]


    EDIT fixed an error in the spreadsheet...
    Last edited by raffriff42; 16th May 2018 at 17:09.
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