Question regarding color densitometer.

Discussion in 'Darkroom Equipment' started by Chan Tran, Jan 26, 2013.

  1. Chan Tran

    Chan Tran Member

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    I have a question regarding color densitometer whether reflection or transmission type. I know all of them only have 3 sensors and only measure the R,G and B. They use the RGB value to compute the V value. I wonder is there a formula to do that? I know if the reading is 1.00, 1.00, 1.00 on all 3 color channels then the V is also 1.00 but what if the RGB values are different? Just and average of the 3 or is there a weight for some channels.
     
  2. ic-racer

    ic-racer Member

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    None of my color densitometers work that way. Is there a specific unit you know of that works that way?
     
  3. Prof_Pixel

    Prof_Pixel Member

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    The luminosity method is a more sophisticated version of the average method. It also averages the values, but it forms a weighted average to account for human perception. We’re more sensitive to green than other colors, so green is weighted most heavily. The formula for luminosity is 0.21 R + 0.71 G + 0.07 B
     
  4. Chan Tran

    Chan Tran Member

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    thanks Prof Pixel
     
  5. Mr Bill

    Mr Bill Member

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    I think that Prof_Pixel is describing the weighting used in a television standard (Rec.709), which I think is only valid for a specific set of phosphors and a certain visual adaptation (ie, to a specified color temperature).

    I don't think it's legitimate to use this for densitometry; they're just different things.

    The ideal thing is to use a densitometer with a proper visual response. I don't believe it is possible to derive the visual density of an arbitrary material from standard tri-color densities (Status A or Status M).
     
  6. Chan Tran

    Chan Tran Member

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    What I am asking is my densitometer like most has 4 readings R, G, B and V but it only uses 3 channel sensor. How did it figure out the V.
     
  7. ic-racer

    ic-racer Member

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    What brand of densitometer is it. I have not seen a densitometer that shows "V". You probably need to contact the manufacturer to find out.
    If "V" is from the HSV system, then there are some converters on-line that can go from RGB to HSV: http://www.cs.rit.edu/~ncs/color/t_convert.html#RGB to HSV & HSV to RGB http://www.javascripter.net/faq/rgb2hsv.htm

    The color densitometry chapter in Todd an Zakia describes Equivalent Neutral Density (END) and mentions that if you know the END and Analytic Spectral Density of a dye, you can take that ratio and use it in the ASD/ISD conversion matrix to calculate END.
     
  8. Chan Tran

    Chan Tran Member

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    The densitometer is the X-Rite 810 reflection/transmission densitometer. The V stands for Visual and it's the same as B&W density.
     
  9. ic-racer

    ic-racer Member

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    Why do you think it calculates the "Visual"? From the manual, it would seem it measures it at the time it measures the 3 colors. Perhaps you have had the measuring head off and saw 3 sensors only?
    Also, common sense indicates the "X-rite Visual" may be similar to a B&W density reading, but did you see in any documentation that it is indeed "the same as B&W density?"
    Anyway, the "X-rite Visual" has my curiosity because my x-rite 890 does not display the "X-rite Visual" output, it just lists the 3 colors. Also, this X-rite document on color standards and measurement does not mention "Visual" or "X-rite Visual": http://www.xrite.com/documents/literature/en/L10-001_Understand_Color_en.pdf
     
    Last edited by a moderator: Jan 28, 2013