Question about reflection densitometer & gray card

Discussion in 'Digital Photography' started by KenP, Jan 15, 2004.

  1. KenP

    KenP Guest

    My question is this: what sort of reading should I expect from a
    standard 18 percent grey card when measuring it with a reflection
    densitometer that gives readings in visual density, cyan, magenta and
    yellow? My readings don't make a whole lot of sense to me, I get:

    V = 0.81D
    C = 0.10D
    M = 3.01D
    Y = 0.37D

    Does this make sense? Why aren't cyan (C), magenta (M) and yellow (Y)
    all equal? And why does density read 0.81 instead of approx. 0.18?

    Thanks for any help,

    -KenP
     
    KenP, Jan 15, 2004
    #1
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  2. The *reflectance*, R, of a grey card is 18% or 0.18

    The Density of a grey card is log (1/R), or about 0.75

    The V density reading you got of 0.81 is not far off, but the colour
    densities are way off. A grey card is not perfectly neutral -
    printing limitations aside, true neutral is not pleasing to the human
    eye - but this is not acceptable.

    When was the densitometer last calibrated? Was it in the right mode?

    For more info on this stuff I recommend:

    Stroebel et al., "Photographic Materials and Processes." Focal Press,
    London, 1986. ISBN 0-240-51752-0
     
    The Black Sheep, Jan 15, 2004
    #2
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  3. Actually, to pick a nit, -log10(1/R). About 0.745 on my calculator.
    Yeah; if you really printed something with those values, it would be
    nowhere near gray.
    Or is the densitometer designed to measure a single ink? Maybe it's
    trying to take a monochrome measurement and deduce what that would be
    if printed in each of the inks.
     
    Stephen H. Westin, Jan 15, 2004
    #3
  4. Uhm, no, the log of (1/0.18) is a positive number. Did you do log
    (1/18)?

    And 0.745 is *about* 0.75, particularly since most people use 2
    significant digits for prints (and 3 for film) when measuring density.

    {sticks tongue out at the nit-picker}
    It would be green, actually. Instead of "not acceptable", I should
    have said "ridiculous".
    Or simply not zeroed? Or the lamp not allowed to warm up? Etc.

    I have also used old densitomers that, when set to B&W mode, still
    gave you CMY density values. Completely useless values.

    Whatever the cause, this is clearly operator error (or some student
    dropped the poor densitometer on the floor).
     
    The Black Sheep, Jan 15, 2004
    #4
  5. KenP

    KenP Guest

    I'm sure it's operator error, it's gotta be. The color densitometer
    measures the individual CMYK cal targets approximately correct
    (according to the printed density values on the cal chart). There's
    something fishy about this 18% gray card measurement, however--must be
    "the mode I'm in."

    And yes, I now understand that an 18% reflectance corresponds to a D
    of about 0.75, since it is a log value (D is mathematically negative).
    Googling around it appears that others also measure about 0.80 or 0.81
    when measuring a gray card, so perhaps there's a small error due to
    paper reflectance. I'm going to try and order that "enhanced" gray
    card from Germany that includes grayed circles of 18% magenta, yellow
    and cyan--it seems like I'm not quite understanding what the
    densitometer is telling me (an X-Rite 408).

    If I can figure out how to use this thing properly I'd like to make
    some fade measurements on 3rd-party inks. That brings up another
    question, however--usually fade is expressed as a percentage of the
    original color. Is that to be interpreted as a percentage of the log
    density value, or does the reading need to be converted back to linear
    values?
     
    KenP, Jan 15, 2004
    #5
  6. Or some other factor, it certainly wouldn't look grey with thise
    values.
    IIRC we were taught that anything between 0.75 and 0.80 was good
    enough. Keep in mind that there will be imperfections and batch
    variations in grey cards, and it may fade with time, etc.

    <Shudder> this is bringing back nightmares of shooting greycards under
    dozens of different light conditions, and having to print the
    resulting negs so the print was V=0.75 to 0.78 and perfectly neutral.
    The numbers it reported simply do not make sense.
    Good question. I would guess - and this is just a guess - that you
    should convert density back to percentage reflection, and compare the
    changes between the linear %R. I have no idea if this is the way
    fading is normally compared, however.
     
    The Black Sheep, Jan 16, 2004
    #6
  7. KenP

    KenP Guest

    Thanks for the reference, I'll look into it. Yes, it was my
    fault--the densitometer unit was set to a memory mode that recalled
    the last dominant color, and now everything seems OK. Only I'm
    noticing that my "gray" cards seem to be skewed toward yellow or cyan
    (and visually they appear so). What do you think "ideal" readings be
    for an 18 percent perfect gray card for the CMY values?
     
    KenP, Jan 16, 2004
    #7
  8. You're welcome. Very technical book, the kind that you can use to
    scare away first-year college students ("But I just wanted to take
    pretty pictures!") and beat off would-be muggers with.
    I don't recall the ideal values, I will try to look that up for you
    tomorrow. Digging into memory, however, I think a Kodak grey card is
    a bit magenta.
     
    The Black Sheep, Jan 16, 2004
    #8
  9. I can't find a source for ideal values, but all 4 numbers should be
    very close to each other, and all close to 0.75.

    For example, my grey card is 0.75, 0.75, 0.77 according to my own
    notation on the back.

    Note that there can be some surface variation in a card, we used to
    measure the 4 corners and the centre, and then tale an average.
     
    The Black Sheep, Jan 16, 2004
    #9
  10. SNIP
    That entirely depends on the ink and paper stock used. Besides, you'd need
    Black in addition to CMY, because the impure CMY pigments result in muddy
    Brown.

    As another besides, measurements are normally expressed as specified RGB
    triplets, or as Lab / XYZ values. CMY is only (?) used in process printing.

    Bart

    Bart
     
    Bart van der Wolf, Jan 16, 2004
    #10
  11. KenP

    KenP Guest

    OK, what I see now from a grey card (Delta 1 brand) on its matte side
    is: 0.79, 0.80, 0.77, 0.80 for CMYK respectively. "K" is actually
    termed "V" by this densitometer, for "visual."

    The card has a reverse side that is also of the same visual appearance
    except for having a semiglossy surface, but no mention of using this
    is included in the instructions. Corresponding measurements on this
    side are: 0.88, 0.90, 0.87, 0.89.

    Interestingly, measurement of a fluorescent red paper sample gave a
    negative result, the first I've seen.
     
    KenP, Jan 16, 2004
    #11
  12. KenP

    KenP Guest

    I'd left off the black (visual) reading, but when measuring my grey
    card I now get about 0.79 for C, M, Y and also K(v). Now I'm curious
    as to how much effect on these values is do to paper surface quality,
    especially a glossy surface vs. one that is completely matte.
     
    KenP, Jan 16, 2004
    #12
  13. It is NOT K, it is not a measure of how much black ink is used.

    Your card has an overall density to light of 0.80, the V value.

    Your card has a density to cyan light of 0.79, to magenta light of
    0.80, and to yellow light of 0.77.

    You can give the denity value of your card as either 0.80, or as 0.79,
    0.80, 0.77

    Do not confuse this with the amounts of inks used in a CMYK printing
    system.
    I would guess this the paper's reflectance was outside the range that
    the device can measure.
     
    The Black Sheep, Jan 16, 2004
    #13
  14. SNIP
    CMYK depends on the reflective color (!) and reflection of the paper stock
    or substrate/support, because it's a subtractive color model. The CMYK is
    only for adjusting the reflection from that base.

    Bart
     
    Bart van der Wolf, Jan 16, 2004
    #14
  15. KenP

    KenP Guest

    Youch! $65, well it's probably a must-have for those working in color
    printing and repro.

    Got another question (now that the densitometer and I are getting
    along better): Where can I find a listing of measured color density
    values for the Macbeth 24-color chart? It's not hard to find the RGB
    values used to create the chart, but the reflected color density
    values expected from a matte example of this chart is what I need
    right now.

    Thanks for any help...KenP
     
    KenP, Jan 17, 2004
    #15
  16. Yeah, its a beast by any measurement. It was a required text in my
    college photography program.
    Good question, do the Macbeth chart makers have a 1-800 number? You
    could just ask them. Or borrow one and read it.
     
    The Black Sheep, Jan 17, 2004
    #16
  17. Bart van der Wolf, Jan 17, 2004
    #17
  18. KenP

    Pard Guest

    Pard, Jan 18, 2004
    #18
  19. KenP

    KenP Guest

    KenP, Jan 18, 2004
    #19
  20. Using the "ColorChecker Calculator", you could take the Log10 of the
    reciprocal of Y from the XYZ coordinate, D=Log10(1/Y) .

    Bart
     
    Bart van der Wolf, Jan 18, 2004
    #20
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