the nature of color, reproduction and perception

Discussion in 'Digital Photography' started by hbarta@comcast.net, Aug 15, 2003.

  1. Guest

    Color is a result of various wavelengths of light from near infra-red
    through ultra-violet. What we see as white light is an equal amount
    of all frequencies. Or is it? Is there any fundamental reason for
    the energy in the spectrum from the sun, for example, to be evenly
    distributed across visible frequencies? It seems like our eyes are
    capable of 'self white balance' without any conscious effort. How
    many people complain about the green tint from fluorescent lighting,
    for example.

    A specific color is some mix of various frequencies. Is this
    typically a combination of various frequencies or is the energy
    distributed across a range of frequencies? e.g. what is red? I
    believe that something like a red LED emits light at a specific
    wavelength. Something like a LER (light emitting resistor ;) emits
    light over a range of frequencies that are commonly known as 'red.'

    What does a digital color sensor measure? Does it measure a specific
    wave length, or is an element sensitive to a range of wavelengths
    that are characteristic of the given color? And regardless, it
    seems like an imaging element provides a single measurement within
    the given color range. There seems to be no way to differentiate
    the actual spectral distribution. Given the previous example of an
    LED or something hot enough to glow red, there is no way to tell
    the difference for the red value. It seems to me that the only way
    to fully characterize a given color is to measure the energy at all
    visible wavelengths from near infra-red to ultra-violet. And yet all
    digital color systems with which I am familiar do this with three
    values. (OK, I'm thinking about RGB encoding. there is also CMYK
    encoding that uses four values. Still not a lot of information for
    what seems to be a potentially complex value.) Increases in color
    fidelity seem to be oriented toward providing finer gradations for
    the three or four channels rather than including more points along
    the distribution.

    Finally, what about reproduction? It seems to me that three or four
    values for a given point can only approximate the spectral energy
    distribution that represents a color at any given point. A CRT works
    with three (RGB) inputs and so does a an LCD screen. I suspect that
    the CRT has broader spectra for each of the three colors than the
    LCD. Could that be why CRTs seem to be favored for work where color
    fidelity is important? Printers use the same three or four color
    values to produce their output. While some printers have more than
    three colors of ink, that is to work around the problem that pigments
    and dyes don't mix as easily as colored light.

    If anything I've said is not correct or skirts the facts, please
    follow up with corrections or clarifications.

    I guess my basic question is the approximation of color by three
    values. How come that seems to work so well?

    thanks,
    hank
     
    , Aug 15, 2003
    #1
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  2. Rafe B. Guest

    On Fri, 15 Aug 2003 02:57:49 GMT, wrote:


    > I guess my basic question is the approximation of color by three
    > values. How come that seems to work so well?



    Because that's how human eyes work?

    Actually, the 4th axis is intensity, which really provides
    most of the detail. Color is just frosting on the cake.


    rafe b.
    http://www.terrapinphoto.com
     
    Rafe B., Aug 15, 2003
    #2
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  3. wrote:

    > Color is a result of various wavelengths of light from near infra-red
    > through ultra-violet. What we see as white light is an equal amount
    > of all frequencies. Or is it? Is there any fundamental reason for
    > the energy in the spectrum from the sun, for example, to be evenly
    > distributed across visible frequencies? It seems like our eyes are
    > capable of 'self white balance' without any conscious effort. How
    > many people complain about the green tint from fluorescent lighting,
    > for example.


    White Light is not EQUAL amounts of all frequencies.
    Technically, it is the distribution of wavelengths radiated from a "black
    body" at some specified temperature.
    Usually the temperature specified is 5500°K. This corresponds to the
    apparent surface temperature of the sun at noon. The spectral distribution at
    this temperature peaks at about 480-490 nanometers , a kind of blue/green
    and drops off at higher and lower frequencies.
    Some folks specify 6000°K. to allow for the effect of blue skylight.
    The Eye/Brain Complex does an amazing job of "White Balancing".
    I suspect that the eye records the actual color distribution on say an
    overcast day, but the brain interprets the data like it thinks the subject
    should look like from memory of similar scenes. It gets really hairy.
    Bob Williams
     
    Robert E. Williams, Aug 15, 2003
    #3
  4. Samuel Paik Guest

    I started writing a long response, but when it grew to around
    a thousand words I figured it makes more sense to refer you
    to books.

    The short answer: light is a power spectra, color is human
    perception of light through the sensors in the eye, there
    are three kinds of sensors in the eye, (...lots of stuff
    elided here...), you can hence parameterize any color
    with three numbers, which is why RGB is so popular.

    To learn more, you should hit some books, e.g.

    Wyszecki & Stiles; Color Science: Concepts and Methods,
    Quantitative Data and Formulae.

    Some books on computer graphics have sections on color and the
    human visual system, e.g.

    Hall; Illumination and Color in Computer Generated Imagery

    Lots of books on human perception (psychology) have sections
    on the human visual system.

    [my background is in computer graphics, not psychology, so I
    can name the title of a book in that category off the top of
    my head]

    > I guess my basic question is the approximation of color by three
    > values. How come that seems to work so well?


    Because it is rooted in the human visual system. Dogs and aliens
    may need a different number of parameters.

    Sam
     
    Samuel Paik, Aug 15, 2003
    #4
  5. Don Stauffer Guest

    Light is composed of a stream of photons. Each single photon is of a
    very specific frequency/wavelength. However, a wide band emitter like a
    blackbody emits an absolutely staggering number of photons per second
    (billions of billions of billions), and these photons span a wide range
    of frequencies. An LED emits a narrow band of frequencies, but still a
    band. A laser GENERALLY spans an even narrower band of frequencies,
    depending on type. Some produce an extremely narrow band (certain gas
    lasers). Most solid state lasers are relatively broad in spectrum
    compared with gas lasers, but still narrow compared to LED sources. So
    called 'single mode' lasers are more narrow in bandwidth than multi-mode
    ones.

    wrote:
    >
    >
    > A specific color is some mix of various frequencies. Is this
    > typically a combination of various frequencies or is the energy
    > distributed across a range of frequencies? e.g. what is red? I
    > believe that something like a red LED emits light at a specific
    > wavelength. Something like a LER (light emitting resistor ;) emits
    > light over a range of frequencies that are commonly known as 'red.'
    >
    > snip


    > thanks,
    > hank


    --
    Don Stauffer in Minnesota

    webpage- http://www.usfamily.net/web/stauffer
     
    Don Stauffer, Aug 15, 2003
    #5
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