Re: fluorescent colors in colorsync environment
Re: fluorescent colors in colorsync environment
- Subject: Re: fluorescent colors in colorsync environment
- From: Seth <email@hidden>
- Date: Tue, 10 Jul 2001 10:26:43 -0400
Actually, you are both correct. But you are looking at the phenomenon from
different perspectives. Mr. Tobie describes the process of flourescence and Mr.
Asselbergs the result.
CIEXYZ values are determined by multiplying the energy measured in each spectral
band by the appropriate photopic sensitivities (sensitivity of the cones in the
eye), and then summing the resultng values to get the CIE XYZ values for the
color. The CIELab values are a mathematical transform of the XYZ values. The
theory is that if two samples have the same sums, then they are visually
identical.
Even though the energy in some bands is greater than 100% of the energy from the
light source for those bands, as both of you point out, the energy must come from
some other band. The total energy is never more than 100%. But for flourescent
colors, as I mentioned in my first post, the energy that is transferred often
comes from the UV band. Only the very longest wavelength edge of this band is
measured as part of a colorimetric measurement. Any energy transferred from
shorter wavelength UV can push the total energy above 100% of the measured input.
Then it becomes possible to have a color that can not be created in the viewing
light without flourescent ink. (Measuring more of the UV band will not help
because the photopic sensitivity in that area is 0.)
Also, when energy is transferred from one visual band to another, its effect on
the each of the three types of cones changes. If the change is great enough,
although in this case the total energy will not change, the amount affecting one
or two of the cone types can exceed the energy in the light source affecting
those cones.
I note that every visible color has a CIEXYZ (and CIELab) value. The CIE system
tells you what color you have. To determine if it can be rendered with
npn-flourescent ink in a partilcular light, one should compare its XYZ values
with the XYZ values of white for that light. If any one of the color's three
numbers are greater than the corresponding number for white, then it cannot.
Seth Cohen
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>
Igor Asselbergs wrote:
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>
> C. David Tobie wrote:
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> > If I use a good digital camera to shoot fluorescent objects and print to an
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>
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> > Epson inkjet, I get some amazingly bright colors, but "fluorescent", by
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> > definition, means colors beyond what can occur from the amount of light
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> > available in the proper band of the spectrum, and stealing light from other
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>
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> > bands to increase emittance..
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>
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> Actually I have my doubts. Regarded from an environment of measuring and
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> computing you're absolutely right. But regarded from the realm of perception
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> I don't think your statement is necessarely true. As I explained before it's
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> easy to photograph fluorescent colors, it doesn't need any special
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> equipment.
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>
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> BTW: Even the brightest fluorescent colors I can come up with appear to fall
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> within CIELab space, according to colorshop (though I must add that
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> colorshop has a max intensity of 125 percent per waveband, wich is too
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> little for some fluorescent colors). That makes sense to me because while
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> part of the spectrum has a reflection of more than 100 percent, other parts
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> are still absorbed. Overall most fluorescent colors are probably not
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> brighter than white.