Color perception

Lee Poulsen
Fri, 23 Jan 2004 14:59:49 PST
--Lee Poulsen
Pasadena area, California, USDA Zone 9-10On Jan 23, 2004, at 2:24 PM, wrote:
> Printers wrestle with this metamerism problem all the time.  Two 
> objects made
> from two different pigments may look the same under one light source, 
> but
> different under another light source.  Happens all the time to interior
> decorators.

And in a similar vein, from what I've read, humans will perceive two 
colors as being the same if they both lie at the same coordinates in 
the CIE standard color space, no matter how the color is produced. If a 
human sees a source of light consisting of two different frequencies in 
the visible light spectrum, it will appear as a single color that lies 
at the midpoint between the two frequencies locations in the standard 
color space. However, this point can often correspond to a source of 
light consisting of a single frequency. Either one of these sources of 
light will appear the same color to most humans, even though one is a 
spectrally "pure" frequency corresponding to that color and the other 
is really two other colors that have mixed to produce that color.

Why is this important? I've read that if you are trying to attract 
flies to a bright yellow colored object (like sticky fly traps), you 
have to use spectrally pure yellow paper or whatever. If the color of 
the object that appears bright yellow to us really consists of two or 
more pigments whose mixture appears bright yellow to us, the fly eye is 
different and they don't see it as yellow at all. This is also why some 
kinds of blue objects appear different colored in photographs than 
other blue objects like flowers for example, making blue flowers very 
difficult to render correctly to our perception. The photographic 
chemicals react differently to spectral reds and spectral blues than 
our eyes do making the flower often look wrong, even though to us when 
we look at it directly, it looks like it's "pure" blue.

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