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Old March 22nd 08, 11:23 AM posted to rec.photo.digital
Chris Malcolm[_2_]
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Default Photographing Ultraluminous LED-lit Art Projects

Bob Williams wrote:
Pooua wrote:
http://web.mit.edu/neltnerb/www/artwork/index.html features artwork
illuminated by super-bright LEDs, but the photos do not accurately
reflect the colors of the lighting. The artist says that his camera
has trouble picking up the purple lighting, instead showing it washed
out, apparently because it is outside the normal color space of the
imaging sensor. Does that sound likely? What might a photographer do
to take better photos of these tricky lighting situations?


I think the reason is, that NO combination of RGB used in sensors can
produce violet (purple?) light. The visible color spectrum is ROYGBV.
All colors between R and B can be generated by mixing appropriate
amounts of R, G, and B. But Violet is a shorter wavelength than any of
the frequencies captured by an RGB sensor. So no combination of longer
wave lengths can produce a shorter wavelength.


That's true physically speaking, but not in terms of how the human eye
sees colour, which is in terms of three colour filtered bands. Which
is why for us there are three primary colours, and why, for example,
we can't tell the difference between a pure spectral green and the
optical illusion of an appearance of green produced by mixing blue and
yellow. A goldfish with its much larger number of colour sensors would
probably not be fooled by mixing blue and yellow.

The problem with violet is that while it's theoretically possible to
mix up an undetectable different equivalnce to any colour using three
primary colours, in practice it's extremely difficult to find
chemicals which produce exactly the three colours required. Which is
why high quality colour printers usually use more than three. There
will be printers and monitors capable of reproducing any specific
violet, but you might have one of those, and they might be rather
expensive.

The same problem exists in the camera. Just as in the old days of
colour film different manufacturers produced colour films with
different virtues and vices in their not quite perfect capture of
colour, so too with modern digital sensors. They're not perfect, and
in some at least that imperfection is visible as defects in violets.

--
Chris Malcolm DoD #205
IPAB, Informatics, JCMB, King's Buildings, Edinburgh, EH9 3JZ, UK
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