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Old March 22nd 08, 04:36 PM posted to rec.photo.digital
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Default Photographing Ultraluminous LED-lit Art Projects

Pooua wrote:
On Mar 21, 2:47 pm, Bob Williams wrote:
Pooua wrote:
http://web.mit.edu/neltnerb/www/artw...x.htmlfeatures 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.
You can't fix it with Photoshop either because PS also uses an RGB
palette (e.g. Adobe RGB).
Bob Williams


I don't know if that explains the problem.

1) Purple is actually a combination of red and blue.
2) The cones of our eyes only detect red, green and blue (some
extremely rare women can see a 4th color)


Purple is a combination of red and blue, and cameras get that just fine.

But violet can be a combination of red and blue, or its very own
spectral self, from 410 nm down into the near ultraviolet at about 360 nm,
if you are young or have had cataract surgery.

I tested my Canon 30D with the 100 mm f/2.8 macro lens
with monochromatic (2nm wide) light. And what color
is violet?

Well, it is not violet, not blue, in fact it is BLACK, dead BLACK.

At 420 nm the camera is rather weak in response, and produces blue.
At 410 nm and below it is dead as a doornail.

I don't have any other lens with me, so I don't know whether it is
the lens or sensor.

I do add that Photoshop is perfectly capable of turning any color
you wish into the color that 410 or 400 or even 390 nm light looks like,
which is violet. The color that shows on a video screen is not near
as saturated, of course, but it is the correct color. Prints are even
worse saturation, but still you can get correct color.

Doug McDonald