Photographing Ultraluminous LED-lit Art Projects

I wrote:


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 should have realized that I saw into the UV (398 nm) with my eyes,
through the viewfinder. That means that the sensor has a cutoff filter
on it. Not unexpected, of course.

I should add that no real reflective objects depend on reflectivity
in the 380-415 nm region for color. That is because the eye's
sensitivity is too low there. You really need an actual spectrum
to see that violet color. The spectrum from a CD looking at the sun or
a fluorescent light will do.

Doug McDonald

[A complimentary Cc of this posting was sent to
Chris Malcolm
], who wrote in article :
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

Wrong. You assume that *physically possible* colors (those obtained
by mixing pure spectral colors in the range about 400..700nm) form a
triangle in the "linear" color space (more precisely, in the
"projective" color space). They do not.

In particular, any triangle with vertices in the "real color" space
does not cover the "real color" space. (In addition, no triangle
"comes close" to covering all the space.)

Hope this helps,
Ilya

On 2008-03-19 21:39:39 -0700, Pooua said:

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?

There is a device called an Expo/Disc. You hold it over the lens and
point the camera at the light source, take a shot and measure the white
balance. You set the white balance for whatever the Expo/Disc saw.

But the artist is not all wet. You can easily get outside the color
space of the sensor in situations like this. Sometimes it helps to
switch to Adobe RGB in these situations. Sometimes it makes it worse.

--
Waddling Eagle
World Famous Flight Instructor

On 2008-03-20 21:35:48 -0700, Pooua said:

On Mar 20, 9:02*am, Don Stauffer in Minnesota
wrote:
On Mar 19, 11:39 pm, Pooua wrote:

http://web.mit.edu/neltnerb/www/artw...eaturesartwork
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?

Here is yet another problem. *We frequently do not perceive with our
eyes the true color of LED or laser light sources if we view them
directly. *Some of the cones can saturate, really throwing off our
eyeball calibration.

I suppose I could adjust hue in Photoshop. I've just never been very
accurate with that thing.

Any other ideas?

You have to get it right when you take the picture. Photoshop cannot
put back what was never there.
--
Waddling Eagle
World Famous Flight Instructor

"C J Campbell" wrote in message
news:200803221855388930-christophercampbell@hotmailcom...

You have to get it right when you take the picture. Photoshop cannot put
back what was never there.


What? In terms of hue and intensity?
Wanna bet?

--
Jeff R.

Ilya Zakharevich wrote:
[A complimentary Cc of this posting was sent to
Chris Malcolm
], who wrote in article :
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

Wrong. You assume that *physically possible* colors (those obtained
by mixing pure spectral colors in the range about 400..700nm) form a
triangle in the "linear" color space (more precisely, in the
"projective" color space). They do not.

In particular, any triangle with vertices in the "real color" space
does not cover the "real color" space. (In addition, no triangle
"comes close" to covering all the space.)

Interesting point. I wasn't aware of that. Are you referring to the
need to use negative primary values to get to the intensity of
saturation of pure spectral colours outside the triangle?

--
Chris Malcolm DoD #205
IPAB, Informatics, JCMB, King's Buildings, Edinburgh, EH9 3JZ, UK
[http://www.dai.ed.ac.uk/homes/cam/]

[A complimentary Cc of this posting was sent to
Chris Malcolm
], who wrote in article :
Wrong. You assume that *physically possible* colors (those obtained
by mixing pure spectral colors in the range about 400..700nm) form a
triangle in the "linear" color space (more precisely, in the
"projective" color space). They do not.

In particular, any triangle with vertices in the "real color" space
does not cover the "real color" space. (In addition, no triangle
"comes close" to covering all the space.)

Interesting point. I wasn't aware of that. Are you referring to the
need to use negative primary values to get to the intensity of
saturation of pure spectral colours outside the triangle?

Yes, this is one of the manifestations. [Since the pure colors form a
convex curve in the projective color space, ANY pure color which is
not a primary will REQUIRE negative coefficients to express it via the
primaries.]

Yours,
Ilya