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Old Nov 13, 2003, 9:01 AM   #1
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Default Anyone explain this ?

:?: Whilst waiting for the total luna eclipse the other night I decide to try
out my 10D at 1600 ISO. I took an image of a white van under a sodium
streetlight. When you split the image into RGB, in PSP, you get a red and
green channel image and no blue detail, as expected from a sodium
streetlight BUT if you brighten the blue channel and contrast stretch it you
get an image that appears inverted. Is it IR it is recording ?.

Original, RGB and Blue enhanced at www.knighttrain.freeserve.co.uk/1600.htm
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Old Nov 14, 2003, 5:20 PM   #2
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Default IR?

I don't believe this is IR. I thought about this for a while, and I think I at least have a hint as to what you are seeing.

The sensor in the 10D uses a 3 primary color (RBG) filter Bayer pattern, from which the three color planes are synthesized by a matrixing (decoding) operation. I believe that the very low level "negative" image that you are seeing in the blue channel is a "remnant" of the matrixing process, and is merely an artifact of the operation.

The LP sodium lamp should have nearly no light in the blue range, and so what the camera sensor actually "sees" is quite unusual - no (or very, very little) signal in the blue pixels. The matrixing operation creates the ghost in the blue plane as a result of some very low-level overflow in the matrix operations. In this case, these low level artifacts just happen to be inverse to the strongest signals in the other two planes, thereby creating a negative effect.

I don't believe that it is IR at all.

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Old Nov 15, 2003, 10:15 AM   #3
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Thanks for the reply, I tend to agree now in the fact it is not IR, I couldn't see how that would be possible. Your theory on artifacts sounds good. I just couldn't understand how the van appears very dark, ie no blue signal, but doesn't show any noise like other areas of the scene with low light levels.
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