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From: alanr@rd.bbc.co.uk (Alan Roberts)
Subject: Re: correcting for uneven lighting
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Date: Wed, 9 Nov 1994 09:42:51 GMT
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Correction in TV cameras for uneven lighting

A TV camera, whether CCD or tube, produces signals which are linearly
related to light intensity.

When the signals are intended for viewing on a CRT, or for transmission
via a TV system, the signals are "gamma-corrected" according to a law
which is approximately logarithmic. Typically the ITU-R BT.709 law may
be used:
Volts = 1.099 Light ^ 0.45 - 0.099   for Light > 0.018
Volts = 4.5 Light                    for Light <=0.108
this makes the _signals_ nearly linear in human perception terms.

So, you need to know what psocessing the RGB signals have undergone
before deciding how to compensate for uneven lighting.

If the RGB signals are linear, then division is correct.

If the RGB signals are gamma-corrected, then subtraction is approximately
correct.

If that has not made it clear, I suggest you look at Charles Poynton's
recently posted FAQ:

elaine.crcg.edu:/pub/doc/colour/GammaFAQ.pdf
ftp.igd.fhg.de:/pub/doc/colour/GammaFAQ.pdf

You will need an Adobe Acrobat reader, available free from Adobe.

--
************* Alan Roberts **************
* BBC Research & Development Department *
* My views, not necessarily Auntie's    *
*    but they might be, you never know. *
*****************************************
