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Re: Green Light

At 6:07 PM 3/16/98, Charles Poynton wrote:
 The important thing is neither of these. It's the intergal of the light
source spectrum (SPD), weighted by [or seen through] the dye absorption
curves of each of the three dye layers.....
>In other words, you try to arrange for the light source to have no power
>where the dye absorbtion curves overlap substantially. This will assist in
>retrieving the three "records" without having to compensate downstream for
>interaction between the channels.

Mr. Poynton has oversimplified the situation, I believe. Absolutely, the
important thing is the integral of the spectrum (the area under the
spectral curve): a larger intensity or broader spectrum (weighted as above)
will yield more power to shove through the film to the detectors.

But: it *doesn't matter* that the light source has power in the overlapping
sections of the dye curves. This is made irrelevant when the detectors have
filters which tailor their individual responses to well within the three
color records, so that none of the overlap signal is "read". This is at the
heart of the Philips CCD pickup: the detectors sample a narrow portion of
each of the dye curves, well away from the crossover areas.

This is the main reason that colorists like Bob Festa have commented that
the "front end" of the machine just "falls into place". Of course it does!
By comparison, almost no masking is required to compensate for picking up
the interaction between channels. The more intense the light source, the
narrower you can make the analyzing filters for the three channels for the
same signal/noise ratio, and the less crossover you pick up, requiring less
electronic masking and nasty side-effects that have become so tiresome as
we approach the 21st century!

Perhaps we could persuade Dave Bancroft or Steve Russell to publish the
charts which form part of Philips' presentation on the matter. Surely a
topic of interest to all?


Mike O

| "I do not distinguish by the eye, but by the mind,      |
|   which is the proper judge of the man."                |
|         Seneca. 8 B.C.- 65 A.D.                         |

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