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Re: Green Light



Adrian Thomas wrote to the telecine list,

> the important thing is the continuity of the spectrum,
> not where the peak is. 

Sorry to frighten the non-mathematical among you: 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.

- The light source needs to have power in some spectral region where
   each dye absorbs.

- The light source need not (some would argue, "should not") have power
   where no dye absorbs.

- Ideally, you would put illuminant power at each of the three places where
   one dye absorbs maximally and the two others absorb minimally. 

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. To some extent the right places depend upon film stock.

What the light "looks like" is, to the first order of approximation, irrelevant. What's important is how its SPD plot relates to the dye absorbtion plot of the film. The color of a light depends upon its SPD, of course, but here the light's SPD is taking the role of an analysis function - it's more like a CMF than an SPD, for thoise of you who are interested in color science.

For background to this topic, you might want to read the Color FAQ.

  <http://www.inforamp.net/%7fpoynton/ColorFAQ.html>

On the parallel thread "Curves" on the telecine list, you might want to read the Gamma FAQ:

  <http://www.inforamp.net/%7fpoynton/GammaFAQ.html>

Cheers,

C.

--
Charles Poynton 
<mailto:poynton at poynton.com> [Mac Eudora, MIME, BinHex, uu, qpv]
<http://www.inforamp.net/~poynton/>





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