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Constant Luminance

Under the "Subject:" tag Full Bandwidth Color Difference, Martin Euredjian asked the Telecine list for an explanation of  "constant luminace." Here's my one-paragraph summary:

   Since 1953, we have been using the wrong block diagram for color
   video! The principles of color science dictate that we mix linear RGB
   to make true luminance, denoted Y. This is known as the Principle of
   Constant Luminance. But in video we depart from that principle, and
   implement an engineering approximation: We mix nonlinear ("gamma
   corrected") R'G'B' to make what I call "luma," denoted Y'. (Many video
   engineers carelessly call this "luminance.") To form luma, we use the
   theoretical coefficients of color science, but we use them in the wrong
   block diagram: We apply gamma correction before the mixing, instead
   of after. This alteration in the block diagram is more or less
   inconsequential in practice, though the departure from theory is
   apparent in the dark band seen between the green and magenta color
   bars of the standard video test pattern.

Details are available in Chapter 8 of "A Technical Introduction to Digital Video." Information about the book is available through my web page. The book's explanation of Constant Luminance is servicable, but some improvements are in the works. (It was while I was working on improvements in that section that my editor finally screamed, "SUBMIT THAT MANUSCRIPT!")

Gamma has unjustifiably acquired a bad reputation. Three months ago, I presented a paper, "The rehabilitation of gamma," at the SPIE/IS&T conference in San Jose. That paper outlines the Principle of Constant Luminance - in fact, that paper does a better job of introducing Constant Luminance than chapter 8 of the book! However, as you can intuit from its title, that paper concentrates on the reproduction of lightness (which is related to luminance, which is related to luma). It merely outlines the color issues. You can access the Abstract,


From that page, you can access the typeset version of the full paper in Acrobat PDF format. 

I presented the related issue of choosing luma coefficients for conventional video, DTV, ATV, and HDTV, in a SMPTE paper two months ago: " Luminance, luma, and the migration to DTV." The so-called paper is virtual at this moment, having not been actually finished in that medium! However, the abstract of the presentation is available:


For the truly courageous, an audiotape of the session is available through SMPTE. The opening paragraph of this note is the first paragraph of that paper's abstract. 

Some fragments of the paper-in-progress are available. Start with the brief technical note, "Errors due to nonconstant luminance":


If you STILL want to keep going, access the links at the bottom of that page. 

All of this will be tied together within a month or two, and then (eventually) released as the written version of the SMPTE paper.

A detailed discussion of constant luminance is part of my 3-day course on Digital Video Technology, which I'll present at UCLA, the week after NAB. 



Charles Poynton 
<mailto:poynton at poynton.com> [Mac Eudora, MIME, BinHex, uu, qpv]

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