2006.12.06 04:45 "Re: [Tiff] Re: Tiff Digest, Vol 31, Issue 3", by Edward Lam
The standard conversion from RGB to gray, documented in the textbooks, is grey level = .33R+.5G+.17B. So a unitary conversion back to RGB is fine.
And of course this equation is acknowledged to be wrong - you can't combine gamma encoded values like this to give a correct greyscale.
Charles Poynton has a nice note explaining this:
Found from the the wikipedia article, http://en.wikipedia.org/wiki/Luma_%28video%29