2004.01.23 20:52 "Re: [Tiff] How to convert MINISBLACK to PHOTOMETRIC_RGB", by Chris Cox
I am by no means an authority on the TIFF specification. My interpretation of the file in question is that a sensible application should render the first band in greyscale and ignore the 2nd and 3rd since they are extra samples with no meaning attached.
Or load the 2nd and 3rd channels as alpha (not transparency) channels -- as Photoshop does.
It would be incorrect to interpret this image as RGB.
How does Photoshop treat a second alpha channel? Does it actually get used for anything?
Alpha channels are just alpha channels - they get loaded and the user can do what they want with them.
Only transparency (associated alpha) has any special meaning.