1996.12.05 07:48 "(more than 8 bits) TIFF images", by Claire Rossignol

1996.12.05 20:58 "Re: (more than 8 bits) TIFF images", by Cristy

And there is at least one free 64-bit viewer (16 bits each of red, green, blue, and alpha):

   http://www.wizards.dupont.com/cristy/ImageMagick.html

You need to set a define at compile time to get the 16-bit capabilities. See README for details.