Save image as 16bit image with lookuptable

Hi,

i've a question about how to save an image as 16bit depth with a lookup table. I have already an option to save as 8bit bitmap with lookup table but with that option I can only use 256 colors. by that time it was enough but now i have to support more colors. what is the best solution for this problem. I want a lookuptable to use ease a color spectrum. The first idea i had was to use pf16bit instead of pf8bit colordepth and increase the looktable to 1024 elements but this didn't work. so do you have an idea to increase number of colors with a color spectrum.