convert 16 bit tiff to 8 bit tiff

I have a bunch of 16 bit tiff images that need to be converted to 8 bit
tiffs.  The images that need converting are 4 channel satellite images.
Chanels 1 to 4 are blue, green, red and infrared, respectively.  I use
the following code which does convert to 8 bit but the colors in the
resulting image are way off.  I am using Windows XP and IDL 6.2.  Any
help would be greatly appreciated.

	files = FindFile('*.tif')
	kount =  N_ELEMENTS(files)
	For j = 0, kount-1 DO BEGIN
	     image=read_tiff(files[j])
	     CD, '\\Renee\IDLTest\', CURRENT=old_dir
	     write_tiff, (files[j]), TEM****ARY(image),
	    CD, old_dir
	ENDFOR