there is an excellent article in the July/August edition of the Journal of forensic Identification (Vol. 57 #4) entitled "Image enhancement and Adobe Photoshop: Using Calculations to Extract Image Detail". I've fooled around with calculations but I'm not really sure of what I'm doing. Any thoughts?

I haven't read the JFI article yet, so I'll not comment on it specifically. But, calculations is a powerful tool. The primary application in using calculations in forensics has been for what I refer to as 'color isolation techniques.' These are techniques used to isolate some data in the image by manipulating color channels. Unfortunately, calculations are not intuitive, but the Channel Mixer is, and you can do many of the same things using the channel mixer - with the added benefit that 1) it's non-destructive/editable if using adjustment layers; and 2) can be easily explained in court if needed.

To use the Channel Mixer, invoke it as an adjustment layer, click the Monochrome box, then adjust the sliders to either mute or enhance detail based on color information. In RGB, sliding any slider to the left makes it darker, to the right makes it lighter. The opposite is true for CMYK values, of course.

In my workshops I provide exercises for this technique to show detail in areas that previously could not be seen (bleached checks), in eliminating security backgrounds behind fingerprints, for isolating signatures from overwritten endorsement stamps, etc.

I hope this is helpful.

George