Several different choices.
 
The Government Printing Office, I believe, carries two scale sizes in white
plastic, marked in centimeters. These are offered as part of the NPT
Curatorial Services Branch, Washington, DC  20013-7127. It's been several
years since I ordered, so I don't know the price. Very nice scales and they
last forever with reasonable care.
 
Kodak offers their Kodak Color Control Patches. Two in a package. One for B/W
(with a gray scale) and the other for color (with color scales). They are
marked in inches and centimeters. A little long for what you want, but they
are paper and can be easily cut down (you'll lose some of your color
controls, but that shouldn't be too big of a deal for most photographs.
 
You mentioned that you were using a light table, which suggests a third
option. Using film ink you can easily create appropriate scales on clear
acetate.
 
Best,
 
Michael Trinkley, Ph.D.
Director
Chicora Foundation, Inc.
PO Box 8664
Columbia, SC  29202-8664
803/787-6910
Email: [log in to unmask]