Optical monitor tooling (monitor/substrate M/S) can be calculated in MONITOR. One deposits a fairly thick film, say 15 quarter waves, on a monitor glass and subsequently measures the coated substrate in a spectrophotometer. In former times a slide rule helped to determine the number of quarter waves on the coated substrate. This technique, possibly developed at Bausch & Lomb in Rochester, has stood the test of time and is widely utilized.
 
M/S is actually the ratio of the apparent physical thickness on the optical monitor to the physical thickness on the substrate. 'Apparent' because M/S is based on the number of monitor quarter waves deposited at some nonzero angle of incidence. While calculations based on thick single layers will be fairly accurate, M/S for films in a stack  usually requires correction. We need a design which uniquely gives correction factors for two materials.

A technique known to thin film cognoscenti utilizes a half wave ripple design, say 1H 2L 2H 2L 2H 2L 1H. The method has been encapsulated in the Excel sheet shown below. Since tooling correction is a built-in DESIGN function, the circuitous Ripple Test may no longer be required but is nonetheless interesting and instructive. Click here to download RIPPLE2.xls.