Reflectometry measures reflected intensity of light from a thin film. The index of refraction must already be known for the material being measured. Index of refraction can be obtained from the resist vendor data or be measured by ellipsometry.
The Nanospec is a reflectometer and can be used to measure resist thickness.
use the following recipe on the Nanospec:
019 ZEP on Silicon (10x)
037 pmma on silicon
048 Thin HSQ on Si
130 maN-2403 on Silicon Devin