A sensitive and versatile spectrophotometer for differential or absolute reflectance and transmittance measurements is described. Special sample configurations are not required, and the sample may be subjected to almost any type of external perturbation. A transparent refracting chopper is used to obtain separate sample and reference beams. A synchronous electronic circuit is used to measure the reference and sample signals while suppressing the intervening switching transients. The fractional absolute error is less than ±10-2. The peak-to-peak instrument noise is approximately 2 × 10-5 of the reference signal. Thus, changes in reflectance or transmittance as small as a few parts in 105 can be seen. In many cases this approaches the shot noise limit and is comparable to the sensitivity obtainable with modulation techniques. Examples of results for the reflectance of InP near 3.2 eV and GaAs near 1.8 eV are given.
D. D. Sell, "A Sensitive Spectrophotometer for Optical Reflectance and Transmittance Measurements," Appl. Opt. 9, 1926-1930 (1970)