We describe a single beam device that records continuously as a function of wavelength, simple quantities related to the reflectance and transmittance, (1 - R)/(1 + R) and (1 - T)/(1 + T). The unit is easily attached to any spectrometer. It is sensitive to changes in these quantities of about 1 × 10-4, while systematic errors in alignment set an absolute accuracy of about 1 × 10-3.
D. Beaglehole, "A Sensitive Single Beam Device for Continuous Reflectance or Transmittance Measurements," Appl. Opt. 7, 2218-2220 (1968)