Reflectometers

Different optical geometries are used depending on the measurement requirements:

• Relative measurement setups compare the reflected light from the sample to that of a calibrated reference mirror. This is simple but depends on the quality and cleanliness of the reference standard.
• Absolute measurement setups (e.g., the V-W or V-N geometries) use a configuration where the detector can be moved or the beam path reconfigured to measure the source intensity directly and then the reflection, without moving the sample relative to the beam. This eliminates the need for a calibrated reference standard for specular reflection.
• Integrating spheres are used to collect reflected light over a wide angular range. This is essential for measuring diffuse reflectance or total hemispherical reflectance, rather than just specular reflection.

Generally, the reflectance also depends on the optical wavelength. Therefore, a reflectometer ideally allows one to measure the reflectance also as a function of wavelength in a wide spectral range. Such a device may be called a spectroscopic reflectometer, which can be considered as a type of spectrophotometer. However, for some purposes it is sufficient to work with a fixed wavelength or with a fixed wavelength interval.

Spectroscopic reflectometers are needed, for example, for characterizing dielectric mirrors and other kinds of dielectric coatings. Here, spectral resolution is important, since e.g. errors in thin-film growth can spectrally shift or distort reflection bands. Similar applications involve thin-film semiconductor devices such as VECSEL gain structures or SESAMs.

The calibration of a reflectometer becomes critical when reflectance values very close to unity (perfect reflectance) are to be measured. Refined methods can be used to optimize such calibration procedures; for example, one may use an optical setup where one can alternately measure the optical power of light reflected at the sample and the power of light from the same source getting to the same photodetector through some reference path. For very high reflectance values, however, — for example, in the context of supermirrors — one has to resort to other methods, e.g. the cavity ring-down technique.

Reflectometers are often used for specular reflections, but some (called optical backscatter reflectometers) also work with diffusely scattered light. There are other devices for optical scattering measurements, which can distinguish specular reflections from scattered light and provide detailed information also on the latter.

For oblique (non-perpendicular) incidence on the sample, the polarization of the test light can also be relevant. An instrument may work either with linearly polarized or with unpolarized light, or possibly with other defined polarization states.

A related technique is ellipsometry, where one measures not only the reflections, but also the polarization state of the reflected light. Therefore, ellipsometers must contain additional optical hardware for polarization analysis.

The reflectance of a sample can also depend on the position on the sample. Some reflectometers work with tightly focused light and can measure the reflectance as a function of the position with relatively high resolution.

Another potentially useful feature is the ability to work with samples having curved surfaces.