The article focuses on the devices that perform the function of modulation, that is the varying signal used in electronic detectors. When the intensity of light increases and decreases in a regular pattern, some types of spectroscopies work better. Choppers, which are mechanical modulators physically block the light beam. The frequency of modulation can be varied as many choppers come with interchangeable wheels or variable speeds. A shutter performs intermittent chopping. The Faraday effect, a change in the refractive index due to the presence of a magnetic field, is used by a magneto-optical modulator. Materials like flint glass and water show a Faraday effect.
Ball, David W., "Modulators" (2006). Chemistry Faculty Publications. 128.