In every spectrophotometer the light source that is illuminating the cuvette is placed on the fixed height. The height of the beam is measured from. An optical spectrometer (spectrophotometer, spectrograph or spectroscope) is an instrument used to measure properties of light over a specific portion of the electromagnetic spectrum, typically used in spectroscopic analysis to identify materials. The variable measured is most often the. Monochromatic light is usually used for the measurement light beam shown in Fig. To be precise, it has a spectral bandwidth (slit width).