Optical Prisms in Monochromator Spectrometers

A monochromator is a device that produces a beam of radiation with high spectral purity. Its function is to decompose composite light into monochromatic light or spectral bands of a certain width. The monochromator is composed of an entrance slit, a collimating lens, a dispersive element, a focusing lens, and an exit slit.

The optical material of the prism has different refractive indices for different wavelengths of light. Shorter wavelengths have higher refractive indices, while longer wavelengths have lower refractive indices. Therefore, parallel light, after being dispersed, is decomposed into different wavelengths in order of wavelength. After focusing, it forms images at different positions on the focal plane, resulting in a spectrum spread out by wavelength.

Commonly used prisms include the Cornu prism and the Littrow prism, as shown in the figure below.

Cornu Prism: This is a prism with an apex angle α of 60°. To prevent the generation of double images, this 60° prism is composed of two 30° prisms, one made of left-handed quartz and the other of right-handed quartz.

Littrow Prism: This is a 30° prism made of either left-handed or right-handed quartz, with aluminum or silver coating on its longitudinal axis plane. The optical characteristics of the prism can be characterized by its dispersion rate and resolution.