Equilateral dispersing prisms are used for wavelength separation applications. The three sides are of equal length. A light ray is twice refracted passing through the prism with total deviation denoted by q_d in the figure below. Deviation is a function of refractive index, and hence wavelength. Angular dispersion Dq_d is the difference in deviation for light rays having different wavelengths and varies with prism orientation.

• Reflection losses are minimized for un-polarized rays traveling parallel to the base of the prism. This condition is called minimum deviation.
• Minimum deviation occurs when the ray and wavelength angle of incidence at the entrance surface are equal to the angle of emergence (both angles measured with respect to the surface normal).
• Antireflection coatings help reduce polarization at the prism surfaces by increasing total transmittance.

General Specifications: