Pellin Broca Prisms

Pellin Broca prisms are four sided prism with included angles of 135, 90, 75, and 60 degrees  Input light enters between the 90 and 75 degree angles and exits deviated by exactly 90 degrees.  If a broadband light is input, only a single wavelength will be deviated by 90 degrees (plus a small angle from dispersion) and by rotating the prism, you can select individual wavelengths because the 90 degree deviation is constant.  This property is commonly used to separate individual wavelengths from a multi-line laser.  Pellin Broca prisms are commonly used in optical spectroscopy and for separating the second harmonic generation wavelength from the fundamental wavelength.