Pellin–Broca prism
A Pellin - Broca prism is a special dispersion prism (prism, the different wavelengths separated). It is used in order to select from the laser light sources which emit a line spectrum with a plurality of lines of a line. The light beam from a Pellin - Broca prism is deflected by 90 °.
Design and operation
A Pellin - Broca prism consists of a glass body of specific geometry. One can imagine the vitreous as a composition of three individual prisms: two 30 ° -60 ° -90 ° prisms ( The angles correspond to the internal angles of the prism ) and a 45 ° -45 ° -90 ° prism
A collimated light beam takes on the following path in the prism. As shown in the adjacent figure, light falls on the side AB of the prism. In glass as prism material, only a small portion (approximately 4 %) is in accordance with the Fresnel 's equations is reflected at the interface, and the remainder (about 96%) is refracted according to the refraction law, with minimal distraction. Within the prism of the light beam is initially parallel to the prism base AC and meets with an incident angle of 45 ° to the side BC. The light is totally reflected on the basis of the selected prism material and rotated 90 °. The beam is now in the direction of the DC side, where it exits in accordance with the minimum deflection of the prism.
The selection of the wavelength, as is indicated via the " minimum deviation " (English minimum of deviation ). This refers to the case in which a light beam passes through a prism so that equal angles occur at the two refractive surfaces. With a normal ( isosceles ) prism, the incident and therefore extend the beam emerging from the prism to the cutting angle of the two refracting surfaces rotated this angle of intersection is at Pellin - Broca prism 90 °. The effect of minimum deviation is dependent on the incident angle and the refractive index of the prism. Due to dispersion ( change in refractive index of a material as a function of wavelength) that is only a specific wavelength is minimum, exactly 90 °, deflected. All other wavelengths, however, show a greater deflection. With the aid of a slit diaphragm can filter the rotated by 90 ° beam out. If one turns now the prism perpendicular to the image plane, as calculated by the modified angle of incidence and the dispersion of the prism material for a different wavelength, the minimum distraction. So can be selected by a simple rotation of the prism without further changes to the appearance, the desired wavelength.
A further special feature is that a light beam incident on the prism at Brewster angle, this also leaves again at the Brewster angle. When passing through the Pellin - Broca prism, the light is therefore polarized parallel to the plane of incidence, with almost no reflection losses (based on the parallel polarized component). In this case, the Pellin - Broca prism is also a Brewster prism.