Optical Prisms Information
Optical prisms are blocks of optical material with flat, polished sides that are arranged at precisely controlled angles to one another. They are used in optical systems to deflect or redirect beams of light. They can invert or rotate images, disperse light into component wavelengths, and separate states of polarization. There are several basic types of optical prisms. Anamorphic prism pairs are used to correct the asymmetric, elliptical beams produced by laser diodes so that the beams are nearly circular in shape. Corner cube retroreflector prisms are designed to reflect all of the beams that enter an optical prism back onto the prism. Right-angle prisms direct beams of light at 90° from the incident. Dove prisms are truncated right-angle prisms that use total internal reflection to produce inverted images without deviation. Pentagonal prisms redirect an incident beam through 90° without beam inversion or reversal. Roof prisms invert and reverse an image as it is deflected 90°. Equilateral prisms provide better brightness than diffraction gratings and are often used as dispersing elements in applications that require spectral separation. Rhomboid optical prisms are used to control and redirect the optical path without affecting the image orientation. Wedge prisms are used mainly with laser beams, either for the elimination of reflections from a second surface or for beam steering.
Optical prisms differ in terms of physical dimensions, material specifications, and special coatings. Typically, suppliers specify a prism’s largest dimension in millimeters (mm). Several prism materials are commonly available. BK7 glass is a borosilicate crown glass with a wavelength range of 330 nm – 2100 nm and an angular dispersion of 0°42’37”. It is relatively hard and does not scratch easily. F2 glass provides excellent chemical resistance and is often used as a second glass in achromatic lenses. It has a wavelength range of 350 nm – 2200 nm and an angular dispersion of 1°40’45”. SF10 glass is a high-index material with a wavelength range of 400 nm – 2400 nm and an angular dispersion of 2°58’25”. Optical prisms that are made of UV-grade fused silica are also available. They provided better transmission in the ultraviolet (UV) region than crown glass and provide both a wavelength range of 200 – 2500 nm and an angular dispersion of 0°34’01”. Some optical prisms use reflective coatings to help reduce the loss of light due to transmission. Others use antireflective coatings to reduce the loss of light due to reflection.
Selecting optical prisms requires an analysis of surface quality and surface flatness specifications. Scratches are defects whose lengths are many times greater than their widths. Digs are defects that are nearly equal in length and width. Optical prisms with a 10-5 scratch/dig surface quality have scratches with an average length of .10 mm and digs with an average diameter of .05 mm. Optical prisms with a 20-10 surface quality have an average scratch length of .20 mm and an average dig diameter of .10 mm. Products with 40-20, 60-40, and 80-50 scratch/dig quality measurements are also available. Surface flatness, another important specification, is expressed in terms of λ where λ = 633 nm. Typically, the largest variation in surface height is no larger than λ/ 2, λ/ 4, λ/ 5, λ/ 8, λ/ 1 0, or λ/20.