Prisms are used in countless optical systems — from the pair of binoculars in your closet to cutting edge targeting systems to retroreflectors on the Moon. In modern, high-end optical systems, precision optics may be required that push the envelope of manufacturability within given cost constraints. Prisms are an integral component in many of these systems and often require manual fabrication in small quantities; however, they save space, mounting time, and materials, presenting opportunities as well as challenges for optical system designers.

Figure 1. The Prism Manufacturing Process
In their most basic form, prisms are essentially solid pieces of glass with polished faces that have been crafted into more geometrically — and optically — meaningful shapes. Prisms can have many different functions depending on the position, number, and angles of its faces. For example, we have all learned from Newton that an Equilateral Prism is capable of splitting white light into all of the colors of the spectrum. These prisms are called dispersive prisms, and they are integral to instruments such as spectrometers and refractometers.

Figure 2. A right angle prism’s hypotenuse is hypothetically tilted from its proper angle of 45° by an additional 5 arc minutes. After the incoming light reflects off the tilted hypotenuse, the ray is deviated by twice the angular “error” because of the law of reflection (same angle in, same angle out, so the error is doubled). The ray is no longer “square” to the exit face of the prism because of this angle error. At the glass-to-air boundary of the exit face, refraction occurs. This increases our ray deviation by another factor of ~1.5, assuming that 1.5 is the air-to-glass index ratio. A 5' angle error ultimately caused a 15' deviation of our ray in this case. Small errors in prism angle tolerance can mean significant errors for the rest of an optical system.
Perhaps the most common uses for prisms, however, are as replacements for mirror assemblies. Mirrors and prisms can both be used to correct image orientation, to divert light into different angles or around obstacles, to split or combine beams of light, or simply to “fold” optical systems into physically smaller spaces. All reflective prisms have a mirror equivalent, but prisms can offer distinct advantages over mirrors. Rather than using multiple mirrors in various cumbersome, imprecise mounting fixtures, one could often simply use a single prism. Such a substitution may result in a decrease in alignment issues, size, and maintenance, all with an overall increase in accuracy and simplicity. For example, Porro Prisms inside many binoculars take the place of an equivalent four-mirror system per eye. Many prisms do not even require a mirror coating to offer the same reflections as a mirror would provide. Not requiring such mirror coatings (or multiple mirrors) can often result in increased system efficiency.

To be used successfully in a variety of different applications, prisms must be manufactured to a high degree of accuracy and tolerance. As such, the prism manufacturing process is quite complicated. Prism manufacturing is appreciably different from other kinds of optical component manufacturing, which, in turn, is appreciably different from any other industrial manufacturing. Most modern manufacturing processes are highly automated. While lenses, mirrors, and other optical components are the beneficiaries of some forms of automation, prisms are not quite as lucky. Not only do prisms come in a wider variety of shapes and sizes compared to other optics, they almost always have many more surfaces. The more surfaces an optic has, the more operations and iterations must be performed for a given process. The virtually infinite number of possible prism configurations makes large-scale automation all but impossible. Most precision prism orders are also for quantities that are much too low to warrant implementation of unique automated processes.


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