The Evolution of Coating Functions for Right-Angle Prisms

If we think of a right-angle prism as a stage, then coating technology is the endlessly variable "set design" for that stage. By changing the coating materials and structure, the same prism can deliver completely different optical functions. From the simplest reduction of light energy loss to the most complex wavelength separation, the evolution of coating technology has greatly expanded the application boundaries of right-angle prisms.

Anti-reflection coatings: letting more light "pass through"

This is the most basic and most widely used type of coating. When light passes through the right-angle faces of a prism (the surfaces where light enters and exits), the loss caused by interface reflection is not negligible. For a single prism, the reflection losses from the two faces can add up to nearly 10%. In multi-lens systems such as endoscopes and camera lenses, if every element is left untreated, the total light energy loss would be astonishingly high.

Applying anti-reflection coatings to these right-angle faces has therefore become standard practice. Depending on the wavelength band in use, coatings can be applied for ultraviolet (UV), visible (VIS), near-infrared (NIR) or even the entire broad band. For example, a standard visible-light anti-reflection coating gives the prism surface a blue-violet or dark green appearance under reflected light, precisely because the coating uses interference effects to "cancel" reflection at the center wavelength, allowing more light to "pass through." For laser applications, "laser-line anti-reflection coatings" tailored to a single wavelength (such as 532nm or 1064nm) can also be custom-made, reducing reflectance to below 0.1%.

Reflective coatings: steering more light "around"

When a prism needs to function as a mirror, as described earlier, applying a metallic coating or a dielectric high-reflection coating to the hypotenuse is the core process. Metallic reflective coatings (aluminum, silver, gold) are the general-purpose choice thanks to their broad spectral bandwidth. For example, in visible-light systems requiring high reflectance, silver coatings are favored for their superior reflectance, while aluminum coatings are better suited to instruments sensitive to ultraviolet light.

Dielectric high-reflectance coatings are even better. Made of dozens of alternating layers of high- and low-refractive-index materials, they can achieve reflectance above 99.9% in a specific wavelength band through precise thickness control — something no metallic coating can match. Although their reflection bandwidth is relatively narrow, dielectric high-reflectance coatings are the only choice in applications demanding extremely low loss, such as laser resonators.

Beam-Splitting Coatings: Sending Light Down Different Paths

This is one of the more complex applications of coating technology, with dichroic prisms and polarizing beam-splitter prisms as typical examples.

Dichroic prisms are widely used in color imaging systems. They exploit dichroic filter coatings deposited on specific cemented prism surfaces to achieve "wavelength selection." For example, in a typical three-chip CCD/CMOS camera beam-splitter prism assembly, the first coated element reflects blue light and transmits red and green light, while the second reflects red light and transmits green light. After passing through such a prism assembly, white light is split into red, green, and blue monochromatic beams, each captured by a corresponding sensor and ultimately combined into a high-definition color image.

Polarizing beam-splitting coatings take advantage of the polarization properties of light. By depositing a specific multilayer dielectric coating on the hypotenuse of a prism, light incident at Brewster's angle passes almost completely through as P-polarized light, while S-polarized light is strongly reflected, thereby achieving polarization-based beam splitting.

From simple anti-reflection to complex dichroic splitting, every advance in right-angle prism coating technology gives optical system designers greater freedom. It shows that what truly redirects light is not only the prism's geometry, but also the "magic coating" of the microscopic world that clings to it.

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