Optical Prism Buying Guide: How to Choose the Right Prism
Optical prisms redirect, invert, rotate, or split light using refraction and reflection rather than a curved surface. Because the same basic block of glass can do very different jobs depending on its geometry, picking the right prism starts with knowing exactly what you need the beam or image to do.
Key Specifications at a Glance
| Parameter | Common Options |
|---|---|
| Common substrates | N-BK7, UV fused silica |
| Angle tolerance | ±3 arcmin (standard), ±30 arcsec (precision) |
| Surface flatness | λ/4 (standard), λ/10 (precision) at 633 nm |
| Surface quality | 60-40 (standard), 20-10 (precision) |
| Coating | Uncoated (TIR), metallic mirror coating, dielectric coating, AR-coated faces |
Match the Prism to the Job
- Deflect a beam 90° without inverting the image — penta prism
- Deflect or retroreflect a beam with no coating needed — right-angle prism, using total internal reflection
- Rotate or invert an image — dove prism
- Send a beam straight back to its source at any incidence angle — corner cube (retroreflector) prism
TIR vs. Coated Reflection
Several prism designs can reflect light two ways: through total internal reflection (TIR), which needs no coating as long as the angle of incidence exceeds the critical angle, or through a metallic or dielectric coating on the reflecting surface, which works at any angle but adds cost and absorbs a small amount of light. A right-angle prism used for a straightforward 90° deflection at the correct angle can often skip the coating entirely; a penta prism cannot, because its internal angles fall below the critical angle for TIR.
Choosing a Substrate
N-BK7 covers most visible-light applications at the lowest cost. For UV transmission, a higher laser damage threshold, or better thermal stability, move to UV fused silica.
FAQ
Do I need an AR coating on a prism?
If light enters and exits through flat faces — not the TIR or mirror-coated surface — yes, an AR coating on those faces reduces reflection loss the same way it does on a lens.
How much does prism angle tolerance matter?
It depends on the application. Beam alignment and interferometry need tight angle tolerances, in the arcsecond range; general beam-folding in an illumination path can usually tolerate the standard grade.
