Right-Angle Prism: Specs, Uses & How It Works
A right-angle prism is a triangular block with two legs at 90° to each other and a hypotenuse connecting them. It's one of the simplest and most widely used prisms, valued for its ability to fold or reverse a beam without the cost or fragility of a coated mirror.
Key Specifications at a Glance
| Parameter | Typical Value |
|---|---|
| Substrate | N-BK7, UV fused silica |
| Leg length | 5 mm–100 mm |
| Angle tolerance | ±3 arcmin (standard), ±30 arcsec (precision) |
| Surface flatness | λ/4–λ/10 at 633 nm |
| Surface quality | 60-40 (standard), 20-10 (precision) |
| Coating | Uncoated (TIR) / aluminum or silver mirror / AR-coated legs |
How It Works
Light entering one leg of the prism at normal incidence strikes the hypotenuse at 45°, well above the critical angle for common glasses, so it reflects internally without needing a coating — that's total internal reflection. The beam exits through the other leg, deflected by exactly 90°.
Send light in through the hypotenuse instead, and it reflects off both legs in sequence, coming back out through the hypotenuse deflected by 180° — a simple, robust retroreflector for normal-incidence beams.
When to Coat It
TIR only works within a specific angle range and can leak light at the edges of a beam or at non-ideal incidence angles. If your system can't guarantee light hits the hypotenuse within the TIR window, add a mirror coating to the hypotenuse instead — it costs more, but reflects reliably regardless of angle.
Applications
- Beam folding in laser and imaging systems
- 180° beam retroreflection
- Periscopes and beam-height adjustment
- Barcode scanners and rangefinders
Buying Tips
Right-angle prisms are one of the most stocked prism shapes, so standard leg lengths and angle tolerances cover most designs. Confirm whether your beam path uses the legs (90° deflection) or the hypotenuse (180° retroreflection) before ordering — it changes which faces need an AR coating.
