Penta Prism: How It Works, Specs & Applications
A penta prism is a five-sided prism that deviates a beam by exactly 90°, without inverting or reversing the image. Unlike a right-angle prism, a penta prism keeps that 90° deviation constant even if the prism is slightly misaligned — which is exactly why it shows up in alignment and viewfinder optics.
Key Specifications at a Glance
| Parameter | Typical Value |
|---|---|
| Substrate | N-BK7, UV fused silica |
| Aperture | 5 mm–50 mm |
| Deviation angle | 90° ± 3 arcmin (standard), tighter on request |
| Surface flatness | λ/4–λ/10 at 633 nm |
| Surface quality | 60-40 (standard), 20-10 (precision) |
| Coating | Reflective coating required on both internal reflecting faces (aluminum, silver, or dielectric); AR coating on entrance/exit faces |
Why 90° Stays Constant
The two internal reflecting surfaces of a penta prism are set at an angle that makes the total deviation independent of small rotations of the prism around its axis. That property — a fixed, reliable 90° no matter how the prism sits — is the main reason to choose a penta prism over a simpler right-angle prism when precise deviation matters more than cost.
Why It Needs a Coating
The internal angles inside a penta prism are shallow enough that light striking the reflecting faces falls below the critical angle, so total internal reflection doesn't happen on its own. Both reflecting surfaces need a metallic or dielectric mirror coating to work.
Applications
- Camera and rangefinder viewfinders
- Alignment and surveying instruments
- Machine vision systems needing a fixed 90° reference
Buying Tips
Because the coating quality directly sets the prism's throughput, check the reflectivity spec for your wavelength before ordering — a coating optimized for visible light will underperform in the near-IR, and vice versa.
