English
A penta prism is a frequently used prism, characterized by having one right angle. When a light beam enters through the right-angle surface, it undergoes two internal reflections before exiting from the other right-angle surface. The outgoing light beam is deflected at a 90-degree angle relative to the incoming light beam. Below are the optical path diagram and the actual physical image of the pentagonal prism.
The penta prism deflects light by 90º while maintaining the original image orientation without inversion or reversal. Slight movement of the penta prism has minimal impact on the reflected light, making it ideal for defining right angles in optical systems. Its internal reflection does not rely on total internal reflection—two reflective surfaces are coated with reflective film, two transmitting surfaces have anti-reflective coatings, while the fifth face remains optically unused.
| Material | BK7 glass / UV Fused silica |
| Dimension | 3mm~200mm |
| Dimension tolerance | ±0.1mm~±0.05mm |
| Surface quality | 60/40, 40/20, 20/10 |
| Surface flatness | λ/20 – λ/4 |
| Angle tolerance | <30arc sec. ~5arc min |
| Bevel | <0.25mm×45° |
| Coating | As per customized request |
The light entering the pentagonal prism undergoes two reflections within the prism, causing a directional change of 90°. This process not only prevents image inversion but also does not alter the image's handedness. Pentagonal prisms are commonly used in single-lens reflex (SLR) camera viewfinders as reflective devices and can also serve as angle-fixed beam deflectors.
Compared to a single-reflection right-angle prism, a pentagonal prism can precisely deflect the input beam by 90° without being affected by the prism's mounting errors. This means that the direction of the incoming light does not need to be considered, making it more stable than a system composed of two mirrors. Therefore, pentagonal prisms are often used in various image observation instruments and calibration devices, such as plumb lines, collimators, and distance meters.
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