A pentaprism is a specialized five-sided optical prism designed to deviate a beam of light by exactly 90° while consistently maintaining image orientation (erect and correct left-to-right), regardless of the prism’s alignment. This unique capability makes it irreplaceable in applications where precise, non-reversed viewing is critical.

Unlike a right-angle prism, which relies on a single Total Internal Reflection (TIR) and inverts the image, the pentaprism achieves its function through two internal reflections. This double reflection mechanism is what rotates the image back to its correct orientation.

• Constant 90° Deviation: The two reflecting surfaces are typically coated with a highly reflective material (like aluminum or silver) to ensure near-100% reflectivity. The specific 45° angles between the prism’s surfaces are engineered so that the exit beam is always precisely 90° to the incident beam, even if the prism rotates slightly. This makes it very tolerant to alignment errors.

• Erect and Correct Image: The two reflections effectively flip the image twice, resulting in a final image that is identical to the original—it is not inverted or mirror-reversed.

• Roof Pentaprism (Dachkantenprism): A very common variant incorporates a “roof” section (two surfaces at a 90° angle) on one of the reflecting faces. This roof section adds a third reflection, which corrects the image in the second axis. This is the standard prism found in Single-Lens Reflex (SLR) cameras, allowing the photographer to see exactly what the lens sees.

• Materials: Manufactured from high-quality optical glass (e.g., N-BK7) for excellent transmission across the visible spectrum.

• Coatings: The two reflective surfaces are almost always coated with a protective metallic coating and blocking layers to ensure high reflectivity and durability, as TIR is not used for these surfaces.

• Precision: The 90° angle between the entry and exit beams must be held to extremely tight tolerances for applications like surveying. Surface flatness and roof edge accuracy are also critical to avoid degrading image quality.

• Single-Lens Reflex (SLR) Cameras: This is its most famous application. The pentaprism (or roof pentaprism) is the core component of an SLR viewfinder, directing light from the lens to the viewfinder eyepiece and presenting the photographer with a bright, accurate, and right-reading image.

• Surveying and Metrology: Used in the optical systems of theodolites, levels, and rangefinders where a precise 90° deviation of the line of sight is required.

• Precision Optics: Employed in various optical systems, alignment tools, and instruments where a stable, correctly oriented image is necessary.