A pentagonal prism is a prism that accurately reflects 90° of incident light. The reflective surface is coated with aluminum to improve reflectivity. Pentagonal prisms can reflect different incident light at the same angle. This feature eliminates the need for precise positioning of the prism in practical applications. Therefore, the performance is better than a cemented part composed of two lenses. This prism is used in the viewfinder of a camera, and the pentagonal prism is suitable for image observation systems or measuring instruments.