Abstract: An optical assembly is provided which comprises a light source, preferably a laser, and a particularly compensated prism unit, the laser and the prism unit being mounted on a common holder or base. The beam emitted from the laser is reflected 90.degree. by the prism unit, and although this unit is only approximately aligned with the laser the reflected beam will always form a right angle to the incident beam with an extreme accuracy, the deviation from 90.degree. being less than half an angular minute. The accuracy in reflexion independently of prism unit setting is attributed to the compensated prism unit, which comprises a quadrangular prism composed of two semi-quadrangular prisms, a reflecting surface being mounted, preferably in an adjustable position, adjacent one side of the prism.