Kulmasäde

Kulmasäde, in Estonian, literally means "angle ray" and is used to refer to a light or other radiation ray that forms an angle with a reference direction, such as a surface normal or an optical axis. The term is formed from kulm- (angle) and säde (ray). It is a general descriptor rather than a specific, widely standardized concept, and its exact meaning depends on the context.

In optics, a kulmasäde describes any ray that is not perpendicular to a reference plane. The angle is measured between the ray and the chosen reference direction, typically the normal to a surface or the optical axis of an instrument. The angle of incidence, angle of refraction, and angle of reflection are all examples of angular descriptions of rays. Angles are commonly expressed in degrees or radians.

Applications of the concept appear in ray tracing, optical design, and demonstrations of fundamental laws such as Snell's law and the law of reflection. By tracking kulmasäded through lenses, prisms, or waveguides, engineers and students analyze how light propagates, bends, or splits. The term also appears in educational materials to illustrate how changing the angle affects imaging, illumination, and sensor responses.

Related concepts include incidence angle, refraction angle, and the optical axis. While kulmasäde is primarily used in Estonian-language texts, the underlying idea—describing rays by their angle relative to a reference direction—exists across optics and physics.