How lenses work
The lens distributes the light out of the luminaire and creates the desired light distribution.
Controlling the light
Lenses are based on a beam of light passing through the boundary between two media with different densities, such as from air to plastic or glass. The lens is often made of clear PMMA (acrylic) with a high level of transparency. This contributes to high energy efficiency because very little light is absorbed by the material. The lens may have convex and/or concave shapes. A prismatic or etched plastic disc also works in the same way as a lens.
In the lens, there are essentially two optical phenomena that are used to control the light: refraction and total internal reflection.
Refraction
Refraction occurs when the light passes between two media with different refractive indices. If the light travels from a denser medium (e.g. clear acrylic) to a less dense medium (e.g. air), it will be bent at an angle greater than the angle of incidence, and vice versa. Parallel surfaces give rise to a parallel displacement of the light rays.
Total internal reflection
If the light is coming from the optically denser medium and the angle of incidence is sufficiently large, all of the light is reflected back into the optically denser medium.
A ray passing from an optically denser medium is partly reflected and partly transmitted at a larger angle from the normal. When the angle of incidence becomes sufficiently large, all the light is reflected back into the material.
Total internal reflection occurs when:
- The light ray travels from an optically denser medium to a less optically dense medium.
- The angle of incidence exceeds the critical angle for total internal reflection. The critical angle is defined by Snell’s law.