What is TIR?
TIR stands for total internal reflectance, a property of otherwise transparent materials like glass in which light entering the material at an angle nearly parallel to the material's edge will be contained within the material because rays hit the edge at angles beyond the critical angle of the material. In a fiber optic, light is introduced through optics that collimate the light within a range of "acceptance" angles that behave according to TIR. In a light pipe, light enters air inside of a tube or pipe but still employs the critical angle effect of the pipe material. In either case, light bounces along the fiber or pipe until it hits an end optic or diffusing surface to let light out.
Edge lighting is a technique that takes advantage of the natural TIR qualities of sheet glass and acrylic to channel lighting through the sheet from the edges. In edge lighting, the intent is for light to move through the material until it reaches an etched, carved, or sandblasted portion, and then light is finally visible.
To make edge lighting work, consider the following:
- The light source does not have to outline the perimeter, but for even effects it is desirable. A gradation will occur with the center of the panel being darker than the edges.
- The light source needs to be directional to push light through the material.
- The material absorbs and colors the light, and if ordinary glass is used, it will become increasingly green toward the center of the panel.
- Near the edges of the panel, light often leaks out and the source is usually visible. Consider hiding the actual edges to minimize the effect.
TABLE ONE: COLOR FILTERS FOR ARCHITECTURAL LIGHTING APPLICATIONS
TABLE TWO: THE LIGHT BOX: ALTERNATIVE LIGHT PANEL MATERIALS