What is Total Internal Reflection?

TIR is an abbreviation that is used for two different meanings. You have probably come across the abbreviation TIR on the back of a truck. TIR stands for Transports Internationaux Routiers. TIR is also used as an abbreviation in the lighting world. TIR stands for Total Internal Reflection. TRALERT® tells you everything you need to know about Total Internal Reflection.


To understand what total internal reflection is, it's first important to understand what reflection is. Reflection is quite simple to explain. It is the change in the direction of light when it strikes a particular object. In the case of light, this object must have a shiny polished surface. These surfaces are also called reflective media. This can, of course, also be a reflector designed for reflection. Within a reflection, four terms are important:

  1. Incidental beam
  2. Point of incident
  3. Normal radius
  4. Reflected beam

The incident beam is the beam that goes towards the surface. The point of incident is the place where the incident ray hits the surface. The normal ray is a perpendicular line on the surface at the point where the incident ray hits the surface. The reflected ray is the ray that leaves the surface again.

Then there are two different light reflections. The mirrored reflection and the diffuse reflection. Reflective reflection occurs when a light ray reflects in parallel. This occurs on smooth surfaces. Diffuse reflection occurs when the surface reflects the incident ray in many different directions. This occurs with uneven surfaces.

Total Internal Reflection

But when exactly does one speak of a total internal relflection? There are a number of issues related to this. First of all, total internal reflection only occurs when the light beam passes from a medium with a high refractive index to a medium with a low refractive index. In addition, the angle of incidence of the light beam must always be greater than the critical angle. This critical angle is derived from Snellius' Law. When these two issues apply, one speaks of a total internal reflection.