April 16th, 2018
Use of Light Pipes and Light Guides as TIR (Total Internal Reflection) Optics
In recent years there has been an exponential growth in the usage of light pipes and light guides in lighting systems for both external (forward and rear lighting) as well as internal vehicle illumination. Light pipes and light guides act as TIR (Total Internal Reflection) optics. With this type of optic, light is transmitted through an optically transmissive material in order to either transmit the light to a new location or distribute that light along a path. The advent of powerful single source LED light sources has allowed an entirely new generation of concept design and styling to occur by using light pipes and light guides.
When it comes to the particular science involved with TIR consider that light as it enters and exits a material is dependent on a couple of key factors. One is the transmissibility of the material, or in other words how efficient is that material at passing light. This factor relates to the degree of transparency and clarity of the material. For example, a good optical acrylic plastic would typically be at least 92% transmissive, where a perfectly opaque material would be 0% transmissive. The second key factor is the index of refraction of the material, which is unique to each material and whose value determines the extent to which light is refracted when entering or leaving a substance. A typical index value for clear acrylic is 1.49 whereas for clear polycarbonate 1.57 is common. Specialty high index materials are also available which can provide unique capabilities for certain optical lens situations. This index of refraction then becomes a significant factor in optical designs for TIR and directly relate to a concept known as the critical angle which is defined in physics by “Snell’s Law” relating the ratio of indexes of the two materials in contact to the angle of the light ray striking them. So the phenomenon known as TIR occurs when the light strikes the boundary between our optical material and its surroundings at an angle greater than its critical angle. The net result when this happens is the light gets reflected back inside the material rather than passing out or through the material. Rather than transmitting across the second boundary, light appears to reflect and transmit from the opposite face. TIR is often referred to as the reflection of total incident light at the boundary between two media.
Light pipes and guides are engineered components used to distribute or transport light. The purpose is to create custom illumination where the source of the light is in a different location than the output of the light. These are broken down into two categories, including hollow structures containing light with a reflective lining and transparent solids as described above, which contain and transmit light by TIR. The flow of light through these devices is governed by principles of non-imaging optics. Typically, pipes and guides have a round cross-section without a geometry limitation. Frequently, individual optic features are designed into the pipes to help propagate the light down the pipe or to project outward a uniform distribution of light reflected orthogonally to the length of the pipe.
Nowhere have we seen a faster implementation of light pipes and lighting supplier in recent years than in automotive lighting design. Designers are now using light pipe concepts in order to create a signature look to their lighting designs; with bold linear design elements, distinctive “eye-brow” designs and other new features now becoming common place on our roadways today standing as stark contrast to the simple round illuminations common in previous design.
Light guides are also very useful as internal components in instrument clusters, control panels and other button/interface aspects of a vehicle. The expanding use of LED technology will only grow this usage further in electronic devices of all kinds for directing illumination from a circuit board’s LEDs.