22 EN SVĚTLO 2013/3 luminaires and light apparatuses Nowadays, light guides are common optical parts used in exterior or interior automotive lightings. They are used in ap-plications where a light from light source is needed to be distributed in space in a way to highlight important design parts like contours around chambers of each light functions, edges and lamp contours. Light guides offer many possibilities to create attractive design of lamp and they also have important place in vehicle’s in-terior lighting system.Light guide is optical device used to guide a light coupled from light source to point or set of points with minimal en-ergy loss. Guiding of light in light guides is based on total reflection of light. Light guides are usually made from optical transparent materials like a clear plastic. Light coupled to the light guide from light source; usually LED is guided along optical axis by light guide using total re-flection from light guide’s cylinder wall. In case of breaking condition of total re-flection (there is an uncoupling optical element e.g. a streak or teeth in the light guide) light is uncoupled from light guide to the viewer. Placing the teeth system caus-es uniform light-up of whole light guide.Light guides used in automotive ex-terior lighting represent signal lighting function, in the most cases front posi-Concept of light guides for automotive application tion light and rear tail light. With more powerful light sources light guide start to represent signal function with higher requirement on intensity of emitted light like Day Time Running light in Head-lamp and stop or turn light in real lamp.From styling point of view light guides have different geometrical shape, for ex-ample follow the shape of chamber of another light function in lamp or follow complete headlamp or rear lamp or can fill part of the lamp.Principal of light spreading in light guides Coupled light from light source is guided in light guide by Total Internal Reflection from wall of the light guide. Total reflection occurs on interface of two environments with different refrac-tive indexes. If ray of light is spread at the angle (Φi) that is bigger than critical angle (Φc) from optical thicker environ-ment, it means from environment with higher refractive index to optical thin-ner environment (lower refractive index environment), it leads to total reflection. This means that 100% efficiency is reflect-ed back to optical thicker environment. This situation is shown in Fig. 3.Value of critical angle (Φc) is evaluat-ed by equation arcsin ()1 2 c n n (1) (1)Where (n1) is value of refractive index of optical thicker environment and (n2) is refractive index of optical thinner en-vironment. Refractive indexes of most materials used in light guides for automotive ap-plications have value about 1.5 and light guide is surrounded by air with refractive index value 1. In this case, light guide that lands at angle (Φi) bigger than 42° is returned back into origin environment. Uniform decouple light from light guide As mentioned before, ray of light in the light guide is guided by total reflection from the walls of light guide. System of teeth situated along the light guide is changing direction of ray so the total reflection condition is bro-ken. This is used to decouple the light from the light guide. Ray is falling on the decouple teeth of Fig. 1. Varroc rear lamp with light guide for rear tail Fig. 2. Example of styling solution to highlight contour of lamp chamber using light guide Φn2 Φt Φi n Φr n1 Fig. 3. Reflection and refraction of ray in light guide en-vironment, total reflection for Φi > Φc, all energy of ray is reflected back to origin environment Mgr. Jan Martoch, Varroc Lighting Systems, s. r. o.ray refracted to another environment incoming ray of light ray reflected back to environment