Illumination engineering would be hard to imagine without light-emitting diodes, or LEDs for short. They boast low power consumption, longevity and a high light output. This is why they are suitable both for light design, e.g. in the automobile sector, for interior lighting and for stagecraft. In these contexts, coloured light that can be dynamically mixed from red, green and blue (RGB) is an attractive means of design. Basically, all that LED light mixing with light-conducting elements requires is an optical fibre and the coloured LEDs that are to be mixed. The light rays are coupled into the fibre and superimposed through multiple reflexion from the inner surfaces. In conventional technologies, this reflexion is supported by the optical fibre's special surface structure. Once the light ultimately emerges from the optical fibre, it has been blended. However, with existing solutions, the differently coloured light rays drift apart again at a longer distance - the mixed light becomes inhomogeneous. Scientists of KIT have solved this undesirable kaleidoscope effect with an improved optical fibre. The basic shape of this light transmission bar is an oblong with sides curved inwards that looks a bit like a star. The entire surface of the LEDs is covered by the optical fibre's cross-section and thus absorbs the light as surface radiation. The LEDs can be arranged in different ways. The light rays emitted spread in different directions in the light transmission bar and are reflected in an unpredictable manner - according to a geometric system called chaotic billiards. The clever combination of surface radiation and special geometry with convex side surfaces enables particularly fast blending and spatially constant, coloured light via LED control. The highly effective light transmission bars made of transparent plastic or glass can be manufactured using simple procedures such as die casting and are compatible with conventional LED systems.