Lightpipe The lightpipe uses the principle of total internal reflection to guide light inside the length of a hollow pipe and allow some of the light to leave the pipe along the way. This technology is usually used for building illumination but in our case it makes a very nice ‘colour mixer’. Because the light bounces around in the pipe we can use the lightpipe to combine the output of the different LED and create a very uniform colour mix. Total Internal Reflection (TIR) occurs when light hits a boundary between two media with different indices of refraction at an angle past the critical angle. The critical angle Theta is given by the Snell’s Law as Theta = arcsin(n1/n2) Where n1 and n2 are the indices of refraction of the media at the boundary. For our lightpipe the media inside the pipe will be air (n2=1) and the other material will be polycarbonate (n1=1.58).     In principle the lightpipe consists of a tube of optical lighting film which is a 3M manufactured microstructured film with small prismatic ridges on one side. The light from the LED cluster will enter this tube and strike the microstructure on the wall at a range of angles that support total internal reflection along the direction of the tube. In this fashion the light continues to reflect inside the tube until it reaches the end. The end of the tube is covered with a mirror (usually a reflective film such a 3M Radiant Mirror Film or SilverLux but in our case simple aluminium foil will do). The light reverses at the mirror and proceeds to bounce towards the LED cluster again. In principle this would allow near perfect transport of the light but in order to extract some light from the lightpipe we need to insert a strip of white material (the extractor) into the tube. For a 20cm long tube of 5cm diameter it will be sufficient to make an extractor strip that is 20cm long and triangular in shape with a base with of 2cm and tapering to a point on the other end. The wider end has to go into the mirror side of the tube. The purpose of the extractor is to diffuse light hitting it into a range of angles such that it does not undergo TIR when it hits the opposite side of the lightpipe. This portion of the light escapes and we get a glowing tube that is uniformly illuminated along its length. The tapering of the extractor compensates for the loss of light due to extraction earlier along the tube.