#Y2021 #定义  Example 1: This is a particular problem for Led based illumination systems as the source dies often have an inherently high etendue. a common construction is to build a large array of Led emitters where each has a very wide, almost Lambertian, hemispherical light output spread. we now have the worst case: a large source and a large angle, which combine to give you just about as high an etendue as you can get! What you’ll typically see immediately after the source is a large, TiR based reflector or lens that collimates that broad beam down to a more usable beam angle. now you know about etendue, you can immediately understand why the output end of those TiR reflectors has to be so large. it has to be large enough that etendue can be conserved, as a narrower angle beam means you must have a larger emitting area. if the output side wasn’t this large then the lens would be very inefficient and waste a lot of light. unfortunately, this large lens size dictated by etendue conservation puts a limit on how closely together you can cram those Led emitters.  LED array在下面，小area但是非常大的angle。narrow down output angle最后造成aperture变大。