In physics, particularly E&M, I had associated "etendue" with this concept of counting rays given area and solid angle. However, with some light research, it appears that "throughput" is preferred in the specific field of radiometry. After doing some light research, it appears there is no difference in the two terms or concepts, and that they can be used interchangeably. Is there, in actuality, a subtle difference between these two terms?
(I always liked the word "etendue" after first learning it! Also, nice double meaning of "light research".)
I believe as well that it is the same thing, with the one detail that etendue has the squared index of refraction of the medium in there as well. In turn, this means that it corresponds to "basic throughput", which is throughput with a squared i.o.r. factor.