Alright, let's see what ideas everyone comes up with on this question. I'm no optics guru, so I'm hoping you have better ideas than me. Here's the situation: I have a column of hot gas wherein every gas molecule emits radiation over 4π sr. At one end of the column I have a window where emitted energy can pass through to be collected. The end goal is to collect as much energy as possible from this hot gas column and focus it into a fiber optic. I have a fiber coupler that accepts collimated light and focuses it into the fiber. So, the question becomes what is the best way to collimate as much energy as possible from this diffuse source so it will go into my fiber? The coupler has some acceptance angle θ (it accepts a column + some small divergence.