Er, this Spark lamp thingy...
I don't reckon that torch-derived headlamps are necessarily the way ahead (at the moment).
A lot of the development by the Chinese in headlamps seems to be designed to give monstrous throw with quite a tight beam - I suspect the fact that a lot of torches seem to be aimed at the hunting market is behind this*.
Thus, a headlamp which effectively uses a torch head / reflector will tend to be optimised for distance rather than spread - which in my opinion is not what you're looking for, despite some vendors trying to tell you that a 6" spot at 20 yards is what you need.
That said, an orange peel reflector will help things somewhat - as the comparison beam-shot on CPF demonstrates.
I guess it's horses for courses - but at the moment my thinking is that if you want a headlamp for underground use, you're best bet is to get something that's intended for underground use.
That said, there are a lot of Chinese Miners Lamps on the market which promise the earth but are neither well-made, nor are they anywhere near as bright as they should be given recent developments in LED technology...
* for example, the Fenix TK15 is equipped with springs at both ends of the battery compartment, solely to prevent the light from being switched off by recoil when it's rifle-mounted! 😮
Hello again darkness, my old friend...