> I have seen some of these lights (for illuminating driveways etc)
> and am wondering "why not?"
Dennerle sell fittings that use the low voltage dichroic bulbs so they
can't be all that bad. I've used a domestic low voltage track fitted
with 50 watt dichroics with success on a planted tank but it DID heat
up the tank noticeably and it DID need quite a lot of watts per gallon
(about 7 watt per (Imperial) gallon). If I ever try them again I'll
buy the special bulbs with a very narrow beam angle and mount them on
the ceiling above the tank on the theory that the heat radiation will
disperse more than the light and more of it will miss the tank. I'm
not too worried about energy costs - the watts I burn above my
fishtank pale into insignificance compared to the kilowatts I burn in
Jack Hardie in Winter England where any light is better than the grey
stuff that comes through the windows.