Guy just blew my mind a bit on a Reduser thread about the upcoming 10kw Tungsten equivalent LED fresnel that Mole Richardson have coming out.
I'd always worked under the assumption that an HMI fixture is going to give you roughly 4x the light output per watt of an equivalent tungsten fixture. i.e. 1.2kw HMI = 5kw of tungsten, 2.5kw of HMI = 10kw of tungsten etc.
Now if you compare an 1800w M18 to a 2000w Arrilite 2000+ (both of which use arrimax open-face reflectors), this does appear to ring true. At 30' full flood (63° beam compared to a 62° beam, so very close) you're looking at very close to 4x the output from the M18 compared to the 2000+.
However looking through the photometrics for fresnels (on the Arri calculator), the HMIs don't seem to fair much better than 2x the lumen efficiency of their tungsten counterparts.
Now I almost never get to work with bigger fresnels (excluding occasional 5ks in studios) as everything's open faced lights into frames most of the time, so I've never really encountered this discrepancy before.
So what I'm wondering is, why does HMI lose so much of its efficiency advantage compared to tungsten when going through a fresnel lens?