Following on from discussions elsewhere in the forums, I have complied figures for the various efficiencies in terms of Lumens/W for different lamp types. The higher the Lm/W, the more efficient the light source. The data is based on web info for Philips lamps.
SOXBased in info from
Lamptech35W SOX+: 130
35W SOX PSG: 126.3
26W SOX E: 141
36W SOX E: 163
55/90W SOX: 120
66W SOX E: 164
135W SOX: 135
180W SOX: 150
PL-L36W: 80.5
55W: 87
CDO-TTMetal halide which can be used as a direct replacement for SON on same gear, all come out at approx 90 Lm/W for ratings 70-250W.
Cosmopolis 60-140W approx 115 Lm/W, 45W is 95.
CDM-EliteThese are the metal lamps being used by the Highways Agency for retrofit Iridiums to replace SON lighting on trunk roads.
210W: 110
315W: 114
CDM-MasterThe more "traditional" CDM lamp.
35W: 94
70W: 94
150W: 86
SON Master PIA 70W: 94
100W: 107
150W: 116
250W: 133
400W: 141
600W: 150
BGP322/3 LED lanternsThe data on Philips web page is somewhat contradictory for Lumen figures and circuit wattage, possibly since the lantern is new and some of the figures may be provisional and subject to revision. However for the 4200 Lm version the efficiency for the entire lantern comes out at 59.1 Lm/W.
For the lanterns used on the A5 its 84 Lm/w. Even then the circuit power of 199W doesn't tie up with the Philips contact value - he stated 192W.
So there you have it. With the exception of the 400/600W SON, SOX is still the most efficient discharge light source.
CDM-Elite/Cosmopolis are comparable in efficiency to the medium SON lamps, CDO-TT and CDM are the less efficient types of MH lamp.
This of course doesn't give the full story. SOX gear is more lossy than SON. 35W SOX circuit loss is very similar to 50W SON. 135W SOX circuit loss is similar to 150W SON.
PL lanterns have very efficient gear and by running the lamp at high frequency (more light at high frequency operation) the lamp can be under run so that total circuit loss is often only a few watts more than lamp rating.
The power saving comes from white light in theory allowing a lower level of lumens to be used due to perceived brightness. However in many cases "lumens are king" - white light lumens match orange/yellow lumen figures, which results in over lighting.