I'm wondering if anyone can speak in generalities about lighting efficiency. My recollection is that High Pressure Sodium is the highest efficiency, with the most photons in the visible spectra per watt. Where do flourescents, CFL, metal halide, etc. fit in?

Let's ignore spectra of the light. (I recognize that spectra could influence perceived efficiency. For example, a light source that peaks in the blue green region, would be positioned to take advantage of the non-linear photo responsivity of the human eye, with I seem to recall peaks at 530nm.)

A trivia item... the human eye is a tri stimulus photo receptive device. Most raptors and birds have quad stimulus photo responsivity, which probably contributes to their ability to select camoflouged lunch from the landscape.

Bb

Based on flashlights, I'd guess that leds have the most light output for power input.

The efficiency you seek is referred to as:
Efficacy
A measurement of how effective the light source is in converting electrical energy to lumens of visible light. Expressed in lumens-per-watt (LPW) this measure gives more weight to the yellow region of the spectrum and less weight to the blue and red region where the eye is not as sensitive.

However, is not in any way related to color temperature, or CCT, Color Corelated Temprature, measured in Degrees Kelvin.For, example, 2700DK is what an incandescent lamp emits in color temeprature, and 4100DK is the Cool White fluorescent, and 5000-6000DK is Daylight. This value is published as CRI, or Color Rendering Index,as a percent compared to 2700DK as the benchmark.

The commercially available standard lamps in order of Efficacy, best to worst, are:

• LED, thou I have not seen any specs, will produce the most lumens per watt consumed, and are fairly new to market.
• Low Pressure Sodium, poor CRI, produces mostly orange light. Not used much any longer in USA due to the CRI, but the highest LPW.
• High Pressure Sodium, very popular, yellowish light, excellent LPW, reasonable CRI for areas where a higher CRI is desired, such as street lighting.
• Metal Halide, excellant CRI, various colors close to pure white light, good LPW close second to HPS.
• Mercury Vapor, reasonable CRI, mostly white light, LPW next behind MH. Is the grandfather of HID lighting, was very popular for parking lots and street lighting until MH and HPS came on the market.
• Flourescent, next in line for LPW, good CRI, with various colors available to match interior decor.
• Incandescent, whcih includes halogen and quartz have the worst LPW, but warm 2700 D K temperature that is benchmark of the Color Rendering Index,typical for homes, restaurants.

Thou LPW, getting the most light for the wattage consumed is important, there are other factors that are to be considered during the design process of lighting systems, such as rated hours of life of lamp, ambient temperature, the CRI compared to the area being illuminated.

This may be a bit more information than you were looking for, but wanted to illustrate that efficiency is not a stand alone value to consider.

Hope this helps.

You're absolutely right... efficacy is the lighting term. And I was able to find a few data points. I've generalized them here:

Metal Halide, the subject source I asked about, I found a reference stating 24%.
CFL, 15 to 20%, depending upon phosphors, etc.
HPS, 35
LPS, 40 to 55%

Somehow when I think of efficacy, I think of the Human Visual System response, and the usability of the light. Efficacy is not the accepted term for that. Nor is CRI, which is a color recognition capability index. For that matter, the nominal black body temperature of the light isn't a good metric of the power spectrum of the light. A recent measure of daylight with a spectralradiometer (yes, I have those at work) showed a nominal 4700K reading at noontime. The HVS has a photoresponsivity which accounts for the responsiveness of rids and cones, at light levels where they are active. Also the human eye responds with different sensitivity to different spectra, at different light levels.

The best data I have for LEDs say 15 to 20%, although about 9 months ago I saw a reference to some which were a few percentage points higher... at 23%. They were being marketed for aircraft interiors, among other applications.