Published in the PLOS (Public Library of Science), a comparison of HID, Florescent and LED light sources for plant growth. From the abstract: "Here we report the photosynthetic (400–700 nm) photon efficiency and photon distribution pattern of two double-ended HPS fixtures, five mogul-base HPS fixtures, ten LED fixtures, three ceramic metal halide fixtures, and two fluorescent fixtures. The two most efficient LED and the two most efficient double-ended HPS fixtures had nearly identical efficiencies at 1.66 to 1.70 micromoles per joule. These four fixtures represent a dramatic improvement over the 1.02 micromoles per joule efficiency of the mogul-base HPS fixtures that are in common use. The best ceramic metal halide and fluorescent fixtures had efficiencies of 1.46 and 0.95 micromoles per joule, respectively. We also calculated the initial capital cost of fixtures per photon delivered and determined that LED fixtures cost five to ten times more than HPS fixtures. The five-year electric plus fixture cost per mole of photons is thus 2.3 times higher for LED fixtures, due to high capital costs. Compared to electric costs, our analysis indicates that the long-term maintenance costs are small for both technologies. If widely spaced benches are a necessary part of a production system, the unique ability of LED fixtures to efficiently focus photons on specific areas can be used to improve the photon capture by plant canopies. Our analysis demonstrates, however, that the cost per photon delivered is higher in these systems, regardless of fixture category. The lowest lighting system costs are realized when an efficient fixture is coupled with effective canopy photon capture." So, while they don't outperform double ended 1000 watt HPS, LED's do seem to hold up to their promise of being more efficient producers of PAR than traditional HPS (though still not living up to claims they are 200% or more efficient than HPS), and still hold a bit of an edge over CMH in terms of efficiency. However, their higher price tag makes them a less economical choice than double ended in the long run. As LED prices drop you can expect them to become more cost effective in this regard, but not until the day you see a true 1000 watt LED with all the bells and whistles (5 watt diodes, secondary lenses, most light in 630-660 nm) selling for about the same as a 1000 watt double ended system will it become a contender for replacing the double ended 1000 watt HPS as the reigning king of commercial greenhouse lighting. I can add from experience that LED's are useful in tight spaces because they have a narrower profile (less height needed to hang) and they can run closer than a HPS (1 ft from canopy for LED vs 3 ft for double ended 1000w HPS) so if height is an issue in your grow space you may want to take that into account when choosing lighting. Tried to paste the study but it exceeds the 50,000 character limit for posts. Here's a link to the original online: Economic Analysis of Greenhouse Lighting: Light Emitting Diodes vs. High Intensity Discharge Fixtures