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LED fixtures are being marketed as a replacement for high pressure sodium fixtures in greenhouse lighting. Here we compare the cost per photon for LED and HPS fixtures based on their ability to convert electrical energy into photons delivered to a horizontal surface below the fixture. Some LED fixtures now exceed the efficiency of the best HPS fixtures by 23%, but the initial capital cost per photon delivered is 5 to 10 times greater. HPS fixtures with electronic ballasts and optimized luminaires (reflectors) are 27% more efficient than widely-used HPS fixtures with magnetic ballasts. Our analysis, however, demonstrates that light distribution and radiation capture are more important than the electrical efficiency of the fixture. No single fixture is optimal for all applications. The lowest cost per photon is realized when an efficient fixture is coupled with effective radiation capture, but the value of uniform plants may outweigh the cost of wasted photons. Just as precision irrigation can improve water use efficiency, precision lighting can improve electrical use efficiency.