Based on our results, the best light bulb for promoting photosynthesis in plants is an LED plant bulb. This bulb produces a strong output in both the blue and red wavelengths, with very little additional light in other regions to cause heat build-up. All of the other light sources have very little output in the blue range.
Halogen and incandescent bulbs have extremely broad output ranges from green to deep into the red portion of the spectrum, but with little to nothing in the blue range.
The least suitable lamp for photosynthesis is a standard CFL bulb. While they emit some light in both the blue and red ranges (with several peaks in between), the intensity of this type of bulb was the weakest when compared to all the other lamps.
LED plant lights are available from a variety of online merchants and home and garden stores. They have become very affordable, and work well for experiments that investigate photosynthesis.