Since these lamps are point sources, it is more difficult to obtain uniform radiation distribution. Metal halide has good distribution in the 400 - 700 nm band widths but spectral shifts can occur with age. Lamps of 400 watts have an average life of 20,000 hours, whereas 1000 watt lamps have an average life of 12,000 hours. High-pressure sodium have low emission in the blue spectrum, however plants seem to do well at very high intensities since enough blue is then provided. In addition, the irradiance level is poor.
Diodes have very narrow wavebands.
The ones commonly used are 620 - 680 nm, which has maximum photon effectiveness for photosynthesis. These diodes in combination with blue diodes can provide adequate light for some plants. Arrays have been built and testing continues. Heat does not leave thru the casing but rather the back, thus heat can be better kept from the plant growth area. Currently LEDs are less efficient than HID lamps but are seemingly greater than fluorescent.