High-pressure sodium (HPS): HPS lamps deliver (single and double ended) primarily green-yellow region of light spectrum, 67% of total light energy, most of which are rejected by green plants. HPS deliver 120 to 140 lumen/watt. HPS grow lamps are nominally rated for 10,000 hours or less.
Metal halide (MH): MH lamps deliver primarily blue region of light spectrum and are used primarily for vegetative phases of plant growth. MH deliver 80 to 120 lumens/watt. MH grow lamps are nominally rated for 10,000 hours or less.
LED: LED growlight luminaires are designed to emit specific parts of the light spectrum that activate certain photoreceptors in the plant. For example, blue LED light is used to promote phototropism and cryptochromes which help germination and elongation of the plant, while red LED light is used to stimulates phytochromes which help the plant to flower. Although blue and red spectrum play dominant role for plant photosynthesis, however, some of the other colors in the light spectrum are also necessary for optimum plant growth. Formulating optimal LED mixing that best mimic the photosynthetic action for all types of plants remains a challenge.
LEDs are made of semiconductors. A critical issue with all semiconductor devices is junction temperature, which must not exceed its maximum rated value. Most of the electricity in an LED becomes heat, about 75% heat and 25% light. If this heat is not removed, junction temperature of an LED may reach and exceed its critical level causing decreased light output and physical defects. Thus, unlike other light sources, thermal management of LEDs is a must. Even with fan cooling, a 500W LED growlight requires a very large heatsink.
Although the delivered energy efficiency of LEDs continue to increase, however, the current efficiency of Blue as well as Red LEDs is less than 70 lumen/watt.
Two distinct advantages of LED grow lights are 1) they can provide a direct light without the need for a reflector and 2) they are mercury free.
T-5 Fluorescent: Like MH, T5 lamps also provide mostly blue region of light spectrum and they are primarily used for vegetative phases of plant growth. T5 delivered lumen/watt is 70 to 80. T5 grow lamps are nominally rated for 20,000 hours or less.
Induction: Induction lamps are inefficient, deliver 60-70 lumen/watt. Light spectrum consists of primarily blue region and therefore suitable for vegetative phases of plant growth.