Both high pressure sodium and metal halide lamps are considered HID (high intensity discharge). These bulbs, which are mostly used in outdoor settings, offer different amounts of light output and operate in different color spectrums. Both types come with unique advantages and disadvantages.
Light intensity is measured in lumens. Since both types of lamp are HID, they are designed to produce generous amounts of light, or lumens. A typical 250 watt high pressure sodium bulb will produce around 87 lumens per watt; 21,750 lumens total. A metal halide bulb will produce around 58 lumens per watt; 14,500 lumens total.
Mercury is a dangerous substance found in all high intensity discharge light bulbs. Mercury should be kept out of landfills both because it is dangerous and because it is easily recyclable. A standard 250 watt metal halide lamp contains around 38 mg of mercury, a 250 watt high pressure sodium lamp contains around 15 mg.
A typical high pressure sodium lamp will last longer than a metal halide lamp of similar wattage. A 250 watt high pressure sodium lamp can last from 12,000 to 24,000 hours. A 250 watt metal halide lamp will last from 10,000 to 15,000 hours.
Metal halide lamps produce light in the white and blue spectrums, while high pressure sodium lamps produce light in the yellow, orange and red spectrums. High pressure sodium lamps last longer than metal halide; they also produce more lumens per watt. Because of their mercury content, neither type of lamp is particularly good for the environment. Metal halide lamps produce more of a glare compared to high pressure sodium lamps.
Certain types of light produced by HID bulbs can affect your health. Blue light is said to affect hormone levels and slow melatonin production. Exposure to metal halide can contribute to muscular degeneration.
Both types of HID lamps work well for encouraging plant growth, due to their high lumen outputs. Metal halide lamps promote vegetative growth in plants because they emit a bluish white light ideal for vegetative growth. High pressure sodium lamps help promote flowering in plants. This type of lamp emits an orange-red glow that is ideal for flowering.