The intensity of light output is measured in lumens. Both types of lamps are HID and are designed to put out generous amounts of lumens per watt. A common high-pressure sodium, 250 watt bulb will put out around 87 lumens per watt of power used or 21,750 lumens total. A typical 250 watt mercury vapor lamp will put out around 48 lumens per watt or 12,000 lumens total.
Mercury, which is found in all HID lamps, is a dangerous material to handle. Mercury should never make its way into landfills. Used high pressure sodium and mercury vapor lamps should be recycled. A typical 250 watt high-pressure sodium lamp can contain around 15 mg of mercury.
High intensity discharge lamps are designed to last. Mercury vapor and high-pressure sodium lamps will both usually last anywhere from 12,000 to 24,000 hours, depending on the wattage.
High-pressure sodium lamps produce light from the yellow, red and orange spectrums. Mercury vapor lamps produce light from the blue and green spectrums. Neither lamp is necessarily good for the environment because of their mercury content. High pressure sodium lamps put out more lumens than mercury vapor lamps while lasting about the same amount of hours before they need to be replaced. Light produced by high intensity discharge lamps can affect health in some instances. Light in the blue spectrum can slow melatonin production and affect hormone levels.
Most HID lamps encourage growth in plants because of their high output of lumens. High-pressure sodium lamps are ideal for promoting flowering because of their red and yellow light output. Mercury vapor lamps are ideal for promoting vegetative growth because of their blue and green light output.