A sodium-vapor lamp is a gas-discharge lamp that uses sodium in an excited state to produce light. There are two varieties of such lamps: low pressure and high pressure.
Low-pressure sodium lamps are highly efficient electrical light
sources, but their yellow light restricts applications to outdoor
lighting such as street lamps. High-pressure sodium lamps have a broader spectrum of light than the low-pressure lamps, but still poorer color rendering than other types of lamps. Low-pressure sodium lamps only give monochromatic yellow light and so inhibit color vision at night.
A mercury-vapor lamp is a gas discharge lamp that uses an electric lamp through vaporized mercury to produce light.Mercury vapor lamps are more energy efficient than incandescent and most fluorescent lights with luminous effects of 35 to 65 lumen/watt. Their other advantages are a long bulb lifetime in the range of 24,000 hours and a high intensity, clear white light output
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