Light Sources

Author: E.E. Kimberly

The general popularity of the common incandescent gas-filled lamp is due chiefly to its pleasing light and relatively low cost. Lamps, such as the mercury-vapor, sodium-vapor, and neon-tube lamps, in which a luminous vapor or gas is used as the light source, give a light of marked characteristic color which is undesirable for most purposes. The mercury-vapor lamp gives off no red rays, and red objects appear black under its illumination. The arc lamp in various forms is used for production of ultra-violet rays, photography, and projection purposes.

In an attempt to produce light approximating daylight in quality, some lighting units are provided with both incandescent lamps and mercury-vapor lamps. This is called mixed lighting. Mixed-lighting units cost much more than incandescent units with comparable lumen capacity, and cannot be economically justified for most purposes, except where the color of the light is highly important. Next to the incandescent lamp, the fluorescent lamp is the most popular for general illumination.