We can feel infrared energy as heat but we cannot see it directly, so for lighting a room it is wasted energy. The incandescent panel feels warmest, the mercury-vapor panel a bit less so, and the fluorescent and sodium panels feel cool. The low-pressure sodium lamp emits four times more visible light and uses only half the energy of the incandescent lamp, which is why it feels cooler. The other two lamps also produce more light than heat. We judge the efficiency of a lamp by the amount of visible light it produces from the energy it consumes.
Different lamps emit different combinations of color.
The photograph can only reflect the colors produced by each lamp. Low-pressure sodium does not produce blue light, so the sky in the photo appears gray. The 1962 photo to the right demonstrates the same effect. All three models are identically dressed and are in identical surroundings, but they appear different because each is illuminated by a different type of lamp. Because of color characteristics, a lamp may be ideal for one use and not for another.