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Color of Discharge Lamps

Color of Discharge Lamps

Discharge lamps, in contrast to heat radiators like filaments of incandescent lamps, do not have a continuous spectrum but one which consists of a large number of lines. This is a quasi-continuous spectrum similar to that of the sun which has a color temperature of 60000K.

Only if the spectrum of the discharge lamp closely resembles that of an ideal radiator can its color appearance be described by color temperature. This is then referred to as correlated color temperature, which means that its color appearance is close to that of a light with a continuous spectrum of the same color temperature.

The color appearance of a discharge lamp can also be expressed in terms of the color location in the chromaticity diagram .

A non-continuous spectrum of a discharge lamp.

Colors are defined by coordinates. The color of a typical discharge lamp has x=0.33, y=0.33, which is close to the bluish white color of daylight.

Different discharge lamps emit different colored light. Some emit only a very limited part of the spectrum, such as metal halide lamps which are commonly used for street lighting. In architectural lighting, these colors are often exploited to achieve different hues when lighting the exterior of a building .
High quality film and television recording require a close match between the individual color components of the light. This is described by the Ra color rendering index which has maximum value of 100. The Color Rendering Index (Ra) describes how closely the color of a light source matches the color a continuous spectrum light source of the same color temperature. Film and television require an Ra which is greater than 85. The Ra of discharge lamps is typically over 90, which means they have a high color fidelity.