Fluorescent Light Sources

Abstract

IN a paper before the Illuminating Engineering Society on April 14, Mr. J. N. Aldington pointed out that two types of fluorescent lamp have been developed within the last decade, both of which employ the mercury vapour discharge as the source of primary radiation. In the first type, envisaged so long ago as 1900 by P. Cooper Hewitt, a high-pressure mercury arc operates with a high luminous efficiency, and fluorescent coatings on the outer bulb containing the arc tube produce colour modulation of the emitted light. Ultra-violet and violet radiation of a wavelength above about 3,000 A. is absorbed by suitable inorganic sulphides and is re-emitted in the visible region to supplement the dominant mercury light. The change in overall efficiency brought about by this energy transformation is negligible, but the effect on the colour-rendering properties of the emitted light is appreciable and useful. The second, type of fluorescent light source is exemplified in the 80 watt MCF/U lamp, which was put on the market in March 1940, and is being used extensively for industrial lighting. A fluorescent powder layer coated on the inside surface of the tube containing a low-pressure mercury discharge tube is so vigorously excited by the resonance radiation from mercury that it produces about ten times the light given by an uncoated tube of similar size. The light is white and has colour-rendering properties sufficiently close to those of daylight for most industrial applications. The fluorescent layer is a mixture of inorganic powders which if used separately would give blue, yellow and red light and they are most efficiently activated by radiation of wave-length 2,537 A.