An investigation was undertaken of phosphors capable of converting primary ultraviolet radiation (Hg line, 2537 angstroms) to radiation in the erythemal or sun tan ultraviolet region (2900–3200 angstroms). Several types of alkaline earth silicates and phosphates activated with lead or thallium were found to be practical. Among these was a calcium phosphate activated with thallium and containing about 1.3 moles of calcium per mole of phosphate. This phosphor not only showed a good initial erythemal emission, but when used in an experimental fluorescent lamp maintained a high percentage of this emission for the several hundred hours tested. The lamp was constructed of special glass and coated with the above phosphor, but otherwise was similar to commercial fluorescent lamps. In addition, the investigation showed some similar types of phosphors to be efficient in the neighboring “black-light” ultraviolet region (approximately 3400–3800 angstroms), and to give more emission in that region than the ceriumactivated phosphors commonly used at present. Some discussion of sun tan lamp and fixture design is also included.
RICHARD H. CLAPP and ROBERT J. GINTHER, "Ultraviolet Phosphors and Fluorescent Sun Tan Lamps," J. Opt. Soc. Am. 37, 355-355 (1947)