Study on the Wall Blackening of a High Intensity Discharge Lamp

A ceramic metal halide HID lamp was operated with short duration turn-on/turn-off procedure to enhance the effect due to electrode sputtering during lamp ignition. Time dependent change of optical emission spectrum showed that the intensity of W I line spectra were prominent only during the time of lamp ignition. The tested lamp showed more homogeneous blackening compared with a lamp operated with a longer turn-on period. Meanwhile, the lamp operated with a longer turn-on period had shown a blackening thicker toward the direction of gravity. Distributions of light intensities and those of optical emission spectra were measured by producing images of the test discharge tubes through a pinhole. Spatial distributions of the line spectrum intensity of Dy I at 421.18 nm and those of light absorption by Al at 394.40 nm indicated that the densities of these atoms increased toward the direction of gravity.