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Abstract
The efficacy of high pressure sodium lamps is adversely affected by scattering and absorption of light in the arc tube material. A theoretical model is presented which explains these effects. The theory predicts that both total and forward-diffuse transmittance influence lumen output. However, a change in total transmittance is about three times as effective as a change in forward-diffuse transmittance. Experimental results on 400-W HPS lamps show close agreement to the theory.