On Background Correction in Spectrum Line Photometry

Abstract

The process of subtracting background exposure from line exposure, as a means of correcting for the effect of background, has been investigated. Of two lines of equal exposure, one in a clear ground (and presumably requiring no correction) and the other in a measurable background, which had been subtracted, the energy of the former is greater. This difference is constant and independent of both line and background densities. As the addition of photographic exposures is not open to question, the probable cause of this difference is the less than inertial exposure the emulsion received, here called the subliminal exposure, either as scattered light in the spectrograph or during manufacture. Its effect on the emulsion characteristic is to reduce the contrast. This subliminal exposure is ordinarily not subtracted. Although the resulting error from these two causes is small for dense lines, for light lines it can amount to 50%. In view of these findings, changes in the standard photometric procedure are suggested.