Abstract

The absolute spectral intensity distribution at the center of the focused image of a carbon arc was determined in the region of 3000–9000 A. The basic optical system (used in studies of thermal radiation damage) consisted of a high intensity carbon arc at the focus of a 36-in. paraboloidal searchlight mirror, and a similar condensing mirror which focused the reflected beam in a cone of vertex angle 120°. The intensity at the focus of the condensing mirror was approximately 100 cal/cm2/sec. The high intensity and convergence of the beam at the focus led to the adoption of a new technique for the spectral analysis. A magnesium-oxide-smoked plate was inserted into the beam at the focus, and the light reflected normally from the plate was focused upon the slit of a spectograph. Absolute intensities were obtained by comparison with a standard pyrometric arc. The results are compared with absolute black-body measurements of the total intensity at the focus. The observed differences in spectrum between arc and image are discussed.

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