Effect of Sample Wedge on Transmittance Measurements in the 2- to 15-μ Range*

Abstract

Difficulties experienced in obtaining reproducible infrared transmittance measurements of various optical materials stimulated this investigation of the effect of specimen wedge on spectrophotometric measurements. Disks, 25 mm in diameter by 6.3 mm thick, of fused quartz, arsenic trisulfide, silicon, and germanium were finished with wedges of 0, 0.001, 0.002, 0.005, and 0.010 rad across the diameters. The transmittances of the disks were measured in the 2- to 15-μ range with four different orientations of the wedge relative to the slits of Perkin–Elmer Models 21 and 221 infrared spectrophotometers. Similar measurements were made at 2μ with a Cary Model 14 spectrophotometer. The wedge angle produces a deflected beam within the monochromator, which results in an optical misalignment and an erroneous measurement. Wedge angle and refractive index have been correlated with indicated transmittance to show the parallelism requirement of specimens for accurate transmittance measurements. Quartz, which has a comparatively low refractive index, does not exhibit detectable errors with a 0.010-rad wedge, whereas measurements of a 0.001-rad wedge of germanium, which has a high index, are subject to appreciable errors.