Equipment for routine spectral transmission and reflection measurements

This paper presents a description of an assembly of apparatus which has been in use at the Bureau of Standards since August, 1924, for routine spectral transmission and reflection measurements. The Koenig-Martens spectrophotometer is used with different light sources, each source being designed especially to meet the required condition of illumination on the sample, in the measurement of the following quantities as functions of wave length: 1. The unidirectional normal transmission of an optically homogeneous (nondiffusing) material in the form of a plate with plane-parallel surfaces. 2. The normal brightness of a sample by reflected light, relative to the same for a standard material, when both sample and standard are under equal and completely diffused illuminations. The sources for transmission measurements are three separate white-lined inclosures, each just large enough to contain, respectively, two 400-watt gasfilled incandescent lamps, two helium lamps, and a quartz mercury-vapor lamp. These sources are conveniently and quickly interchangeable, so that anyone of them may be used separately for specific purposes. A uniform distribution of 156 small gas-filled incandescent lamps on a white diffusely reflecting hemispherical surface provides a source of illumination for reflection measurements which is shown to be equivalent to the desired ideal completely diffused condition. Accessory equipment includes ventilation apparatus, convenient devices for handling the transmission and reflection samples, standard sectored disks for use with the spectrophotometer, and apparatus for temperature control of liquid transmission samples. The theory of the Koenig-Martens spectrophotometer is given, along with a discussion of some possible sources of error in the spectrophotometric measurements. Tests of the accuracy in the measurement of the specified quantities are described.