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Integrating sphere for use with a spectroradiometer.

In many optical systems it is of great importance to know the spectral distribution of light at remote or practically inaccessible locations in the optical path of the system. A spectroradiometer used for measuring the spectral distribution at such locations is illustrated in Fig. 1. The purpose of the diffuser in Fig. 1 is twofold: (1) It ensures that a constant fraction of the light incident on the diffuser passes through the entrance slit of the monochromator and impinges on the monochromator grating. (2) The diffuse light at the entrance slit of the monochromator ensures that the entire surface of the monochromator grating is illuminated. In order to obtain absolute measurements of spectral distributions, it is necessary that both of the above conditions be met. The first condition is required because in general the geometry of the calibration source differs from that of the source to be measured. In our work, for example, the standard source used to calibrate the spectroradiometer is an unfocused tungsten source of known spectral irradiance, while the distribution to be measured is at the focal plane of a low ƒ/number optical system. The second condition is required so that errors due to any irregularities in the grating may be minimized. In commercially available spectroradiometers, the diffuser of Fig. 1 consists of a plastic button or a piece of opal glass, and accurate spectral measurements can be obtained as long as the intensity of the incident light is relatively high. At lower intensities (of the order of 10 nW/cm/nm) the dark current of the phototube becomes an important factor in the accuracy of the measurements. This lower limit on the intensity that can be measured is determined in large part by the fact that most of the light incident on the diffuser is scattered out of it and consequently does not pass down the fiber optics probe to the entrance slit of the monochromator. In attempting to increase the amount of light incident on the entrance slit of the monochromator, a small integrating sphere was designed for use with the spectroradiometer of Fig. 1. This approach was taken because the integrating sphere not only dif-