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Abstract

In order to reduce the size of a spectrophotometer having a rotatable diffraction grating and to compensate for longitudinal chromatic aberration and to provide for triangular bandwidth (where the image of the entrance aperture through which the light to be analyzed enters the spectrophotometer is equal in width to the width of exit slit through which the spectral components of each band are passed to a photodetector) thereby enabling high, accurate resolution of spectral line locations and edges of colors to be obtained, the diffraction grating is pivoted about an axis laterally displaced from the grating in the tangential plane, which plane contains substantially all of the light paths in the spectrophotometer. The exit aperture rotates with the grating and has a displacement from the grating in a direction transverse to the direction of displacement of the pivot, which displacement may be in the sagittal plane of the spectrophotometer. The displacement of the exit aperture may be in the direction of the lines of the grating and the width of the exit aperture intercepts light from the entrance slit which is directed from the grating to a mirror having power such that the power of the grating and mirror is less than one. Then the image of the entrance aperture at the exit aperture is minimally displaced in the tangential plane thereby compensating for longitudinal chromatic aberration and the projection of the entrance aperture on the exit aperture stays approximately constant over each of the spectral bands, thereby providing the spectrophotometer with a triangular bandwidth characteristic. The triangular bandwidth characteristic may be maintained without sacrificing illumination intensity by utilizing a refractive element such as a prism which is rotatable with the exit aperture and the grating. The element is in the light path to the exit aperture.

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