Abstract: A single beam wavelength scanning spectrophotometer comprises means for passing radiation from a radiation source (1) to a radiation detector (4) over a defined path which includes a monochromator (2) capable of being scanned across a selected wavelength range, means (7) for storing a first set of signals representative of the radiation falling on the radiation detector, the first set of signals being spread across the selected wavelength range to form a first spectral response, means for inserting a sample (3) in the defined path, means for producing a second set of signals representative of the radiation falling on the radiation detector when the sample (3) is inserted in the defined path, the second set of signals being spread across a selected wavelength range to form a second spectral response, and means (7) for calculating the transmittance or absorbance of the sample at desired wavelengths from the first and second spectral responses.

Abstract: Configurations of optical systems using two concave mirrors to image an off-axis slit are used advantageously in monochromators or spectrographs in which the coma produced by one mirror is cancelled by that produced by the other mirror. However, such systems produce a curved line image of a straight line slit. This curvature is substantially removed in a two concave mirror optical system in which the off-axis angle and the separation 2L between the mirrors are chosen so that the principal ray OP.sub.1 from the center of the line object to the first mirror and the principal ray P.sub.2 I from the second mirror to the center of the line image are both substantially perpendicular to an axis of symmetry of the optical system which passes through the centers of curvature C.sub.1, C.sub.2 of the two concave mirrors. The image line is then curved in a plane normal to the image plane and appears substantially straight in the image plane.