Abstract: A double monochromator having a principal stage and a pre-monochromator or first stage that includes a coarse reflecting plane diffraction grating and a concave mirror for converging the object beam on the grating and for imaging the selected band of diffracted light from the grating on the entrance slit of the principal or second stage. The first stage spectral bandwidth is much larger than the second stage, thus permitting use of a grating having a groove spacing approximately ten times that of the second stage grating, and the use of linear wavelength adjusting means. The imaging mirror results in a shorter image and object distance and therefore a smaller convenient instrument.

Abstract: In a spectrophotometer having a cuvette, a photoelectric detector having a photosensitive surface, a light source arranged for passing a light beam through the cuvette onto the photosensitive surface, an imaging optical system arranged between the cuvette and the photosensitive surface or a rear image thereof being located in a focal plane of the optical system outside the image plane of the light source, whereby the light beam impinges on the photosensitive surface as a relatively large light spot of substantially uniform illumination and, as a result, the location and size of the light spot is not adversely affected by parallel offset, and the demands as to accuracy and alignment of the cuvette are reduced.