An image monochromator is arranged to receive light at an input thereof simultaneously from an entire object field to be viewed and to produce and transmit a viewable image of the entire object field simultaneously through an output thereof at a selected wavelength band. The monochromator has inner and outer optical systems, the inner system comprising an input slit which is projected on an output slit through imaging means such as lenses or mirrors, via dispersive means such as a diffraction grating or prism operable to transmit light at a tunable narrow wavelength band. The outer optical system comprises an objective system, in which a picture of the object field is imaged on a picture plane by imaging means such as lenses or mirrors, with the input and output imaging means so arranged that the inner optical system receives and transmits to the output optical component light rays from each object point which have an angular relationship to each other within a range from slightly divergent to slightly convergent.