Abstract: An optical scanning device, such as a chromatoscanner, includes a spectroscope having a rectangular outlet slit for producing a light beam in the shape thereof. A rotatable disk having a spiral slit is disposed in the path of the light beam to produce light pulses along radial lines of the disk which are directed to the surface of a specimen. The absorbancy of the light pulses at discrete positions of the specimen is measured and stored in a memory in accordance with the corresponding location on the specimen as derived from measuring the rotation angle of the disk.