Abstract: A densitometer which measures reflection optical density and transmission optical density has a lens barrel comprised of a fixed portion and a movable portion. The movable portion is slidably engaged with the fixed portion and movable up and down. When the density is measured, the lower end of the movable portion is put into contact with the surface of a sample to be measured. The fixed portion and the movable portion of the lens barrel is comprised of an inner cylinder and an outer cylinder extending coaxially with each other. A flux of light for illuminating the surface of the sample is guided between the inner and outer cylinders and condensed on the surface of the sample. The light reflected by the sample is guided through the inner cylinder.

Abstract: A density check plate comprising a support having thereon a plurality of light absorbing filters, each of which has a light transmission limit wavelength at a different spectral wavelength and which completely and sharply absorbs light of a wavelength shorter than the wavelength of the light transmission limit wavelength.