This instrument is used to measure the concentration of pencil lead. That is to measure the reflected optical density of the scratch mark of the lead on the designated sample paper (use the symbol “d” to represent the reflected optical density: d=Lg1/R450, where R450 is the diffuse reflection brightness factor of the sample for blue light with a wavelength of 450nm, that is, under certain lighting. Under certain conditions, in a given direction, the ratio of the sample’s reflected light brightness to the ideal diffuse reflected light brightness under the same conditions)