| abstract |
(57) [Summary] [Purpose] An object of the present invention is to provide a method and apparatus for measuring chromate film thickness of chromate-treated steel sheet suitable for on-line production and having high measurement accuracy. [Structure] A surface of a chromate-treated steel sheet is irradiated from a projector 3 of a light source including a light in a range of ultraviolet rays to visible light, passes through an integrator 2, and diffuse reflection light thereof is interfered with by interference filters 5a and 6a. Depending on 350-390nm and 450-700nm Photoelectric conversion element 5 by selecting one wavelength in each range b, 6b, and the light reflected by the semitransparent mirror 4 provided in the light projecting path is selected by the interference filters 8a, 9a to select one wavelength from each of the same wavelength range, and the photoelectric conversion element 8b, 9b, the respective outputs are digitally processed to measure the film thickness of the chromate film on the chromate-treated steel sheet. |