| 摘要 |
PURPOSE:To detect the life of a photosensitive body having a protective layer on a photosensitive body by comparing the value of the quantity of the reflected light by a light receiving element with a reference value and detecting the change in the thickness of the protective layer of the photosensitive element. CONSTITUTION:A light projecting element 4 which projects light on the surface of the photosensitive body 1 and the light receiving element 5 which receives the reflected light of the element 4 from the surface of the body 1 are provided. Since the value of the quantity of the reflected light by the element 5 changes with the thickness of the protective layer of the body 1, the life of the body 1 is known by comparing the value of the quantity of the reflected light with the reference value. The life of the body 1 having the protective layer is thereby known. The state of the protective layer which gradually loses its thickness until the layer disappears is thus precisely recognized and the time for exchanging the body 1 is displayed. |
