Abstract: In an copying apparatus of the cartridge type having a photosensitive drum, a staticization charger, a destaticizer and any other peripheral processor units, a variable width slit formed above the drum and in the optical path between the light source and the photosensitive drum for regulating the amount of light reaching the drum. The variable width slot acts as a compensation devise to compensate for varying sensitivity of the photosensitive drum. The width of the slot is altered by changing the position of a sliding adjustor member so as to vary the amount of light reaching the drum to compensate for the variation of the sensitivity of the drum resulting from the various manufacturing parameters thereby enhancing the reliability of the manufactured apparatus.