Developing device having a developer transport roller with a magnetic field distribution that eliminates streaking and fogging

摘要

A developing device includes a movable sleeve confronting a photosensitive member at a first point. A magnet member provided within the sleeve produces a magnetic force to maintain developer on an external surface of the sleeve, with the developer being transported in accordance with movement of the sleeve. A regulation member confronts the external surface of the sleeve at a second point to regulate the thickness of the developer on the external surface of the sleeve. This second point is upstream of the first point with respect to the direction of developer transportation. A distribution curve of the radial magnetic force produced by the magnet member has a peak point in an area between the first point and the second point, and first and second inflection points in this area on the same side of the peak point with respect to the direction of developer distribution. The first inflection point is nearer to the peak point than the second inflection point.