Image forming apparatus and image forming method for forming color images by gray-level image forming technique

An image forming apparatus which comprises:

• a photosensitive member comprising a photosensitive layer of Tp thickness on a surface thereof;
• a charging unit configured to uniformly charge the surface of the photosensitive member;
• an image exposure unit configured to form an electrostatic latent image by exposing the photosensitive member with a light beam;
• a gray-level image device configured to transmit a drive signal to the image exposure unit, and to produce a gray-level image in accordance with a gray-level image signal based on image data including gray-level information;
• a developing unit configured to develop an electrostatic latent image formed on the photosensitive member;
• and
• a transfer unit configured to transfer a developed image from the photosensitive member to a recording medium;
• wherein a relationship between the thickness Tp of the photosensitive layer on the photosensitive member and an exposure diameter Db of the light beam is defined as
• 2Tp<Db,
• where the exposure diameter Db is a minimum diameter at 1/e2 with respect to a peak value in an exposure distribution determined by integrating an energy distribution of the light beam on the photosensitive member with an exposure time, and
• the image exposure unit is configured to set a maximum exposure within the exposure diameter Db of the light beam at a value that makes a differential sensitivity of the photosensitive layer of the photosensitive member a third of a maximum value of the differential sensitivity or smaller.

An image forming apparatus, comprising:

• a photosensitive member comprising a photosensitive layer on a surface thereof;
• an image exposure unit configured to form an electrostatic latent image by exposing the photosensitive member with a light beam having a light energy more than twice as large than a light energy that can cause a surface potential of the uniformly charged photosensitive member to dampen to a half of a potential thereof;
• a gray-level image device configured to transmit a drive signal to the image exposure unit, and to produce a gray-level image in accordance with a gray-level image signal based on image data including gray-level information;
• a developing unit configured to develop an electrostatic latent image formed on the photosensitive member by a positive development process;
• and
• a transferring unit configured to transfer a developed image from the photosensitive member to a recording medium.

An image forming method for forming a gray-level image by an electrophotographic process, comprising the steps of:

• defining a relationship between a thickness Tp of a photosensitive layer on a photosensitive member and an exposure diameter Db of a light beam as
• 2Tp<Db,
• where the exposure diameter Db is a minimum diameter at 1/e2 with respect to a peak value in an exposure distribution determined by integrating an energy distribution of the light beam on the photosensitive member with an exposure time;
• and
• setting a maximum exposure within the exposure diameter Db of the light beam at a value such that a differential sensitivity of the photosensitive layer of the photosensitive member is sufficiently small in order that a gray-level image will be produced as a result of damping characteristics of the photosensitive member.

An image forming method for forming a gray-level image by an electrophotographic process, comprising the steps of:

• defining a relationship between a thickness Tp of a photosensitive layer on a photosensitive layer on a photosensitive member and an exposure diameter Db of a light beam as
• 2Tp<Db,
• where the exposure diameter Db is a minimum diameter at 1/e2 with respect to a peak value in an exposure distribution determined by integrating an energy distribution of the light beam on the photosensitive member with an exposure time;
• and
• setting a maximum exposure within the exposure diameter Db at a value that makes a differential sensitivity of the photosensitive layer a third of a maximum value of the differential sensitivity or smaller.