In a sensor, a light emitter outputs light with which a toner pattern on an image carrier or a surface material of the image carrier is irradiated, and a photodetector receives reflection light from the toner pattern or the surface material. The sensor light intensity control unit provides a control voltage to the light emitter and controls light intensity thereof. The density determining unit determines a toner density on the basis of output of the photodetector. Further, the density determining unit determines a reference control voltage of the light emitter to set as a predetermined value the output of the photodetector corresponding to the reflection light from the surface material, determines a correction parameter corresponding to the reference control voltage, determines a correction amount corresponding to the correction parameter and the toner density, and corrects the toner density on the basis of the correction amount.

A drive transmission device, which is included in an image forming apparatus and detachably attached to an apparatus body configured to include a driven body to which a driving force is transmitted, includes a first opposing body and a second opposing body disposed facing each other and forming a gap therebetween, a gear rotatably disposed in the gap formed between the first opposing body and the second opposing body, and a rotary shaft configured to receive the driving force from a drive source and rotate in the gap. One rotational axial end of the rotary shaft being passed through the second opposing body and rotatably supported by the second opposing body in the gap.

An electrophotographic photoreceptor includes a conductive substrate and a single-layer-type photosensitive layer on the conductive substrate. The single-layer-type photosensitive layer contains a binder resin, a charge generating material, an electron transporting material, and a hole transporting material. The single-layer-type photosensitive layer has a concentration ratio (A/B) of 0.7 or more and 1.0 or less, where the concentration ratio (A/B) is a ratio of a concentration A of the electron transporting material relative to the binder resin measured from a surface of the photosensitive layer remote from the conductive substrate to a concentration B of the electron transporting material relative to the binder resin measured from a surface of the photosensitive layer close to the conductive substrate.

An electrophotographic photoreceptor includes an intermediate layer, a photosensitive layer, and a surface layer laminated on a conductive support in this order, wherein a cured product of a polymerizable monomer contains an electron transport agent and n-type metal oxide fine particles to which a reactive organic group is bonded in the surface layer.

An electrophotographic photoreceptor includes an intermediate layer, a photosensitive layer, and a surface layer laminated on a conductive support in this order, wherein a cured product of a polymerizable monomer contains an electron transport agent and n-type metal oxide fine particles to which a reactive organic group is bonded in the surface layer.

An image formation apparatus includes: a replaceable part attached to an image formation device and including an information storage unit having unique identification information stored therein; an error detector which detects an occurrence of an anomaly; an error code storage unit having a first error code and a second error code stored therein, each code indicating a content of error display concerning the anomaly; and a controller which determines whether the replaceable part is of a first type or a second type based on the identification information. The controller outputs a first notification signal indicating a content corresponding to the anomaly of the first error code if the replaceable part is of the first type when the error detector detects the anomaly, and outputs a second notification signal indicating a content corresponding to the anomaly of the second error code if the part is of the second type.

An electrophotographic photoreceptor includes a conductive substrate including an outer peripheral surface treated with a silazane; and a photosensitive layer on the outer peripheral surface of the conductive substrate. The photosensitive layer includes a charge generating material and a charge transporting material.

An image forming apparatus includes an apparatus main body, a conductive member, a contact spring, a hollow cylinder, and a drum unit attachable to and detachable from the apparatus main body. The conductive member is disposed in the drum unit. The contact spring is disposed in the apparatus main body and comes in electrically conductive contact with the conductive member. The hollow cylinder is disposed in the apparatus main body. The contact spring has a contact part and a helical spring part. The contact part is provided on the helical spring part. The contact part is retractable in and out of the hollow cylinder. The helical spring part is contained in the hollow cylinder and urges the contact part in a direction for causing the contact part to protrude from the hollow cylinder. The contact part has a longitudinal shape elongated along an axis of the helical spring part.

An image forming apparatus configured to form images on a first surface and a second surface of a sheet includes an apparatus body, an image forming unit, a feed tray, and a return unit configured to return the sheet having an image formed on the first surface of the sheet toward the image forming unit. The return unit includes a first guide, a second guide, and an adjuster. The first guide is detachably attached to the second guide and includes a first roller and a positioned portion. The second guide is positioned in the apparatus body and includes a second roller facing the first roller, and a positioning portion configured to contact the positioned portion of the first guide, thereby positioning the first guide relative to the second guide. The adjuster is configured to adjust an orientation of the second guide relative to the apparatus body.

An electrophotographic photoreceptor includes: a conductive substrate; a charge generation layer that is disposed on the conductive substrate and contains a resin, a phthalocyanine pigment, and a compound represented by the following Formula (BP); and a charge transport layer that is disposed on the charge generation layer and contains a charge transport material,

##STR00001##

wherein R.sup.B1, R.sup.B2 and R.sup.B3 each independently represent a hydrogen atom, a halogen atom, a hydroxyl group, an alkyl group having from 1 to 10 carbon atoms, an alkoxy group having from 1 to 10 carbon atoms, or an aryl group having from 6 to 10 carbon atoms.