Abstract: A toner sensor measures the amount of toner attached onto a photosensitive drum upon development by a developing unit. A controller compares the measured toner attaching amount with a predetermined reference value, and executes processing for changing at least one of image forming conditions, for example, the charging amount by a charger, the developing bias voltage of the developing unit, the exposure amount of an optical system, and the toner density of the developing unit, on the basis of the comparison result.

Abstract: A toner sensor measures the amount of toner attached onto a photosensitive drum upon development by a developing unit. A controller compares the measured toner attaching amount with a predetermined reference value, and executes processing for changing at least one of image forming conditions, for example, the charging amount by a charger, the developing bias voltage of the developing unit, the exposure amount of an optical system, and the toner density of the developing unit, on the basis of the comparison result. Besides the above, a shielding member is provided and is opened only during a measurement mode of the toner sensor in order to prevent the toner sensor from being contaminated by toner during a non-measurement mode.

Abstract: An image forming apparatus having a humidity detector which is detachably attached to the main apparatus body that accommodates an image forming member. The humidity detector detects a humidity in the vicinity of the image forming member, converts the detected humidity into a relative humidity and outputs it. Based on the value of the output relative humidity, image forming conditions, such as the amount of exposure, are set. The humidity detecting unit contains a time detecting unit for detecting the time for replacement.

Abstract: In an optical unit, a light beam is emitted from a light source toward a given object. Part of the light beam is reflected by a reflecting mirror and is returned to a monitor diode which is incorporated in the light source. The reflecting mirror is arranged in an optical path along which the light beam is directed from the light source to the object, but is located outside of an image region containing an optical axis and expanding in the main scanning direction. By use of the light beam returned to the monitor diode, the deflection start position from which the light beam emitted from the light source is deflected is detected. In other words, the deflection start timing is detected. A variation in the monitor current produced by the monitor diode is utilized for controlling the light source, to thereby match the horizontal synchronization.