1. Field of the Invention
The present invention relates to an image forming apparatus such as a copy machine or a laser beam printer, for example.
2. Description of the Related Art
Image formation conditions (for example, a gamma correction table, laser power, fixing temperature, electric charge of toner) used in an image forming apparatus to form a high-quality image need to be changed as appropriate depending on an installation environment (for example, temperature and humidity) of the image forming apparatus, or operating time thereof.
Japanese Patent Laid-Open No. 2002-072574 proposes a technique for performing color adjustment by forming a measurement image such as a patch or a pattern image on a transfer member, detecting the amount of applied toner of the measurement image using a regular reflection sensor, and feeding back the detected amount applied toner to a lookup table. Thus color stability is maintained without bothering a user.
Meanwhile, Japanese Patent Laid-Open No. 2003-215981 proposes a technique for controlling the amount of used recording material at an optimum value by operating outputs of an irregular reflection sensor and a regular reflection sensor.
Meanwhile, the abovementioned conventional techniques still have the following problem, and there is a room for improvement.
Generally, a measurement image detecting portion detects light reflected by the measurement image, and outputs a signal corresponding to a received reflected light amount (reflection output) to a density conversion circuit. Because the reflection output is a kind of luminance signal, the density conversion circuit converts the reflection output into a density signal. Usually, a relationship exists where as an attached toner amount becomes larger and an image density becomes higher, a reflection output becomes smaller. The density conversion circuit converts the reflection output having such a characteristic into an image density at the time of being formed on a recording material.
Incidentally, the image formation conditions are adjusted in every use of an image forming apparatus, and therefore change from time to time. For example, a target density and a target potential are maintained at appropriate values by density adjustment, potential control, or the like. However, it has been found that when the image formation conditions change, a correspondence between the reflection output and the image density held by the density conversion circuit becomes different from the actual relationship. If the correspondence between the reflection output and the image density becomes different from its initial state, accurate density control or potential control cannot be performed. If the conversion between the reflection output and the image density cannot be performed, a high-quality image will not be able to be formed.