In an image forming apparatus, a developing unit for developing a latent image on a photosensitive drum is detachably mounted in the apparatus and is provided with a identification signal generator for each particular developing unit. When the developing unit is replaced by a new one, an identification...http://www.google.ca/patents/US5012286?utm_source=gb-gplus-sharePatent US5012286 - Image forming apparatus having device for detecting concentration of developing agent
