Device for measuring toner concentration in developer comprising toner and carrier

A toner concentration measuring device used in a developing system that houses developer comprising toner and carrier, said toner concentration measuring device comprising:

• a transparent detection window which faces the interior of the developing system;
• a light-emitting unit which illuminates the developer inside the developing system through the transparent detection window;
• a light-receiving unit which receives the light emitted from the light-emitting unit and reflected off the developer through the transparent detection window and which outputs a signal corresponding to the amount of the light thus received;
• and
• a computing unit which computes the toner concentration of the developer based on the signal output by the light-receiving unit,
• wherein said transparent detection window comprises a first transparent plate member located on the side of the light-emitting unit and light-receiving unit and a second transparent plate member located on the side of the developer and movably attached to the first transparent plate member, and wherein said first transparent plate member is made of a material which becomes charged, through its contact with the second transparent plate member, with the opposite polarity from the charge polarity of the toner, and said second transparent plate member is made of a material which becomes charged, through its contact with the carrier, with the same polarity as the charge polarity of the toner.