Toner density sensor, and ink jet head, developing unit and image forming apparatus in which toner density sensor is used

A toner density sensor, comprising:

• a light emitting element;
• a light receiving element for receiving light emitted from said light emitting element;
• a liquid developer tank for accommodating a liquid developer therein;
• a diluting liquid tank for accommodating a diluting liquid therein;
• a mixing chamber for mixing the liquid developer and the diluting liquid at a certain fixed ratio to create a diluted liquid developer;
• a passage for forcedly circulating the diluted liquid developer;
• a transparent cell element disposed in said passage between said light emitting element and said light receiving element for allowing the light emitted from said light emitting element to be transmitted through the diluted liquid developer in said passage so that an amount of the light transmitted through the diluted liquid developer may be measured by means of said light emitting element and said light receiving element;
• and
• means for detecting a density of the liquid developer based on the amount of the light transmitted through the diluted liquid developer in said passage which is represented by an output of said light receiving element.