Abstract: A method of calibrating an optical density sensor comprising calculating a first pigment solid density value of an ink solution using a current first electrical output signal value from a photodetector, a current second electrical output signal value from a photodetector, and a current lens gap value, calculating a second pigment solid density value of the ink solution using a previously measured first electrical output signal value, a previously measured second electrical output signal value, and a previously measured lens gap value, comparing the current first electrical output signal value, the current second electrical output signal value, and the current lens gap value with the previously measured first electrical output signal value, the previously measure second electrical output signal value, and the previously measured lens gap value, and comparing the first pigment solid density value with the second pigment solid density value.