Image forming apparatus

An image forming apparatus comprising:

• an image carrying element;
• a writing device to form, on said image carrying element, latent images representative of first and second images;
• a developing device to develop said first and second latent image to produce corresponding first and second visible images, respectively;
• a transfer device to transfer said first visible image from said image carrying element to a recording medium;
• a sensor mounted on an optical path unobstructed by elements that are subject to wear to detect and output an amount of reflected light (1) from said second visible image, (2) a surface of the image carrying element and (3) said recording medium;
• a memory storing reference values;
• and
• a controller to control an image forming operation based on the reference values stored in the memory and the time-varying signal received from the sensor, wherein the controller reads the time-varying signal from the sensor and compares the time-varying signal read to the reference values to determine a condition of the image forming device.

A computer program product comprising:

• a computer readable medium and a computer program code mechanism embedded in the computer storage medium for causing a processor control an image forming device subsystem, the computer program code mechanism comprising:
• a first computer code device configured to read a first charge value from a sensor as the sensor senses a photoconductive element having a background condition at a first time;
• a second computer code device configured to drive a transfer sheet at a second time;
• a third computer code device configured to read a second charge value from the sensor as the sensor senses a toner pattern on the photoconductive element at a third time;
• and
• a fourth computer code device configured to detect a paper jam based on relative timings of the first, second and third times.