HP Color LaserJet 8500 Service Manual - Page 162

Color calibration density sensor The density sensor inside the right upper door senses the density detection pattern for each color formed on the transfer drum during a calibration cycle. The sensor emits light from the LED, and the density detection pattern on the transfer drum reflects the light. The sensor then receives the light with the photo diode and that information is converted to digital values and stored on the controller board. The controller board uses this information to control appropriate developing bias and primary charging bias values to stabilize density of each color. (process marks) Figure 40. Note Density sensor Calibration occurs at the following intervals:
After the power is turned on
After 50 pages in one-page mode are printed (after initial power- on)
After 95 pages in one-page mode are printed
After the printer is revived from Power Save (if Power Save has been on for more that 30 minutes)
When a toner cartridge is installed
When an imaging drum cartridge is installed
When environment is changed by the environment change control The density control will only be executed if all of the printer doors are closed and all of the printer components are detected. 160 Chapter 5 - Theory of operation EN