Optical dirty cell sensor for an electronic air cleaner

In a electrostatic air filter having a plurality of substantially flat plates including first and second outer plates to be electrically charged and between which air having electrically charged particles may be passed, to thereby cause said particles to deposit themselves on said plates, an improvement for determining when a predetermined amount of said particles have been deposited on said plates, comprising

• a) on each of said plates, a test area having an aperture permitting light to pass through said plate from a first side of the plate to a second side of the plate, the aperture in each plate's test area in alignment with every other plate's aperture, wherein a preselected one of the test areas has a gauge aperture substantially smaller than the aperture in each of the other test areas;
• b) a light source mounted adjacent to the outer side of the first outer plate and aligned with the test area to direct at least a portion of light from the light source toward the test area of the first outer plate, wherein the light source is mounted to direct light through the aperture in every test area;
• c) a light sensor having a sensor surface and responsive to light falling on said sensor surface, providing a sensing signal whose magnitude is a function of the intensity of the light falling on the sensor surface, said light sensor mounted adjacent to the outer side of the second outer plate with its sensor surface in alignment with the test area of the second outer plate to receive on its sensor surface, light from the light source;
• and
• d) a level detector receiving the sensing signal and providing a status signal having a first level responsive to the level of the sensing signal exceeding a predetermined level, and a second level otherwise.