A filter cartridge (10) is provided with filter papers (14) which are formed into layers within a case (12). A light source (20) is provided at a position within the filter papers to radiate light toward one side of the case, the light being blocked at the...

An air sensor (8) with an electronic evaluation circuit (28) determines the degree to which an automotive filter is blocked. The sensor has a memory component (32) holding and adding data from the sensor by an integrator (34). The integrator output signal ...