Dust sensors also contain an IR emitter (LED) and a detector and are able to measure particle concentration in the air. The emitter and detector are arranged diagonally, with a slit in the middle where air can pass through. If no particle is in the slit, the detector receives almost no light from the emitter. As soon as particles penetrate into the slit, the light from the emitter is diffused and received by the detector. With increasing particle concentration, the amount of light which is received by the detector also increases, which will then be forwarded as an equivalent output voltage. The dust sensor can only detect a relative change of the particle concentration referenced to a calibration condition.