Photon-counting-based dust monitor

Abstract

Most of the dust monitors are based on optical principle: the scattered light intensity is registered. The classical approach is using the multiple photon optical signal intensity and processing. Single photon detection -- photon counting is exploited in Satellite Laser Ranging and was implemented in space born application for Mars Surveyor Program 98, as well. The main advantage of single photon detection is an extreme sensitivity, the entire digital approach, no analog signal is treated. All the light intensity information is acquired on the basis of statistical data treatment. The dust detector consists of the LED diode based transmitter, single photon solid state diode detector and the digital data processing unit. The light beam from the LED diode passing the dust column is detected in a photo detector. The detector employs the avalanche Silicon photodiode 40 micrometers diameter active area and is operated in a passive gating and active quenching mode above its breakdown voltage. The detector provides uniform digital pulses, one for each photon detected. The light intensity measurement is converted into the photon flux counting -- frequency or event counting. The microcomputer controlled data processing electronics counts the detector output pulses, accounts for detector dark count rate, calibration constants, and computes the corresponding dust concentration averaged over desired period. The second LED located close to the detector is used as a reference light source to eliminate the temperature, aging and sensor contamination influence. The laboratory measurements show the long term and temperature stability of the scale within 1%. The setup was tested at the cement plant smoke stack and compared to the commercial optical dust concentration monitor operating on analog multiphoton principle. Due to an extreme optical sensitivity of the photon counting detector, the energy balance is favorable to realize lightweight equipment by a factor of ten in comparison to analogue-based device. The photon counting principle along with the self-calibration setup does not require re-calibration. The optical apertures involved are of the order of millimeters what reduces the problems of contamination of the device by the dust monitored. As no collimating optics is used, the no optical alignment is required.