After a long period of development and improvement, there are more than 200 measurement methods based on different working principles. Light scattering has gradually attracted people’s attention and has been favored since 1980, and has been widely developed in recent years. Its production practice technology has also been substantially improved. With the continuous enhancement of integrated chip functions, a dust concentration sensor has been initially formed based on light scattering theory. The sensor is small, light, sensitive, and easy to be used. With the advent of the Internet and the era of intelligence, the application of light scattering principles combined with research on the laser and microelectronics technology to achieve lightweight and reliable sensors has always been the goal that the industry highly praises and pursues.
1. Optical detection of particles
(1) Direct imaging
The so-called direct imaging is a method that analyzes and calculates the concentration of particulate matter through image processing technology after collecting pictures of the detection area, and the detection range is about 1 to 1000pm. This method is greatly influenced by humans, and the wavelength of light and the magnification of the lens play the most important role in the resolution of the microscope. Therefore, the smaller the particle size is, the greater the error becomes, and the sampling rate is low. Therefore, the detection of distribution and concentration of the particle size is not easy. The high price of optical professional equipment limits the development of its application.
(2) The optical holography
Compared with ordinary photography, optical holography uses the reflected light of the object to form an inverted real image through an optical lens and records it on the photosensitive film while recording the complete wave field on the plane. The amplitude and phase of the light wave are also accurately recorded. Gabor encodes identifiable quantities into a photographic film by converting the phase to a background wave, a recording known as a hologram. By illuminating the hologram with a suitable light wave, the recorded wavefield will fully appear. The observer looks at the light beam as if he can see the real object in three dimensions. Holography can be used to detect aerosol particles with a detection range of sub-micron to millimeter order of magnitude.
(3) Light scattering
Light scattering is based on the scattering that occurs when electromagnetic waves encounter atmospheric particles in the propagation process, and the particle distribution and particle size concentration are obtained through the analysis of the scattering spectrum by the Mie scattering theory. Scientists in China have studied light scattering since the 1900s. It is a widely recognized detection technology among various particle detection technologies. It can realize real-time detection, continuous measurement, and long-distance measurement in the area. Therefore, light scattering is currently one of the most popular detection methods for the concentration of atmospheric particulate matter.