As an indispensable visibility ground-based observation instrument in atmospheric environment monitoring, the forward scatterometer currently adopts the standard scatterer external field calibration method, that is, multiple standard scatterers with different scattering characteristics are placed in the working light path in turn, so as to transfer the measured values of the forward scatterometer to the scattering characteristics calibration results of the standard scatterer. For the calibration of standard scatterers, the calibration method based on the field comparison test between the forward scatterometer and the atmospheric transmission meter is widely used internationally at present, which can trace the calibration results of standard scatterers to the calibration accuracy of the neutral density attenuator used to calibrate the atmospheric transmission meter. However, this method has complex process, many error transmission links, serious influence from weather conditions, long calibration time, and low efficiency. Moreover, the calibrated standard scatterer can only be used for the field calibration of a specific type of forward scatterometer, which has poor universality. At the same time, the field calibration results of the forward scatterometer are subject to the traceability chain of the measured values of the atmospheric transmittance, and because the calibration light sources for calibrating the neutral density attenuator of the atmospheric transmittance are mostly tungsten halogen lamps, xenon lamps and monochrome or white LEDs, the spectral distribution is different from that of the 2700 K color temperature incandescent lamps in the definition of meteorological optical range (MOR), the field calibration results of the forward scatterometer cannot be traced to the definition of MOR. In this paper, a calibration method of standard scatterers traceable to the definition of MOR is proposed, and a calibration optical system of standard scatterers used to calibrate the forward scattering visibility meter is designed. The synchronous calibration of multiple scattering angles is realized, the calibration time is greatly shortened, and the universality is strong. At the same time, the problem that the calibration of standard scatterers cannot be traceable to the definition of MOR is solved, providing a technical basis for improving the transmission and traceability system of visibility values.
According to the standard scatterer calibration method, a standard scatterer multi-angle synchronous calibration optical system architecture is determined, and a standard scatterer calibration optical system is designed for calibrating the forward scattering visibility meter. The standard scatterer calibration system is divided into an illumination optical system and a light field measurement optical system. The illumination optical system includes a light source, a shaping lens, a double-row fly-eye lens and a collimating lens group. The optical system for light field measurement is mainly composed of standard scatterer, hemispherical dome, aspheric reflector, panoramic imaging system and charge-coupled device (CCD) cameras. The illumination optical system and light field measurement optical system are optimized, and a calibration system energy calibration method is studied. The energy calibration error of the calibration system is simulated using standard Lambert scatterers.
The spectral distribution simulation based on the ideal 2700 K color temperature incandescent lamp verifies the calibration accuracy of the standard scatterer calibration method traceable to the definition of MOR. The designed standard scatterer calibration optical system realizes multi-angle synchronous calibration with pitch angle of 20°-50° and azimuth angle of 0°-360° (Fig. 4). The study on the energy calibration method of the calibration system shows that when the standard scatterer calibration system conducts energy calibration within the range of 20°-50° of the scattering angle, the maximum circumferential average relative error is 2.28% when the scattering angle is 31° (Fig. 15), which proves the improvement in calibration accuracy of the standard scatterer. The simulation verification shows that the magnitude range of MOR represented by the standard Lambert scattering angle of 20°-50° is 6.54-45.74 m, which conforms to the definition of MOR by the World Meteorological Organization, and the maximum absolute error is -2.41 m (Fig. 16).
Using the standard scatterer calibration method that can be traced back to the definition of MOR, a standard scatterer calibration optical system used to calibrate the forward scattering visibility meter is successfully designed, and the multi-angle synchronous calibration with pitch angle of 20°-50° and azimuth angle of 0°-360° is realized. The maximum absolute error of the standard scatterer calibrated by the calibration method is better than 1/10 of the measurement accuracy requirement of the forward scatterometer specified by International Civil Aviation Organization (ICAO), providing theoretical basis and technical support for the calibration of the standard scatterer traceable to the definition of MOR.
