As early as the 1950s, scientists found that the sunlight with wavelength less than 285 nm was almost completely absorbed by the ozone layer during the process of penetrating the atmosphere to the sea level, which made it almost impossible to detect the sunlight in this band from the sea level to 20 kilometers above sea level. Therefore, this band is called the sun-blind ultraviolet band.
Taking advantage of the characteristics of ozone in the earth’s atmosphere, the sun-blind filter has a strong absorption and scattering effect on the natural light in the range of 200~280nm, so it is difficult to reach the earth’s surface at the range of 200~280nm (referred to as the “sun-blind zone”). Therefore, many research institutions have the advantages of low background noise, low false alarm rate, strong concealment, simple structure and wide use. The solar blind filter is used to detect the weak ultraviolet signal under the sunlight. The solar blind filter has good performance, high transmittance and deep cut-off rate in the visible and near-infrared bands.
The sun-blind ultraviolet filter is the core device of the sun-blind ultraviolet detection system. It is used to filter the interference of the radiation of other spectral bands in the detection environment, so that the detector can better detect the ultraviolet radiation of the sun-blind zone, thus improving the signal-to-noise ratio of the detection system and reducing the false alarm rate. Therefore, it is extremely important to test the out-of-band cut-off depth of the sun-blind ultraviolet filter, and its requirements are increasingly strict. At present, the instruments for measuring the cut-off depth of the filter mainly include spectrophotometer, DF reflectometer, acoustooptic modulation detection system, etc. Among them, spectrophotometer and DF reflectometer are suitable for measuring filters with cut-off depth less than 2-OD. In 2009, the acoustooptic modulation detection system appeared. The system uses acoustooptic modulator, transmissive attenuator, integrating sphere and other equipment to extend the measurement range of the cut-off depth of the filter to 6-OD. However, the dynamic measurement range of the cut-off depth of the sun-blind ultraviolet filter with the out-of-band depth cutoff and part of the spectral band up to 10-OD is still insufficient.
At present, sun-blind filters are widely used in power lines, corona detection, high-voltage equipment corona discharge detection, forest fire detection, remote sensing detection equipment, atmospheric environment research, ultraviolet imaging, space-based early warning, guidance, ultraviolet communication and other optical fields. Biochemical analysis, biomedical analysis, public security reconnaissance and many other fields have great commercial and military application value.
