Visibility is a basic meteorological parameter and is regarded as a weather index to understand atmospheric stability and vertical structure. Unlike horizontal visibility, slant visibility is a crucial parameter that pilots are actually concerned about, and it directly determines the safety of aircraft take-off and landing in the aviation field. In addition, slant visibility is an important parameter for space target recognition and plays an important role in the field of weather analysis, sea, land, and air traffic, astronomical observation, sea fog warning, and so on. Therefore, slant visibility, as a highly concerned atmospheric optical and meteorological parameter in recent years, has shown important scientific research significance and application value in atmospheric research, civil aviation, space exploration aerospace, military, and other fields.

With the development of lidar technology, few lidar visibility meters have appeared in recent years, with single-wavelength Mie-scattering lidar as the core, and two types of inversion algorithms have been developed. One is the slant visibility inversion method based on Koschmieder's visibility law, in which the measurement of slant visibility only depends on the inversion of atmospheric aerosol extinction coefficients; the other is the inversion method based on optical thickness, in which the atmospheric optical thickness obtained by multi-elevation angle lidar detection is used to estimate the slant visibility. However, the main limitation lies in the neglect of the influence of scattered radiance and the uniform path assumption. As a result, these inversion methods have certain inversion defects. In addition, previous research has begun to pay attention to the atmospheric scattered radiance. However, most of them focus on theoretical modeling and simulation analysis, and thus in-depth study and further exploration are greatly required.

To solve the difficulty of atmospheric scattered radiance, the research team of Xi'an University of Technology recently developed a new slant visibility measurement method by lidar and the radiative transfer model (Fig. 6). By taking full advantage of laser remote sensing, aerosol lidar detection was carried out with the high spatial-temporal resolution and high-precision, and the real-time aerosol information including optical, micro-physical, and scattering parameters was provided. The radiative transfer model realized the path distribution of actual atmospheric scattered radiance (Fig. 9), which fundamentally solved the current technical bottleneck of slant visibility measurements. Moreover, a reflectance measurement system was supplemented to provide the intrinsic contrast of the object and the background, and the slant visibility measurement considering the correction of atmospheric scattered radiance was ultimately achieved (Fig. 10).

Atmospheric visibility can be measured by visual methods or visibility instruments. Both the forward-scattering-type and the transmission-type visibility meters can measure horizontal visibility. However, they are unable to provide information on slant visibility.

We comprehensively review the main techniques and research progress of laser remote sensing in slant visibility measurements, and several slant visibility inversion methods are sorted out. The shortcomings and limitations of the existing techniques are investigated as well. In view of the difficulty of actual atmospheric scattered radiance, a new remote sensing technique combining lidar and the radiative transfer model is mainly introduced, which has effectively broken through the bottleneck of slant visibility measurements. In the future, with the development of satellite remote sensing technology, it is envisioned to achieve the global map of slant visibility by laser remote sensing technology.