Study On Imaging Quality Through Turbid Atmosphere

Atmospheric aerosols and molecules degrade the quality of optical images collected over long distances due to light scattering and absortion.The scattered lights(including scattered direct sunlight,atmospheric diffused light,and light reflected by Earth’s surface)and the emitted thermal light are added to the target and neighboring pixels,thus cause reduction in the image contrast.An imaging degradation model described by transmittance and atmospheric scattering light was widely used in related applications.The point spread function(PSF)or the modulation transfer function(MTF)quantitatively characterizes the imaging quality.The turbid medium MTF based on small angle assumption(SAA)could be applied for low frequency and high frequency.Recently,the equivalence of the turbid medium MTF and the transmitted radiance from the medium under isotropic diffuse illumination was demonstrated,then the whole spatial frequency characteristic of the turbid atmosphere MTF can be acquired.In practice,the image quality is not only affected by the turbid medium MTF but also related to the background radiation.A formalized MTF is proposed based on the turbid medium MTF and and the background radiation.Main work and related results are as follows:1)Experiment and simulation confirmation on the equivalence principle of MTF of a turbid medium;Experiment and Monte-Carlo simulation verification were carried out to confirm the validity of the equivalence principle.The differences between the equivalence MTF and SAA MTF were analyzed,and the validity of SAA MTF was revealed.2)Proposal of a methodology that measures the atmospheric transmittance based on a single frame of multi-spectral image;Based on atmospheric optical image degradation model and the dark channel prior statistical theory,the atmospheric transmittance of specific band can be measured from radiometric images.The experimental comparison indicates fairly good correlations between the transmittances measured by this method,and thosed obtained with a visibility meter and a lidar.3)The description of the effects induced by turbid atmosphere on image quality in the whole spatial frequency domain;Atmospheric aerosols and molecules reduce the inherent brightness of a target pixel due to the effects of absorption and scattering.The multiple scattering light of adjacent pixels blurs the edge of the target pixel.The scattered lights(including scattered direct sunlight,atmospheric diffused light,and light reflected by Earth’s surface)or the emitted thermal light are added to the target and neighboring pixels.The formalized MTF is proposed by considering all the effects mentioned above.4)Development of a new method to evaluate image quality in spatial frequency domain;Image blur simulations show that the results from the formalized MTF are more consistent with actual scenes than results only from turbid medium MTF.Thus,the formalized MTF can describe the image degradation process through atmosphere comprehensively.The image restoration results indicate that the formalized MTF method performs better than dark channel prior method.The Spectrum area(AS)is proposed to evaluate image restoration methods effectively in spatial frequency domain5)Wavelength dependence of formalized MTF in turbid atmosphere;Equivalence principle shows the turbid medium MTF depends on atmospheric optical parameters,i.e.τ,ω,and g.The wavelength dependence of atmospheric optical parameters determines the spectral characteristcis of the formalized MTF.Analysis resuts demonstrate that the mid-high frequency of turbid medium MTF in VIS and NIR is higher than that in MIR,while the zero frequency of formalized MTF in VIS and NIR is lower than that in MIR.6)Band selection for optimal imaging;Imaging simulations in rural atmosphere were performed.Based on the atmospheric formalized MTF spectral characteristics,better image quality means that high spatial frequency of formalized MTF is higher and zero spatial frequency of formalized MTF is lower.The image data of the spaceborne ESA-mission CHRIS(Compact High Resolution Imaging Spectrometer)on-board PROBA-1 are analyzed.Results show that the actual imaging effect of different wavebands is agreement with the prediction based on formalized MTF spectral characteristics.In addition,the MTFs of different component in turbid and turbulent atmosphere and elec-optical imaging system are analysed.