| As the important component in optical systems, the scattering characteristics ofoptical mirror surface are closely related to performance of optical systems. Under theauspices of the cooperative projects between our laboratory and Research Center of LaserFusion in Chinese Academy of Engineering Physics, in the first place, this paper builds aBidirectional Reflectance Distribution Function (BRDF) model used for describing thescattering characteristics of particulate contaminants deposited on optical mirror surfaceby using Mie scattering theory. In the next place, making use of MATLAB simulationsoftware compiles simulating procedure to calculate the Bidirectional ReflectanceDistribution Function (BRDF) of particulate contaminants on optical mirror surfaceunder three incident laser wavelengths such as the visible light0.6328μm, near infraredlight1.053μm and thermal infrared light10.6μm. Eventually, scattering characteristics ofoptical mirror surface under different pollution conditions are researched following thecalculation results when laser is incident. The calculation results indicate that:(1) There is a very clear relationship between the BRDF of optical mirror and thewavelength of the incident laser. As the wavelength of the incident laserbecomes gradually shorter, the BRDF of optical mirror gradually increases. It isshown that the scattering circumstances of optical mirror become morecomplicated when incident laser wavelength is shorter.(2) For a clean rough optical mirror surface, the BRDF of optical mirror isobviously connected with mirror surface roughness and autocorrelation length.On the condition of certain autocorrelation length, if mirror surface roughness isfar less than the incident laser wavelength (σ<<λ), scattering magnitude of cleanrough optical mirror surface is proportional to roughness σ and inverselyproportional to incident laser wavelength λ.(3) Scattering magnitude of particulate-contaminated optical mirror surface relatesto quantities of particles on the mirror surface closely when optical mirrorsurface is contaminated by particulate contaminants. The greater quantities of particles are, the higher values of BRDF are, and the more complex scatteringcase is.(4) Scattering magnitude of particulate-contaminated optical mirror surface is alsowith size of particles on the mirror surface closely. If there are larger sizeparticles on optical mirror surface, wave of BRDF curve will become veryviolent. Scattering becomes more complicated.Usually, the pollution condition of contaminated optical mirror surface is describedby surface “cleanliness level”. Take advantage of the established BidirectionalReflectance Distribution Function (BRDF) model, the BRDF of optical mirror surfaceunder three cases of mirror surface cleanliness levels such as basic clean (cleanlinesslevel200), slight pollution (cleanliness level500) and severe pollution (cleanliness level750) is simulated and computed in the light of foregoing three incident laser wavelengths(0.6328μm,1.053μm,10.6μm) situations. It is shown that:(1) If the wavelength of incident laser is determined, along with more larger opticalmirror cleanliness level, the value of BRDF is more and more greater and itresults in more stronger scattering.(2) The BRDF caused by lower optical mirror cleanliness level is corresponding tothe BRDF of clean rough optical mirror surface. It means that a significantinfluence on laser scattering characteristics is already made by a few particulatecontaminants on optical mirror surface, and this phenomenon will be moreobvious in pace with more and more longer incident laser wavelength.Based on the above theoretical analysis, in the first place, a common method ofmeasuring BRDF is described. In the next place, a exploitative experiment is put intoeffect by making use of Laser Beam Profiler according to the imaging principle of CCD.The experimental results was studied preliminarily. It is shown that the reflected lightbeam energy of clean mirror is more concentrated than the reflected light beam energy ofpolluted mirror. Due to the presence of particulate contaminants on the optical mirror, itenhances the scattering energy in all directions, but in the main direction of reflection, itreduces the light beam energy. In other words, the overall reflection energy distribution of polluted mirror is more decentralized, the loss of laser transmission of optical mirrorcaused by particulate contaminants is increased.All in all, if particulate contaminants on optical mirror surface distribute morecomplex, the more significant scattering will be made. Hence one can see that particulatecontamination is a major source of optical mirror surface scattering, and useful help willbe provided to estimate the pollution of optical mirror and to clean the mirror surface inoptical engineering. |
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