Research On On-board Calibration Light Source Of Solar Reflection Spectrum Based On Large Aperture Freeform Reflector

With the progress of radiometric calibration technology,optical remote sensing technology plays an increasingly important role in the fields of social economy,national defense and security,environmental monitoring,engineering and information technology.In the process of transmission and on-board operations,the calibration system would be affected by the weather,the sun in the incident Angle,the influence of complex factors such as equipment aging,so that different remote sensor could not be traced back to a unified orbit radiation scale,leading to different satellite or the same satellite lack of comparability between remote sensor data,thereby limiting the space remote sensing in all aspects of the application.At present,there were mainly two methods for on-board calibration: one was to use solar as the on-board calibration light source,but the diffuse plate was direct exposed into the ultraviolet environmen,it may cause unpredictable attenuation,and it was difficult to measure the solar incidence angle.The other was to use the standard lamp as the calibration light source,mainly in the form of "standard lamp + diffuse reflector plate" and "standard lamp + integral sphere".In the system of "standard lamp + diffuse reflector",the light intensity and stability change with the change of the field angle of incident light,which was easy to introduce response error caused by the uneven incident light,incident angle and position.In the "standard lamp + integral sphere" system,if you want to get a large area of uniform illumination,according to the characteristics of the integral sphere,the opening of the integral sphere needs to be very large,so as the volume and weight,which is not conducive to on-board calibration.Therefore,by combined the characteristics of the two methods,we proposed to use a small opening integral sphere for uniform light,and then carry out optical design to obtain a collimating and uniform irradiance light source.The main innovative work of this paper is as follows:1.It is proposed to use the integral sphere with standard lamp as the calibration light source and the freeform reflector is used to achieve uniform illumination output.Only a single reflector was used,it can shorten the transmission links and saves satellite resources.The lambert source from the integrating sphere is collimated through a freeform reflector,so that the transfer radiometer and the imaging spectrometer can observe the same target in the same light path,the error caused by the difference of light source is eliminated.2.By analyzing the properties of the Jacobian matrix,the mathematical model of the freeform surface was established,the initial model of freeform surface was discreted by using Chebyshev points.It makes the energy correspondence of each part clearer and two perspectives of energy utilization rate and uniformity was considered.The Monge-Ampére equation was used to construct the on-axis and off-axis lighting systems.Through simulation analysis: in the on-axis lighting system,the utilization rate of energy can reach 93.1%,the irradiance uniformity of the illuminated area can reach99.1%,The energy utilization rate increased by 2.11 times,in the off-axis illumination system,the energy utilization rate can reach 93.7%,the irradiance uniformity of the target plane can reach 99.5%,and the energy utilization rate is increased by 1.95 times.Through the construction of the 3D model,the division between the illumination area of the target surface and the energy emitted from the light source is more precise,and the design of the lighting system with high uniformity is realized.3.Design integral sphere light source by analyzing actual requirements.A freeform surface design method suitable for extended light source is proposed.The design of the freeform surface for integral sphere light source is completed.Through the analysis of optical software,it can be seen that after the light source is collimated and folded the uniformity of the illumination area is above 99%.The results show that the instability of the integral sphere light source is 0.6%,the irradiance uniformity of the target plane is 98.8% and the radiance uniformity is 98.6%.The irradiance uniformity is above 97.7%within 25 mm in front of and behind the target.The analysis shows that the designed freeform reflector meets the requirements of the calibration light source that this method can be applied to the solar reflection spectral source calibrated on an orbiting star.