光源光谱特性对空间相机调制传递函数检测的影响

Influence of spectral characteristics of light sources on measuring space camera modulation transfer function

调制传递函数(modulation transfer function,MTF)检测是评价空间相机像质的重要手段.空间相机光学系统透过率和色差、探测器量子效率均与波长相关,采用不同光谱特征的光源所得到的MTF会出现偏差,光源光谱特性的影响不可忽略.针对这一问题,提出了一种分析光源光谱特性对空间相机MTF检测结果影响的方法,设计了空间相机光谱响应率和单色PSF标定装置及方法.利用所提方法及标定结果,计算了五种光源检测空间相机MTF时的理论值,发现氙灯和其他四种光源的MTF值偏差较大.对比卤钨灯和氙灯检测MTF时的理论值,发现卤钨灯所得MTF在全频段内均大于氙灯所获取的MTF,二者之间的偏差在中高频处最大,最大偏差为0.075.搭建了实验装置,分别采用卤钨灯和氙灯作为光源,利用倾斜刃边法检测MTF,发现二者所得MTF在各个频率点处的分布特征及偏差与理论计算结果相同,且最大偏差为0.057.理论及实验结果表明,本文方法能够准确评估光源光谱特性对空间相机MTF检测的影响.

Modulation transfer function(MTF)measurement is a major means to evaluate the imaging quality of a space camera.The influence caused by the spectral characteristic of light source on the MTF results is not negligible,because the transmittance and color aberration of optical systems,and quantum efficiency of the space camera detectors are all spectrally related.Thus,MTF results tested by different light sources are different from each other.To address this problem,we propose a method to analyze the influence of spectral characteristics of light sources on measuring the MTF of space cameras.In addition,the devices and methods are designed to calibrate the spectral response and monochrome point spread function(PSF)of space camera.A Sigma lens(focal length:1000mm,F number:5.6)and a Cannon EOS 5DSR camera(pixel size:4.14μm)are combined into an experimental space camera,whose spectral response is calibrated with a monochromator(Omno30300,NBeT)and a spectral radiometer(FieldSpec,ASD).We calibrate the monochrome PSF of the Sigma lens with the same monochromator and a CCD(PIXIS 1024,Princeton Instruments,pixel size:13μm)micro-measuring system(20X objective).During the calibration of spectral response and monochrome PSF,the same collimator(focus:5000 mm,F number:10)is used.With using the proposed method and those calibrating data,we compute the theoretical values of the MTF of a space camera measured separately with five different light sources.The results indicate that MTF measured by a xenon lamp is greatly different from those MTFs measured by the other four light sources.Comparisons of those theoretically calculated MTFs,separately,show that the MTF measured by a tungsten halogen lamp is greater than the MTF measured by a xenon lamp at each spatial frequency.The deviation between those two lamps reaches a maximum value of 0.075 in the medium-high frequency zone.Furthermore,in order to verify those theoretical conclusions,a platform including a collimator and the previous space camera is constructed.The MTFs measured by a tungsten halogen lamp and a xenon lamp are computed with the slanted-edge method respectively.The results demonstrate that the distributions and deviations of the MTFs tested by those two lamps are identical to those theoretical results at each spatial frequency,with the maximum deviation being 0.057.The theoretical and experimental results demonstrate that the suggested method can accurately calculate the influence of spectral characteristics of light sources on measuring MTF of space cameras.The proposed method can also be adopted to investigate the influence of spectral characteristics of light sources on MTF of optical systems in the design or test stages.