大口径平行光管作为实验室测试和标定必不可少的设备，已经普遍应用于各种光学设备的标定与检测中。但是光管因为震动、温度的梯度变化和空气扰动等影响因素出现离焦现象时，会导致平行光管出射光发散角增大，测量结果可信度降低。针对以上问题，本文提出了一种利用五棱镜作为光反馈元件的小型焦面监测装置，采用 CCD作为图像采集装置，利用软件实现系统离焦量的计算，可对大口径平行光管的焦面进行长时间高精度的监测。经过计算机仿真与试验表明：本系统检焦分辨力可以达到 45 μm以内，满足实验室检测设备精度要求

Large-aperture collimator was widely used in calibration and testing for various optical devices as a essential equipment for testing and calibration in laboratory. However, when collimator was defocused because of vibration, gradient change of temperature and airflow, divergence of beam which was emitted from collimator was increased and result credibility of test was decreased. To solve the above problems, a new miniature auto-focusing system was presented which used pentaprism for a light feedback element. CCD was used as image receiving device, and software was applied to realize calculation of the system defocusing amount, which can monitor and test the focal plane of collimator on a high precision level for a long time. Computer simulation and test show that the system can reach 45μm auto-focusing resolution and fully meet the precision requirements for laboratory testing equipment.