CCD图像灰度与照度的转换标定方法

Method for conversion calibration between CCD image gray value and illumination

本文提出了一种基于CCD摄像机利用图像灰度获得实际照度的新方法,该方法适用于摄像机的任意曝光和增益参数,且考虑了摄像机安装位置和角度对测量的影响。CCD摄像机照度测量法的实际应用主要涉及两个过程:一是标定过程,即应用CCD摄像机进行照度测量前,标定CCD图像灰度与照度之间的转换参数和转换模型;二是照度测量过程,即通过CCD摄像机拍摄图像,将图像灰度代入已定的转换模型,获得测量对象的照度值。其中,灰度与照度转换模型的标定是利用CCD摄像机实现照度测量的基础和关键,文中主要讨论了该模型的推导过程以及标定方法。具体为:首先通过理论推导获得图像灰度与CCD传感器感应的相对辐照度的关系,然后借助以均匀光源搭建的实验系统标定CCD传感器相对辐照度与物面实际照度之间的关系,进而获得图像灰度与物面实际照度的转换关系。在转换过程中讨论了曝光时间、增益以及CCD相机与照度测量点之间的距离和角度对转换模型的影响,为CCD摄像机照度测量法的实际应用奠定了基础。实验结果表明本测量及标定方法适用于摄像机任意曝光和增益条件,经过本方法标定后,应用CCD摄像机进行照度测量,测量结果的相对误差在4.5%以内。

This paper puts forward a new method that uses image gray value to obtain the actual illumination based on CCD camera,the method is suitable for any exposure and gain parameters of the camera,and the effects of the camera installation position and angle on measurement are considered.The practical application of the illumination measurement method with CCD camera mainly involves two aspects：one is the calibration process,which is calibrating the conversion parameters and conversion model between CCD image gray value and illumination before using CCD camera to measure illumination.The other is illumination measurement process,which is using CCD camera to capture image,then the illumination of the measurement object can be obtained through substituting the image gray value into the conversion model.The calibration of the conversion model between gray value and illumination is the foundation and key to realize illustration measurement using CCD camera.The derivation process and calibration method of the model is emphatically discussed in this paper.The detailed process is like this：through theoretic deduction,the relation between the image gray value and the relative irradiance of the CCD sensor is obtained; then,the relation between the CCD sensor relative irradiance and actual illumination is calibrated in the experiment system with uniform light source; and last,the conversion relationship between the image gray value and actual illumination is obtained.In the conversion process,the effects of the exposure time,the gain,the distance and angle between CCD camera and illumination measurement points on the conversion model are discussed,which establishes the basefor the practical application of illumination measurement with CCD camera.The experiment result shows that this measurement and calibration method is suitable for any exposure time and gain conditions; and after calibrating with this method,the relative error of the illumination measurement result obtained using CCD camera is less than 4.5%.