AUAOMATED CALIBRATION OF A ZOOM LENS CCD IMAGE SYSTEM FOR VIDEOGRAMMETRY

This paper presents the automated calibration method and the calibration results of the off-the-shelf zoom lens CCD block camera. The CCD camera interior and exterior parameters with respect to the total-station were determined at every step of the focal length settings. The repeatability of the calibrated parameters against zooming action was assessed at five steps of the focal length settings. Both the accuracy and the repeatability for the automated calibration were analyzed. 1 INSTODUCTION Zoom lenses have many desirable features but appear to be geometrically unstable and difficult to calibrate. It is common knowledge that the zooming camera parameters change with zoom position. The calibration needs to be performed at various focal settings within the zoom range. If the zoom lenses are used in a measurement application at the setting from which the calibrations had been carried out, the camera parameters need to be derived from the calibrated values. The problem with the use of zoom lenses is the identification of calibration parameters at every used focal setting. For any vision system, geometric distortions caused by system electronics, focal plane construction, and imperfect lenses, lead to a less than perfect two-dimensional representation. In order to achieve precise three-dimensional positioning of objects, it is necessary to calibrate the vision system and correct for the effects of the above mentioned distortions. Changing the focal length alters the interior geometry of the camera, and consequently changes the magnitude of virtually all of interior orientation parameters. Calibration of zoom lenses at every used focal length by manual processes is complicated and very time consuming. An auto-calibration method needs to be developed for calibrating zoom lenses for photogrammetric applications. The paper presents an auto-calibration routine for zoom lenses that has been developed from the camera-on-theodolite method by Huang and Harley. The results of auto-calibration using the Sony EVI-371 were presented. The repeatability of auto-calibration results was tested on numerous focal lengths to check the stability of the camera against zoom. The warm-up effect of imaging system was also assessed. 2 THE SYSTEM REQUIREMENTS The system is comprised of the videotheodolite, an image grabber and a desktop computer. This videotheodolite is set at a number of stations during the research. At each station, the theodolite controlled by the desktop computer can precisely rotate at every angular step for the CCD camera to capture images to cover the designed target. The theodolite angular readings were recorded on capturing of each image to calculate the central position and bias error of the designed target in the image. The CCD camera of the system is the Sony EVI-371D. It is a 1/3inch CCD image sensor with the 752(H) x 582(V) sensing area and its cell size is 6.35(H) x 7.4(V) microns. The zoom lens has the focal length from 5.4mm to 64.8mm, auto focus and 12 X zoom lens. For this calibration, the focus was fixed at a certain distance and the iris was fixed for the solid exposure.