Techniques for Fisheye Lens Calibration using a Minimal Number of Measurements T. Nathan Mundhenk, Michael J. Rivett, Xiaoqun Liao, Ernest L. Hall 1 University of Cincinnati Center for Robotics Research Dept. of Mech. Ind. and Nuc. Eng. University of Cincinnati Mail Location 72, Cincinnati, Ohio 45221 ABSTRACT A method is discussed describing how different types of Omni-Directional “fisheye” lenses can be calibrated for use in robotic vision. The technique discussed will allow for full calibration and correction of x,y pixel coordinates while only taking two uncalibrated and one calibrated measurement. These are done by finding the observed x,y coordinates of a calibration target. Any Fisheye lens that has a roughly spherical shape can have its distortion corrected with this technique. Two measurements are taken to discover the edges and centroid of the lens. These can be done automatically by the computer and does not require any knowledge about the lens or the location of the calibration target. A third measurement is then taken to discover the degree of spherical distortion, This is done by comparing the expected measurement to the measurement obtained and then plotting a curve that describes the degree of distortion. Once the degree of distortion is known and a simple curve has been fitted to the distortion shape, the equation of that distortion and the simple dimensions of the lens are plugged into an equation that remains the same for all types of lenses. The technique has the advantage of needing only one calibrated measurement to discover the type of lens being used. Keywords: omni, vision, fisheye, circular, regression, correction, distortion, nikon 1. INTRODUCTION The omni-directional lens or as it is also called, a fisheye lens can have many advantages over traditional lenses when used for such things as tracking and surveillance. However, use of the fisheye lens brings with it its own problems. A fisheye lens is able to see half of the world. It can give a camera a complete view of 180° along the horizon and 180° vertically when pointed forward. However, this supreme view of the world comes with a price. The image taken from an omni-directional lens is distorted. The picture below shows what a picture taken from a fisheye lens looks like. As mentioned, this picture shows half of the world. To make these fisheye images a Nikon Nikkor 8mm 1:2.8 243304 lens was used 1 Corrispondance: T. Nathan Mundhenk nathan@mundhenk.com Michael Rivett rivettmj@rivett.org Xiaoqun Liao liaoxn@email.uc.edu Ernest L. Hall ernie.hall@uc.edu Center for Robotics Research http://www.robotics.uc.edu