ELIMINATION OF COLOR FRINGES IN DIGITAL PHOTOGRAPHS CAUSED BY LATERAL CHROMATIC ABERRATION

The effects of (monochromatic and chromatic) lens aberrations in optical imaging are well documented in the literature. These geometrical imperfections are caused by the physical parameters of the optical system (lens) of the photographic camera and apply to both analog and digital cameras. All these aberrations produce lateral distortions (geometric errors) and/or longitudinal distortions (image blur) in one way or another. In this paper we focus on the elimination of the effect of lateral chromatic aberration within a post-processing step after image acquisition. This task has already become a vital topic with the advent of digital (consumer) cameras. Many references can be found in the World Wide Web. Several methods, from simple heuristic to more stringent ones, are proposed by the user community. Since the usage of digital consumer cameras (SLR or compact cameras) in documentation and mapping of cultural heritage is becoming more and more wide-spread, the present topic should be discussed in more detail. Color fringes are inherent to all analog and digital (color) photographs taken by cameras for which chromatic aberration is not sufficiently corrected for. The width of color fringes, mainly introduced by lateral chromatic aberration, is smallest around the image center and greatest in the corners of the photographs. The authors have developed a computer-based procedure to precisely determine the geometric distortions of the red and blue image channel (plane) in comparison to the green reference channel. Least-squares matching is employed at distinct corner points found by an interest operator in order to measure point displacements. The paper also describes how these measurements can be carried out using a commercial software, i.e. PhotoModeler 5.0. In a first approximation the three RGB color channels differ in scale, i.e. they are radially displaced. Originally, the DistCorr software has been developed in order to compensate for lens distortion to obtain perfect central-perspective images. The software mentioned was readily modified for correction of lateral chromatic aberration. As a result, the geometrically re-scaled red and blue image planes are registered to the green one. The amount of image displacement of the two color channels can be specified as an additive correction to the linear parameter of the radial-symmetric lens distortion formula. Lateral color fringes can thus be eliminated to a great extent with this simple method. This paper also presents examples of practical investigations. Three lenses (17 mm, 20 mm, 50 mm) of a digital consumer camera, i.e. a Nikon D100 SLR with 6 Megapixels, were analyzed. The results obtained are presented numerically and graphically. An outlook on further improvements in the elimination of color fringes is given at the end of the paper.