Pattern Recognition Letters I I (1990) 771-777 November 1990 North-Holland Calibration of a video camera digitising in the CIE L*u*o* colour space system N.J.C. STRACHAN, P. NESVADBA and A.R. ALLEN* Ministry of Agriculture, Fisheries and Food, Torry Research Station, 135 Abbey Road, Aberdeen AB9 8DG, UK Received 5 February 1990 Abstract: A method is described for calibrating a colour video camera using the Macbeth colour chart under fixed lighting conditions in the L*u*u* colour space. An assessment of the stability of the equipment is given and the techniques discussed should be useful for comparing and analysing colour images. Key words: Colour calibration, colour image analysis, Macbeth colour chart, colour video camera, colour spaces, colorimeter. Introduction Colour has been used in the image analysis and pattern recognition fields, for example to identify spray paint caps (Berry, 1987), to detect the colour codes on resistors (Bajon et al., 1988) and to guide a robot to selectively pick up petri dishes (Massen et al., 1988). Colour has also been used to segment images into sets of uniform colour regions (Tominaga, 1988; Celenk, 1987; Klinker et al., 1988) and to detect edges in images (Nevatia, 1976). Clearly, colour calibration is needed for these applications. However none of the papers above give a detailed description of how to calibrate for colour. The EBU (European Broadcasting Union, 1987) in an effort to assess the overall performance of colour television cameras have recommended two methods of measuring the colorimetric fidelity of the cameras and these are discussed in (Dalton, 1987 and 1988). The first, called the spectrophoto- metric method, is complicated and needs the use of * University of Aberdeen, Department of Engineering, Kings College, Aberdeen AB9 2UE, UK. a spectrophotometer. The second, the real samples method, measures the actual camera reproduction of a set of calibrated colours in the CIE (Commis- sion Internationale de l'Eclairage) L*u*u* colour space (CIE, 1978). Lee (1988) gives an algorithm for colorimetric calibration of a video digitising system using the Macbeth colour chart (McCamy et al., 1976). For this algorithm the spectral irradiance of the illumi- nant and the spectral reflectance of the sample must be known. This information can only be ob- tained using a spectroradiometer. For someone working in the pattern recognition field this is not practicable because this work is very time consum- ing and they probably do not have the equipment or the expertise. The Macbeth colour chart is a subset of the Munsell colour standards (Nickerson, 1940) and it is only one of many colour charts (atlases), a bibliography of which can be found in Billmeyer (1985). The aim of this paper is to describe a method of calibrating a video digitising system using the Macbeth colour chart in the CIE L*u*v* colour space which will be of practical use land interest to people working in the image analysis and pattern recognition fields. 0167-8655/90/$03.50 © 1990 -- Elsevier Science Publishers B.V. (North-Holland) 771