Practical camera characterization for colour measurement

Abstract

This paper reviews the major issues involved in the use of digital cameras to derive the CIE X, Y and Z tristimulus values of objects in real scenes. Both practical and theoretical investigations have been carried out to gain experience in this specialized field of imaging that is finding application in, for example, machine vision, product quality assessment by panel observation and digital archiving of art objects. The practicalities of camera characterization described include: lighting—spectral power and uniformity; test target—choice and number of colours; camera signal processing—linear or gamma corrected; colour analysis—filter transmittance and infrared filtration; characterization method—linear or higher order; quality measure—CIELAB, CMC, CIE94 colour difference; quality statistic—mean, median, etc. The potential impact of these parameters is discussed by the use of a computer model and from practical experience.It is shown, as might be expected, that the choice of colour separation filtration is the most sensitive variable. If a ‘colour’ camera is used, then it needs to be carefully selected; a more adaptable choice, however, may be a monochrome camera with external filters. In addition, the illumination uniformity of the test target used for characterization is shown to be important: that it is never perfectly uniform must be considered in the characterization process. With careful selection of system components, a median value of less than 1.0 CIELAB colour difference can be obtained between independently measured colorimetry and that calculated from the camera output.