Practical camera characterization for colour measurement

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 applicatio n in. for example, machine vision, product quality assessment by panel obse rvation 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 filt ration: characterization method-linear or higher order; quality measure-CIE LAB. 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 filt ration is the most sensitive variable. If a 'colour' camera is used, then i t needs to be carefully selected, a more adaptable choice, however. may be a monochrome camera with external filters. In addition, the illumination un iformity of the test target used for characterization is shown to be import ant: that it is never perfectly uniform must be considered in the character ization process. With careful selection of system components, a median valu e of less than 1.0 CIELAB colour difference can be obtained between indepen dently measured colorimetry and that calculated from the camera output.