Practical Camera Characterisation for Colour Measurement

摘要

This paper reviews the major issues involved in the use of digital cameras to derive the CIE X, Y, Z tristimulus values of the objects in real scenes. Both practical and theoretical investigations have been carried out to gain experience in this specialised field of imaging. The practicalities of camera characterisation 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 infra-red filtration, characterisation method - linear or higher order, quality measure - CIELAB, CMC, CIE94 colour difference, and quality statistic - mean, median etc. It is shown 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 is shown to be important: that it is never perfectly uniform must be considered in the characterisation process. With careful selection of system components, a median value of less than 1.0 CIELAB colour difference between the required and the predicted colorimetry can be obtained.