M.R. Pointer

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.

Minimum Perceptible Differences in the Colour Reproduction of Photographic Prints

AbstractThe aim of this work was to investigate observer perceptibility of small colour differences in the colour reproduction of reflection prints produced using a negative/positive photographic system. Comparison of the experimental results, in the form of hue-oriented and non hue- oriented ellipses, was made with MacAdam ellipses, CIELAB unit ellipses and ellipses derived from the CMC (l:c) colour difference formula, in the 1976 CIELAB colour space. It was found that the CMC (l:c) ellipses fitted the hue-oriented ellipses more closely than the CIELAB unit el-lipses, there being no significant differences between the hue-oriented and the non hue-oriented ellipses. There was, however, a significant difference between MacAdam ellipses and the non-hue oriented experimental ellipses. It was therefore concluded that the CMC (l:c) colour difference formula could be used to predict the tolerance ellipse for any colour centre, under the conditions used in this work. The minimum perceptible difference was found ...

Food colour appearance judged using images on a computer display

Abstract Pixel-based images of food products are prepared and systematically perturbed in colour and lightness, and the changes quantified. Observations are reported in terms of CIELAB and pixel value, with colour differences corresponding to acceptability and perceptibility thresholds for each subject and each perturbation applied. Conclusions are drawn concerning the usefulness of digital imagery for product panel assessment, the relative importance of colour and lightness variations of the food products investigated and the applicability of a mathematical function to the results of the visual assessments carried out. The relationship of acceptability to imperceptibility of image perturbations is quantified.

The Application of Colour Reproduction Indices to Photographic Reflection Prints

AbstractA possible correlation between colour reproduction indices derived from a recent model of colour vision, and subjective rankings of photographic reflection prints, was sought. Sets of reflection prints consisting of a normal print and a colour ring-around were produced for four scenes. Observers placed in rank order the prints in each set according to the quality of colour reproduction. Ten observations were made for each set of prints; viewing was made in a viewing booth using standard daylight conditions. From colorimetric measurements of the Macbeth ColorChecker Chart set of prints, colour reproduction indices were calculated. A reasonable correlation between the implied rank order derived from the colour reproduction indices and experimental rank order was found.

Perceived colour differences in displayed colours Part 1: hard copy to soft copy matching

Abstract An investigation is reported into visual comparisons between object colours and simulations on a cathode ray tube (CRT) monitor display. Experiments are described in which observers viewed a bipartite field in which two coloured areas were separated by a narrow division and viewed against an unlit grey background. One coloured area comprised a surface colour patch illuminated using a light source which was a close colorimetric match to CIE Illuminant D65. The second area was simply an aperture held against the faceplate of the computer CRT monitor, operated with a white point corresponding again to CIE Illuminant D65. The illumination of the reflection patch was arranged so that there was a visual match between a white patch and the monitor white. The monitor patch was controlled in appearance from the keyboard and was specified in terms of red, green and blue pixel values. Observers were asked to match the illuminated reflection patch by adjusting the (unseen) specification of the monitor patch. Separate experiments concerning the ability of observers to detect colour perturbations of matches are also described. Conclusions are drawn as to the concordance of repeated colour matches by individual observers, and between all the observations. The magnitudes of perceptible colour differences are also evaluated.

Observer responses to colour perturbations in reflection prints

Abstract The results of investigations into observer response to perturbations of colour prints with known CIELAB colour differences from a standard reproduction are assessed. The responses in terms of acceptability and perceptibility of colour differences are related to percentage imperceptibility, being defined as 100—perceptibility. The acceptability and imperceptibility of colour reproduction are evaluated as functions of CIELAB colour difference, giving good fits to a single simple mathematical function but with differing parameters. A near-constant ratio of acceptable to imperceptible colour difference is found to apply over the entire range of observer responses.

Photography with non-standard light sources and reversal colour films

AbstractThe influence of non-Planckian light sources on the photographic colour reproduction of daylight reversal materials has been investigated. Various scenes were illuminated with such sources, and transparencies for projection produced. In order to ascertain the possibility of improving any impairment of the colour reproduction, corresponding transparancies were produced with experimentally derived filter packs placed over the camera lens. A subjective evaluation of the acceptability of the colour reproduction of the projected transparencies has been carried out, with subsequent statistical analysis of the results.

Perceived colour differences in displayed colours Part 2: spatial differences in soft copy imaging

Abstract This paper follows and extends previously reported investigations into the ability of observers to match surface colour samples by synthesis using red, green and blue stimuli provided by a cathode ray tube (CRT) monitor, controlled by a computer and adjusted by either an observer or the experimenter. Investigations are described into the ability of observers to perceive small colour perturbations applied to a colour patch, in the simultaneous presence of a reference and five other closely related patches, all the patches being displayed on a CRT computer display. Consistency is found between results obtained by different sets of observers at three locations, using local equipment for display. The results obtained are found to indicate sensitivities to perturbations that depend on the hue and chroma of the reference, and the hue direction perturbations. The surround to the colour patches is shown significantly to influence the sensitivity of observers to the perturbations, and this finding is related to predictions made using a colour appearance model. The data obtained are fitted by the observer response model hitherto successfully employed by the authors in previously reported investigations, and the goodness of fit is quantified statistically. Conclusions are drawn concerning the red, green and blue bit ranges and CIELAB colour differences of perturbations likely to be perceptible to observers in the presence of an unperturbed reference.

Measured colour difference and the acceptability of photographic colour prints

The relationships between an overall Colour Reproduction Index (CRI), a mean colour difference, CIELAB ΔE ab * , and the acceptability of colour prints are investigated and quantified for two scenes at a range of controlled hue and lightness excursions from standard print criteria. This paper extends the analysis of experimental results to show linear relationships between CRI and CIELAB ΔE ab *, and between each of these measures and the acceptability of reproductions. The relationships are, however, both complex and noisy. Print lightness is shown to be a more significant quality parameter than is allowed for in the calculation of the overall CRI and CIELAB ΔE ab *. It is also shown that hue is a scene-specific determinant of print acceptability. Conclusions are drawn concerning the relative usefulness of the two different measures of colour difference : CRI and CIELAB ΔE ab *.

Colour reproduction indices and acceptability of prints at controlled density levels

The correlation between a Colour Reproduction Index (CRI) and a single stimulus, forced-choice, scaling of the acceptability of colour prints is examined. The prints were obtained from normally-exposed negatives, each scene photographed included a calibrating Macbeth ColorChecker Chart for colorimetric purposes. Each negative was printed to give a standard ideal reproduction, according to objective criteria, and a systematic array of controlled colour perturbations, a ring-around. Further sets of prints were made by reprinting additional ring-arounds at two lighter and two darker controlled density levels. Prints were assessed by twenty observers, using a standard viewing booth, with the included Macbeth Chart hidden from the observer by an opaque grey mask. The reproduction of the Macbeth Chart in each print was also measured spectrophotometrically and CRI values were calculated. A linear relation exists between CRI and the level of acceptability of each print in a constant lightness set. Comparisons are made with previously published results in estimating weightingsfor subsidiary indices in the calculation of CRI values.