Phil Green

Consistency and Standardization of Color in Medical Imaging: a Consensus Report

This article summarizes the consensus reached at the Summit on Color in Medical Imaging held at the Food and Drug Administration (FDA) on May 8–9, 2013, co-sponsored by the FDA and ICC (International Color Consortium). The purpose of the meeting was to gather information on how color is currently handled by medical imaging systems to identify areas where there is a need for improvement, to define objective requirements, and to facilitate consensus development of best practices. Participants were asked to identify areas of concern and unmet needs. This summary documents the topics that were discussed at the meeting and recommendations that were made by the participants. Key areas identified where improvements in color would provide immediate tangible benefits were those of digital microscopy, telemedicine, medical photography (particularly ophthalmic and dental photography), and display calibration. Work in these and other related areas has been started within several professional groups, including the creation of the ICC Medical Imaging Working Group.

Gamut mapping for the Perceptual Reference Medium Gamut

A requirement for a baseline algorithm for mapping from the ICC Perceptual Reference Medium Gamut to destination media in ICC output profiles has been identified. Before such a baseline algorithm can be recommended, it requires careful evaluation by the user community. A framework for encoding the gamut boundary and computing intersections with the PRMG and output gamuts respectively is described. This framework provides a basis for comparing different gamut mapping algorithms, and a number of candidate algorithms are also described.

Test target for defining media gamut boundaries

A new target for defining media gamut boundaries is described. The use of measured gamut boundary data using provided by this target is capable of improving the accuracy of gamut boundary predictions in both device model and gamut mapping computations. The accuracy of existing methods of calculating gamut boundaries can be improved by use of an enlarged gamut boundary training set to derive the gamut boundary descriptor. An alternate, computationally simple, method of calculating gamut boundary intersections is described, and its performance in calculating such intersections and in media mapping is described. There is good agreement between this method and FSLGB in predicting boundary intersections if the same data is used to derive the gamut boundary descriptor. The gamut boundary target described in this paper is thus proposed as a means of obtaining greater information about media gamut boundaries.

False-colour palette generation using a reference colour gamut

Monochrome images are often converted to false-colour images, in which arbitrary colours are assigned to regions of the image to aid recognition of features within the image. Criteria for selection of colour palettes vary according to the application, but may include distinctiveness, extensibility, consistency, preference, meaningfulness and universality. A method for defining a palette from colours on the surface of a reference gamut is described, which ensures that all colours in the palette have the maximum chroma available for the given hue angle in the reference gamut. The palette can be re-targeted to a reproduction medium as needed using colour management, and this method ensures consistency between cross-media colour reproductions using the palette.

Characterizing coated paper surface for modeling apparent dot area of halftone prints.

A new model is proposed for predicting the apparent dot area of simulated halftone prints on coated paper surface without requiring printing. It is based on Hotellings multivariate T(2) statistic which is shown to provide a measure of lateral light scattering. The T(2) statistic is computed from colorimetric coordinates obtained from of a knife shadow image response on the paper surface. The proposed method offers superior prediction of halftone dot area compared to current light scattering models. A method for characterising peaks on the coated paper surface is introduced in this work. The effect of the paper coating layer thickness and the surface peak height on lateral light scattering and printed dot size are shown.

Baseline Gamut Mapping Method for the Perceptual Reference Medium Gamut

A need for a baseline algorithm for mapping from the Perceptual Reference Medium Gamut to destination media in ICC output profiles has been identified. Before such a baseline algorithm can be recommended, it requires careful evaluation by the user community. A framework for encoding the gamut boundary and computing intersections with the PRMG and output gamuts respectively is described. This framework provides a basis for comparing different gamut mapping algorithms, and a candidate algorithm is also described.

Colour perception with changes in levels of illumination

The perceived colour of a stimulus depends on the conditions under which it is viewed. For colours employed as an important cue or identifier, such as signage and brand colours, colour reproduction tolerances are critically important. Typically, such stimuli would be judged using a known level of illumination but, in the target environment, the level of illumination used to view the samples may be entirely different. The effect of changes in the viewing condition on the perceptibility and acceptability of small colour differences should be understood when such tolerances and associated viewing conditions, are specified. A series of psychophysical experiments was conducted to determine whether changes in illumination level significantly alter acceptability and perceptibility thresholds of uniform colour stimuli. It was found that perceived colour discrimination thresholds varied by up to 2.0 ΔE00. For the perceptual correlate of hue however, this value could be of significance if the accepted error of colour difference was at the threshold, thereby yielding the possibility of rejection with changes in illumination level. Lightness and chroma on the other hand, exhibited greater tolerance and were less likely to be rejected with illuminance changes.

Color characterization methods for a multispectral camera

With advances in sensor technology, the availability of multispectral cameras and their use are increasing. Having more information compared to a three-channel camera has its advantages but the data must be handled appropriately. In this work, we are interested in multispectral camera characterization. We measure the camera characterization performance by two methods, by linear mapping and through spectral reconstruction. Linear mapping is used in 3-channel camera characterization and we use the same method for a multispectral camera. We also investigate whether instead of linear mapping, spectral reconstruction from the camera data improves the performance of color reproduction. The recovery of reflectance spectra is an under-determined problem and certain assumptions are required for obtaining a unique solution. Linear methods are generally used for spectral reconstruction from the camera data and are based on training on known spectra. These methods can perform well when the test data consists of a subset of the training spectra, however, their performance is significantly reduced when the test data is different. In this paper, we also investigate the role of training spectra for camera characterization. Five different spectral reflectance datasets are used for training the camera characterization models. Finally we provide a comparison between the linear mapping and spectral reconstruction methods for multispec-tral camera characterization and also test the camera characterization framework on hyperspectral images of natural scenes.

Introducing iccMAX: new frontiers in color management

ICC has announced a preliminary specification for iccMAX, a next-generation colour management system that expands the existing ICC profile format and architecture to overcome the limitation of the fixed colorimetric Profile Connection Space and support a much wider range of functionality. New features introduced in iccMAX include spectral processing, material identification and visualization, BRDF, new data types, an improved gamut boundary descriptor and support for arbitrary and programmable transforms. The iccMAX preliminary specification is accompanied by a reference implementation, and will undergo a period of public review before being finalized.

Adaptive color rendering of maps for users with color vision deficiencies

A map is an information design object for which canonical colors for the most common elements are well established. For a CVD observer, it may be difficult to discriminate between such elements - for example, it may be hard to distinguish a red road from a green landscape on the basis of color alone. We address this problem through an adaptive color schema in which the conspicuity of elements in a map to the individual user is maximized. This paper outlines a method to perform adaptive color rendering of map information for users with color vision deficiencies. The palette selection method is based on a pseudo-color palette generation technique which constrains colors to those which lie on the boundary of a reference object color gamut. A user performs a color vision discrimination task, and based on the results of the test, a palette of colors is selected using the pseudo-color palette generation method. This ensures that the perceived difference between palette elements is high but which retains the canonical color of well-known elements as far as possible. We show examples of color palettes computed for a selection of normal and CVD observers, together with maps rendered using these palettes.