The LLAB model for quantifying colour appearance

A reliable colour appearance model is desired by industry to achieve high colour fidelity between images produced using a range of different imaging devices. The aim of this study was to derive a reliable colour appearance model capable of predicting the change of perceived attributes of colour appearance under a wide range of media/viewing conditions. The research was divided into three parts: characterising imaging devices, conducting a psychophysical experiment, and developing a colour appearance model. Various imaging devices were characterised including a graphic art scanner, a Cromalin proofing system, an IRIS ink jet printer, and a Barco Calibrator. For the former three devices, each colour is described by four primaries: cyan (C), magenta (M), yellow (Y), and black (K). Three set of characterisation samples (120 and 31 black printer, and cube data sets) were produced and measured for deriving and testing the printing characterisation models. Four black printer algorithms (BPA), were derived. Each included both forward and reverse processes. A 2nd BPA printing model taking into account additivity failure, grey component replacement (GCR) algorithm gave the most accurate prediction to the characterisation data set than the other BPA models. The PLCC (Piecewise Linear interpolation assuming Constant Chromaticity coordinates) monitor model was also implemented to characterise the Barco monitor. The psychophysical experiment was conducted to compare Cromalin hardcopy images viewed in a viewing cabinet and softcopy images presented on a monitor under a wide range of illuminants (white points) including: D93, D65, D50 and A. Two scaling methods: category judgement and paired comparison, were employed by viewing a pair of images. Three classes of colour models were evaluated: uniform colour spaces, colour appearance models and chromatic adaptation transforms. Six images were selected and processed via each colour model. The results indicated that the BFD chromatic transform gave the most accurate predictions of the visual results. Finally, a colour appearance model, LLAB, was developed. It is a combination of the BFD chromatic transform and a modified version of CIELAB uniform colour space to fit the LUTCRI Colour Appearance Data previously accumulated. The form of the LLAB model is much simpler and its performance is more precise to fit experimental data than those of the other models

Evaluation and improvement of the workflow of digital imaging of fine art reproduction in museums

Fine arts refer to a broad spectrum of art formats, ie~painting, calligraphy, photography, architecture, and so forth. Fine art reproductions are to create surrogates of the original artwork that are able to faithfully deliver the aesthetics and feelings of the original. Traditionally, reproductions of fine art are made in the form of catalogs, postcards or books by museums, libraries, archives, and so on (hereafter called museums for simplicity). With the widespread adoption of digital archiving in museums, more and more artwork is reproduced to be viewed on a display. For example, artwork collections are made available through museum websites and Google Art Project for art lovers to view on their own displays. In the thesis, we study the fine art reproduction of paintings in the form of soft copy viewed on displays by answering four questions: (1) what is the impact of the viewing condition and original on image quality evaluation? (2) can image quality be improved by avoiding visual editing in current workflows of fine art reproduction? (3) can lightweight spectral imaging be used for fine art reproduction? and (4) what is the performance of spectral reproductions compared with reproductions by current workflows? We started with evaluating the perceived image quality of fine art reproduction created by representative museums in the United States under controlled and uncontrolled environments with and without the presence of the original artwork. The experimental results suggest that the image quality is highly correlated with the color accuracy of the reproduction only when the original is present and the reproduction is evaluated on a characterized display. We then examined the workflows to create these reproductions, and found that current workflows rely heavily on visual editing and retouching (global and local color adjustments on the digital reproduction) to improve the color accuracy of the reproduction. Visual editing and retouching can be both time-consuming and subjective in nature (depending on experts\u27 own experience and understanding of the artwork) lowering the efficiency of artwork digitization considerably. We therefore propose to improve the workflow of fine art reproduction by (1) automating the process of visual editing and retouching in current workflows based on RGB acquisition systems and by (2) recovering the spectral reflectance of the painting with off-the-shelf equipment under commonly available lighting conditions. Finally, we studied the perceived image quality of reproductions created by current three-channel (RGB) workflows with those by spectral imaging and those based on an exemplar-based method

The time-course of colour vision

Four experiments are presented, each investigating temporal properties of colour vision processing in human observers. The first experiment replicates and extends an experiment by Stromeyer et al. (1991). We look for a phase difference between combined temporal modulations in orthogonal directions in colour space, which might null the often-claimed latency of signals originating from the short-wavelength sensitive cones (S-cones). We provide another estimate of the magnitude of this latency, and give evidence to suggest that it originates early in the chromatic pathway, before signals from S-cones are combined with those that receive opposed L- and M-cone input. In the second experiment we adapt observers to two stimuli that are matched in the mean and amplitude of modulation they offer to the cone classes and to the cardinal opponent mechanisms, but that differ in chromatic appearance, and hence their modulation of later colour mechanisms. Chromatic discrimination thresholds after adaptation to these two stimuli differ along intermediate directions in colour space, and we argue that these differences reveal the adaptation response of central colour mechanisms. In the third experiment we demonstrate similar adaptation using the same stimuli, measured with reaction times rather than thresholds. In the final experiment, we measure the degree to which colour constancy is achieved as a function of time in a simulated stimulus environment in which the illuminant changes periodically. We find that perfect constancy is not achieved instantaneously after an illuminant chromaticity shift and that constancy of colour appearance judgements increases over several seconds

Anomalous trichromacy : external enhancement of colour signals, individual differences and diagnosis

Anomalous trichromacy is known to conceal a substantial range in perceptual ability, but this is not typically considered in assessments of corrective aids and diagnostic tests. In addition to the diversity caused by genetic polymorphisms, perceptual ability is thought to be influenced by little understood postreceptoral mechanisms, adding to the need for research focusing on this population. A modelling and behavioural investigation establishes the effectiveness of EnChroma filters in enhancing anomalous colour vision. Paper 1 (not yet published) employs a physiologically accurate model of colour vision to estimate the enhancements in cone-opponent signals conferred by the filters. Paper 2 (not yet published) presents behavioural validation of the model’s predictions, showing that notch filters can result in enhanced perceived saturation for deuteranomalous observers, with effects for suprathreshold perception and partial effects at absolute threshold. Paper 3 (submitted for publication) uses the physiologically accurate model of colour vision to investigate the impact of variation in edition and lighting conditions on the effectiveness of the Ishihara plates test in identifying those with mild anomalous trichromacy. The model predicts a significant impact of plate and illuminant, but no influence of edition, which is supported by the findings of a behavioural investigation. This thesis provides the first direct evidence that altering the input to the visual system using filter-based aids can impact the cone-opponent signals available to anomalous trichromats, and that this change in signal is useable by the visual system, resulting in changes to perceived saturatio

Phototherapy in the treatment of skin disease in Scotland

This thesis discusses some aspects of the use of ultraviolet rays to treat skin disease. The early history of dermatological phototherapy in Scotland is summarised as an introduction to the more recent developments described in a chapter on the Scottish phototherapy and photochemotherapy audit of 1996/1997 (funded by the Clinical Resource and Audit Group, the Scottish Office). This audit revealed some aspects of the phototherapy service that could be improved, and identified areas of particular importance for future research. Firstly, what type of UVB phototherapy lamp is most effective: the narrow-band (311-313nm) TL-01 lamp, or the broad-band UVB lamp? A meta-analysis conducted as part of this thesis gave a clear answer. TL-01 UVB is much more effective than broad-band UVB. The second included study was a randomised, controlled study that has contributed to deciding the optimal treatment frequency for TL-01 UVB phototherapy of psoriasis. Although 5x weekly treatment cleared psoriasis slightly more quickly than 3x weekly treatment, the difference in speed of clearance was too small to warrant the significantly greater frequency of acute erythema during treatment, and the greater number of exposures and dose required. The final question, answered by a randomised, controlled trial, in conjunction with a systematic review of the previous literature, was: for chronic plaque psoriasis, is TL-01 UVB or psoralen-UVA photochemotherapy to be preferred? The study conducted for this thesis showed TL-01 UVB to be more effective. Heterogeneity in findings of the studies addressing this question highlighted the importance of the particular treatment regimens selected for comparison, but the overall conclusion was that TL-01 UVB is the first choice of these two therapies