Stephen Westland

Characterization of trichromatic color cameras by using a new multispectral imaging technique.

We investigate methods for the recovery of reflectance spectra from the responses of trichromatic camera systems and the application of these methods to the problem of camera characterization. The recovery of reflectance from colorimetric data is an ill-posed problem, and a unique solution requires additional constraints. We introduce a novel method for reflectance recovery that finds the smoothest spectrum consistent with both the colorimetric data and a linear model of reflectance. Four multispectral methods were tested using data from a real trichromatic camera system. The new method gave the lowest maximum colorimetric error in terms of camera characterization with test data that were independent of the training data. However, the average colorimetric performances of the four multispectral methods were statistically indistinguishable from each other but were significantly worse than conventional methods for camera characterization such as polynomial transforms.

Spectrophotometric Analysis of All-ceramic Materials and Their Interaction with Luting Agents and Different Backgrounds

In this study, two All-Ceramic (AC) materials—Empress 2 (EMP) (Ivoclar Vivadent AG, Schaan, Liechtenstein) and In-Ceram ALUMINA (ICA) (Vita Zahnfabrik, Bad Säckingen, Germany)—were analyzed, along with the effects of 3 luting agents—viz. Zinc Phosphate cement (ZNPO, PhospaCEM PL, Ivoclar Vivadent AG, Schaan, Liechtenstein), Glass Ionomer Cement (GIC, Ketac-Cem Radiopaque, ESPE Dental AG, Seefeld, Germany), and Compolute (COMP, ESPE Dental AG, Seefeld, Germany)—on the final color, using the CIELab system. Color differences (DeltaL, Deltaa, Deltab, and DeltaE) were calculated for samples with luting agents and for samples without luting agents with standard white and black backgrounds, with the use of a spectrophotometer, Luci 100 (Dr. Lange, Berlin, Germany). One-way ANOVA for DeltaL, Deltaa, Deltab, and DeltaE within both the AC systems, with and without luting agents, showed significant contributions of the background (p < 0.05). EMP was seen to be more translucent than ICA. Darker ceramics showed less color variation. Luting agents altered the final color of the restoration. ZNPO was least translucent, followed by GIC and COMP. Marginal increases in thicknesses of ICA samples (0.4 mm) do not show a statistically significant color difference. No method exists to predict the outcome of an AC restoration based on consideration of the luting agent and the background color.

Invariant cone-excitation ratios may predict transparency

Cone-excitation ratios for pairs of surfaces are almost invariant under changes in illumination and offer a possible basis for color constancy [Proc. R. Soc. London Ser. B 257, 115 (1994)]. We extend this idea to the perception of transparency on the basis of the close analogy between the changes in color signals that occur for surfaces when the illumination changes and the changes in color signals when the surfaces are covered by a filter. This study presents measurements and simulations to investigate the conditions under which cone-excitation ratios are statistically invariant for physically transparent systems. The invariance breaks down when the spectral transmission of the filters is low at some or all wavelengths. We suggest that cone-excitation ratios might be useful to define the stimulus conditions necessary for the perception of transparency.

Multiple groups of orientation-selective visual mechanisms underlying rapid orientated-line detection

Visual search for an edge or line element differing in orientation from a background of other edge or line elements can be performed rapidly and effortlessly. In this study, based on psychophysical measurements with ten human observers, threshold values of the angle between a target and background line elements were obtained as functions of background–element orientation, in brief masked displays. A repeated–loess analysis of the threshold functions suggested the existence of several groups of orientation–selective mechanisms contributing to rapid orientated–line detection; specifically, coarse, intermediate and fine mechanisms with preferred orientations spaced at angles of approximately 90°, 35°–50°, and 10°–25°, respectively. The preferred orientations of coarse and some intermediate mechanisms coincided with the vertical or horizontal of the frontoparallel plane, but the preferred orientations of fine mechanisms varied randomly from observer to observer, possibly reflecting individual variations in neuronal sampling characteristics.

Colour statistics of natural and man‐made surfaces

The reflectance spectra of natural and man‐made surfaces are highly constrained. Statistical analyses have been conducted that confirm that the surface reflectance spectra form a set of band‐limited functions with a frequency limit of approximately 0.02 cycles/nm. The reflectance spectra can be represented by a linear‐model framework and are adequately described by 6‐12 basis functions. However, the spectral properties of surfaces are not so constrained as to allow the human visual system to recover the surface properties from cone excitations. Furthermore, trichromatic colour devices such as scanners and cameras can only capture illumination‐specific colour information.

Discoloration of Teeth after Avulsion and Replantation: Results from a Multicenter Randomized Controlled Trial

INTRODUCTION There is evidence to suggest that Ledermix, placed as an intervisit root canal dressing, might improve periodontal healing after replantation of avulsed teeth. As a part of a multicenter randomized controlled trial, we aimed to compare the effect of 2 root canal medicaments, Ledermix and Ultracal XS, on the discoloration of replanted teeth. METHODS Discoloration was investigated by using 3 methods: patient satisfaction with the color of replanted teeth, clinical photographs taken at baseline and 12-month reviews, and estimation of color change by using CIELAB scores for baseline and 12-month photographs. RESULTS Twenty-two patients (27 teeth) were recruited. Ten patients (12 teeth) were randomized to the Ledermix group and 12 patients (15 teeth) to the Ultracal XS group. At 12 months, 8 patients were concerned with the discoloration of their teeth. Seven came from the Ledermix group and 1 from the Ultracal XS group. This difference was significant (Fisher exact test, P = .009). Standardized photographs were taken for the patients recruited at one center only (17 patients). There was significant discoloration of teeth from baseline with Ledermix, causing a darkening and gray-brown discoloration (mean change from baseline to 12 months, L∗ = -5.1, a∗ = 0.3, b∗ = -1.2, and ΔE = 8.1) and Ultracal XS, causing a yellowing and lightening of teeth (L∗ = 1.9, a∗ = 0.3, b∗ = 3.3, and ΔE = 5.4). There was a significant difference for the L∗ and b∗ variables (independent t test) between the 2 groups. CONCLUSIONS Both root canal medicaments cause discoloration, with Ledermix proving less acceptable to patients.

Kubelka-Munk or neural networks for computer colorant formulation?

Traditionally Computer Colorant Formulation has been implemented using a theory of radiation transfer known as Kubelka-Munk (K-M) theory. Kubelka-Munk theory allows the prediction of spectral reflectance for a mixture of components (colorants) that have been characterised by absorption K and scattering S coefficients. More recently it has been suggested that Artifical Neural Networks ANNs) may be able to provide alternative mappings between colorant concentrations and spectral reflectances and, more generally, are able to provide transforms between color spaces. This study investigates the ability of ANNs to predict spectral reflectance from colorant concentrations using a set of data measured from known mixtures of lithographic printing inks. The issue of over-training is addressed and we show that the number of hidden units in the network must be carefully selected. We show that it is difficult to train a conventional neural network to the level that matches the performance that can be achieved using the K-M theory. However, a hybrid model is proposed that may out-perform the K-M model.

Color Adjustment Potential of Resin Composites

The study objective was to explore a new method for quantifying the color adjustment potential originating from physical translucency on a set of 7 resin composites, and then for testing the hypothesis that color adjustment potential is dependent on the composites and shades studied. Two-composite specimens (an outer base shade with an inner hole filled with inner test shades) and single-composite specimens of all shades were made. A 1-mm circular area, with its center in the middle of the specimen (P0mm), was measured by means of spectroradiometry. A newly developed equation for quantification of the color adjustment potential was tested. Color adjustment potential at P0mm ranged from −0.19 (negative color adjustment/contrast) to 0.61. Within the limitations of this study, a newly developed concept and equation have proved the existence of the physical component of color adjustment of translucent dental materials. Color adjustment potential was dependent on composite and shade.

Comparison of linear spectral reconstruction methods for multispectral imaging

This paper compares the performance of a number of linear methods for reflectance estimation from digital camera responses. Methods based upon smoothness maximisation, linear models of reflectance and least-squares fitting are compared using two simulated 6-channel camera systems. In our experiments, the smoothness methods were generally found to deliver the best performance on test data. Furthermore, they deliver equivalent performance on training data, even compared to those methods that make explicit use of a priori knowledge of the training data.

Conditions for perceptual transparency

We review the conditions that are necessary for the perception of transparency and describe the spatiochromatic constraints for achromatic and chromatic transparent displays. These constraints can be represented by the convergence model and are supported by psychophysical data. We present an alternative representation of the constraints necessary for transparency perception that is based on an analogy with a model of colour constancy and the invariance of cone-excitation ratios. Recent psychophysical experiments are described that suggest that displays where the cone-excitation ratios are invariant produce a stronger impression of transparency than displays where the cone excitations are convergent. We argue that the spatial relations in an image are preserved when a Mondrian-like surface is partially covered by a transparent filter and therefore show an intriguing link between transparency perception and colour constancy. Finally, we describe experiments to relate the strength of the transparency percept with the number of unique patches in the image display. We find that the greater the number of surfaces in the display that are partially covered by a transparent filter the stronger the impression of transparency.