Kaida Xiao

The achromatic locus: Effect of navigation direction in color space

An achromatic stimulus is defined as a patch of light that is devoid of any hue. This is usually achieved by asking observers to adjust the stimulus such that it looks neither red nor green and at the same time neither yellow nor blue. Despite the theoretical and practical importance of the achromatic locus, little is known about the variability in these settings. The main purpose of the current study was to evaluate whether achromatic settings were dependent on the task of the observers, namely the navigation direction in color space. Observers could either adjust the test patch along the two chromatic axes in the CIE u*v* diagram or, alternatively, navigate along the unique-hue lines. Our main result is that the navigation method affects the reliability of these achromatic settings. Observers are able to make more reliable achromatic settings when adjusting the test patch along the directions defined by the four unique hues as opposed to navigating along the main axes in the commonly used CIE u*v* chromaticity plane. This result holds across different ambient viewing conditions (Dark, Daylight, Cool White Fluorescent) and different test luminance levels (5, 20, and 50 cd/m(2)). The reduced variability in the achromatic settings is consistent with the idea that internal color representations are more aligned with the unique-hue lines than the u* and v* axes.

Colour-opponent mechanisms are not affected by age-related chromatic sensitivity changes

The purpose of this study was to assess whether age‐related chromatic sensitivity changes are associated with corresponding changes in hue perception in a large sample of colour‐normal observers over a wide age range (n = 185; age range: 18–75 years). In these observers we determined both the sensitivity along the protan, deutan and tritan line; and settings for the four unique hues, from which the characteristics of the higher‐order colour mechanisms can be derived. We found a significant decrease in chromatic sensitivity due to ageing, in particular along the tritan line. From the unique hue settings we derived the cone weightings associated with the colour mechanisms that are at equilibrium for the four unique hues. We found that the relative cone weightings (wL/wM and wL/wS) associated with the unique hues were independent of age. Our results are consistent with previous findings that the unique hues are rather constant with age while chromatic sensitivity declines. They also provide evidence in favour of the hypothesis that higher‐order colour mechanisms are equipped with flexible cone weightings, as opposed to fixed weights. The mechanism underlying this compensation is still poorly understood.

Color reproduction for advanced manufacture of soft tissue prostheses

OBJECTIVES The objectives of this study were to develop a color reproduction system in advanced manufacture technology for accurate and automatic processing of soft tissue prostheses. METHODS The manufacturing protocol was defined to effectively and consistently produce soft tissue prostheses using a 3D printing system. Within this protocol printer color profiles were developed using a number of mathematical models for the proposed 3D color printing system based on 240 training colors. On this basis, the color reproduction system was established and their system errors including accuracy of color reproduction, performance of color repeatability and color gamut were evaluated using 14 known human skin shades. RESULTS The printer color profile developed using the third-order polynomial regression based on least-square fitting provided the best model performance. The results demonstrated that by using the proposed color reproduction system, 14 different skin colors could be reproduced and excellent color reproduction performance achieved. Evaluation of the systems color repeatability revealed a demonstrable system error and this highlighted the need for regular evaluation. The color gamut for the proposed 3D printing system was simulated and it was demonstrated that the vast majority of skin colors can be reproduced with the exception of extreme dark or light skin color shades. CONCLUSIONS This study demonstrated that the proposed color reproduction system can be effectively used to reproduce a range of human skin colors for application in advanced manufacture of soft tissue prostheses.

Investigation of Chinese skin colour and appearance for skin colour reproduction

A Chinese skin colour database is established based on measurements taken from nine bodies areas of 202 Chinese individuals to move towards accurate skin colour reproduction. The colour appearance of each skin point is predicted, and a comprehensive colour gamut for Chinese skin is determined. A consistent colour shift between facial and arm colours is identified, in which facial colour tends to be more reddish, more colourful, and darker than arm colour. Moreover, Chinese females are found to have lighter, paler, and more yellowish skin than Chinese males. Variations in Chinese skin colour are quantified, and body area differences and gender differences are shown to have significant effects on Chinese skin colour.

Investigation of Elastomer Infiltration into 3D Printed Facial Soft Tissue Prostheses

Objectives: 3D colour printing, a method of additive manufacturing, has been developed and utilised to produce facial soft tissue prostheses. This was achieved by layered fabrication of a biocompatible powder held together by an aqueous binder containing a resin and coloured inks, followed by infiltration with a medical grade silicone polymer. The aim of this study was to investigate the elastomer infiltration depths within the 3D printed models. Methods: Three sets of 30 cubes ? 20x20x20 mm ? were used to investigate the infiltration depth of Sil-25 maxillofacial silicone polymer (an MSP) under atmospheric pressure, 2 bar and 3 bar pressure for 5, 10, 15, 20 and 25 min. The investigation was also repeated with two other MSPs ? Promax-10 and M-3428 ? under 3 bar pressure. Following infiltration, the cubes were bisected, the internal aspects stained with dye, and the infiltration depth measured using a travelling microscope. Infiltration quality was also assessed using scanning electron microscopy (SEM). Results: At standard atmospheric pressure, the maximum infiltration depth of Sil-25 was 1.45 mm after 25 min. However, after 25 min at 2 and 3 bars pressure, the infiltration depth increased to 3.9 mm and 8.7 mm, respectively. At 3 bars the infiltration depth of Promax-10 and M-3428 was 2.4 mm and 7.5 mm, respectively. In all samples SEM revealed a disorganised distribution of starch particles within the MSP infiltrate. Significance: Pressure significantly increased the infiltration rate and depth of the MSPs within 3D printed constructs. The infiltration depth obtained is sufficient for prostheses that are less than 16 mm thick.

Visual gamma correction for LCD displays

Abstract An improved method for visual gamma correction is developed for LCD displays to increase the accuracy of digital colour reproduction. Rather than utilising a photometric measurement device, we use observers’ visual luminance judgements for gamma correction. Eight half tone patterns were designed to generate relative luminances from 1/9 to 8/9 for each colour channel. A psychophysical experiment was conducted on an LCD display to find the digital signals corresponding to each relative luminance by visually matching the half-tone background to a uniform colour patch. Both inter- and intra-observer variability for the eight luminance matches in each channel were assessed and the luminance matches proved to be consistent across observers (Δ E 00 E 00 E 00 . We conclude that this observer-based method of visual gamma correction is useful to estimate the OETF for LCD displays. Its major advantage is that no particular functional relationship between digital inputs and luminance outputs has to be assumed.

Improved method for skin reflectance reconstruction from camera images

A improved spectral reflectance reconstruction method is developed to transform camera RGB to spectral reflectance for skin images. Rather than using conventional direct or two-step processes, we transform camera RGB to skin reflectance directly using a principal component analysis (PCA) approach. The novelty in our direct method (RGB to spectra) is the use of a skin-specific colour characterisation chart with spectra closer to human skin spectra, and a new database of skin reflectances to derive the PCA bases. The experimental results using the facial images of 17 subjects demonstrate that our new direct method gives a significantly better performance than conventional, two-step methods and direct methods with traditional characterization charts. This new spectral reconstruction algorithm is sufficiently precise to reconstruct spectral properites relating to chromophores and its performance is within the acceptable range for maxillofacial soft tissue prostheses (error < 3 ΔE*ab units).

Characterising the variations in ethnic skin colours: a new calibrated data base for human skin

Accurate skin colour measurements are important for numerous medical applications including the diagnosis and treatment of cutaneous disorders and the provision of maxillofacial soft tissue prostheses.

Colour management system for displaying microscope images

Abstract A four stages colour management system is developed in order to truly display microscope images on a display and to objectively assess colour properties for cytology samples. By using 35 stained glasses, the colour outputs of two microscopes were simulated. Each stage of the processing was performed and the microscope images captured by two different microscope camera systems were reproduced to a single LCD panel. The management system evaluated was found to be effective in terms of colour image reproduction and could be applied to situations when objective assessment is required.

Gamut Volume Index: a color preference metric based on meta-analysis and optimized colour samples

A novel metric named Gamut Volume Index (GVI) is proposed for evaluating the colour preference of lighting. This metric is based on the absolute gamut volume of optimized colour samples. The optimal colour set of the proposed metric was obtained by optimizing the weighted average correlation between the metric predictions and the subjective ratings for 8 psychophysical studies. The performance of 20 typical colour metrics was also investigated, which included colour difference based metrics, gamut based metrics, memory based metrics as well as combined metrics. It was found that the proposed GVI outperformed the existing counterparts, especially for the conditions where correlated colour temperatures differed.