The colour size effect is an important aspect of colour appearance phenomenon, in which the colour appearance is changed according to different sizes of the same colour stimulus. Although colour appearance has been actively researched over the last 20 years and models have been developed capable of predicting the colour appearance under a wide range of viewing conditions, the colour size effect was not taken into account due to lack of reliable colour appearance data for colours with various sizes. This lead to great difficulty for industry for faithfully reproducing colours between dissimilar sizes. In this study, it is intended to accumulate colour appearance data for various sizes, to analyse the size effect and develop a model to predict size effect in purpose of accurate colour appearance reproduction across dissimilar sizes. Colours in six sizes, including the size of a real room, were divided into two groups and were assessed by a panel of observers. The techniques of magnitude estimation and asymmetric colour matching (by using CRT and physical colours) were applied. Totally, there were 4920 assessments conducted for various field sizes by using different scaling techniques under different viewing conditions. The colour appearance of the room was investigated in two aspects: size effect and mixed illuminant based upon both physical measurement and psychophysical visual experimental results. For the former, colour appearance between the room and 10° size were compared and modelled. For mixed illuminant prediction, a mathematical model was developed to predict the adopted white for a room with any wall colour. Finally, a model for predicting colour appearance for room colours was derived including three parts: the illumination prediction, colour appearance model (CIECAM02) and size effect correction. The size effect was further investigated using the visual results from all different six SIZ es. Based upon the CIB recommended 2° and 10° standard colorimetric observers, the colour appearances of object samples were predicted. The changes of colour appearance for those colour samples from one size to another were analysed. Most importantly, the relative changes of colour appearances of six dissimilar size colours were also revealed based upon proposed experimental results. A clear trend was found that colour appears lighter and more colourful with the increase of stimulus size, there is however little change in hue.Finally, colour appearance models that are capable of predicting colour size effect for any sizes of colours were developed. They were firstly derived based upon both CIELAB uniform colour space and CIECAM02 colour appearance model, which were named as size effect correction model. Another model, named as size effect transform model, was also developed based on human cone response functions. By using those size effect models, the colour appearance for dissimilar sizes can be predicted incorporate with colour appearance model . These models can also successfully predict the visual results.

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A coloured background may induce a contrasting colour in a target figure set against it. This effect, known as 'simultaneous chromatic contrast induction', can strongly influence the way in which we perceive colours and may therefore play an important role in our colour perception. Yet in order to support putative roles in phenomena such as colour-constancy, simultaneous contrast effects must be in some way 'regulated', otherwise they would cause objects to change colour when moved from one setting to another. Here we investigate a number of candidate cues that might be expected to influence the strength of simultaneous chromatic contrast. In experiment 1, we investigate the influence of global illumination cues that can indicate whether a colour change is consistent with a change in the illumination over the entire scene, or with a localised change in the background colour. Global illumination changes have already been shown to facilitate simultaneous contrast induction, and we further find that this effect is diminished by increasing the luminance contrast between the elements that make up the scene. Contrary to our expectations, increasing the number of distinct chromaticities in the surround does not have any effect. Finally, we attempt to discover whether this effect occurs at a monocular or a binocular site, by presenting different elements of the stimuli either to the same eye as the target, or to different eyes. Here, our results suggest that the effect is probably binocular. In experiment 2, we investigate how the strength of simultaneous contrast induction changes when textured backgrounds and targets are substituted for the uniformly shaded backgrounds and targets that have been studied in most recent investigations of simultaneous contrast induction. Texture in the background reduces the strength of simultaneous contrast induction for uniform targets. However, texture in targets set against a uniform background strongly inhibits simultaneous contrast induction. When both background and target are textured, simultaneous contrast induction is partially restored suggesting that texture may exert its influence due to segmentation effects. However, another result shows that simultaneous contrast is primarily determined at a local level, arguing against this interpretation of the effect. We do show that the effects of texture are tightly tuned to the cardinal axes, implicating early visual mechanisms in the functioning of simultaneous chromatic contrast effects. In experiment 3, we investigate two other strong segmentation cues: binocular disparity and differential motion between a target and its immediate surround. Our results show that differential motion may facilitate simultaneous contrast induction, strengthening its effects. However the effect is small and is often inconsistent between observers.

The topic of colour appearance has been studied over the years and various colour-appearance models have been developed for predicting colour appearance under a wide range of viewing conditions. The CIECAM02 model was recommended in 2002 by the CIE as the latest colour-appearance model for industrial applications.

Perceived object colour tends to stay constant under changes in illumination. This phenomenon is called colour constancy. Colour constancy is an essential component of colour perception and is typically studied in the laboratory via asymmetric colour matching experiments, in which the observer views two colours under two different illuminations side by side and makes matches between them. This situation is unlike colour constancy in the real world, which must typically involve a comparison between the colour one views and the colour one remembers - in other words, colour memory must be required. Furthermore, most colour constancy studies use twodimensional Mondrian images as experimental stimuli. These stimuli enable easy computer control of colour but exclude most of the natural perceptual cues such as binocular disparity, 3D luminance shading, mutual reflection, surface texture, glossy highlights, all of which may contribute to colour perception. My aim, in this project, is to study the colour perception of real objects in a more natural environment. To do so, I have developed an experimental setup which preserves the advantages conferred by easy computer-driven control of colour as well as the natural binocular and monocular cues to 3D shape. The setup also permits the use of real solid objects as stimuli, and the manipulation of their apparent surface colour as well as the background illumination. Thus, using this setup, I have been able to employ both 2D and 3D natural objects as stimuli and investigate aspects of colour perception related to colour constancy and colour memory as well as object familiarity. In developing and analysing these experiments, I have also introduced a new index of colour constancy which explicitly incorporates colour memory. My experiments reveal the following main principles: 1) colour constancy relies on colour memory, and is as good as colour memory allows; 2) colour and shape perception interact in both object similarity and discrimination tasks, indicating that colour and shape cannot be studied completely independently of each other; 3) object familiarity affects colour perception, for both foreground and background objects; 4) object familiarity also affects colour perception at perceptual levels, as measured by the reaction times and the range of appropriate colours accepted for an object.

The current thesis presents a series of experiments examining the relationship between language and colour cognition. The main focus is on the categorical perception (CP) of colour, the finding that discrimination of colours that straddle a category boundary is more accurate than within category colour discrimination. It has recently been argued that CP is not a perceptual effect, but rather a direct effect of language due to the comparison of stimulus labels (Roberson & Davidoff, 2000). The current thesis tests this account in a series of colour discrimination tasks with verbal and visual interference, a visual search task and a cross-cultural comparison of speakers of languages that make different categorical colour distinctions. The results of the colour discrimination tasks suggest that the effect of verbal interference on CP is dependent on task design. CP was found to survive verbal interference when the type of interference was not predictable, suggesting that the task was open to encoding strategies. CP did not survive when the colour target was presented with incongruent Stroop interference. An account of CP is proposed in which target name generation is necessary for CP. However, incongruent Stroop interference at test stimuli presentation did not selectively affect CP, suggesting that the process leading to CP is more complex than a simple matching-to-labels account would suggest. Target name generation may activate or reinforce a category code, which in turn facilitates cross-category discrimination. However, as evidence for CP was also found in the visual search task - suggesting that CP can be a perceptual effect - the account of CP proposed here may be specific to certain types of tasks only. The possibility that CP is a memory effect due to a shift towards prototype (Huttenlocher, Hedges & Vevea, 2000) was also considered. The findings presented here suggest that CP is dissociated from a shift towards prototype. The cross-cultural comparison of English and Owambo speakers on a triads and a visual search task allowed a further test of the role of language in colour CP. Language effects were found on both tasks, however, the differences were not all predicted by differences in colour naming. The results suggest that the representation of colour may also differ cross-culturally. Overall, the results presented here suggest that colour perception is not independent of colour language.

The primary goal of this work was to investigate colour perception in a natural environment and to contribute to the understanding of how cues to familiar object identity influence colour appearance. A large number of studies on colour appearance employ 2D uniformly coloured patches, discarding perceptual cues such as binocular disparity, 3D luminance shading, mutual reflection, and glossy highlights are integral part of a natural scene. Moreover, natural objects possess specific cues that help our recognition (shape, surface texture or colour distribution). The aim of the first main experiment presented in this thesis was to understand the effect of shape on (1) memory colour under constant and varying illumination and on (2) colour constancy for uniformly coloured stimuli. The results demonstrated the existence of a range of memory colours associated with a familiar object, the size of which was strongly object-shape-dependent. For all objects, memory retrieval was significantly faster for object-diagnostic shape relative to generic shapes. Based on two successive controls, the author suggests that shape cues to the object identity affect the range of memory colour proportionally to the original object chromatic distribution. The second experiment examined the subject’s accuracy and precision in adjusting a stimulus colour to its typical appearance. Independently on the illuminant, results showed that memory colour accuracy and precision were enhanced by the presence of chromatic textures, diagnostic shapes, or 3D configurations with a strong interaction between diagnosticity and dimensionality of the shape. Hence, more cues to the object identity and more natural stimuli facilitate the observers in accessing their colour information from memory. A direct relationship was demonstrated between chromatic surface representation, object’s physical properties, and identificability and dimensionality of shape on memory colour accuracy, suggesting high-level mechanisms. Chromatic textures facilitated colour constancy. The third and fourth experiments tested the subject’s ability to discriminate between two chromatic stimuli in a simultaneous and successive 2AFC task, respectively. Simultaneous discrimination threshold performances for polychromatic surfaces were only due to low-level mechanism of the stimulus, whereas in the successive discrimination, i.e. when memory is involved, high-level mechanisms were established. The effect of shape was strongly task- dependent and was modulate by the object memory colour. These findings together with the strong interaction between chromatic cues and shape cues to the object identity lead to the conclusion that high level mechanisms linked to object recognition facilitated both tasks. Hence, the current thesis presents new findings on memory colour and colour constancy presented in a natural context and demonstrates the effect of high-level mechanisms in chromatic discrimination as a function of cues to the object identity such as shape and texture. This work contributes to a deeper understanding of colour perception and object recognition in the natural world.