Colour appearance modelling of unrelated self-luminous stimuli

Colour appearance models, i.e. models that attempt to predict the colour appearance of a stimulus by taking the physical properties of the stimulus and its surroundings into account, have been developed and investigated for more than 40 years. Most of these models were developed to handle related colours, i.e. colours perceived in relation to other colours. A typical example is the rsquo; colour of an object as seen in an illuminated scene. However, two models - CAM97u and CAMFu -nbsp;developed to predict the appearance of unrelated colours, i.e. colours perceived in isolation from any other colour (e.g. a traffic light seen at night). Unfortunately, due to the lack of psychophysical data, neither of these two models has been investigated extensively. Before beingnbsp;to extend these models to other types of stimuli and viewing conditions, they need to benbsp;using new visual data. The aim of this doctoral research is to investigate the colour appearance of unrelated self-luminous stimuli. An accurate prediction of the colour appearance of these stimuli through a colour appearance modelnbsp;be a valuable tool: it can assist in the development of requirements for light-emitting diode (LED) signs, in the standardization of the appearance of marine, aviation or traffic lights viewed during a dark night, in the continuous development of colour appearance models for other viewinghellip; In a first series of psychophysical experiments, the brightness of stimulinbsp;a constant luminance has been evaluated by a group of observers. The stimuli were shown in a darkened room, specially designed for this doctoralnbsp;project. In the centre ofnbsp;wall, a circular self-luminous area was present. The colour of this stimulus area wasnbsp;controllable bynbsp;the flux of the R(ed)G(reen)B(lue)W(white) LED behind it. The observers viewed the stimulus area from a distance that ensures a 10° field of view. The brightness evaluation of these stimuli was performed using a magnitude estimation method by scaling the brightness of each test stimulus compared to that of a reference stimulus to which anbsp;value of 50 was attributed. The predictive performance of the CAM97u and CAMFu colour appearance models and four other vision models, specially designed to predict brightness, was investigated. Due to, among others, a severe underestimation of the effect of colourfulness onnbsp;- also known as the Helmholtz-Kohlrausch effect -nbsp;of the models seemed to be able to adequately predict the brightness perceived by the observers. Adapting the CAM97u model by increasing the colourfulness contribution in the brightness attribute, resulted in a modified model, called CAM97um, which allows for a substantially better brightness prediction. The performance ofnbsp;newnbsp;was confirmed by the results of both a matching experiment and an extensive magnitude estimation experiment in which the test stimuli covered a widenbsp;of luminance and chromaticity values. In anbsp;series of psychophysical experiments, in addition to the brightness, the hue and “amount of white” perception of unrelatednbsp;stimuli was also investigatednbsp;a magnitude estimation method. The amount of white is a newly proposed attribute, and basically corresponds to a layperson’s conception of attributes such asnbsp;chroma or saturation. It was introducednbsp;on the results of a preliminary pilot study revealing that laypersons often have difficulty understanding, and hence judging, the colourfulness of a stimulus. Again, unrelated self-luminous 10° stimuli, with anbsp;range of luminance and chromaticity values, were evaluated by observers in the darkened room. Based on the obtained visual data, a new colour appearance model for unrelated self-luminous stimuli, CAM15u, was developed. The main features of the model are the use of the absolute spectral radiance of the stimulus as input, the use of the CIEnbsp;cone fundamentals and a simplified calculation procedure compared to existing models. The model predictsnbsp;brightness, hue, colourfulness, saturation and the amount of white. The CAM15u model is restricted to photopic, non-glary unrelated stimuli having a field of view ofdeg;. The model was validated using the results of an additional experiment. It was found that, despite its simplicity, CAM15unbsp;as well or better than other,nbsp;complicated, CAMs. In a final series ofnbsp;experiments, the brightness perception of different sized,nbsp;self-luminous stimuli was investigated in a magnitude estimation experiment. The stimuli were shown in a darkened room on a wide gamut LCD monitor. A significant, hue independent, effect of stimulus size on brightness was found, effectively modeled by a simple power function. Finally, the dependence of brightness on stimulus size was incorporated into the brightness prediction of the CAM15u model.nbsp;predictive performance of the modified brightness predictionnbsp;validated using the results obtained in an additional experiment in which observers evaluated the brightness of unrelated self-luminousnbsp;stimuli with variable size, chromaticity and luminance. Although further improvements and extensions are still possible, CAM15u has proven itsnbsp;in predicting the appearance of unrelated self-luminous stimuli. It can be a valuable tool for the improvement of existing standards and guidelines for traffic signs, LED billboards,…

Publication year:2015

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