Author: Wu, Jialu
Title: Colour characterization and specification for liquid crystal displays (LCDs) and organic light emitting diode (OLED) displays
Advisors: Wei, Minchen (BEEE)
Degree: Ph.D.
Year: 2023
Subject: Information display systems
Color display systems
Hong Kong Polytechnic University -- Dissertations
Department: Department of Building Environment and Energy Engineering
Pages: xxii, 157 pages : color illustrations
Language: English
Abstract: As a procedure to present images in appropriate quality and accurate colours, display calibration was explored since the invention of the electronic display in the nineteenth century. Its aim is to accurately reproduce colours on the display corresponding to real-world vision or another display. Overall, the procedure involves two stages: colour characterization and colour specification. The first stage involves characterizing the colours presented on displays in the International Commission on Illumination (CIE) colorimetry system based on the input digital count values to evaluate the colours perceived by the human visual system. Thereafter, the values in the CIE colorimetry system were specified in a hypothetical display colour space that is independent of the devices. Thus, the displays were calibrated to produce colours according to the values defined in the hypothetical colour spaces.
However, the conventional models of colour characterization were developed assuming channel independence and channel-chromaticity constancy according to the property of the conventional cathode ray tube (CRT) displays. Those models failed to characterize the newly developed organic light emitting diode (OLED) display and a component of the liquid crystal display (LCD), which did not satisfy channel independence. Although various models have been developed to resolve the issue of channel dependency for these displays, extensive measurements are required to achieve high accuracy for such models.
Other problems with the display calibration involves colour mismatch, which refer to the phenomenon that colours with the same values defined in the hypothetical colour spaces appear as different colours to human observers, or colours observed the same colour appearances but having different values defined in the hypothetical colour spaces. This phenomenon illustrated that the CIE colorimetry system may not explain the colour perception of the human visual system in certain conditions, because the values in the hypothetical colour spaces were calculated based on those defined in the CIE colorimetry system. Moreover, the variations of densities of optical pigments in human eyes varied across individuals, which caused the phenomenon of observer metamerism—a pair of stimuli with a matched colour appearance to one observer exhibits a different appearance to another observer. Thus, the use of one standard observer to represent the colour vision of normal observers may cause failure of characterization for certain observers.
Taking considerations of those problems, in Study 1, this dissertation proposed a display colour characterization model to correct the influences of channel dependency and can be used to accurately characterize OLED displays. We propose a 3D piecewise model, wherein dividing the display RGB space into 27 subspaces and characterizes the interactions between two and three channels based on the measurements of the tristimulus values of 64 RGB combinations of digital count values. The average colour difference in terms of ΔEab in the CIELAB colour space (Section 2.2.4) of 41 test colours between the model predictions and the measurements was around 0.92, in comparison to the average of 10.47 by the widely used model assuming channel independence (i.e., the GOG model as described in Section 2.5.1), across nine OLED displays. As a result, the proposed model performed great improvement compared to the conventional model in the characterization of OLED displays.
In Study 2, two colour matching experiments were reported to explore the performance of the CIE colorimetry system on the characterization of the colours presented on various displays. In the experiments, human observers matched the colour appearance of six colour stimuli produced by four smartphone displays, including one LCD and three OLED displays, to those produced by a reference smartphone OLED display. The matching and reference stimuli had a field of view (FOV) around 4.77° for the first part of Study 2 and 20.2° for the second part of Study 2. The performance of the four CMFs, however, were not significantly changed with the increase of the FOV. The experimental results indicated the failure of the applicability of the CIE colorimetry system in characterization of the colours between LCD and OLED displays, corresponding to the colour perception of the human visual system.
In Study 3, a corrected method of colour mismatch between LCD and OLED displays was proposed. A colour matching experiment was carried out in Study 3. The experiment results indicated the similar performance of the CIE colorimetry system in characterization of the colours between displays, but more serious degrees of colour mismatch and observer metamerism compared with the results derived in Study 2. The colour matching data collected from the experiment were used for correcting the colour mismatch. The results indicated the effectiveness performance of the correction, through which the degree of colour mismatch between LCD and OLED displays was significantly reduced.
In summary, this dissertation aids in understanding the procedure of display calibration applied to various display technologies. The present findings will serve as references to achieve accurate display calibration—both in the processes of the colour characterization and colour specification—for new types of displays such as OLED displays.
Rights: All rights reserved
Access: open access

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