Individual Colorimetric Observers for Personalized Color Imaging

Colors are typically described by three values such as RGB, XYZ, and HSV. This is rooted to the fact that humans possess three types of photoreceptors under photopic conditions, and human color vision can be characterized by a set of three color matching functions (CMFs). CMFs integrate spectra to produce three colorimetric values that are related to visual responses. In reality, large variations in CMFs exist among color-normal populations. Thus, a pair of two spectrally different stimuli might be a match for one person but a mismatch for another person, also known as observer metamerism. Observer metamerism is a serious issue in color-critical applications such as soft proofing in graphic arts and color grading in digital cinema, where colors are compared on different displays. Due to observer metamerism, calibrated displays might not appear correctly, and one person might disagree with color adjustments made by another person. The recent advent of wide color gamut display technologies (e.g., LEDs, OLEDs, lasers, and Quantum Dots) has made observer metamerism even more serious due to their spectrally narrow primaries. The variations among normal color vision and observer metamerism have been overlooked for many years. The current typical color imaging workflow uses a single standard observer assuming all the color-normal people possess the same CMFs. This dissertation provides a possible solution for observer metamerism in color-critical applications by personalized color imaging introducing individual colorimetric observers. In this dissertation, at first, color matching data were collected to derive and validate CMFs for individual colorimetric observers. The data from 151 color-normal observers were obtained at four different locations. Second, two types of individual colorimetric observer functions were derived and validated. One is an individual colorimetric observer model, an extension of the CIE 2006 physiological observer incorporating eight physiological parameters to model individuals in addition to age and field size inputs. The other is a set of categorical observer functions providing a more convenient approach towards the personalized color imaging. Third, two workflows were proposed to characterize human color vision: one using a nomaloscope and the other using proposed spectral pseudoisochromatic images. Finally, the personalized color imaging was evaluated in a color image matching study on an LCD monitor and a laser projector and in a perceived color difference study on a SHARP Quattron display. The personalized color imaging was implemented using a newly introduced ICC profile, iccMAX

Effects of resolution of lighting control systems

Advances in lighting technologies have spurred sophisticated lighting control systems (LCSs). To conserve energy and improve occupants’ wellbeing, LCSs have been integrated into sustainable buildings. However, the complexity of LCSs may lead to negative experiences and reduce the frequency of their use. One fundamental issue, which has not been systematically investigated, is the impact of control resolution (the smallest change produced by an LCS). In an ideal LCS, the resolution would be sufficiently fine for users to specify their desired lighting conditions, but the smallest change would be detectable. Thus, the design of optimal control systems requires a thorough understanding of the detectability and acceptability of differences in illuminance, luminance and colour. The control of colour is complicated by the range of interfaces that can be used to facilitate colour mixing. Four psychophysical experiments investigated the effect of LCS resolution. The first two experiments explored the effect of resolution in white light LCSs on usability and energy conservation. The results suggest that, in different applications, LCSs with resolutions between 14.8 % and 17.7 % (of illuminance) or 26.0 % and 32.5 % (of luminance) have the highest usability. The third experiment evaluated the usability of three colour channel control interfaces based on red, green, blue (RGB), hue, saturation, brightness (HSB) and opponent colour mixing systems. Although commonly used, the RGB interface was found to have the lowest usability. The fourth experiment explored the effect of hue resolution, saturation resolution and luminance resolution on the usability. Generally, middle range resolutions, which are approximately between three and five times the magnitude of the just noticeable difference (JND), for both hue and saturation were found to yield the greatest usability. The interaction between these three variables was characterised. Findings from this research provide a deeper understanding of the fundamental attribute of control resolution and can guide the development of useful and efficient lighting control systems

Characterization and Control of a Multi-Primary LED Light Lab

A new light lab facility has been commissioned at Rochester Institute of Technology with the research goal of studying human visual adaptation under temporally dynamic lighting. The lab uses five-channel LED luminaires with 16 bits of addressable depth per channel, addressed via DMX. Based on spectral measurements, a very accurate multiprimary additive color model has been built that can be used to provide “colorimetric plus” multi-primary channel intensity solutions optimized for spectral accuracy, color fidelity, color gamut, or other attributes. Several spectral tuning and multi-primary solutions are compared, for which accuracy results and IES TM-30-15 color rendition measures are shown

Perception-Aware Optimisation Methodologies for Quantum Dot Based Displays and Lighting

Human colour vision acuity is limited. This presents opportunities to leverage these perceptual limits to achieve engineering optimisations for devices and systems that interact with the human vision system. This dissertation presents the results of few investigations we carried out into quantifying these limits and several optimisation methodologies that we devised. The first step in this process is to quantify the acuity of human colour vision. We obtained a large corpus of colour matching data from a mobile video game called Specimen. We examine what questions about human vision this dataset allows us to answer and explore global statistics about colour vision based on this data on 41,000 players from 175 countries. We show that we can use the information in this dataset to infer potential candidate functions for the spectral sensitivities of each person in the dataset. The human eye acts like a many to one function; quantifiably different spectra can look like the same colour. This is referred to as metamerism. From a device perspective, different spectra consume different amounts of energy to generate. We show that we can use these two properties to elicit the same colour sensation using less energy. In the colour samples we evaluated, we show that we can achieve up to 10 times less power consumption while achieving a colour match. Given that one cannot change the emission spectrum of a display after fabrication, we propose the use of a multi-primary colour display to achieve this. We present two indices for quantifying the metameric capacity of such a display and its ability to save energy. The emission spectrum of a quantum dot (QD) based device is very narrow. Previous work in the literature suggested that narrow bandwidth spectra can lead to observer metameric breakdown; different observers disagreeing on the perceived ‘colour’ of a spectrum. We show that this might not be the case, using modern colour science tools, and show how metameric breakdown in a display could be minimised by carefully choosing the primary emission wavelengths. The limited colour acuity of human vision implies that people cannot notice small differences in colour. This fact has been used to create approximate colour transformation algorithms that subtly change colours in images such that they consume less energy when displayed on an emissive pixel display without causing unacceptable visual artefacts. We conducted a user study to gather information about the effect of one such colour transform called Crayon. We present a method for effectively picking the optimal transform parameters for Crayon, based on the user study results. The method presented calculates these parameters based on the properties of the image being transformed such that the power saving can be maximised while minimising the loss of image quality. The user study results show that we can achieve up to 50% power saving with a majority of the study participants reporting a negligible degradation in image quality in the transformed images. We additionally investigate a hypothesis that was presented stating that images with large amounts of highly luminous pixels cause increased power consumption in OLED displays due to localised display heating. We show that this hypothesis is wrong. We also investigate if sub-pixel rendering in Pentile displays can be used to reduce display power consumption by intentionally turning off random sub-pixels. However, we present a negative result showing that even single-pixel artefacts are observable on the test platform and thus, this cannot be used to improve display power efficiency. The narrow-band optical emissions of QD based devices mixed with their ability to be fabricated through solution processing can be used to mix multiple QDs together to build devices that generate arbitrary spectral shapes. We show how to use this property in an numerical optimisation based design framework to create lighting devices with a high colour rendering index (CRI). We evaluate the effects of different cost functions and initialisation strategies, and show that, we are able to design devices with a CRI > 96 using only four different QD primaries. We use a charge-transport based simulator to asses the electric properties of the designed devices. We also showcase initial work done on a modular software interface and a material library we developed for this simulator.EPSRC DTP studentship award RG84040:EP/N509620/

Real-time optical manipulation of cardiac conduction in intact hearts

Optogenetics has provided new insights in cardiovascular research, leading to new methods for cardiac pacing, resynchronization therapy and cardioversion. Although these interventions have clearly demonstrated the feasibility of cardiac manipulation, current optical stimulation strategies do not take into account cardiac wave dynamics in real time. Here, we developed an all‐optical platform complemented by integrated, newly developed software to monitor and control electrical activity in intact mouse hearts. The system combined a wide‐field mesoscope with a digital projector for optogenetic activation. Cardiac functionality could be manipulated either in free‐run mode with submillisecond temporal resolution or in a closed‐loop fashion: a tailored hardware and software platform allowed real‐time intervention capable of reacting within 2 ms. The methodology was applied to restore normal electrical activity after atrioventricular block, by triggering the ventricle in response to optically mapped atrial activity with appropriate timing. Real‐time intraventricular manipulation of the propagating electrical wavefront was also demonstrated, opening the prospect for real‐time resynchronization therapy and cardiac defibrillation. Furthermore, the closed‐loop approach was applied to simulate a re‐entrant circuit across the ventricle demonstrating the capability of our system to manipulate heart conduction with high versatility even in arrhythmogenic conditions. The development of this innovative optical methodology provides the first proof‐of‐concept that a real‐time optically based stimulation can control cardiac rhythm in normal and abnormal conditions, promising a new approach for the investigation of the (patho)physiology of the heart

Practical Measurement and Reconstruction of Spectral Skin Reflectance

We present two practical methods for measurement of spectral skin reflectance suited for live subjects, and drive a spectral BSSRDF model with appropriate complexity to match skin appearance in photographs, including human faces. Our primary measurement method employs illuminating a subject with two complementary uniform spectral illumination conditions using a multispectral LED sphere to estimate spatially varying parameters of chromophore concentrations including melanin and hemoglobin concentration, melanin blend-type fraction, and epidermal hemoglobin fraction. We demonstrate that our proposed complementary measurements enable higher-quality estimate of chromophores than those obtained using standard broadband illumination, while being suitable for integration with multiview facial capture using regular color cameras. Besides novel optimal measurements under controlled illumination, we also demonstrate how to adapt practical skin patch measurements using a hand-held dermatological skin measurement device, a Miravex Antera 3D camera, for skin appearance reconstruction and rendering. Furthermore, we introduce a novel approach for parameter estimation given the measurements using neural networks which is significantly faster than a lookup table search and avoids parameter quantization. We demonstrate high quality matches of skin appearance with photographs for a variety of skin types with our proposed practical measurement procedures, including photorealistic spectral reproduction and renderings of facial appearance