Road marking BRDF model applicable for a wide range of incident illumination conditions.

To drive safely and comfortably, an adequate contrast between the road surface and road markings is needed. This contrast can be improved by using optimized road illumination designs and luminaires with dedicated luminous intensity distributions, taking advantage of the (retro)reflective characteristics of the road surface and road markings. Since little is known about road markings' (retro)reflective characteristics for the incident and viewing angles relevant for street luminaires, bidirectional reflectance distribution function (BRDF)-values of some retroreflective materials are measured for a wide range of illumination and viewing angles using a luminance camera in a commercial near-field goniophotometer setup. The experimental data are fitted to a new and optimized "RetroPhong" model, which shows good agreement with the data [root mean squared error (R M S E)<0.13, normalized root mean squared error (N R M S E)<0.04, and the normalized cross correlation ratio (N C C)>0.8]. The RetroPhong model is benchmarked with other relevant (retro)reflective BRDF models, and the results suggest that the RetroPhong model is most suitable for the current set of samples and measurement conditions.

CAM18sl brightness prediction for unrelated saturated stimuli including age effects.

Modelling the influence of age on the perception of brightness of visual stimuli is an important topic for indoor and outdoor lighting. As people get older, the transmittance of the ocular media becomes lower, especially in the blue wavelength region. This paper reports on an experimental study aiming to evaluate how the brightness perception of red and blue stimuli is affected by the age of the observer. A matching experiment has been set up in which both young (25 years old on average) and older (70 years old on average) adult observers had to match the brightness of a blue stimulus with the brightness of a red stimulus, both surrounded by a dark background (unrelated stimuli). A significant difference in brightness perception between the two groups of observers was found. In particular, older people report a decrease in brightness perception for the blue stimuli compared to younger people. The results show that the brightness correlate of the colour appearance model CAM18sl (applied with zero luminance background) adequately predicts the matching results of young observers, but failed to predict the results obtained by the older observers. As CAM18sl is built on cone fundamentals which include the transmittance of the ocular media and consider the age of the observer as an input parameter, the authors developed the idea to substitute the cone fundamentals for a young observer by the cone fundamentals for a 70 years old observer. This updated CAM18sl performed very well for the older observer as well, on condition that the transmittance of the ocular media is isolated and kept out of the normalization of the cone fundamentals.

Freeform Fresnel lenses with a low number of discontinuities for tailored illumination applications.

Most work in the field of freeform lens design has been focused on finding design algorithms for continuous freeform lens surfaces which transform an arbitrary ingoing light distribution into an arbitrary outgoing distribution. The shape of the resulting continuous lens surfaces depends fully on the source and target light distribution for which the lenses are tailored. In some cases this results in large, voluminous optical components which depending on the application are not practical. Fresnel lenses can have a much smaller volume, but are not straightforward to design in the case of freeform lenses. This paper demonstrates a new method to design freeform Fresnel lenses based on concentric freeform segments. Such lenses have a much lower number of discontinuities compared to already existing Fresnel-type freeform lenses which are based on an array of facets. Less discontinuities means less stray light due to the unavoidable rounding errors with current manufacturing processes. The new design method is first explained, and then illustrated for a freeform Fresnel lens with a rectangular target distribution in the far-field.

Effect of adapting field size on chromatic adaptation.

The human visual system adapts to changes in white tone of the illumination to maintain approximately the same object color appearance. Chromatic adaptation transforms (CAT) were developed to predict corresponding colors, which are colors that look the same under a wide range of illuminants. However, existing CATs fail to accurately predict corresponding colors, particularly under colored illumination, because of an inaccurate estimation of the degree of adaptation. In this study, the impact of the adapting field size on the degree of adaptation was investigated. A memory color matching experiment was conducted, in a real scene, with the background adapting field varying in the field of view, luminance and chromaticity to provide data for the development of a more comprehensive CAT. Results show that a larger field of view leads to a more complete adaptation, despite a much lower background luminance.

Improving the opto-thermal performance of transmissive laser-based white light sources through beam shaping.

Laser diodes have been proposed as a good replacement for light-emitting diodes in high-luminance white light sources. However, laser diodes typically generate very sharp temperature gradients inside the colour-converting elements (CCE) used to produce white light. This poses a thermal management problem in transmissive configurations, where most of the thermal dissipation occurs at the edges of the CCE. The hot spot in the center of the CCE typically drives the efficiency of the system down due to thermal quenching. In this work, we propose a strategy to tackle this issue that is based purely on optical manipulation. By using a free-form lens, the radiation pattern of the laser diode exciting the CCE is tailored so that its power distribution is skewed towards the periphery of the CCE: the zone with the highest thermal dissipation. With this technique, the maximum temperature inside the CCE can be significantly lower than when uniformly illuminating the CCE. Additionally, by lowering the temperature inside the CCE, this technique excites the CCE with a higher radiant flux, allowing higher luminance to be extracted from the system. These results were obtained with a realistic opto-thermal simulation framework and were then experimentally verified.

Holistic opto-thermal simulation framework for high-brightness light sources based on fluorescent conversion.

Multi-physics approaches are increasingly adopted in the development of efficient, high brightness solid-state light sources, in particular for the realistic modelling of the fluorescent colour conversion element that is typically used to create white light. When a fluorescent material is excited by a high-power laser diode, it will self-heat and reach high temperatures. The efficiency or quantum yield of fluorescent materials lowers as their temperature increases, an effect called thermal quenching. The lower efficiency further increases the amount of phosphor self-heating which can lead to thermal runaway. This effect has been considered by different researchers when modelling the opto-thermal behaviour of the fluorescent colour conversion elements. However, other key fluorescent properties such as the absorption and emission spectrum also depend on temperature, and often also on the radiant flux density. This gives rise to a complex set of interplays between optical and thermal properties which are not considered in the current opto-thermal models but that significantly influence the performance of fluorescent material based solid-state light sources. In this work, we present a holistic opto-thermal simulation framework: a novel comprehensive simulation tool that includes all relevant multi-physics considerations. We show that the framework allows for an accurate and realistic prediction of the performance of high-luminance solid-state white light sources by comparing simulation results to experimentalmeasurements of a laser-based configuration, thereby validating the framework.

Luminance spreading freeform lens arrays with accurate intensity control.

Glare and visual discomfort are important factors that should be taken into account in illumination design. Conventional freeform lenses offer perfect control over the outgoing intensity distribution, thereby allowing optical radiation patterns with sharp cut-offs in order to optimize the unified glare rating index. However, these freeform lenses do not offer control over the near-field luminance distribution. Observing the emitted light distribution from a high-brightness LED through a freeform lens gives a high peak luminance that can result in glare. To reduce this peak luminance, freeform lenses should be used in conjunction with light diffusing structures. However, this diminishes the control over the outgoing intensity distribution what is the main benefit of a freeform lens. Another approach to reduce the observed peak luminance, is by spreading the emitted light over multiple optical channels via freeform lens arrays. This paper proposes a novel method to design luminance spreading freeform lens arrays that offer perfect control over the resulting intensity pattern. The method is based on a non-invertible mapping of a 2D parameter space. This results in a source-target mapping in which multiple ingoing ray directions are mapped onto every position of the target distribution. The case of continuous and discontinuous mappings are both discussed in this paper. Finally, the example of a discontinuous freeform lens array with 7×7 individual lenses is designed and experimentally demonstrated.

Impact of the starting point chromaticity on memory color matching accuracy.

In this study, the impact of starting point chromaticity and number of observers on memory color matching results was investigated. Matching data were obtained for 3 objects (neutral grey cube, yellow lemon and green apple) under a neutral white and a yellow background illumination. Memory color matchings were made for ten starting points of which eight chromaticities were symmetrically distributed along the hue circle and centered at the equal energy white (EEW) chromaticity of the neutral white background illumination; one starting point at the EEW chromaticity and one with the same chromaticity as the background. The matching track from starting point to the memory matched chromaticity was also recorded. It did not tend to cross over the central region towards the complementary hue, especially for experienced observers. The results also demonstrated a significant starting point bias, whereby the matched chromaticities were biased towards the chromaticity of the starting point. Starting point bias can be minimized by selecting three starting points symmetrically distributed around the expected memory color chromaticity. Furthermore, at least, ten observers are needed to achieve stable results for the grey cube and yellow lemon. For the green apple, the results are less conclusive and around 40 observers would be needed to obtain a stable average estimate for the chromaticity of the memory color. The large inter-observer variation may result from cultural differences or from natural variations in the "green" apple appearance. This study provides a well-founded guidance for future application of the memory color matching method.

Ray mapping method for off-axis and non-paraxial freeform illumination lens design.

The ray mapping method for freeform illumination design is an easy and flexible method, but only in the paraxial regime does it result in surface normal fields that are directly integrable into continuous freeform surfaces that provide the desired illuminance distribution. A new mapping scheme is proposed to alter an initial source-target mapping via a symplectic flow of an equi-flux parametric coordinate system. The resulting mapping provides integrable surface normal vector fields for complex off-axis and non-paraxial illumination problems, as demonstrated by two freeform lens examples.

Radiance based method for accurate determination of volume scattering parameters using GPU-accelerated Monte Carlo.

Volume scattering is an important effect in different fields, ranging from biology to lighting. Models for volume scattering usually rely on parameters that are estimated with inverse methods that iteratively fit simulations to experimental data. To obtain accurate estimates for these parameters, the scattered intensity distribution can be used in such fitting methods. However, it has been shown that for samples with long optical path lengths this type of data may result in poor parameter estimates. In this work, an inverse procedure is proposed that fits to scattered radiance distributions. By taking advantage of current generation graphics processing units, the method implemented is sufficiently efficient to allow performing an in-depth simulation study on the difference between using radiance or intensity distributions to estimate the volume scattering parameters of samples. This work shows that for samples with moderate optical path lengths, the intensity distribution contains sufficient information to accurately estimate the volume scattering properties. However, for longer optical path lengths, the descriptive power of the intensity distribution is not enough and radiance distribution based methods, such as the inverse method proposed, are better suited.

Study of chromatic adaptation using memory color matches, Part I: neutral illuminants.

Twelve corresponding color data sets have been obtained using the long-term memory colors of familiar objects as target stimuli. Data were collected for familiar objects with neutral, red, yellow, green and blue hues under 4 approximately neutral illumination conditions on or near the blackbody locus. The advantages of the memory color matching method are discussed in light of other more traditional asymmetric matching techniques. Results were compared to eight corresponding color data sets available in literature. The corresponding color data was used to test several linear (von Kries, RLAB, etc.) and nonlinear (Hunt & Nayatani) chromatic adaptation transforms (CAT). It was found that a simple two-step von Kries, whereby the degree of adaptation D is optimized to minimize the DEu'v' prediction errors, outperformed all other tested models for both memory color and literature corresponding color sets, whereby prediction errors were lower for the memory color sets. The predictive errors were substantially smaller than the standard uncertainty on the average observer and were comparable to what are considered just-noticeable-differences in the CIE u'v' chromaticity diagram, supporting the use of memory color based internal references to study chromatic adaptation mechanisms.

Study of chromatic adaptation using memory color matches, Part II: colored illuminants.

In a previous paper, 12 corresponding color data sets were derived for 4 neutral illuminants using the long-term memory colours of five familiar objects. The data were used to test several linear (one-step and two-step von Kries, RLAB) and nonlinear (Hunt and Nayatani) chromatic adaptation transforms (CAT). This paper extends that study to a total of 156 corresponding color sets by including 9 more colored illuminants: 2 with low and 2 with high correlated color temperatures as well as 5 representing high chroma adaptive conditions. As in the previous study, a two-step von Kries transform whereby the degree of adaptation D is optimized to minimize the DEu'v' prediction errors outperformed all other tested models for both memory color and literature corresponding color sets, whereby prediction errors were lower for the memory color set. Most of the transforms tested, except the two- and one-step von Kries models with optimized D, showed large errors for corresponding color subsets that contained non-neutral adaptive conditions as all of them tended to overestimate the effective degree of adaptation in this study. An analysis of the impact of the sensor space primaries in which the adaptation is performed was found to have little impact compared to that of model choice. Finally, the effective degree of adaptation for the 13 illumination conditions (4 neutral + 9 colored) was successfully modelled using a bivariate Gaussian in a Macleod-Boyton like chromaticity diagram.

Selecting the optimal synthesis parameters of InP/CdxZn1-xSe quantum dots for a hybrid remote phosphor white LED for general lighting applications.

Quantum dots can be used in white LEDs for lighting applications to fill the spectral gaps in the combined emission spectrum of the blue pumping LED and a broad band phosphor, in order to improve the source color rendering properties. Because quantum dots are low scattering materials, their use can also reduce the amount of backscattered light which can increase the overall efficiency of the white LED. The absorption spectrum and narrow emission spectrum of quantum dots can be easily tuned by altering their synthesis parameters. Due to the re-absorption events between the different luminescent materials and the light interaction with the LED package, determining the optimal quantum dot properties is a highly non-trivial task. In this paper we propose a methodology to select the optimal quantum dot to be combined with a broad band phosphor in order to realize a white LED with optimal luminous efficacy and CRI. The methodology is based on accurate and efficient simulations using the extended adding-doubling approach that take into account all the optical interactions. The method is elaborated for the specific case of a hybrid, remote phosphor white LED with YAG:Ce phosphor in combination with InP/CdxZn1-xSe type quantum dots. The absorption and emission spectrum of the quantum dots are generated in function of three synthesis parameters (core size, shell size and cadmium fraction) by a semi-empirical 'quantum dot model' to include the continuous tunability of these spectra. The sufficiently fast simulations allow to scan the full parameter space consisting of these synthesis parameters and luminescent material concentrations in terms of CRI and efficacy. A conclusive visualization of the final performance allows to make a well-considered trade-off between these performance parameters. For the hybrid white remote phosphor LED with YAG:Ce and InP/CdxZn1-xSe quantum dots a CRI Ra = 90 (with R9>50) and an overall efficacy of 110 lm/W is found.

Flexible design method for freeform lenses with an arbitrary lens contour.

A method is presented that allows the design of freeform lenses with an arbitrary contour in a flexible and robust manner. The method is based on the generation of two equi-flux grids representing the source and target beams, with two separate curl-free mappings from an equi-spatial rectangular grid. Because the source and target grids are generated independently from one another, one can map arbitrary complex source beams with certain contours onto arbitrary complex target beams within other contours with high convergence probability. The method is illustrated by calculating a triangular freeform lens that reshapes a triangular beam from a Lambertian source into a uniform pentagonal irradiance distribution on a target plane.

Determination of volume scattering parameters that reproduce the luminance characteristics of diffusers.

The extension of a well-known inverse technique, inverse adding-doubling (IAD), is investigated for determining the volume scattering properties of diffusers for display and lighting applications. The luminance characteristics of volume scattering diffusers are vital for these applications. Through a simulation study, it is shown that fitting solely to the scattered (angular) intensity information with the extended IAD method, results in a volume scattering characterization that also reproduces the correct (spatial and angular) luminance characteristics for a wide range of samples. The gap between the simulation work and the experimental application of the investigated fitting procedure is bridged by considering the effect of experimental error in the scattered intensity distributions. This does not significantly alter the presented conclusions.

Impact of cross-regional differences on color rendition evaluation of white light sources.

In a study, involving laboratories from seven geographic regions, the memory colors of eleven familiar objects were investigated. Based on that study, one global and seven regional memory color rendition indices (MCRIs) are created and the impact of cross-regional differences on the evaluation of color rendition was investigated. A first analysis focuses on the impact on MCRI index values by comparing the regional index values, calculated for 401 light sources, with those of the global index. A second analysis examines the impact on predictive performance in terms of the visual appreciation and naturalness of rendered objects colors as evaluated in respectively twenty-one and fifteen experiments published in literature. Both analyses show that, although there are small differences in absolute level of color rendition, the regional metrics are generally comparable in terms of predicting light source rank order and correlation with visual data. Therefore, ànd considering between-region variability to be smaller than or of the same size as the within-region variability, a globally valid memory color rendition metric can be proposed without introducing substantial errors. Finally, Smet's Rm index, obtained using real objects, is suggested as a good approximation to that globally valid metric.

Chromaticity of unique white in illumination mode.

The chromaticity of unique white viewed in illumination mode and under dark adapted conditions was investigated for 3 luminance levels (200, 1000 and 2000 cd/m2) using a unique white setting method. Unique white was found to encompass a rather large region in color space located slightly below the blackbody locus and centered around a CCT of 6600 K. Luminance level was found to have no significant effect on the mean unique white chromaticity. The high and low end points of the CIE class A and B white regions respectively under- and overestimate the chromaticity region perceived as white. Agreement along the Duv direction was quite good. However, another Duv related limit associated with white lighting (|Duv|≤5.4e-3) was found to be on the small side, especially for chromaticity values below the blackbody locus. The results for unique white viewed in illumination mode were compared to those reported for object mode presentation. Overall they were very comparable, although a statistical analysis does show a (just) significant effect of stimulus presentation mode for high (il)luminance levels. However, no such effect could be established at the individual observer level. Therefore, it was concluded that unique white chromaticity is essentially the same for both illumination and object mode stimulus presentation, at least under dark adapted viewing conditions.

Brightness prediction of different sized unrelated self-luminous stimuli.

In a series of magnitude estimation experiments, the effect of the size of a circular stimulus varying from 1° to 30° field of view on the perception of brightness has been investigated for unrelated self-luminous stimuli. A clear, hue independent, size effect on brightness was found. Based on a simple modification of the recently developed Color Appearance Model CAM15u, the brightness of different sized unrelated self-luminous stimuli was adequately predicted. The modified brightness prediction performs much better than existing predictions and has been validated by a separate validation experiment.

Determination of the optimal amount of scattering in a wavelength conversion plate for white LEDs.

Luminescent materials are widely used in white LEDs to convert part of the blue LED light into light with a longer wavelength, resulting in white light when both colors are well mixed. One way to integrate the luminescent material in the LED package is to deposit a thin luminescent layer on a planar carrier or disperse luminescent particles in the carrier material and then position the resulting wavelength conversion plate above one or more LEDs. It is very important that these wavelength conversion plates have the right properties to ensure homogeneous white light with a high efficiency and desired correlated color temperature (CCT). Key properties are the absorption and emission spectrum and the scattering and absorption coefficients. These properties strongly influence the color of the resulting light, but also the efficiency and the angular uniformity. This work describes an extensive study of the effect of the scattering and absorption coefficients in terms of the desired CCT. A computationally efficient extended Adding-Doubling method is used for the simulation of the light distribution and conversion in the planar wavelength conversion element. Ultimately an optimal combination with a high efficiency and low angular color deviation is desired. Different systems are investigated and optimal coefficients are found. With these findings a more targeted approach can be used in the manufacturing of wavelength conversion plates for white LEDs. The addition of scatterers or non-scattering luminescent particles can be used to obtain optimal scattering properties of the wavelength conversion plate.

Rayfiles including spectral and colorimetric information.

To obtain realistic results in lighting simulation software, realistic models of light sources are needed. A near-field model of a light source is accurate, and can be obtained by a near-field goniophotometer. This type of goniophotometer is conventionally equipped with a V(λ)-filter. However, the advent of new light sources with spatial- or angular color variations necessitates the inclusion of spectral information about the source. We demonstrate a method to include spectral information of a light source in ray tracing. We measured the relative angular variation of the spectrum of an OLED using a spectroradiometer mounted on a near-field goniophotometer. Principal component analysis (PCA) is exploited to reduce the amount of data that needs to be stored. Also a photometric ray file of the OLED was obtained. To construct a set of monochromatic ray files, the luminous flux in the original ray file is redistributed over a set of wavelengths and stored in separate ray files. The redistribution depends on the angle of emission and the spectral irradiance measured in that direction. These ray files are then inserted in ray tracing software TracePro. Using the OLED as a test source, the absolute spectral irradiance is calculated at an arbitrary position. The result is validated using a spectroradiometer to obtain the absolute spectral irradiance at that particular point. A good agreement between the simulated and measured absolute spectral irradiance is found. Furthermore, a set of tristimulus ray files is constructed and used in ray tracing software to generate a u'v'-color coordinate distribution on a surface. These values are in agreement with the color coordinate distribution found using the spectral ray files. Whenever spectral or color information is desired at a task area, the proposed method allows for a fast and efficient way to improve the accuracy of simulations using ray tracing.