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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 (RMSE)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.

An unanimous framework for surface gloss perception does not exist yet, although an orthogonal two-dimensional gloss space (distinctness-of-image and contrast gloss) has already been proposed. Haze, a rarely studied attribute of glossiness, is considered in this study and introduced into this existing space. A set of samples with surface haziness is psychophysically and optically characterized. The perceived glossiness correlates with the visual dimension of contrast gloss, and with metrics based on “contrast” - between the specular highlight and sample background – obtained from measurements with different optical instruments. The suitability and advantages of the image-based gloss meter (iGM), introduced earlier by the authors [J Coat Technol Res. 19, 1567 (2022)10.1007/s11998-022-00630-0OCPOBG2770-0208], are thereby illustrated in comparison to the conventional gloss meter standards.

The Unified Glare Rating (UGR) and the modified version (UGR’) have been developed and widely accepted in multiple standards for measuring the discomfort glare of a luminaire in typical indoor environments; however, a standardized glare metric for non-uniform outdoor luminaires is still missing. In this paper, the possibility to apply UGR and UGR’ to an outdoor residential luminaire with a non-uniform spatial luminance distribution is explored. The luminaire was characterized in a large near-field goniophotometer (NFG) and luminance images were captured at four angles specified in the CIE 232:2019 document. Some practical issues of applying the UGR’ for a non-uniform residential luminaire are discussed, such as selecting the luminous area, the blurring parameter, the viewing angles, and the background luminance. In addition to these practical issues, possible solutions and suggestions are explored, such as a different blurring parameter, viewing angle, and background luminance. In the end, employing a human visual system to evaluate the amount of discomfort glare for both indoor and outdoor applications might be preferred. ispartof: Sustainability vol:14 issue:20 status: Published online

Gloss is one of the main attributes to describe the appearance of surfaces and objects, as it contributes to the general quality perception. Gloss is a multidimensional quantity of which ‘specular gloss’ is the most commonly applied attribute. Specular gloss meters are standardized and widely used in industry. However, their readings correlate only partially to the general visual gloss impression, which also comprises distinctness-of-the-reflected-image (DOI), haze, contrast and surface-uniformity attributes. This study presents a more profound image-based gloss meter (iGM) which incorporates a CMOS camera detector. This concept is not new, but limited research has been conducted on the inclusion of various image processing evaluations for gloss attributes. The designed iGM is compatible to 60° specular gloss meter standards. The CMOS detector captures the reflected source image, which is processed to measure four perceptual attributes of surface gloss. The obtained results validate the 60° specular gloss evaluation and indicate a promising capability in characterizing DOI, haze, and contrast. Contrast is an important attribute that is not available yet in industrial gloss meters. It is measured using a diffuse aspecular light source. Generally, this iGM maintains the hardware principles of specular gloss meters, while evolving toward a representative gloss perception meter.

11 pags., 11 figs., 3 tabs., app. -- OCIS codes: (290.1483) BSDF, BRDF, and BTDF; (120.1840) Densitometers, reflectometers; (120.5700) Reflection., The field of spectral radiance factor (SRF) measurements has seen growing interest in recent years. Scale conformity has so far only been established between the national metrology institutes (NMIs) of Germany and the USA. This study aims at a bigger, multilateral scale comparison. For this purpose, a total of six NMIs participated in a scale comparison of goniospectrophotometers based on neutral and colored diffusely reflecting ceramics samples. In addition, two universities, providing a home-built gonioreflectometer and two widely used commercially available color measurement instruments, respectively, were involved. The wavelength range of the scale comparison covers the visible wavelength range from 380 nm to 780 nm. Results indicate systematic issues and that the uncertainty evaluation of the NMIs requires further work; although for the greatest part of the covered spectral range the agreement is good., This work has been realized within the European Metrology Research Program (EMRP) project JRPIND52. The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union. We thank the European Commission and EURAMET e.V. for financial support.

In this paper, we present a combined morphological and optical characterization of aluminum-coated thermoplastic polymer surfaces. Flat plastic substrates, obtained by means of an injection molding process starting from plastic granules, were coated with ultra-thin aluminum films evaporated in vacuo, on top of which a silicon-based protective layer was plasma deposited in order to prevent oxidation of the metal reflective surface. Different sample treatments were studied to unravel the influence of substrate chemistry, substrate thickness, aluminum and protective layer thickness, and surface roughness on the actual optical reflectance properties. Bidirectional reflectance distribution function measurements, corroborated by surface morphological information obtained by means of atomic force microscopy, correlate reflectance characteristics with the root-mean-square surface roughness, providing evidence for the role of the substrate and the thin films’ morphology. The results unravel information of interest within many applicative fields involving metal coating processes of plastic substrates as an example in the case of automotive lighting.

This paper reports on the development of a measurement instrument to perform gloss measurements using an image-based detector. The image-based gloss meter was built according to the specifications of the optical layout of a specular gloss meter in a 60° measurement geometry, as described in ASTM D523-14. The photodiode detector was thereby substituted with a CMOS detector. The optical layout of the system was designed and validated by the use of ray tracing software. A series of 16 matte to high-gloss test samples, with nominal gloss values ranging between 3 and 90 gloss units, was used to compare specular gloss measurements obtained with the developed instrument and a commercial specular gloss meter. An average and maximum deviation of only 1.2 and 2.7 gloss units, respectively, was obtained, confirming the suitability of the system to perform standard specular gloss measurements. The potential benefits of the image-based approach were then studied. By way of example, the optical characterization of orange peel and contrast gloss by the use of the system was discussed, corroborating the fact that the proposed instrument offers important opportunities for a more global characterization of the total gloss impression. ispartof: Journal of Coatings Technology and Research vol:16 issue:4 pages:913-921 status: published

In order to numerically quantify and to ensure consistency of the visual appearance of the end product, many industries perform color and gloss measurements, through the results of which they define maximum tolerances during quality control. For this, they may primarily rely on precision data for repeatability and reproducibility as reported by instrument manufacturers, who develop their instruments according to standardized measurement geometries defined in international standards. This paper reports on an investigation of the inter-instrument agreement between specular gloss meters which conform to universally adopted gloss standards. Six commercially available instruments, manufactured by three different companies, and twenty-five gloss artifacts, with specular gloss values ranging between 2 and 110 gloss units (60° geometry), were selected for use in the study. The repeatability and reproducibility of the instruments were assessed according to the criteria described in ASTM D523-14 and ISO 2813:2014, and to the specifications reported by the instrument manufacturers. Ray tracing simulations were performed by use of the bidirectional reflectance distribution function measurement data of two samples, and based on the standardized optical design of the specular gloss meter, in order to get a better insight on the expected tolerances of the gloss readings from a theoretical point of view. On average, the practical results indicate that both the repeatability and reproducibility values are higher than those specified in the recommendations and by the manufacturers. Ray tracing simulations confirm this finding. While specified repeatability and reproducibility thresholds are based on ideal standards, in practice, there are parameters that can lead to significant higher deviations. Care should therefore be taken when analyzing measurement results obtained from different instruments, and when defining tolerances for evaluation of gloss measurements.

Remote phosphor LED modules could offer advantages over the intimate white phosphor converted LEDs in terms of phosphor operation temperature, light extraction efficiency and angular color uniformity. Existing commercial devices show a large variety with respect to the dimensions of the mixing cavity, which raises the question towards the optimization of the topology. A simplified simulation model applying a two wavelength approach and considering the remote phosphor as one virtual surface to which three bidirectional scattering distribution functions are attributed (respectively describing the blue-blue, blue-yellow and yellow-yellow interaction) is developed and validated. This model has been used to analyze the impact of the cylindrical mixing cavity parameters such as the absolute reflectance, the diffuse-to-specular reflectance ratio and the height of the mixing cavity- and the pitch and angular full-width at half-maximum of the LEDs on the extraction efficiency, the yellowto- blue ratio and the irradiance uniformity. It can be concluded that in order to increase the efficacy substantially, the recuperation of the backward emission of the converted light can only be increased by avoiding further interaction with the phosphor plate. To this extent, other topologies than cylindrical mixing cavities must be considered. ispartof: IEEE Photonics Journal vol:7 issue:5 pages:1-14 status: published

A novel meta-heuristic approach for minimizing nonlinear constrained problems is proposed, which offers tolerance information during the search for the global optimum. The method is based on the concept of design and analysis of computer experiments combined with a novel two phase design augmentation (DACEDA), which models the entire merit space using a Gaussian process, with iteratively increased resolution around the optimum. The algorithm is introduced through a series of cases studies with increasing complexity for optimizing uniformity of a short-wave infrared (SWIR) hyperspectral imaging (HSI) illumination system (IS). The method is first demonstrated for a two-dimensional problem consisting of the positioning of analytical isotropic point sources. The method is further applied to two-dimensional (2D) and five-dimensional (5D) SWIR HSI IS versions using close- and far-field measured source models applied within the non-sequential ray-tracing software FRED, including inherent stochastic noise. The proposed method is compared to other heuristic approaches such as simplex and simulated annealing (SA). It is shown that DACEDA converges towards a minimum with 1 % improvement compared to simplex and SA, and more importantly requiring only half the number of simulations. Finally, a concurrent tolerance analysis is done within DACEDA for to the five-dimensional case such that further simulations are not required. ispartof: Optics Express vol:24 issue:26 pages:29380-29405 ispartof: location:United States status: published

Computer assisted optimal illumination design is crucial when developing cost-effective machine vision systems. Standard local optimization methods, such as downhill simplex optimization (DHSO), often result in an optimal solution that is influenced by the starting point by converging to a local minimum, especially when dealing with high dimensional illumination designs or nonlinear merit spaces. This work presents a novel nonlinear optimization approach, based on design and analysis of computer experiments (DACE). The methodology is first illustrated with a 2D case study of four light sources symmetrically positioned along a fixed arc in order to obtain optimal irradiance uniformity on a flat Lambertian reflecting target at the arc center. The first step consists of choosing angular positions with no overlap between sources using a fast, flexible space filling design. Ray-tracing simulations are then performed at the design points and a merit function is used for each configuration to quantify the homogeneity of the irradiance at the target. The obtained homogeneities at the design points are further used as input to a Gaussian Process (GP), which develops a preliminary distribution for the expected merit space. Global optimization is then performed on the GP more likely providing optimal parameters. Next, the light positioning case study is further investigated by varying the radius of the arc, and by adding two spots symmetrically positioned along an arc diametrically opposed to the first one. The added value of using DACE with regard to the performance in convergence is 6 times faster than the standard simplex method for equal uniformity of 97%. The obtained results were successfully validated experimentally using a short-wavelength infrared (SWIR) hyperspectral imager monitoring a Spectralon panel illuminated by tungsten halogen sources with 10% of relative error.

To obtain realistic results in lighting simulation software, accurate models of light sources are needed. The advent of new light sources with spatial and angular color variations necessitates the addition of spectral information about the source. Including this information in the model of a light source, allows for spectral irradiance maps to be calculated and colorimetric information to be derived. Existing methods to include spectral information about the source require a large set of ray-files to be traced: for each wavelength step in the spectral range, a ray-file should be subsequently traced. Thereafter, the spectral irradiance map can be calculated at an arbitrary surface. Principal component analysis (PCA) allows us to characterize the angular variation of the spectral irradiance by a linear combination of parameters and base functions. Another possibility would be to construct “PCA ray files”, all of them exact copies of the photometric ray-file, yet four times replacing the luminous flux of each ray-file by a parameter resulting from the PCA. Assuming that only spectrally neutral materials are introduced between the source and the detection area, subsequent ray-tracing of these PCA ray files, allows their distribution to be determined on any task area. From this information, the absolute spectral irradiance at any point and in any direction can be constructed using a limited number of additional simulations. Whenever spectral 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.

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. ispartof: Optics Express vol:23 issue:7 pages:A361-A370 ispartof: location:United States status: published

To perform adequate simulations of luminescent cascade systems, a hybrid method combining a commercial ray tracer and a programming tool is presented. True Monte Carlo algorithms for luminescent materials, treating each ray individually, are adapted to allow wavelength conversion of ray sets. Two solutions for the wavelength conversion of ray sets are discussed: a random approach, where absorption events are randomly selected to create emission events, and a combined approach, where information from multiple absorption events is combined to create emission events. Both methods are applied to simulate the performance of a virtual remote phosphor light-emitting diode module. When using the combined approach, the required computation time to achieve sufficient accuracy is a factor 2 lower, compared to the time required when applying the random approach. ispartof: Optics Express vol:22 issue:20 pages:24852-24593 ispartof: location:United States status: published

Light-emitting diodes (LEDs) are becoming increasingly important for general lighting applications. The remote phosphor technology, with the phosphor located at a distance from the LEDs, offers an increased extraction efficiency for phosphor converted LEDs compared to intimate phosphor LEDs where the phosphor is placed directly on the die. Additionally, the former offers new design possibilities that are not possible with the latter. In order to further improve the system efficiency of remote phosphor LEDs, realistic simulation models are required to optimize the actual performance. In this work, a complete characterization of a remote phosphor converter (RPC) consisting of a polycarbonate diffuser plate with a phosphor coating on one side via the bi-directional scattering distribution function (BSDF) is performed. Additionally, the bi-spectral BSDF which embraces the wavelength conversion resulting from the interaction of blue light with the RPC is determined. An iterative model to predict the remote phosphor module power and photon budget, including the recuperation of backward scattered light by a mixing chamber, is introduced. The input parameters for the model are the bi-spectral BSDF data for the RPC, the emission of the blue LEDs and the mixing chamber efficiency of the LED module. A good agreement between experimental and simulated results was found, demonstrating the potential of this model to analyze the system efficiency with errors smaller than 4%. ispartof: Optics Express vol:22 issue:13 pages:A1079-A1092 ispartof: location:United States status: published

Monte Carlo ray tracing is an important simulation tool in applications where fluorescence is present, e.g. in bio-medical applications and in the design of luminaires and luminescent solar concentrators. A frequently used ray tracing procedure for fluorescence is the ‘dual stage’ approach. In this approach, first, all sources are traced through the system and the rays absorbed in the fluorescent components are stored. Next, the emission from the fluorescent components is traced. This approach does not allow for subsequent re-absorption and re-emission effects in fluorescent materials with a spectral overlap between excitation and emission spectra. In this work, a ‘multi stage’ ray tracing procedure for the simulation of luminescence is presented. Herein, wavelengths are traced from short to long separately and no distinction is made regarding the origin of emission (either a fluorescent component or a source). The presented approach can be easily implemented in existing commercial ray tracing software thus reducing the programming efforts for the new ray tracing algorithm and taking advantage of the strength of the selected ray tracing package concerning the modelling of complex geometrical systems. Both techniques are compared to investigate the influence of the selected ray tracing approach on the efficiency and colour prediction of a remote phosphor LED module.

Within near-field goniophotometry, measurement results of both an imaging luminance measurement device and a photometer detector are combined to generate the luminous intensity distribution of a light source. The simultaneous use of these two detectors may engender incorrect measurement results, due to their difference in dynamic range. In this paper, near-field and far-field based luminous intensity distribution measurements of two luminaires are presented, in order to exemplify the problem. Results demonstrate that the distributions obtained from near-field measurements may deviate from the correct intensity distribution, by an amount of up to 16% of the total luminous flux of the luminaire. A method to check for the correctness of the luminous intensity distribution from the near-field measurement, the so-called sanity check, is discussed. To conclude, some possible solutions to eliminate the dynamic range mismatch induced errors are treated. ispartof: Optics Express vol:23 issue:3 pages:2240-2251 ispartof: location:United States status: published

Ray files offer a very accurate description of the optical characteristics of a light source. This is essential whenever optical components are positioned in close proximity (near-field) of the light source in order to perform accurate ray tracing simulations. However, a ray file does not allow for a direct simulation of the spatial luminance distribution, i.e. luminance map, by off-the-shelf ray tracers. Simulating luminance maps of light sources or luminaires is especially important in general lighting in order to predict their general perception when viewed by the observer, and more specific, the perception of glare of luminaires having a non-uniform luminance distribution. To enable the simulation of luminance maps while maintaining the high accuracy offered by a ray file, a sampling method is presented. To validate the approach, near-field goniophotometer measurements of two planar light sources were performed. From these measurement data, ray files were extracted to which the sampling method was applied in order to obtain a set of surface sources. This approach was validated by comparing measured luminance images with simulated luminance images. A good agreement was found, validating the presented method. ispartof: Optics Express vol:21 issue:20 pages:24099-24111 ispartof: location:United States status: published

The use of a ray file to model the optical characteristics of a light source is a well-known and popular method to achieve accurate results when simulating luminous intensity distributions of luminaires, especially if the source is interacting with optical components at a close distance. However, lighting industry becomes more and more interested in the spatial luminance distribution of the luminaire itself. Luminance maps offer a tool to predict the degree of discomfort glare early in the design process, especially when developing fixtures using small and intense LED light sources. The generation of luminance maps is commonly based on the reverse ray tracing technique, requiring one or more surfaces to be defined as light sources. However, ray files can not be considered as surface sources, but as a collection of ray data that model the near field of a light source. Despite the fact that ray files are constructed from experimental data they do not explicitly contain the geometry of the light source. This excludes the use of reverse ray tracing. For this reason the implementation of brute force forward ray tracing to obtain luminance maps was investigated. To be able to compare the results of both techniques an inhomogeneous surface source was defined. Luminance maps were then generated using both the brute force ray tracing approach and the conventional reverse ray tracing approach. A good agreement was obtained. A reduction in simulation time was achieved by parallel ray tracing computation and digital enhancement techniques.

The reliability of ray tracing simulations is strongly dependent on the accuracy of the input data such as the bidirectional reflectance distribution function (BRDF). Software developers offer the possibility to implement BRDF data in different ways, ranging from simple predefined functions to detailed tabulated data. The impact of the accuracy of the implemented reflectance model on ray tracing simulations has been investigated. A light-emitting diode device including a frequently employed diffuse reflector [microcellular polyethylene terephthalate (MCPET)] was constructed. The luminous intensity distribution (LID) and luminance distribution from a specific viewpoint were measured with a near-field goniophotometer. Both distributions were also simulated by use of ray tracing software. Three different reflection models of MCPET were introduced, varying in complexity: a diffuse model, a diffuse/specular model, and a model containing tabulated BRDF data. A good agreement between the measured and simulated LID was found irrespective of the applied model. However, the luminance distributions only corresponded when the most accurate BRDF model was applied. This proves that even for diffuse reflective materials, a simple BRDF model may only be employed for simulations of the LID; for evaluation of luminance distributions, more complex models are needed. ispartof: Optical Engineering vol:52 issue:9 pages:095101-1 status: published

Optical simulations are a common tool in the development of luminaires for lighting applications. The reliability of the virtual prototype is strongly dependent on the accuracy of the input data such as the emission characteristics of the light source and the scattering properties of the optical components (reflectors, filters and diffusers). These scattering properties are characterized by the bidirectional scatter distribution function (BSDF). Experimental determination of the BSDF of the materials is however very sensitive to the characteristics of the measuring instrument, i.e. the dimensions of the illumination spot, the detector aperture, etc. These instrumental characteristics are reflected in the instrument function. In order to eliminate the influence of the instrument function the use of a Bayesian deconvolution technique is proposed. A suitable stopping rule for the iterative deconvolution algorithm is presented. The deconvolution method is validated using Monte Carlo ray tracing software by simulating a BSDF measurement instrument and a virtual sample with a known bidirectional transmittance distribution function (BTDF). The Bayesian deconvolution technique is applied to experimental BTDF data of holographic diffusers, which exhibit a symmetrical angular broadening under normal incident irradiation. In addition, the effect of applying deconvolved experimental BTDF data on simulations of luminance maps is illustrated.