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Designing micro-optics for light-emitting diodes must take into account the near-field radiance and relative spectral power distributions of the emitting LED die surfaces. We present the design and application of a near-field goniospectroradiometer for this purpose.
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... can therefore measure the luminance of a ray anywhere along its length. Now, consider a planar or volumetric light source surrounded by an imaginary sphere ( Figure 2). Every ray of light emitted by the light source will have to intersect this sphere at some point. ...
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... Vol. 6342 634215-8 Having determined which triangle the random ray intersects, we know which three captured images and associated spectra are closest to the point where the ray intersects the imaginary sphere (Figure 2). Knowing the selected point o on the LED die from which the ray originates, it is a straightforward geometric calculation to determine the pixels 1 P , 2 P , and 3 P in the images corresponding to the physical point and their corresponding measured luminances 1 L , 2 L , and 3 L . ...
Citations
... As the steady state condition in photometry neglects temporal variations, R is reduced to a 6-dimensional function. Near field goniophotometry [10][11][12] can be used to directly measure a 5-dimensional R(X , Y , Z , φ, ϑ). Only the spectral information is lost due to its integration on an imaging luminance measurement device (ILMD). ...
Precise spectral and colorimetric simulations in commercial ray tracing software require realistic light source models, which provide spectral information as a function of angle and spatial dimension. We describe and validate a general workflow to create hyperspectral LED models as a linear combination of spectral models. The workflow only requires user defined precisions and rayfiles obtained with different optical filters. The rayfiles are transformed into histogram based models, whose precision is evaluated by normalized cross-correlation values of their intensity distributions in the near-, mid- and far field. Additionally, the concept is evaluated with a spatial and spectral well defined test device.
... The perfect light source model or rayfile would describe spatially and angular varying luminance and spectra. This information can be created by combining spectral and goniophotometric measurements [1][2][3]. Rykowski [4] proposed different methods to combine goniophotometric measurements and spectral measurements. A state of the art technique, which describes conventional phosphor converted white LEDs, uses two spectrally sharp separated rayfiles for the LED and the phosphor [5]. ...
... Therefore an optimization variable is necessary. Our optimization variable is defined as an optionally spectral weighted residual sum of squares (RSS) as described in Eq. (2). The weighting factors w j (λ ) may be set to w j (λ ) = 1 if all wavelengths have the same importance. ...
To simulate and optimize optical designs regarding perceived color and homogeneity in commercial ray tracing software, realistic light source models are needed. Spectral rayfiles provide angular and spatial varying spectral information. We propose a spectral reconstruction method with a minimum of time consuming goniophotometric near field measurements with optical filters for the purpose of creating spectral rayfiles. Our discussion focuses on the selection of the ideal optical filter combination for any arbitrary spectrum out of a given filter set by considering measurement uncertainties with Monte Carlo simulations. We minimize the simulation time by a preselection of all filter combinations, which bases on factorial design.
... Near-field goniophotometry is used to create a near-field representation of a light source [1][2][3][4][5][6][7][8][9]. In a near-field goniophotometer (NFG), a luminance camera, also termed imaging luminance measurement device (ILMD), and a photometer revolve around the light source [11]. ...
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.
... Since the beginning of the 1990s, near-field goniophotometry (NFG) has been introduced as an alternative method to measure the LID [3][4][5]. While far-field goniophotometry (FFG) relies on illuminance data captured by a photometer, NFG starts from the luminance distribution acquired by use of an imaging luminance measurement device (ILMD) [6,7]. To this end, the ILMD revolves about the DUT, and captures luminance images for all directions in which light is emitted. ...
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.
... For many optical systems however, light sources need to be characterized very accurately, both angularly and spatially, especially when they are located in the proximity of other optical components [6]. Such an accurate light source representation can be achieved with ray files obtained from near-field goniophotometer measurements [7][8][9][10]. While no standardized file format for ray files is available at the moment [11], ray files traditionally contain a predefined number of rays, each ray typically represented by a starting position, a direction vector, and a luminous flux. ...
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.
... As an example, radiant intensity distribution in each point of a surface light source is defined by the same IES file, and thus identical. Among the known methods of light source acquisition, representing the near-field characteristics, two main measurements can be distinguished, by means of spatial filters [7][8] or the goniophotometric [9]. Various methods of utilising the measured radiant intensity data for virtual reality are known [10][11][12], however none of them is available in the rendering software. ...
The article presents implementation and verification of algorithms that extend the LuxRender rendering engine capabilities to model of light sources by using rayfiles. The algorithms were developed for various ray tracing methods such as Direct Lighting, Bidirectional Path Tracing and Photon Mapping. Furthermore, these were optimized for speed and energy conservation. Using the developed algorithms, light sources were modelled on the basis of generated rayfiles of real light sources obtained from measurements of characteristics with the CCD goniophotometer in the near field. Results of modelling for the new light sources were compared with the results of structural and radiant intensity modelling methods. A method was also developed to compare accuracy of ray tracing of light sources in the software renderer, as well as to collate and analyze the results of algorithms with those of optical design simulation software. The studies identified usefulness of the application rayfiles for mapping rays of light sources in the near field, in particular their radiant intensity characteristics and the radiance distribution in optical simulation software for visualization and virtual reality.
... Since light distribution patterns immediately above the LED surface were not able to be approximated with either the extended surface light or point source, miniaturized optical component performance was likewise not predictable without concrete LED optical characteristic analysis [12,13]. In this region, CSP LED is ideally considered as a volume light source due to the generation of light from multiple quantum wells (MQWs). ...
... As the distance between light source and BSL was five times less than source diameter, i.e., "near-field" in illumination optics, the LED was designed according to volume light source and optical performance, as well as chip packaging [12][13][14][15]. We previously demonstrated a LED fabrication method by Monte Carlo ray tracing that functions almost identically as commercial LEDs [16]. ...
In this study, we presented a light-emitting diode-based (LED) miniaturized optical pattern imager for slim mobile phone application. To meet volume constraints, we designed a miniaturized compact illuminating and imaging optical component. The objective was to minimize optical loss using several nano- and micro-fabrication methods. After integration into a single optical body, the prototype imager-with dimensions of 6.8 × 2.2 × 2.5 mm and a weight of 0.4 g-demonstrated clear feasibility in measuring 2D micropatterns with widths of 50 and 10 μm.
... The use of ray files to model optical characteristics of a light source allows for accurate simulations of the luminous intensity distributions of luminaires because the interaction with optical components in the near field is taken into account 3,4 . Ray files are constructed by measuring the near field of the light source using a near field goniophotometer 5,6 . ...
... A major disadvantage of this approach is that when the light source is modeled by a ray file, there is no surface available associated with the source and the reverse ray tracing method does not allow the generation of luminance maps. However, the use of ray files to model the near field of the source is absolutely essential whenever optics are situated close to the source, which is almost always the case when using the popular LED sources 3,4 . Several solutions are conceivable to circumvent this issue. ...
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.
... Andersen et al. [ASGB09] par exemple, ont récemment développé un instrument de mesure des matériaux, qui repose sur la capture par une caméra fisheye, de la BRDF d'un échantillon, projetée sur un écran hémielliptique. Ashdown et Salsbury [AS06] ont fabriqué un instrument de mesure des sources, fondé sur le déplacement en champ proche autour de la source, d'une caméra couplée à un spectrophotomètre. Enfin, pour la reconstruction des champs de lumière discrets, Noé et Péroche [NP00] puis Albin et Péroche [AP03], respectivement, pour des données de matériaux et de sources, ont proposé des algorithmes fondés sur des moyennes pondérées à support local. ...
... Ashdown et Salsbury [AS06] ont récemment amélioré l'instrument, substituant un spectrophotomètre au luxmètre, pour la mesure de la luminance spectrique. ...
... En photométrie en champ proche, le domaine géométrique du solide photométrique étendu est une simple surface englobant la source. Les bancs d'acquisition des échantillons de luminance lumineuse se distinguent essentiellement par le type du détecteur : caméra couplée à un luxmètre dans l'instrument de Ashdown [Ash93a] [Ash93b], à un spectrophotomètre dans celui de Ashdown et Salsbury [AS06], ou luminancemètre imageant pour les bancs commerciaux SIG-300 et RiGO801. ...
Dans l'ouvrage [DBB06], Dutré et al. énumèrent dix problèmes d'Illumination Globale non résolus. Parmi ceux-ci, huit sont liés à la mesure et à l'intégration de données réelles dans les simulations. Cette thèse, en quatre parties et trois annexes, s'intéresse à cette problématique. Tout d'abord, les simulations de la propagation de la lumière dans les domaines du rendu physiquement réaliste, de l'éclairage, de la télédétection, de la conception de systèmes optiques... sont envisagées comme résolutions numériques d'un problème d'optique, fédérées par la notion de " photosimulation ". Dans le cadre de l'équation de rendu, les modèles réalistes de sources et le principe de la mesure goniophotométrique en champ lointain sont alors rappelés. La représentation des interactions lumière-matière est ensuite introduite par une exploration de l'apparence macroscopique, qui amène au rappel de la Fonction de Distribution de la Réflectance Bidirectionnelle et Spectrale (SBRDF), de ses principales propriétés et modèles. Le problème de la mesure pratique de la SBRDF d'une surface dans le visible est ensuite exploré. Id est, une taxonomie des méthodes de la littérature est établie ; qui allègue en faveur du développement de nouvelles approches. Un dispositif innovant, multispectral imageant, est alors présenté. Il se fonde sur la capture de la projection de la SBRDF sur un écran lambertien 3/4-sphérique, par une caméra multispectrale grand angle, assemblage d'un objectif fisheye, d'un filtre dynamique LCTF et d'une caméra CCD 12 bits. L'extraction des images capturées de l'information de la SBRDF repose sur un modéle radiométrique, qui explicite la transformation de la lumière en niveaux des pixels, dans le formalisme physique. Ce modèle soulève des problèmes de reconstruction multispectrale et d'interréflexions, pour lesquels de nouveaux algorithmes de résolution sont implantés. Les mesures de SBRDF produites semblent très prometteuses. Dans la troisième partie, le problème de la reconstruction d'une fonction directionnelle, identifié comme fondamental en photosimulation, est traité dans le cadre de la reconstruction de SBRDF discrètes. Pour cela, les propriétés mathématiques souhaitables des fonctions reconstruites sont envisagées. Puis, à l'aune de ce corpus, les approches de la littérature sont discutées ; justifiant la recherche d'algorithmes plus performants. Une nouvelle méthode est alors proposée, fondée sur une double triangulation sphérique des échantillons, et une généralisation à la surface d'un triangle sphérique de l'interpolant spline cubique de Hermite. La fonction reconstruite est interpolante, dérivable, quasi-déterministe, ne présente pas l'artéfact bidirectionnel, intègre la métrique sphérique non euclidienne, et prend en compte le difficile problème du masquage. Qualitativement comme quantitativement, les résultats obtenus semblent plaider en faveur du surcroît de complexité théorique qu'induit cette approche. Dans la dernière partie, cet algorithme de reconstruction est appliqué au problème de la " photométrie en champ proche ", ou, la caractérisation d'une source réelle par un solide photométrique étendu. La supériorité théorique du modèle étendu est d'abord démontrée. Puis, un état de l'art de la photométrie en champ proche est réalisé ; justifiant la mise en œuvre d'une nouvelle approche. Un nouveau dispositif est alors présenté. Il repose sur le déplacement d'un vidéoluminancemètre, couplage original d'une caméra CCD 12 bits avec un luxmètre, le long d'une trajectoire hémisphérique relative à la source. Les procédures de calibrage de l'instrument - géométrique, radiométrique, et photométrique - sont explicitées. Les sources de lumière caractérisées par cette approche ont été jugées suffisamment probantes pour être intégrées aux photosimulations spectrales du CSTB
... This is because a simple approximation of the chip and its packaging status can only be validated in far-field. Hence, further empirical verification of the near-field angular light distribution's accuracy is necessary [23]. ...
This paper presents a novel method in near-field beam shaping based on the precise optical modeling of a gallium nitride light-emitting diode (GaN LED). A Monte Carlo ray tracing simulation has been utilized to calculate the spatial photon distribution near the LED's top. By analyzing the ray data in near-field, the miniaturized lens profile is created and machined with aspherical surfaces and total internal reflection (TIR) Fresnel facets. The prototype lens reduce the viewing angle of the LED from 150 degrees to 17.5 degrees at full width half maximum (FWHM) while increasing the peak luminous intensity 10 times. The array of proposed lens with CSP LEDs exhibits feasibility of ultra thin uniform illumination in near-field.
