Evaluating colour preference by using multidimensional approaches

Colour preference is a key factor in the design and evaluation of lighting systems, particularly with the emergence of multichannel LED systems which allow for greater control over the spectrum of light emitted and therefore the colour appearance of the illuminated objects. To more accurately and objectively measure colour preference, there has been a growing interest in the development of multidimensional evaluation algorithms that consider multiple dimensions of colour rendering, such as chroma and hue shift. The purpose of this study was to compare and evaluate the performance of different multidimensional evaluation algorithms for colour preference in lighting applications. Using computer-generated images of a coloured object displayed on a computer monitor under a fixed white point, we simulated the colour shifts of the object under different light sources and test subjects evaluated the results using a range of multidimensional methods. Our analysis revealed that there are significant differences in the performance of these algorithms, with some providing more accurate and reliable measures of colour preference than others. Considering all relevant criteria, genetic algorithms seem to provide the most promising approach, as they lead to a result quickly and reliably. These findings have important implications for the selection and use of multidimensional algorithms for evaluating colour preference in lighting, particularly in the context of multichannel LED systems, and can inform future research in this area

Untersuchung der Duv-Präferenz in Abhängigkeit von korrelierter Farbtemperatur (CCT), Farbgamut und betrachteten Objekten

Der Weißpunkt einer im Innenraum verwendeten LED-Lichtquelle hat einen entscheidenden Einfluss darauf, ob Menschen die Beleuchtung akzeptieren oder nicht. Daher wurden in den letzten Jahren vermehrt Präferenzuntersuchungen zum Weißpunkt durchgeführt, um die Innenraumbeleuchtung mit LED zu optimieren. Die Studienergebnisse zeigten, dass die Testpersonen bei verschiedenen CCTs negative Duv-Werte präferieren. Allerdings wurde bei diesen Studien weder eine Einordnung in einen bestimmten Kontext vorgenommen, noch die Auswahl vorhandener und beleuchteter, farbiger Objekte begründet. Deshalb wurde im Rahmen einer eigenen Probandenstudie untersucht, welchen Einfluss die korrelierte Farbtemperatur, der Farbgamut und die betrachteten Objekte auf den präferierten Duv-Wert und somit Weißpunkt haben. Die Untersuchung ergab, dass für die CCT 4000 K Duv-Werte zwischen -0,0045 und -0,0015, also nahe des Planck’schen Kurvenzugs, präferiert wurden. Farbige Objekte hatten keinen signifikanten Einfluss auf die Präferenz. Der Einfluss des Farbgamuts konnte nicht vollständig geklärt werden

High‐resolution depth measurements in digital microscopic surgery

Fully digital microscopes are becoming more and more common in surgical applications. In addition to high‐resolution stereoscopic images of the operating field, which can be transmitted over long distances or stored directly, these systems offer further potentials by supporting the surgical workflow based on their fully digital image processing chain. For example, the image display can be adapted to the respective surgical scenario by adaptive color reproduction optimization or image overlays with additional information, such as the tissue topology. Knowledge of this topology can be used for computer‐assisted or augmented‐reality‐guided microsurgical treatments and enables additional features such as spatially resolved spectral reconstruction of surface reflectance. In this work, a new method for high‐resolution depth measurements in digital microsurgical applications is proposed, which is based on the principle of laser triangulation. Part of this method is a sensor data fusion procedure to properly match the laser scanner and camera data. In this context, a strategy based on radial basis function interpolation techniques is presented to handle missing or corrupt data, which, due to the measuring principle, can occur on steep edges and through occlusion. The proposed method is used for the acquisition of high‐resolution depth profiles of various organic tissue samples, proving the feasibility of the proposed concept as a supporting technology in a digital microsurgical workflow

Processing RGB Color Sensors for Measuring the Circadian Stimulus of Artificial and Daylight Light Sources

The three main tasks of modern lighting design are to support the visual performance, satisfy color emotion (color quality), and promote positive non-visual outcomes. In view of large-scale applications, the use of simple and inexpensive RGB color sensors to monitor related visual and non-visual illumination parameters seems to be of great promise for the future development of human-centered lighting control systems. In this context, the present work proposes a new methodology to assess the circadian effectiveness of the prevalent lighting conditions for daylight and artificial light sources in terms of the physiologically relevant circadian stimulus (CS) metric using such color sensors. In the case of daylight, the raw sensor readouts were processed in such a way that the CIE daylight model can be applied as an intermediate step to estimate its spectral composition, from which CS can eventually be calculated straightforwardly. Maximal CS prediction errors of less than 0.0025 were observed when tested on real data. For artificial light sources, on the other hand, the CS approximation method of Truong et al. was applied to estimate its circadian effectiveness from the sensor readouts. In this case, a maximal CS prediction error of 0.028 must be reported, which is considerably larger compared to daylight, but still in an acceptable range for typical indoor lighting applications. The use of RGB color sensors is thus shown to be suitable for estimating the circadian effectiveness of both types of illumination with sufficient accuracy for practical applications

Measurement of Circadian Effectiveness in Lighting for Office Applications

Featured Application: In-field spatially resolved light measurements for the characterization of non-visual effects and proper prediction of the circadian effectiveness for human-centric lighting design. Abstract: As one factor among others, circadian effectiveness depends on the spatial light distribution of the prevalent lighting conditions. In a typical office context focusing on computer work, the light that is experienced by the office workers is usually composed of a direct component emitted by the room luminaires and the computer monitors as well as by an indirect component reflected from the walls, surfaces, and ceiling. Due to this multi-directional light pattern, spatially resolved light measurements are required for an adequate prediction of non-visual light-induced effects. In this work, we therefore propose a novel methodological framework for spatially resolved light measurements that allows for an estimate of the circadian effectiveness of a lighting situation for variable field of view (FOV) definitions. Results of exemplary in-field office light measurements are reported and compared to those obtained from standard spectral radiometry to validate the accuracy of the proposed approach. The corresponding relative error is found to be of the order of 3–6%, which denotes an acceptable range for most practical applications. In addition, the impact of different FOVs as well as non-zero measurement angles will be investigated

Heterogene Farbregistrierung von typgleichen Spektralsensoren

Intelligente Beleuchtung in Innenräumen, welche dynamisch auf die Bedürfnisse des Menschen eingeht, benötigt Sensoren, um die aktuelle Beleuchtungssituation im Raum zu erfassen. Zur Regelung des Beleuchtungsfarbortes sind dabei optische Sensoren mit mindestens drei Kanälen erforderlich, um das trichromatische Sehen des Menschen technisch nachzubilden. Bereits typgleiche Farbsensoren weisen produktionsbedingte Differenzen in ihren spektralen Kanalempfindlichkeiten auf. Sollen also in einem farbortgeregelten Beleuchtungssystem mehrere Farbsensoren parallel eingesetzt werden, ist sicherzustellen, dass die Unterschiede in der Farbregistrierung der Sensoren für die Regelung mathematisch korrigiert werden. Diese Anforderung gilt für weitere Anwendungsbereiche der Sensoren wie Farbauswahl und -abgleich, Authentifizierung sowie Farb- und Spektralanalyse von Materialien und Flüssigkeiten in gleichem Maß. Das standardisierte Vorgehen zur Farbkorrektur von Sensorantworten in den CIE-XYZ -Farbraum nach ISO 17321-1:2012 sieht die Bestimmung und Anwendung einer Transformationsmatrix zwischen dem Sensorfarbraum und dem XYZ-Farbraum vor. Dabei kann über die Berechnung von Sensorantworten bei bekannten spektralen Empfindlichkeiten oder über standardisierte Reflexionstargets vorgegangen werden. In diesem Beitrag werden zunächst die realen spektralen Empfindlichkeiten von 24 typgleichen Farbsensoren den im Datenblatt angegebenen Kurven gegenübergestellt. Ausgehend davon werden beispielhaft die Farbabstände für Testfarborte eines Colorcheckers berechnet, welche sich ergeben, wenn die über die Kurven eines Referenzsensors bestimmte Transformationsmatrix für alle 24 Sensoren verwendet wird. Um in der Praxis nicht tatsächlich für jeden Sensor eine eigene Transformationsmatrix bestimmen zu müssen, wird abschließend die Realisierbarkeit eines Binning-Verfahrens für die untersuchten Sensoren eruiert. Intelligent indoor lighting that responds dynamically to the human users’ needs in terms of illumination levels and comfort requires sensors to detect the current lighting situation in the room. Optical sensors with at least three channels are required to control the chromaticity of the lighting in order to technically simulate the trichromatic vision of humans. Color sensors of the same type already exhibit production-related differences in the spectral sensitivity of their channels. Therefore, if several color sensors are to be used in parallel in a chromaticity-controlled lighting system, it must be ensured that the differences in the color registration of the sensors are mathematically corrected. This requirement applies equally to other application areas, such as color selection and matching, authentication, as well as color and spectral analysis of materials and liquids. The standardized procedure for color correction of sensor responses into the CIE XYZ color space according to ISO 17321-1:2012 is to determine and apply a transformation matrix between the sensor color space and the XYZ color space. This can be done by calculating sensor responses in case that the spectral sensitivities are known, or by using standardized reflectance targets. In this paper, the real spectral sensitivities of 24 color sensors of the same type are first compared with the curves given in the data sheet. From these sensitivities, the resulting color distances for multiple test chromaticity coordinates of a color checker are calculated by using the same transformation matrix determined from a single reference sensor for all 24 sensors. In order to avoid having to determine a separate transformation matrix for each sensor in practice, the feasibility of a binning procedure is explored for the investigated sensors

Tackling Heterogeneous Color Registration: Binning Color Sensors

Intelligent systems for interior lighting strive to balance economical, ecological, and health-related needs. For this purpose, they rely on sensors to assess and respond to the current room conditions. With an augmented demand for more dedicated control, the number of sensors used in parallel increases considerably. In this context, the present work focuses on optical sensors with three spectral channels used to capture color-related information of the illumination conditions such as their chromaticities and correlated color temperatures. One major drawback of these devices, in particular with regard to intelligent lighting control, is that even same-type color sensors show production related differences in their color registration. Standard methods for color correction are either impractical for large-scale production or they result in large colorimetric errors. Therefore, this article shows the feasibility of a novel sensor binning approach using the sensor responses to a single white light source for cluster assignment. A cluster specific color correction is shown to significantly reduce the registered color differences for a selection of test stimuli to values in the range of 0.003–0.008 ∆u'v', which enables the wide use of such sensors in practice and, at the same time, requires minimal additional effort in sensor commissioning

Sky-like interior light settings: a preference study

This paper explores human observer preferences for various sky-like interior lighting scenarios realized by a combination of a blue-enriched indirect uplight component with a correlated color temperature (CCT) of 6,500 K up to 30,000 K and a 4,000 K or 5,500 K direct downlight component. Variations in the natural sky were mimicked by the indirect uplight component reflected from the ceiling of the experimental room. The settings for the direct lighting component, on the other hand, were selected based on the reported outcomes of previous preference studies in the field of interior lighting. The resulting lighting conditions were evaluated by a total of 29 observers, from which subjective ratings of brightness, sky-likeness, satisfaction, pleasantness, and general appeal were collected in an office workplace environment. In this experimental setting, the most preferred lighting conditions exhibited a direct-to-indirect lighting ratio of 50:50 with a CCT of 4,000 K in the direct component and 6,500, 7,500, and 9,000 K in the indirect component. For all examined combinations, none was rated as truly sky-like. Nonetheless, the study results showed that only the combination of a warmer CCT in the direct component and a cooler, blue-enriched CCT in the indirect lighting component leads to a maximum in the subjects’ preference ratings. In summary, the subjects preferred light settings with a white appearance on the work surface without any intense or noticeable blue cast or tint

Sky-like interior light settings: a preference study

This paper explores human observer preferences for various sky-like interior lighting scenarios realized by a combination of a blue-enriched indirect uplight component with a correlated color temperature (CCT) of 6,500 K up to 30,000 K and a 4,000 K or 5,500 K direct downlight component. Variations in the natural sky were mimicked by the indirect uplight component reflected from the ceiling of the experimental room. The settings for the direct lighting component, on the other hand, were selected based on the reported outcomes of previous preference studies in the field of interior lighting. The resulting lighting conditions were evaluated by a total of 29 observers, from which subjective ratings of brightness, sky-likeness, satisfaction, pleasantness, and general appeal were collected in an office workplace environment. In this experimental setting, the most preferred lighting conditions exhibited a direct-to-indirect lighting ratio of 50:50 with a CCT of 4,000 K in the direct component and 6,500, 7,500, and 9,000 K in the indirect component. For all examined combinations, none was rated as truly sky-like. Nonetheless, the study results showed that only the combination of a warmer CCT in the direct component and a cooler, blue-enriched CCT in the indirect lighting component leads to a maximum in the subjects’ preference ratings. In summary, the subjects preferred light settings with a white appearance on the work surface without any intense or noticeable blue cast or tint