Light Booth Research Articles

The Impact of Target Object Size and Dimensionality on the Degree of Adaptation to Global Illumination

ABSTRACT Chromatic adaptation is an important visual phenomenon that specifically influences color appearance under various illumination conditions. There were extensive studies investigating the impact of the illumination condition and the surrounding condition on chromatic adaptation. However, past studies hardly consider the properties of the stimulus object such as color, size, dimensions, etc. In this study, the impact of the size and dimensionality of the stimulus (target object) on the degree of adaptation was investigated. A series of achromatic matching experiments were conducted in a lighting booth, with the adapting field varying in chromaticity and the object varying in size and dimensionality, to collect corresponding color data. Results indicate that the properties of the stimulus, such as the dimensionality and size, can significantly influence the adaptation state, independent of illumination chromaticity. But to apply the results in the real application may be restricted to the specific setup (used in this study) with a separately illuminated stimulus, which needs further investigation.

Poster Session: Chromatic Adaptation to Heterochromatic Illumination.

Heterochromatic viewing enviroments-where scenes are illuminated by two or more colors of light-are a common tool for experiments studying chromatic adaptation and color (in)constancy. This study presents a new method for separating sensory (retina-based) chromatic adaptation from cognitive mechanisms in a heterochromatic viewing environment. Using a large, bipartite light booth lit by two separate 7-channel LED systems, observers were adapted to uniform and mixed illuminations of simulated illuminant A, illuminant D65, and 12000 K daylight. A psychophysical staircase method determined observers' white points and their state of sensory adaptation along the daylight and Planckian loci under each heterochromatic or single-color lighting condition. Under heterochromatic conditions, observers' state of chromatic adaptation fell in between the points of chromatic adaptation when adapted to each individual light source, although there was significant variation between observers. These results suggest that observers' white points should be individually determined for studies of color perception in heterochromatic environments using methods similar to those used in this study. Further research will then allow us to model how cognitive discounting of the illumination color sits on top of the sensory adaptation measured here.

Effect of surrounding objects in the adapting scene on chromatic adaptation.

Most of the existing chromatic adaptation transforms (CATs) were developed for flat uniform stimuli presented in a uniform background, which substantially simplifies the complexity of the real scene by excluding surrounding objects from the viewing field. The impact of the background complexity, in terms of the spatial properties of the objects surrounding the stimulus, on chromatic adaptation is ignored in most CATs. This study systematically investigated how the background complexity and color distribution affect the adaptation state. Achromatic matching experiments were conducted in an immersive lighting booth, with the illumination varying in chromaticity and the adapting scene varying in surrounding objects. Results show that compared to the uniform adapting field, increasing the scene complexity can significantly improve the degree of adaptation for the Planckian illuminations with low CCT levels. In addition, the achromatic matching points are substantially biased by the color of the surrounding object, implying the interactive effect of the illumination color and the dominant scene color on the adapting white point.

Experimental setting and protocol impact human colour preference assessment under multiple white light sources.

Psychophysical experiment is the most straightforward and reliable way to investigate the impact of lighting on visual colour perception. In this study, a series of experiments were conducted in order to investigate the impact of experimental setting and protocol on the obtained conclusions in visual tests regarding human preference on object colour in applied lighting research. Four light sources of 5,500 K, with Duv values of -0.01, 0, 0.015, and 0.02, were used to illuminate different kinds of objects including blue jeans, fruit and vegetables, bread, artware, fresh pork, and skin tones. The use of those experimental light sources and objects was to provide control study for our former research by deliberately changing certain experimental setup and protocol and testify the robustness of our former conclusions. The results show that some of our former findings, like the dominant impact of lighting on colour preference, the visual cognition process of light booth experiments as well as the correlation between the whiteness of lighting and colour preference, were found to be valid in typical light booth experiment. The impact of experimental object turned out to be much stronger under the newly designed protocol and the significance of sex difference on colour preference judgment was found to vary with experimental setup. These new findings highlight the influence of experimental setting and protocol on the validity of research findings, which we believe, could provide deeper understanding for the psychophysical results of current colour preference studies.

A Tunable LED Daylight Luminaire for Textile and Printing Light Booth Application with Optimum LEDs

Color quality testing in the light booth is one of the essential requirements of manufacturing industries. Due to conventional daylight simulators incompetence, there is a need for improvise of light sources used in these booths. Also, solid-state lighting advancements has given a breakthrough for provisioning a light source that is energy efficient, long life, tunable, providing stable and modifiable illuminance. This research work focusses on the optimal design of a polychromatic LED luminaire, mimicking D50, D65 and D75 daylight spectrum with CRI > 90 and average illuminance 2000 lx. A multi-objective algorithmic approach towards LED selection has publicized daylight simulations feasibility with seven emitter combinations of red–green-blue-amber-lime-white (5070 K and 6570 K). The proximity of chromaticity coordinates of the test lamp with reference illuminant ascertains the quality of the designed luminaire. Further, the test lamps spectral fit and illumination consistency in the light booth ensure its suitability for color quality testing chambers.

Experimental methods to investigate time-course of chromatic adaptation

Two different experimental methods, method of adjustment and yellow/blue forced choice, were tested to investigate the time-course of chromatic adaptation. Inside the lighting booth, 2cmx2cm square color stimulus was displayed on the LCD display and the surface of the display was covered with gray paper except for the stimulus area. The lighting of the booth was controlled to have 3,000 K or 6,500 K with 800 lux at the bottom of the booth. During the adjustment method experiment, the observers adjusted the stimulus to preserve an achromatic appearance. In the forced-choice experiment, observers are asked to identify whether the stimuli are yellow or blue. In all experiments, evaluations were performed once every 5 seconds to track color appearance over time. CCT of the booth lighting was changed from 6,500 K to 3,000K or 3,000K to 6,500K every two minutes. The results showed that the observers had difficulties tracking the neutral colors using the adjustment method while the forced-choice experiment showed the more consistent results.

A Comparison of Two Different Light Booths for Measuring Color Difference of Metameric Pairs

A standardized source of light is essential for visual color assessments, which is why lighting booths were developed. For the best results in visual assessment, it is important to consider the right choice of light source, the right viewing conditions, and the variability of the viewer. To date, many light booth technologies have been introduced to meet user demands. Since most of the light sources on the market are characterized by the designer or manufacturer, the resulting variations from booth-to-booth remain. In this study, we compared the performance of two standard light booths to assess the color difference of eleven metameric pairs. In this study, we checked an earlier technology-based light booth that is still used in the textile industry and contains illuminant A (Tungsten lamp) with CCT 2700 K, TL84 (tri-band fluorescent tube) with CCT 4000 K, and simulator D65 (CCT 6500 K) with a different light booth whose original light sources have been replaced by currently available LED retro kits from equivalent CCTs. As an inexperienced customer or industrial user, our question was, how important is this replacement? The results revealed that two different standard lighting technologies with similar CCTs cannot reproduce the same estimates because the light sources produced different SPDs. It is illustrating that caution is necessary when comparing results obtained from two different light booths containing light sources with similar CCTs but different SPDs. This comparative study suggested that the variability of the light sources’ SPDs or the observer or the sample should be modeled considering light booth’s technology to estimate its contribution to the overall variability. The close relationship between perceived and CAM02-UCS suggests that if both booths are used after the light sources have been calibrated, a formula based on color appearance models must be used to predict color appearance. To obtain better agreement between perceived and calculated color difference, one must need to avoid light booths with nominally white light sources.

Accurate physics-based digital reproduction of effect coatings.

We built an improved 3D rendering framework to accurately visualize the complete appearance of effect coatings, including metallic effects, sparkle and iridescence. Spectral reflectance measurements and sparkle indexes from a commercially available multi-angle spectrophotometer (BYKmac-i) were used together with physics-based approaches, such as flake-based reflectance models, to implement efficiently the appearance reproduction from a small number of bidirectional measurement geometries. With this rendering framework, we rendered a series of effect coating samples on an iPad display, simulating how these samples would be viewed inside a Byko-spectra effect light booth. We validated the appearance fidelity through psychophysical methods. We asked observers to evaluate the most important visual attributes that directly affect the appearance of effect coatings, i.e., the color, the angular dependence of color (color flop) and the visual texture (sparkle and graininess). Observers were asked to directly compare the rendered samples with the real samples inside the Byko-spectra effect light booth. In this study, we first validated the accuracy of rendering the color flop of effect coatings by conducting two separate visual tests, using flat and curved samples respectively. The results show an improved accuracy when curved samples were used (acceptability of 93% vs 80%). Secondly, we validated the digital reproduction of both color and texture by using a set of 30 metallic samples, and by including texture in the rendering using a sparkle model. We optimized the model parameters based on sparkle measurement data from the BYK-mac I instrument and using a matrix-adjustment model for optimization. The results from the visual tests show that the visual acceptability of the rendering is high at 90%.

Novel approach in sensory color masking: Effects of colored environments on chocolates with different cocoa content

In sensory practice, the color masking procedure is executed almost exclusively with red light, regardless of the color of the product. In this work the practical application of a novel approach for sensory masking of color differences, hence the applicability of a spectrally tunable light booth was investigated. The aim of the research was to find the most appropriate masking environment, which i) can be considered as the most effective mask and ii) provides the best conditions regarding chromatic adaptation.The main idea of the novel approach in sensory color masking is that the instrumental characterization of all parameters influencing the perception of the sensory assessor. The comparison of the potential masking illuminations based on instrumental results (considering masking effect and chromatic adaptation) and the definition of the most appropriate mask should be accomplished based on sensory tests. The sensory effects of colored environments were tested on chocolate samples containing different amount of cocoa. CIELAB dE color differences between the samples were calculated based on their spectral reflectance. Four monochromatic (red, green, blue, amber) and six combined masking illuminations were developed in a spectrally tunable light booth and were instrumentally characterized by their spectral power distribution and luminance values. The chocolates were ranked according to their darkness by the assessors. The masking environments were compared based on the analysis of sequence correctness, decision time, and their combination (ANOVA, Survival-analysis).In Experiment 1 the monochromatic Blue light was not only more effective, than the commonly used Red light, but it was the most effective mask regarding color differences: the sequence error increased to 61% compared to the standard white (D65) illumination, and the decision time increased to more than double (15.74 s → 35.7 s). In Experiment 2 green and amber lights were combined with the blue mask, increasing the degree of chromatic adaptation: decision time decreased while the effectiveness of the mask did not decrease based on the sequence errors. Following the novel approach product specific masking environment considering effectiveness and chromatic adaptation was realized verified by statistical analysis of sensory test results.

Visual and instrumental assessment of interference pigments

AbstractDiscrepancies often exist between visual assessment and instrumental assessment. These are due to different geometries with which samples are viewed or measured: In visual observation at the window or in the light booth, the sample panels are usually moved up and down. This changes the angle of illumination and observation. The current measuring devices have a fixed illumination.

Perceptibility and Acceptability of Surface Gloss Variation Under Different Illuminants

The purpose of this study was to evaluate the influence of different illuminants on the perceptibility and acceptability of surface gloss variations and to determine limiting values. Eight composite resin specimens and one human tooth specimen were polished to obtain composite resin specimens with different gloss units (GU) of 10, 20, 30, 40, 50, 60, 70, and 80 and a human tooth specimen of 80 GU. Sixty observers compared the surface gloss of the specimens in a light booth. For perceptibility testing, the specimens were randomly positioned two at a time. The acceptability of the gloss variation was determined by comparing the composite resin specimens with the tooth specimen. The observers answered specific questions to determine the level and perceptibility and acceptability limits of gloss variations. All analysis was done with two illuminants (D65 and fluorescent light) used randomly. Data were submitted to a nonlinear probit model and nonlinear probit regression estimation (α=0.05). Significant differences in illuminants were observed for perceptibility (p<0.001) and acceptability (p=0.045). The perceptibility limit for D65 was 7.0 GU and 6.8 GU for fluorescent illuminant. The acceptability limit for D65 was 34.2 GU and 37.1 GU for fluorescent illuminant. More accurate perceptibility and acceptability judgments of the surface gloss of composite resin were made when the specimens were illuminated with D65 light.

Visual validation of the appearance of chromatic objects rendered from spectrophotometric measurements.

We validate a physically based and spectral rendering framework with improved color reproduction. With a recently developed model, we take into account both the colorimetric specifications of the rendering display as well as the spectral and angular characteristics of lighting and also the spectral reflectance of the objects. Therefore, it should provide much better color reproduction than those based on the common standard red, green, blue (sRGB) color space. In addition, it allows real-time rendering on modest hardware and displays. We evaluated the color reproduction of the new rendering framework by psychophysical tests using spectrophotometric measurements of 30 chromatic paint samples. They were rendered on an iPad display, as viewed inside the Byko-spectra effect light booth. We asked 16 observers to evaluate the color match by directly comparing the rendered samples with the physical samples, using two different psychophysical assessment methods. The color reproduction was found to be strongly improved with respect to results obtained with default sRGB color encoding space. The average color reproduction match was found to be equivalent to ΔE00=1.6, which is a small but noticeable color difference. In 80% of the visual assessments, the color reproduction was described as being at least as good as between "difference visible but still acceptable" and "difference visible, doubtful match."

Color perceptibility and validity of silicon carbide–based protective coatings for dental ceramics

Statement of problemA silicon carbide (SiC) protective coating has been developed for dental ceramics, but whether the coated ceramics can match the classical VITA shades is unclear. PurposeThe purpose of this observational in vitro study was to evaluate the color adaptability of SiC-coated dental ceramics by testing the hypotheses that SiC-coated disks can be fabricated to match standard tooth shades and have a perceptible color match rate of at least 50% for disks with a color difference (ΔE)<2.0. The effects of ΔE, shade hue, shade value, observer sex, years of experience, profession, and shade guide orientation on color perception were studied. Material and methodsSiC-coated disks were fabricated to color match (ΔEab<3.3) all 16 VITA classical shades. Uncoated disks of each VITA shade were used as the reference materials to determine whether the SiC-coated disks were color matched to the classical VITA shade guide. Three ΔE formulas (76, 94, and 2000) were applied and compared. Participants (N=120) with an average of 22 years of experience included dental school students, dental faculty members, and dental assistants. Pseudoisochromatic plate and the Farnsworth D-15 Panel test for assessing color deficiency and color blindness were administered. Participants then attempted to match SiC-coated disks to standard shade guides arranged by value or hue. All spectrophotometer readings and color matching were conducted in a light booth with standardized daylight illumination. Statistical analysis used the Fisher's exact test to determine factors associated with improved matching performance (α=.05). ResultsA significant difference in color match rate was found between disks with ΔE<2.0 (63.9%) and ΔE≥2.0 (41.7%) (P<.001). Arranging shade by value (72.2%) instead of hue (67.2%) produced better color matching (P<.001). Sex (P=.430), profession (P=.708), and years of experience (P=.902) had no significant influence on color matching. ConclusionsSiC-coated disks were successfully fabricated to match all VITA classical shades, and clinical visual color matching results confirmed that ΔE was a useful metric in optimizing color matching for the SiC-coated dental ceramics.

Physics-based modeling of a light booth to improve color accuracy of 3D rendering

Computer Aided Design (CAD) is increasingly used as a tool in industries varying from automotive to interior design. Digital visualization allows users to design their working and living spaces, and to select materials and colors for future products. The rendering software that is currently available often suggests photorealistic quality. However, visual comparisons of these images with the physical objects they represent reveal that the color accuracy of these methods is not good enough for critical applications such as automotive design. Therefore, we recently developed a spectral pipeline for rendering gonio-apparent materials such as effect coatings. In order to accurately render objects as they appear in a physical environment, this new approach requires a physics-based representation of the illumination surrounding the objects. In the present article we investigate how to physically represent one, well-defined lighting environment. Therefore, we investigated the lighting inside a recent, commercially available light booth that is widely used in the paint and graphical industry. We determined the spatial dimensions of the X-Rite SpectraLight QC light booth, and built a digital geometrical model of this light booth in the Open Source software Blender. We then measured the spectral radiance emitted by the various light sources that are integrated in the luminaire of this light booth, as well as the illuminance on a grid of measurement spots on the platform below the luminaire. Using these measurement data, we were able to develop an accurate physical simulation of the light field inside the light booth. We plan to use the physics-based model of the lighting inside the light booth to set up visual tests in which physical objects inside the physical light booth are visually compared to images showing virtual objects inside the virtual light booth. These visual tests will form the basis for developing improved models for displaying the color and texture of gonio-apparent materials.

Interactive effect of illuminance and correlated colour temperature on colour preference and degree of white light sensation for Chinese observers

The aim of this study was to explore the interactive effect of Illuminance (E) and Correlated Colour Temperature (CCT) on the visual appreciation of the colour rendition of lighting (termed as colour preference), as well as on a newly defined visual attribute termed the degree of white light sensation, which describes the Chinese understanding on a loosely-defined but widely-used concept: ‘white light’. Thirty E−CCT combinations, that included six E values from 50 lx to 1800 lx and five CCT values from 2500 K to 6500 K, were used to illuminate a pair of highly coloured Jiaozhi artefacts, as well as the neutral space of an empty light booth. Thirty observers, fifteen males and fifteen females, were invited to take part in the visual experiments to rate the colour preference and the white light sensation. The results indicated that both E and CCT significantly influenced both visual attributes. For colour preference, the impact of E was greater while for the degree of white light sensation, stronger influence of CCT was observed. Based on the psychophysical results, the existing metrics that are used to evaluate the colour preference of E−CCT combinations were validated and a new model was proposed to characterize the degree of white light sensation. In addition, gender difference was investigated and it was found that male and female observers behaved similarly in assessing white light but unlike male observer, female observers did not prefer light sources with the highest degree of white light sensation.

Evaluating the color preference of lighting: the light booth matters.

On the topic of color preference of lighting, it is commonly believed that the neutral interior of a light booth has minimum impact on the color perception of the experimental object. Meanwhile, agreement has not been reached on which objects should be placed in the booth. In this study, based on a meta-analysis of eight groups of psychophysical data, we demonstrate that the "perceived color preference" obtained by consecutive visual judgement in a light booth is closely related to the lit neutral environment, while the use of different experimental objects does not markedly influence the results for lighting quality assessment. Such a finding might be attributed to the inherent features of the visual cognition process of light booth experiments and it should be fully recognized by future work.

Assessing gloss under diffuse and specular lighting

AbstractThe visual sensation of gloss is built on cues deduced from the interaction between light, surfaces sunder evaluation, and surrounding conditions. Gloss is a second‐order attribute of the visual appearance, this means that its perception is not directly encoded on biological sensors but constructed from the global scene in the field of view of the observer. It is then a complex quantity to measure. When most studies based on simulated samples stress on the importance of realistic observation conditions, we measure the effect of environment complexity over perception of real samples. We test two different lighting conditions: either diffuse or a combination of diffuse and collimated lighting in order to approach natural complex illumination patterns. Under both lighting conditions, we test two environments: a standard black light booth, designed according to the ASTM D4449, and a realistic office cubicle. Samples consist in a seven‐level gloss scale ranging from full matt to high gloss. These are presented to observers through pair comparison protocol, according to a maximum likelihood difference scaling algorithm. Our results confirm that gloss constancy is maintained even if the convergence of illumination varies. We however measure that the constancy is lost for matt samples perception.

Experimental method for measuring color appearance shifts in high-dynamic-range luminance conditions.

We present an experimental method to determine color appearance shifts under high-dynamic-range conditions. A couple of light booths with variable luminance provide high-dynamic-range luminance conditions, and a perceptual color shift between the two booths is determined using color appearance matching. For red, green, yellow, and blue groups of four surface color samples, color shifts were measured for nine subjects under a dual illumination at background luminance levels of $100\,\,{{\rm cd/m}^2}$100cd/m2 and $4700\,\,{{\rm cd/m}^2}$4700cd/m2. We observed significant perceptual hue shifts toward blue with magnitudes of 2.5 to 3.9 and 5.0 to 6.9 CIELAB units, for the red and green samples, respectively, and decreases in chroma for most samples when changed from low to high luminances.

Perceptibility and Acceptability of Surface Gloss Variations in Dentistry.

To assess the visual perception of observers regarding perceptibility and acceptability of surface gloss variations of resin composites and to determine the limit of perceptibility and acceptability of gloss variations. Eight resin composite specimens and one human tooth specimen were fabricated. The resin specimens were polished to reach a surface gloss, in gloss units (GUs), of 10 GU, 20 GU, 30 GU, 40 GU, 50 GU, 60 GU, 70 GU, and 80 GU, and the human tooth specimen had a surface gloss of 80 GU. Sixty observers were selected to compare the surface gloss of the specimens in a light booth. For the perceptibility assessment, specimens were randomly displayed two at a time. Each observer performed a total of 144 observations. Observers answered two specific questions for determining the level and limit of perceptibility of gloss variations. The acceptability limit of gloss was determined by comparing the resin specimens with the tooth specimen. The observations were performed with dental practice scenarios (illuminant conditions, visualization field, and observers' education). Data were submitted to a nonlinear probit model and nonlinear regression estimation probit (5%). Differences in perceptibility and acceptability were observed for surface gloss variations (ΔGU) (p<0.001). Perceptibility increased with ΔGU (10<20<30<40<50=60=70), while acceptability decreased with ΔGU (0=10>20>30>40>50>60=70). Acceptability and perceptibility limits were 6.4 GU and 35.7 GU, respectively. Perceptibility and acceptability of surface gloss are influenced by gloss variations. A variation of 6.4 GU was required for 50% of observers to notice gloss variations. Concerning acceptability, observers required a variation of 35.7 GU to consider differences in gloss not acceptable.

White lighting and colour preference, Part 1: Correlation analysis and metrics validation

In this work, the correlation between the whiteness of lighting and the preference for the colour of lighting was comprehensively investigated based on four psychophysical studies. Different sets of experimental light sources, two of constant correlated colour temperatures (CCT) and two of different CCTs were adopted to illuminate multiple experimental objects with different illuminance levels (200 lx or 500 lx). The observers were asked to rate their colour preference towards the illuminated objects and rate their white perception towards the illuminated environment of the empty light booth. It was found that for the studies with same CCT but different chromaticities, people indeed preferred whiter illumination. However, for the cases with multiple CCTs, that statement is only valid between 2500 K and 5500 K, since too white a light source impaired colour preference. In addition, based on the whiteness rating data, the performance of three typical metrics for white lighting was assessed. The findings of this study may contribute to a deeper understanding of colour and whiteness perception in lighting, especially for lighting designers and manufacturers.