A novel green-emitting luminescent materials with broad-band emission for high-quality warm white LED indoor lighting

Following the rapid growth of lighting technology, and in order to satisfy the higher requirement of people for lighting, the high-quality warm white lighting for indoors has stolen much limelight of researchers. Herein, a novel green phosphor Ce³⁺-doped Ca₂GdHf₂(AlO₄)₃ with garnet structure which can be used for warm white LED was developed by solid-state method. Under 410 nm light excitation, the phosphor exhibits broad emission spectrum from 450 to 700 nm with FWHM more than 120 nm, and it also owns high internal quantum efficiency of 83.6%. The energy transfer among the activators was identified according to the concentration quenching and decrease of decay time, which belongs to dipole-dipole interaction. The calculated activation energy is about 0.255 eV, indicating the moderate thermal stability. The fabricated white LED device with low CCT (3807 K) and high CRI (R_a = 89.2) demonstrates its superiority and possibility of practical application in high-quality warm white LED fields for indoor lighting fields.     

Chromaticity‐matched but spectrally different light source effects on simple and complex color judgments

As light‐emitting diode (LED) light sources mature, lighting designers will be able to deliver white light with a variety of spectral power distributions and a variety of color rendering properties. This experiment examined the effects of three spectral power distributions (SPDs) that were matched in illuminance and chromaticity on three measures of color perception: one objective (performance on the Farnsworth‐Munsell 100 hue test) and two subjective (judgments of the attractiveness of one's own skin, and preferences for the saturation of printed images). The three SPDs were a quartz‐halogen (QH) lamp and two LED sources that were matched to the QH lamp in terms of both illuminance and chromaticity; the three light sources were nominally CCT = 3500 K, x = 0.40, y = 0.39 and ～ 400 lx. LED A used three channels (red, green, blue), and had very poor color rendering (R_a = 18). LED B used four channels (red, amber, cyan, white) and had very good color rendering (R_a = 96, whereas the QH had R_a = 98). Secondary hypotheses addressed the effects of age and skin and eye color on the dependent measures. As expected, LED A delivered very different color perceptions on all measures when compared to QH; LED B did not differ from QH. The results show that it is possible for LED sources to match the familiar incandescent sources. However, although it is possible to deliver what appear to be millions of colors with a three‐chip (RGB) device, there is the risk of creating a very poor luminous environment. © 2013 National Research Council Canada and Wiley Periodicals, Inc. Col Res Appl, 39, 263–274, 2014; Published Online 12 April 2013 in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/col.21811     

Effects of correlated color temperature on focused and sustained attention under white LED desk lighting

Since the Kruithof's work on general illumination, the relationships between correlated color temperatures (CCTs) and human behavior are well documented. In the recent years, because of the high growth of light‐emitting diode (LED) lighting products, studies on the effects of LED lighting on human behavior have emerged. This study examines the effects of CCTs on focused and sustained attention under white LED desk lighting. Three CCT conditions (2700, 4300, and 6500 K) are examined, and the Chu Attention Test was used to measure focused and sustained attention. Results show that CCTs affect attention. In specific, the 4300 K condition resulted in a significantly better focused and sustained attention. Comfortableness and clarity are rated by the subjects. The self‐reported comfort shows no significant differences among the three conditions, but clarity shows significant differences. Gender differences are also discussed. It is found that the self‐reported clarity by males show no significant differences among the three lighting conditions, whereas the self‐reported clarity by females is significantly lower when CCT is 2700 K. © 2014 Wiley Periodicals, Inc. Col Res Appl, 40, 281–286, 2015     

A high efficacy and tunable white light-emitting diode cluster with both color fidelity and nonvisual performances close to natural lights

A luminous efficacy model for a white light-emitting diode (LED) cluster was developed, and five α-opic efficacy ratios (γ_α,Vs) of intrinsically photosensitive retinal ganglion cells were adopted in order to quantify its nonvisual performance relative to natural light. A tunable C/A/R LED cluster, which consists of the cyan LEDs with green phosphor excited by a blue LED, the amber LEDs with green and orange phosphors excited by same blue chip, and the red LEDs, was proposed by maximizing the average luminous efficacy at correlated color temperatures (CCTs) of 2700 to 6500 K under the condition of color fidelity index (R_f) above 96 and five values of γ_α,V of 0.95 to 1.05. The real tunable C/A/R LED cluster with a R_f of 95 to 96, five values of γ_α,V of 0.97 to 1.01, and a luminous efficacy of 114 to 120 lm/W at a CCT of 2670 to 6456 K were demonstrated. Furthermore, five values of γ_α,V can be adjusted according to the distance from the Planckian or daylight locus on the CIE 1960 uv chromaticity diagram at different CCTs. Thus, the tunability of γ_α,V value of the C/A/R LED cluster at a specific CCT could be applied to different requirements for nonvisual performance.     

Soluble silica assisted synthesis and luminescent characteristics of yellow emitting CaSrSiO4:Eu2+ phosphors for warm white light production

To meet the challenges and additional requirements towards the development of white LED׳s with utmost efficacy, a sol–gel approach is adopted wherein a water soluble silicon compound is used as a silica source. The phosphor material obtained is subjected to detailed structural, morphological and luminescent studies. The results obtained show that the XRD patterns of Eu²⁺ doped CaSrSiO₄ phosphors are in good agreement with the CaSrSiO₄ (ICSD no. 49660) whose structure is orthorhombic. The output of the luminescence studies clearly portrays a broad yellow emission between 450 and 750 nm with a peak at ~600 nm under the broad excitation range. This confirms its efficient emission towards the development of a warm white light using blue LEDs. A red shift in the peak emission wavelength was observed for the prepared samples. This longer shift in wavelength can be credited to the sol–gel method adopted and is not offered by the conventional solid state reaction method. A warm white emitting LED was fabricated by combining near ultraviolet LED (400 nm) chip with our sol–gel synthesized CaSrSiO₄:Eu²⁺ phosphor. The CIE chromaticity coordinates (0.44 and 0.41), color correlated temperature (CCT) <4000 K, color rendering index (CRI) >80 provide their emission potentiality in the warm white light region.     

Dependence of the color appearance of some flowers on illumination

Seven flower colors perceived by five color experts using visual color measurement under 2800 K warm white fluorescent lamps, 3500 K plant growth lamps, and 6500 K light‐emitting diodes (LEDs) were compared with those under 6500 K fluorescent lamps, which represented illuminants in florist shops. Fluorescent lamps (6500 K, 1000 lx) were found to be effective for displaying flower colors and were used as the standard condition. The colors of flowers generally shifted in the same direction as those of the illuminants in CIELAB space. The color differences were highest under the 3500 K fluorescent lamp at both 500 and 2000 lx. At 500 lx, the ΔE values under the 6500 K LED were higher than those under the 2800 K lamp. The C* and ΔE values revealed that the 2800 K lamp was unsatisfactory for purple‐blue and purple flowers and was more suitable for floral displays at lower illuminance. Under the 3500 K lamp, the highest color distortion occurred in cool‐colored flowers, but C* increased for purple‐blue and purple flowers. The 6500 K LED tended to decrease C* for warm‐colored flowers under both illuminances, but it was effective for displaying purple‐blue and purple flowers with increased C*. © 2012 Wiley Periodicals, Inc. Col Res Appl, 39, 28–36, 2014     

Silica nanoparticles assisted preparation of reddish-yellow emitting Eu2+ activated remote-type CaSrSiO4 phosphor for warm white LED applications

A reddish-yellow emitting silicate-based remote phosphor has been developed via the wet-solid phase reaction technique. By employing silica nanoparticles (200 nm), Eu²⁺ doped CaSrSiO₄ phosphor was developed and its efficacy has been examined thoroughly. The developed remote phosphor can get excited over a broad region of the spectrum ranging from ultraviolet to blue (250–500 nm) and as generates a reddish-yellow emission peaked at 580 nm covering a broad range of spectral components (450–800 nm) with a quantum efficiency of 52%. The thermoluminescence studies of developed remote phosphor exhibit 50% of the stable emission up to 200 °C without any shift in the emission wavelength. The developed remote phosphor was then utilized for the making of a proto-type LED using 450 nm blue-emitting commercial LED. The output emission from the proto-type LED confirms the production of efficient warm white light with CCT <5000 K and CRI >85.     

Investigating the use of an adjustment task to set preferred colour of ambient illumination

An experiment was carried out to examine the method of adjustment when determining user preferences for the colour appearance of ambient lighting. A booth was lit using luminaires containing an array of white and coloured light emitting diodes (LEDs), allowing continuous variation of correlated colour temperature (CCT) between 2736 and 4014 K. It was predicted that the preferred CCT would be influenced by the available stimulus range, the anchor (initial setting before adjustment), adaptation time before onset of adjustment action and type of control dial. Preference was estimated for three different CCT stimulus ranges within the available range. All three ranges led to significantly different results for preferred CCT: 3288, 3490 and 3671 K. The experimental results confirmed that stimulus range, anchor and adaptation time have significant effect on the preferred CCT determined using the adjustment task, and thus, the importance of considering and reporting this information in studies that employ the method of adjustment. © 2011 Wiley Periodicals, Inc. Col Res Appl, 2013     

Categorical color constancy under RGB‐LED light sources

The light‐emitting diode (LED)‐based light sources have been widely applied across numerous industries and in everyday practical uses. Recently, the LED‐based light source consisting of red, green and blue LEDs with narrow spectral bands (RGB‐LED) has been a more preferred illumination source than the common white phosphor LED and other traditional broadband light sources because the RGB‐LED can create many types of illumination color. The color rendering index of the RGB‐LED, however, is considerably lower compared to the traditional broadband light sources and the multi‐band LED light source (MB‐LED), which is composed of several LEDs and can accurately simulate daylight illuminants. Considering 3 relatively narrow spectral bands of the RGB‐LED light source, the color constancy, which is referred to as the ability of the human visual system to attenuate influences of illumination color change and hold the perception of a surface color constant, may be worse under the RGB‐LED light source than under the traditional broadband light sources or under the MB‐LED. In this study, we investigated categorical color constancy using a color naming method with real Munsell color chips under illumination changes from neutral to red, green, blue, and yellow illuminations. The neutral and 4 chromatic illuminants were produced by the RGB‐LED light source. A modified use of the color constancy index, which describes a centroid shift of each color category, was introduced to evaluate the color constancy performance. The results revealed that categorical color constancy under the 4 chromatic illuminants held relatively well, except for the red, brown, orange, and yellow color categories under the blue illumination and the orange color category under the yellow illumination. Furthermore, the categorical color constancy under red and green illuminations was better than the categorical color constancy under blue and yellow illuminations. The results indicate that a color constancy mechanism in the visual system functions in color categories when the illuminant emits an insufficient spectrum to render the colors of reflecting surfaces accurately. However, it is not recommended to use the RGB‐LED light source to produce blue and yellow illuminations because of the poor color constancy.     

Designing white‐light LED lighting for the display of art: A feasibility study

When displaying art, several criteria must be balanced when designing illumination including the artist's intention, damage, energy efficiency, viewing experience and understanding, and for commercial galleries and sales. The most common lighting for art includes natural daylight and incandescent spotlights. Neither source is optimal for all criteria; thus there is considerable interest in the use of white‐light light‐emitting diode (LED) lighting. A feasibility study was conducted to address two questions. First, was it possible to design a three‐primary LED source that yielded the same color rendering as common museum lighting? Second, could one design the lighting to achieve specific color appearance attributes? Three‐primary lights using a Gaussian function were optimized matching the chromaticity of D65 and minimizing color differences for a set of acrylic dispersion paints. The optimal wavelengths depended on bandwidth. Lights were also optimized that either maximized or minimized average chroma. A set of real LEDs was selected that produced similar results when evaluated computationally. A source that increases chroma may be useful when used to illuminate works of art with high light sensitivity: very low illuminances are necessary and such a source will compensate for the reduction of colorfulness and visual clarity. A source that decreases chroma may be used to render art in similar fashion to low‐light conditions such as churches and caves. In general, white LED lighting is advantageous for art conservation because they do not emit UV and IR radiation and their visible radiation is reduced when compared with their continuous spectrum equivalent. © 2010 Wiley Periodicals, Inc. Col Res Appl, 2011     

Effect of text‐background lightness combination on visual comfort for reading on a tablet display under different surrounds

Many tablets are designed to change display brightness or color with surround for enhancing visual comfort. Although both color and brightness of a surround may vary a lot, few studies investigated how text‐background lightness combination of a tablet display and surround jointly affect visual comfort, and how display white point affects visual comfort. In this study, 20 observers evaluated visual comfort of 20 text‐background lightness combinations of a 9.7‐inch tablet display through paired comparisons under five surrounds—a dark surround and four ambient lighting conditions comprising two levels of correlated color temperature (CCT)—3500 and 6500 K—and illuminance—300 and 3000 lx. The combination of a black text and a light‐gray background (i.e., L^*_background = 75.33; L^*_text = 1.6) was evaluated the most comfortable when there was ambient light regardless of CCT and illuminance. It was also evaluated the third most comfortable under the dark surround. The observers also evaluated the visual comfort of a dark text on five different white backgrounds under 3500 and 6500 K at 1000 lx. The color of the background that was judged as the most comfortable neither had the whitest appearance nor matched the color of the ambient light. The simultaneous adjustment of the display white point and the text‐background lightness combination merits further investigations.     

Optimization of LED‐based light blendings for object presentation

This study looks at the perceived quality of light‐emitting diode (LED)‐based lighting of various colors. The objective was to find out whether LEDs could provide better (i.e., more relevant and acceptable) lighting than that which is obtained with standard halogen or fluorescent sources. The perception of objects was assessed under different lighting schemes. Subjects were invited to add red, cyan and/or amber to white LED‐based light to match the halogen and fluorescence rendering on specific targets: a color chart and a painting. They were also asked to rate the difference between the two, and to express their preference. The results obtained for the perception of LED‐based lighting were quite positive. Color blendings of LED light were found to provide illuminated situations similar to halogens or fluorescent sources. These blendings were well accepted, and indeed often preferred, although the color rendering index (CRI) was always low. This indicates that the CRI as it stands is inadequate to characterize the color rendering of solid‐state light sources, and needs to be updated. LED‐based lighting systems seem to have considerable potential for use in shops and display units, where they may well outperform existing lighting systems. © 2009 Wiley Periodicals, Inc. Col Res Appl, 34, 310–320, 2009     

2D photonic crystal layer assisted thiosilicate ceramic plate with red‐emitting film for high quality w‐LEDs

In this work, we have succeeded in obtaining high quality warm w‐light‐emitting‐diodes (LEDs) by adopting hybrid two‐dimensional (2D) structure of SiN_x photonic crystal layer (PCL) assisted cyan‐emitting ceramic‐plate thiosilicate SrLa₂Si₂S₈:Ce³⁺ with red‐emitting film SrLiAl₃N₄:Eu²⁺ phosphor on a 430 nm blue LED chip at 350 mA. 2D SiN_x PCL was capped with thiosilicate is because it can enhance the luminous efficacy and maintain the low correlated color temperature (CCT) and high color‐rendering index (CRI). High luminous efficacy (82.3 lm/W), high special CRI (R₉=75) as well as the low CCT (5431 K) of the optimal w‐LED was obtained due to the assistances of 2D SiN_x PCL and narrow‐band red‐emitting phosphor with the doping percentage at 10 wt%. The synthesis processes, structural analysis, optical properties and LED device performances were detailed investigated to find out the relationship between the optimum composition and good optical properties. Based on intriguing luminescence properties by the 2D SiN_x PCL and red‐emitting film phosphor introducing, we proclaim this method could also have high potential application in other phosphor‐converted w‐LEDs.     

蓝光吸收剂在PC中的应用探究

针对白光LED光源的富蓝化现象,采用蓝光吸收剂改性聚碳酸酯(PC)基材,利用蓝光吸收剂吸收高能量的蓝光并转化为热能或无害低能辐射释放出来,以消除白光LED光谱中的高能蓝光波段。结果表明,蓝光吸收剂改性PC材料后,可以有效减少各波长处的蓝光透过率,并且随着蓝光吸收剂添加比例的增加蓝光吸收效果增强;蓝光波段的吸收会造成光谱的缺失,造成光源的显色指数、色温、光通量的下降;不同类型的蓝光吸收剂改性PC树脂的蓝光吸收效果存在差异。     

Chromaticity changes of inorganic pigments in Chinese traditional paintings due to the illumination of frequently‐used light sources in museum

Optical radiation can cause permanent damage to the color of Chinese traditional paintings, which have extremely high responsivity in the museum illumination. In order to obtain quantitative influence laws of artificial illumination on inorganic pigments in Chinese traditional paintings and provide basis for choosing light sources, a long periodic illumination experiment was carried out to examine how prolonged exposure of three light sources (tungsten halogen, metal halide and white light emitting diodes), frequently‐used in museums, affect the chromaticity of five inorganic pigments [azurite (blue), cinnabar (red), orpiment (yellow), ancient graphite (black), clam shell powder (white)], commonly used in Chinese traditional paintings. Through the experiment, we found that the illumination causes a color change to inorganic pigments, and the degree of which is related to the photochemical stability of pigments and the high energy shortwave radiation in the light source spectrum. We also obtain the color change laws of the five pigments and the quantitative relationships of color damage for different painting types. These results can provide data basis for related research on Chinese traditional painting illumination and choice basis for light sources in museum lighting designs.     

Digital color reconstructions of cultural heritage using color‐managed imaging and small‐aperture spectrophotometry

There are many examples of cultural heritage having optical properties that have changed with the passage of time. Examples include the yellowing, darkening, and fading of paints and varnishes caused by light exposure and atmospheric pollution. When it is infeasible to treat an object, an image simulation can provide a view to the past, known as a color reconstruction. A technique is described that relies on a color‐managed image, spectral reflectance factor measurements of the object, an optical model of colorant mixing, an optical database of artist materials, spreadsheet software, and image editing software. Spectral calculations are used to create adjustment curves where segmented portions of an object's image are translated in color. This approach has been used to produce color reconstructions of paintings by Vincent van Gogh and Georges Seurat. This colorimetric translation methodology is described and an example shown for the Chicago version of Vincent van Gogh's Bedroom. The methodology is compared with pixel‐based processing.     

Effects of light quality on the accumulation of oil in a mixed culture of Chlorella sp. and Saccharomyces cerevisiae

BACKGROUND: Chlorella strains rather than terrestrial oil crops having higher oil content and shorter generation time have been considered as promising candidates for alternative biodiesel. Since the influence of light quality on oil formation of microalgae in either monoculture or mixed culture has been shown to be either inconsistent or ambiguous, a light‐emitting diode (LED) photo‐bioreactor with different light sources and intensities was used in this study to investigate a cost‐effective lipid production process. RESULTS: The oil accumulation in a mixed culture of Chlorella sp. and Saccharomyces cerevisiae was higher than that in the monoculture under the different light sources used. Results of the influence of light quality on the mixed culture indicated that the optimal light wavelength and intensity for biomass formation was red LED light at 1000 lux, whereas the optimum for oil formation was blue LED light at 1000 lux. A novel two‐stage LED photo‐bioreactor was thus proposed and the highest P_max and productivity in this study were obtained as 261 mg L^?1 and 8.16 mg L^?1 h^?1, respectively. CONCLUSION: A novel two‐stage LED photo‐bioreactor using a mixed culture to optimize microalgal oil production was proposed and successfully demonstrated in this study. Copyright © 2011 Society of Chemical Industry     

Chromatic adaptation to illumination investigated with adapting and adapted color

The state of chromatic adaptation was investigated by using the two‐room technique. This technique involves a subject in a room who looks a white board in a separate test room through a window and judges the color of the window using the elementary color naming method. When the subject room is illuminated with a colored light and the test room with a white light, the window appears to be a very vivid color, for which the apparent hue depends on the color of the subject room. The color is referred to as the adapted color. The subject also evaluated the appearance of the illumination color of the subject room, which is called the adapting color. Two types of illuminating light in the subject room, fluorescent lamps with 7 colors and LED lamps with 19 colors, were employed. The adapting and the adapted colors were plotted on a polar diagram that was used in the opponent color theory, from which the hue angles were obtained. The hue angle difference between the two colors did not appear to be 180° except for one pair of adapting and the adapted colors, which implies that chromatic adaptation does not follow the opponent color concept. An improvement was achieved to explain the results by introducing complementary colors relation between the adapting and adapted color.     

Texture affects color emotion

Several studies have recorded color emotions in subjects viewing uniform color (UC) samples. We conduct an experiment to measure and model how these color emotions change when texture is added to the color samples. Using a computer monitor, our subjects arrange samples along four scales: warm–cool, masculine–feminine, hard–soft, and heavy–light. Three sample types of increasing visual complexity are used: UC, grayscale textures, and color textures (CTs). To assess the intraobserver variability, the experiment is repeated after 1 week. Our results show that texture fully determines the responses on the Hard‐Soft scale, and plays a role of decreasing weight for the masculine–feminine, heavy–light, and warm–cool scales. Using some 25,000 observer responses, we derive color emotion functions that predict the group‐averaged scale responses from the samples' color and texture parameters. For UC samples, the accuracy of our functions is significantly higher (average R² = 0.88) than that of previously reported functions applied to our data. The functions derived for CT samples have an accuracy of R² = 0.80. We conclude that when textured samples are used in color emotion studies, the psychological responses may be strongly affected by texture. © 2010 Wiley Periodicals, Inc. Col Res Appl, 2010     

Influence of lighting conditions on the appearance of typical interior materials

Two studies investigated how lighting influences perception of various materials. Experiment 1 focused on preferences of cold or warm light. Participants' task was to indicate preference for either coolish or warmish light for 20 different materials. The selection included materials typically used for wall or floor of interior spaces. Different hues and surface characteristics were present. Specifically two materials of blue color (cold hue) were preferred in a cold light while red carpet (warm hue) and synthetic turf materials were preferred in warm light. This latter choice was explained by automatic reference of synthetic turf to the appearance of natural grass. There were no clear light source preferences for other materials. Experiment 2 explored how the same 20 materials are evaluated under LED lighting in comparison to three other lighting situations (halogen lamp, high‐intensity discharge lamp, and compact fluorescent lamp). The results of this explorative study were not conclusive. The materials chosen for this experiment differed greatly in their structure and optical properties. In conclusion, light influence on the evaluation of materials needs systematic investigation in future research. © 2013 Wiley Periodicals, Inc. Col Res Appl, 40, 50–61, 2015