Planckian Locus Research Articles

Evaluation and application strategy of low blue light mode of desktop display based on brightness characteristics

Long-term use of desktop displays may increase the burden of the visual system and users can use low blue light mode for eye protection in terms of circadian effect. In this work, we investigated its influence from two aspects of brightness-visual effect, namely efficacy and circadian effect, and color quality, namely color difference Δu’v’ (chromaticity coordinate offset of two colors), and Duv (deviation from blackbody locus). The decrease of brightness is accompanied by the increase of efficacy while diminishing circadian effect. The blue, cyan, and magenta have the largest Δu’v’, and the lower the saturation, the greater the Δu’v’. The lower the correlated color temperature (CCT), the greater the Duv and the farther it deviates from the Planckian locus. We summarize three low blue light mode adjustment strategies based on red, green, and blue three-channel ratio of spectrum, and propose an optimized mode using genetic algorithm, which has two optional CCT ranges of 3500–5000 K and 2700–3000 K. Furthermore, we establish the relationship between brightness and gamut coverage to refine the screen brightness range for low blue light mode. This research provides valuable insights into low blue light mode application and their implications for human-centric healthy displays.

White Light Emission and Superior Color Stability in a Single-Component Host with Exceptional Eminent Color Rendering and Theoretical Calculations on Duv for Color Quality

Correlated color temperature (CCT) is widely used to describe the chromaticity of white light sources, although chromaticity is only two-dimensional, and the distance from the Planckian locus is typically absent....

Visual and melanopic performance of a tropical daylight-mimicking lighting: a case study in Thailand

This paper designed and developed a tropical daylight-mimicking lighting system based on photometric, radiometric and International Commission on Illumination (CIE) standard melanopic performances from natural lighting cycles in Thailand. Spectral power distribution (SPD) during daylight in summer and winter were recorded to create a dynamic artificial lighting system that best matches the natural daylight characteristics. Two set-ups light emitting diode (LED) (LED-A and LED-B) were screened, developed, validated and compared with different chromaticity layouts of the correlated color temperatures (CCTs) allocated on Planckian locus and later converted to x-y co-ordinates in a chromaticity diagram. Based on CCT and Duv deviations between two developed setups, LED-A could mimick circadian points on the chromaticity diagram better than LED-B did. CCT and Duv values of LED-A (dCCT=3.75% and dDuv=17.36%) can match closer to the daylight than those of LED-B (<span lang="EN-US">d</span>CCT=5.0 % and <span lang="EN-US">d</span>Duv=56.84%). For CIE-standard melanopic performances (melanopic efficacy of luminous radiation (mELR), melanopic equivalent daylight (D65) illuminance (mEDI) and melanopic daylight efficacy ratio (mDER)), LED-A is suitable to use indoor with averages of 1.16 W×lm-1, 236 lx and 0.84, respectively, while LED-B is good to use outdoor with averages of 1.53 W×lm<sup>-1</sup>, 266 lx and 1.06, respectively. The proposed design can be used as a guideline to establish a daylight-mimicking LED lighting system from actual measurement data.

Recommended Method for Determining the Correlated Color Temperature and Distance from the Planckian Locus of a Light Source

ABSTRACT Correlated color temperature (CCT) is one of the primary metrics used to characterize the visual appearance of light and is most informative when coupled with distance from the Planckian locus (Duv). Given a set of chromaticity coordinates—which are calculated from a light source’s spectral power distribution—it is possible to compute CCT and Duv with varying levels of accuracy. Over the last six decades at least a dozen methods have been proposed to compute CCT while balancing accuracy with calculation complexity. CCT values become inconsistent at some level of precision when calculated using different methods, which in turn can lead to discrepancies in dependent or subsequent calculations used by lighting professionals and may be problematic in software. Although methods are now documented that can provide extremely high accuracy, no consensus body has recommended a preferred method. This analysis examines both the calculation speed and the CCT and Duv accuracy of previously proposed and new methods. With consideration of the calculation accuracy, computational burden, calculation complexity, and considerations of practical implementation, we recommend a recent modification of the Robertson method.

Weighted Geometric Mean (WGM) method: A new chromatic adaptation model.

The human visual system has undergone evolutionary changes to develop sophisticated mechanisms that enable stable color perception under varying illumination. These mechanisms are known as chromatic adaptation, a fundamental aspect of color vision. Chromatic adaptation can be divided into two categories: sensory adaptation, which involves automatic adjustments in the visual system, such as retinal gain control, in response to changes in the stimulus, and cognitive adaptation, which depends on the observer's knowledge of the scene and context. The geometric mean has been suggested to be the fundamental mathematical relationship that governs peripheral sensory adaptation. This paper proposes the WGM model, an advanced chromatic adaptation model based on a weighted geometric mean approach that can anticipate incomplete adaptation as it moves along the Planckian or Daylight locus. Compared with two other chromatic adaptation models (CAT16 and vK20), the WGM model is tested with different corresponding color data sets and found to be a significantly improvement while also predicting degree of adaptation (sensory and cognitive adaptation) in a physiologically plausible manner.

Fine art under low illuminance: Gamut and tint

Museum conservation guidelines restrict illuminance for sensitive artwork to levels that can cause objects to be perceived as less colourful, a phenomenon known as the Hunt effect. Previous colour rendering research identified red saturating gamuts that consistently increased perceived saturation and personal preference. A study was conducted to evaluate the visual experience of fine art illuminated by a red saturating gamut family constrained to be uniquely identified by their TM-30 gamut scores (denoted as [Formula: see text]) and position above or below the blackbody locus ( Duv). [Formula: see text] and Duv were systematically varied according to response surface methodology, designed to map second-order terms and interactions, with 96 ⩽ [Formula: see text] ⩽ 124 and −0.0212 ⩽ Duv ⩽ 0.0036, all at 3000 K and 50 lx. Thirty-one naïve participants each evaluated a pair of paintings in a mock art gallery under nine independently presented scenes along semantic scales corresponding to preference, saturation and naturalness. The study identified a response surface for preference that maps an interaction between [Formula: see text] and Duv, predicting Duv = −0.013 was preferred at [Formula: see text] and Duv = −0.005 was preferred at [Formula: see text]. Increasing [Formula: see text] consistently increased both personal preference and perceived saturation.

Multifunctional optical materials based on transparent inorganic glasses embedded with PbS QDs

Quantum dot (QD)-embedded glass, which is a high-performance multifunctional optical material, has the combined advantages of excellent optical properties of QDs and easy processing of glass. In this study, an inorganic PbS QD embedded multicomponent glass, which produces variable emission at different ultraviolet (UV) excitation wavelengths, is synthesized. The samples are fabricated via a facile melt-quenching method and their structural, optical, photoluminescent, and morphological properties are discussed. The QD glass exhibits broad-band UV excitation in the range of 250–400 nm, and the full width at half maximum of the emission peaks of the QD glasses reaches 177.8 and 158.5 nm at room temperature under an excitation of 365 and 310 nm, respectively. Notably, white light emission with a high color rendering index is obtained under UV excitation at 350 and 365 nm by doping blue light emitting rare earth (RE) Tm3+ ions into the system. The doped samples exhibit excellent color stability and their CIE coordinates shift along the Planck blackbody locus with increasing temperature. Further, based on the differences in the luminescence mechanisms of the PbS QDs and Tm3+ ions, the application potential of the RE-doped PbS QD-embedded glasses in optical thermometry is discussed. The results demonstrate the application potential of these optically multifunctional inorganic QD glasses in the fields of optical conversion, anti-counterfeiting, white light illumination, and non-contact optical temperature measurement.

Investigation into the stability condition of correlated color temperature of white illumination sources based on trichromatic light-emitting diodes

We present a simple method to obtain a stable correlated color temperature (CCT) for the variation in light output power (LOP) of a light-emitting diode (LED) comprising a trichromatic LED-based white light source. A mathematical model was developed to determine the condition for the stable CCT operation using colorimetric analyses, and the stable CCT condition can be obtained by taking the derivative of the CCT with respect to the power ratio between the LEDs. Specifically, the stable CCT point that is invariant to the change in the LOP of a green LED can be uniquely determined at a certain value of the power ratio between red and blue LEDs. It was found that the chromaticity coordinate of the mixed white light moves along the isoCCT line of the Planckian locus as the LOP of a green LED varied under the stable CCT condition. The existence of the stable CCT point was also verified experimentally. The proposed method is expected to be advantageously employed in optimizing trichromatic LED-based white illumination sources for improved color stability.

Enhancement of significant colour properties through Ta2O5 incorporation into ZnO–TeO2 binary glasses: An effective method for purposeful utilizations in industry and technology

The fact that multicomponent glass systems offer tuneable optical properties according to different additive rates increases their potential for use in daily industrial and optoelectronic applications that require high optical and colour performance. Therefore, it is vital to determine the optical and colour performances of glass systems in detail. The purpose of this research is to examine the monotonic effects of Ta2O5 on a set of glass samples and the resulting changes to colour concepts to better understand how to acquire these changes at their optimum level and make the most of the optimization possibilities available with Ta2O5. Accordingly, many optical properties such as Average Visible Transmittance (AVT), colour, Colour Render Index (CRI), Correlated Colour Temperature (CCT) that change with Ta2O5 doping ratio in binary Zinc-Tellurite oxide glasses-based systems are examined and their potential to be used for effective optoelectronic applications are presented. With the addition of Ta2O5 into (25ZnO·75TeO2)y(Ta2O5)x (x = 0–7 mol% and y = 100-x mol%) glassy system, all optical and colour properties were improved significantly. The Ta2O5 contribution increases the AVT and changes the colour coordinates of the glass system along the Planckian locus curve from the yellow region to the D65 colour coordinates. In addition, a comprehensive analysis of 15 coloured samples is provided, focusing on the cases in which Ta2O5 enhances the CRIext value. An enrichment was observed for TCS07 and TCS14, and a value of 100 was obtained specifically for TCS09 with a 3% Ta2O5 contribution. It can be concluded that the Ta2O5 is a promising multi-functional tool for colour enhancement of ZnO–TeO2 binary glasses making them more suitable for daily applications as well as applications in advanced technological purposes.

Weighted Geometric Mean (WGM) Method: A New Chromatic Adaptation Model

The geometric mean has been suggested to be the fundamental mathematical relationship that governs peripheral sensory adaptation. This paper proposes the WGM model, an advanced chromatic adaptation model based on a weighted geometric mean approach that can anticipate incomplete adaptation as it moves along the Planckian or Daylight locus. Compared with two other chromatic adaptation models (CAT16 and vK20), the WGM model shows more accuracy in predicting previous visual data.

Enhancement of color and photovoltaic performance of semi-transparent organic solar cell via fine-tuned 1D photonic crystal

Semi-transparent organic solar cells’ (ST-OSCs) photovoltaic and high optical performance parameters are evaluated in innovative applications such as power-generating windows for buildings, automobiles, and aesthetic designs in architectural and industrial products. These parameters require the precision design of structures that optimize optical properties in the visible region and aim to achieve the required photon harvest in UV and IR. These designs can be realized by integrating wavelength-selective photonics-based systems into ST-OSC to increase localized absorption in wavelengths greater than 600 nm and NIR and provide modifiable optical properties. In this study, methodologically, we followed highly detailed light management engineering and transfer matrix method-based theoretical and experimental approaches. We discussed the optimal structures by evaluating color, color rendering index, correlated color temperature, and photovoltaic performances for ST-OSCs, including one-dimensional photonic crystal (1D-PC) designed at different resonance wavelengths (λB) and periods. Finally, by integrating fine-tuned (MgF2/MoO3)N 1D-PC, we report the inherently dark purple-red color of the P3HT:PCBM bulk-heterojunction-based ST-OSC neutralizes with the optimal state was 0.3248 and 0.3733 by adjusting close to the Planckian locus. We also enhanced short current density from 5.77 mA/cm2 to 6.12 mA/cm2 and PCE were increased by 7.34% from 1.77% to 1.90% designed for the N = 4 period and λB = 700 nm.

Effects of chromaticity difference from Planckian locus duv of lighting on tinted color of illumination and brightness in space (part 1): Experiment using a scale model with uniform luminance distribution

AbstractThe purpose of this study was to identify the combined effects of illuminance, correlated color temperature, and chromaticity shift from the Planckian locus duv on the perception of the tinted color of illumination and spatial brightness. All 60 conditions with 3 levels of floor illuminance, 4 conditions of correlated color temperature, and 5 levels of duv were tested. Consequently, it was identified that the effects of the duv shift on the tinted color of illumination differed depending on the correlated color temperature. The spatial brightness was significantly lower in the case of positive duv.

Improved Methods for Computing CCT and D uv

ABSTRACT Recently, Ohno proposed a method for computing the correlated color temperature (CCT) of a light source and the shortest distance (with sign) between its chromaticity and that of the Planckian locus (D uv). In Ohno’s method, firstly a lookup table is chosen, and then a hybrid method based on the triangular and parabolic solutions is used to predict the CCT and D uv of a given light source. In this article, we found that spline interpolation is better than the parabolic solution for predicting CCT and D uv. Therefore, we propose two new hybrid methods based on the triangular and either spline interpolation or third-order interpolation using two local function values and two local second-order derivative values. Comprehensive tests using the lookup table with both 1% and 0.25% increments relative to the current selected color temperature demonstrate that the proposed methods outperform Ohno’s method.

Functional optical design of thickness-optimized transparent conductive dielectric-metal-dielectric plasmonic structure

Dielectric/metal/dielectric plasmonic transparent structures play an important role in tailoring the high-optical performance of various optoelectronic devices. Though these structures are in significant demand in applications, including modification of the optical properties, average visible transmittance (AVT) and colour render index (CRI) and correlated colour temperature (CCT), obtaining optimal ones require precise thickness optimization. The overall objective of this study is the estimation of the optimal design concept of MoO3/Ag/WO3 (10/dAg/dWO3 nm) plasmonic structure. To explore the proper use in optoelectronic devices, we are motivated to conduct a rigorous optical evaluation on the thickness of layers. Having calculated optical characteristics and achieved the highest AVT of 97.3% for dAg = 4 nm and dWO3 = 6 nm by the transfer matrix method, it is quite possible to offer the potential of the structure acting as a transparent contact. Notably, the colour coordinates of the structure are x = 0.3110 and y = 0.3271, namely, it attributes very close to the Planckian locus. This superior colour performance displays that MoO3/Ag/WO3 shall undergo rapid development in neutral-colour windows and LED technologies. Structure with dAg = 6 nm and dWO3 = 16 nm exhibits the highest CRI of 98.58, thus identifying an optimal structure that can be integrated into LED lighting applications and imaging technologies. Besides the colour of structure with dAg = 4 nm and dWO3 = 8 nm is equal for D65 Standard Illuminant, the study reports that the range of CCTs are between 5000 and 6500 K. This optimization makes the structure employable as a near-daylight broadband illuminant. The study emphasizes that optimal MoO3/Ag/WO3 plasmonic structures can be used effectively to boost optoelectronic devices' performance.

Locus filters.

In this paper, directly following from Gage [J. Opt. Soc. Am.23, 46(1993)10.1364/JOSA.23.000046], we study the design of a particular theoretical filter for photography, that we call the locus filter. It is built in such a way that a Wien-Planckian light (of any temperature) is spectrally mapped to another Wien-Planckian light. We provide a physical basis for designing such a filter based on the Wien approximation of Planck's law, and we prove that there exists a unique set of filters that have the desired property. While locus filtered Wien-Planckian lights are on the locus, the amount they shift depends both on the locus filter used and on the color temperature of the light. In experiments, we analyze the nature of temperature change when applying different locus filters and we show that real lights shift more or less as if they were Planckians in terms of the changes in their correlated color temperatures. We also study the quality of the filtered light in terms of distance from the Planckian locus and color rendering index.

Predictive performance of the standard and the modified von Kries chromatic adaptation transforms.

To investigate chromatic adaptation and develop chromatic adaptation transforms (CATs), many psychophysical experiments have been conducted to collect corresponding colors (CC) under various illumination conditions. Most modern CATs have been developed based on a database of CC sets collected in the 20th century. More recently, several additional CC sets have been collected by Smet et al., Wei et al., and Ma et al. using memory color matching or achromatic matching methods. The analysis of these CC data indicates that for yellowish (located on or close to the Planckian locus) and greenish illuminations, the short-wave (S) sensitive cones show a lower degree of adaptation compared to the long-wave (L) and medium-wave (M) sensitive cones. This can result in a large prediction error of the standard von Kries CAT, which adopts a single degree of adaptation value for all three cone types. A modified von Kries CAT is proposed that accounts for these differences between the L-, M- and S-cone signals by applying a compression to the rescaling factor for the S-cones. It outperforms the standard von Kries CAT for the Breneman-C, Smet, Wei, and Ma data, while for other data sources the two CATs have similar performance.

Improved Method for Evaluating and Specifying the Chromaticity of Light Sources

ABSTRACT This article describes a method for calculating and specifying light source chromaticity using the International Commission on Illumination (CIE) 2015 10° color matching functions (CMFs), which, according to analysis of existing psychophysical experiment data, can reduce visual mismatch compared to specifications based on the traditional CIE 1931 2° CMFs in architectural lighting applications. Specifically, this work evaluates, documents, and recommends for adoption by lighting standards organizations a supporting system of measures to be used with the CIE 2015 10° CMFs: a new uniform chromaticity scale (UCS) diagram with coordinates (s, t), a measure of correlated color temperature (CCTst), and a measure of distance from the Planckian locus (Dst). It also presents options for updating nominal classification quadrangles. A complete method of this nature has not yet been standardized, which may be contributing to the slow uptake of the CIE 2015 CMFs. The proposed tools are analogous to u, v, CCT, Duv, and the American National Standards Institute (ANSI) C78.377 chromaticity specifications that are all currently defined in the CIE 1960 UCS diagram using the CIE 1931 2° CMFs. While conceptually equivalent, the differences between the current standard method and the proposed st system are important for reducing unintended visual mismatch in the chromaticity of light. The implications of changing chromaticity specification methods are identified by a comparison over a diverse set of real light source spectral power distributions.

A Simulation Study on Color Rendering Characteristics of CCT-tunable LED Lightings Composed of WW-LEDs and CW-LEDs in Tracing the Planckian Locus by Adding Single-wavelength LEDs

A Simulation Study on Color Rendering Characteristics of CCT-tunable LED Lightings Composed of WW-LEDs and CW-LEDs in Tracing the Planckian Locus by Adding Single-wavelength LEDs

Luminosity thresholds of colored surfaces are determined by their upper-limit luminances empirically internalized in the visual system.

We typically have a fairly good idea whether a given object is self-luminous or illuminated, but it is not fully understood how we make this judgment. This study aimed to identify determinants of the luminosity threshold, a luminance level at which a surface begins to appear self-luminous. We specifically tested a hypothesis that our visual system knows the maximum luminance level that a surface can reach under the physical constraint that a surface cannot reflect more light than any incident light and applies this prior to determine the luminosity thresholds. Observers were presented with a 2-degree circular test field surrounded by numerous overlapping colored circles and luminosity thresholds were measured as a function of (i) the chromaticity of the test field, (ii) the shape of surrounding color distribution, and (iii) the color of the illuminant of the surrounding colors. We found that the luminosity thresholds peaked around the chromaticity of test illuminants and decreased as the purity of the test chromaticity increased. However, the loci of luminosity thresholds across chromaticities were nearly invariant to the shape of the surrounding color distribution and generally resembled the loci drawn from theoretical upper-limit luminances and upper-limit luminance boundaries of real objects. These trends were particularly evident for illuminants on the black-body locus and did not hold well under atypical illuminants, such as magenta or green. These results support the idea that our visual system empirically internalizes the gamut of surface colors under natural illuminants and a given object appears self-luminous when its luminance exceeds this internalized upper-limit luminance.

Wireless Controlled Wide Range Tuneable CCT Lighting System Using Three Component Colour Mixing

This paper proposes a novel approach of wireless controlled, tenable correlated colour temperature (CCT) lighting system with high colour rendering index (CRI) using the concept of colour mixing. Using Grassmann’s colour mixing theory, phosphor coated warm white LED source (CCT 2800 K) is mixed with the blue and green LED light sources to meet the requirement of high CRI and high value of red object identification parameter R9. A detailed mathematical formulation is derived for easy controlling of variable CCT and illuminance and a flowchart of wireless control scheme is incorporated and experimentally validated in this article. As a result, the locus of the blended light source is very close to the Planckian locus on CIE1931 chromaticity diagram with a minimum uv D value of +0.0005. This proposed system provides a very wide range CCT starting from 2800 K to 15000 K with a high CRI value of 94 at CCT 5000 K. Another feature of this blended light source is the true colour identification parameter of red object (R9), which is also increased to 93 at CCT 8000 K. This system has very wide application in industries like textile, food processing etc., in museum, art gallery, and medical application like surgical lighting for its high CRI and R9 value with wide range of CCTs.