Databases for spectral color science

In color science, spectral representation and analysis of colors have become a common approach to study color‐related problems, e.g., accurate industrial color measurement or analysis of color images. In developing algorithms for spectral color science, one often relies on existing databases of reflectance color spectra. Since a number of these databases are easily available, the same databases are commonly used by different research groups. During year 2003 the most popular one of our publicly available spectral reflectance databases was visited over 600 times. In the present article, we describe these color spectra databases and analyze their utility for spectral color science. However, the article does not take the complexity of fluorescent surfaces into account. The aim of this article is to set a solid ground for the comparisons of different methods in the spectral color science. The databases presented here include measured color spectra of natural and man‐made objects as well as spectra of some sets of standard colors. In addition to the commonly used data sets, some new data sets, including a set of standard calibrated colors and a set of natural colors, measured with 10 nm spectral resolution are introduced. © 2006 Wiley Periodicals, Inc. Col Res Appl, 31, 381–390, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.20244     

Dictionary‐based estimation of spectra for wide‐gamut color imaging

With the widespread use of commercialized wide‐gamut displays, the demand for wide‐gamut image content is increasing. To acquire wide‐gamut image content using camera systems, color information should be accurately reconstructed from recorded image signals for a wide range of colors. However, it is difficult to obtain color information accurately, especially for saturated colors, if conventional color cameras are used. Spectrum‐based color image reproduction can solve this problem; however, bulky spectral imaging systems are required for this purpose. To acquire spectral images more conveniently, a new spectral imaging scheme has been proposed that uses two types of data: high spatial‐resolution red, green, and blue (RGB) images and low spatial‐resolution spectral data measured from the same scene. Although this method estimates spectral images with high overall accuracy, the error becomes relatively large when multiple different colors, especially those with high saturation, are arranged in a small region. The main reason for this error is that the spectral data are utilized as low‐order spectral statistics of local spectra in this method. To solve this problem, in this study, a nonlinear estimation method based on sparse and redundant dictionaries was used for spectral image estimation—where the dictionary contains a number of spectra—without loss of information from the low spatial‐resolution spectral data. The estimated spectra are represented by a mixture of a few spectra included in the dictionary. Therefore, the respective feature of every spectrum is expected to be preserved in the estimation, and the color saturation is also preserved for any region. Experiments performed using the simulated data showed that the dictionary‐based estimation can be used to obtain saturated colors accurately, even when multiple colors are arranged in a small region. © 2011 Wiley Periodicals, Inc. Col Res Appl, 2013     

Light trapping structures in wing scales of butterfly Trogonoptera brookiana

The fine optical structures in wing scales of Trogonoptera brookiana, a tropical butterfly exhibiting efficient light trapping effect, were carefully examined and the reflectivity was measured using reflectance spectrometry. The optimized 3D configuration of the coupling structure was determined using SEM and TEM data, and the light trapping mechanism of butterfly scales was studied. It is found that the front and back sides of butterfly wings possess different light trapping structures, but both can significantly increase the optical path and thus result in almost total absorption of all incident light. An optical model was created to check the properties of this light trapping structure. The simulated reflectance spectra are in concordance with the experimental ones. The results reliably confirm that these structures induce efficient light trapping effect. This functional "biomimetic structure" would have a potential value in wide engineering and optical applications.     

The optimal number of sensors for a digital imaging system from the perspective of metamer mismatching

Metamer mismatching occurs in both the human visual system and digital imaging systems. Increasing the number of sensors in digital imaging systems is an efficient method to reduce the degree of metamer mismatching. This study investigates the number of sensors that are needed for the digital imaging systems to have a similar ability to the human sensor system in distinguishing colors from the perspective of metamer mismatching. Optimal spectral reflectance was generated, and a dataset with more than 47 million previously collected practical spectral reflectance functions was used to derive the metameric color pairs. Different sets of Gaussian-shaped functions were designed to model the spectral sensitivity functions of the digital imaging systems. Both the metamer mismatching volumes and color differences were used to characterize the degree of metamer mismatching for pairs of samples, which were metameric to the imaging systems but appeared different to the human sensor system. The results show that the practical metamer mismatching volumes are substantially smaller than the theoretical ones. The results also show that both the metamer mismatching volumes and color differences are significantly reduced by increasing the number of sensors from three to five for the digital imaging system but are only slightly reduced by further increasing the number of sensors from five to seven. This indicates that five sensors is an efficient and optimal solution for an imaging system with Gaussian-shaped sensors to have a similar ability in distinguishing colors compared to the human sensor system.     

Interference colors of oxidized titanium metal surfaces

The range of interference colors that become apparent when thin layers of titanium oxide are electrolytically produced on the surface of the metal are described in terms of simple visual assessments, Munsell designations, and CIELAB color space values. the observed colors change to some extent with the angle of illumination and viewing. the form of the visible reflectance spectra of the surfaces also depends on the optical configuration of the spectrophotometer. Measurements made with an integrating sphere with a 0°/8° geometry provide CIELAB values that are more representative of the appearance of the colors than the values calculated from spectra obtained by 0°/45° diffuse reflectance.     

Reconstruction of reflectance spectra using weighted principal component analysis

The weighted principal component analysis technique is employed for reconstruction of reflectance spectra of surface colors from the related tristimulus values. A dynamic eigenvector subspace based on applying certain weights to reflectance data of Munsell color chips has been formed for each particular sample and the color difference value between the target, and Munsell dataset is chosen as a criterion for determination of weighting factors. Implementation of this method enables one to increase the influence of samples which are closer to target on extracted principal eigenvectors and subsequently diminish the effect of those samples which benefit from higher amount of color difference. The performance of the suggested method is evaluated in spectral reflectance reconstruction of three different collections of colored samples by the use of the first three Munsell bases. The resulting spectra show considerable improvements in terms of root mean square error between the actual and reconstructed reflectance curves as well as CIELAB color difference under illuminant A in comparison to those obtained from the standard PCA method. © 2008 Wiley Periodicals, Inc. Col Res Appl, 33, 360–371, 2008     

Color-rendering capacity of light

Two types of questions, both related to color-rendering properties of light, can be identified. Type A: Given a spectral reflectance chip under illumination of a certain spectral power distribution, What color can appear? Does the appearing color, in comparison with a reference one, look right or distorted? Type B: Given a great many chips of different spectral reflectance functions under illumination of a certain spectral power distribution, How many different colors, no matter what colors, can appear? Are there a great many different colors or just a few different colors appearing? Questions of type A reflect an important aspect of color-rendering properties and can be tackled with an established measure, the CIE color-rendering index. Questions of type B, related to quite another aspect of color-rendering properties, have particular relevance when illumination's function of making things visible is of most concern. This article discusses a new measure, the color-rendering capacity (CRC), developed for dealing with questions of type B, and explains the measure's derivation, calculation, and implications.     

Structural color films with lotus effects, superhydrophilicity, and tunable stop-bands

The structural blue color of a Morpho butterfly originates from the diffraction of light and interference effects due to the presence of the microstructures on the wing of the butterfly. Structural color on the surface of a damselfish reversibly changes between green and blue. Inspired by these creatures, we have been trying to prepare high-quality and functional structural color films. We describe our efforts in this Account. A useful technique to prepare such structural color films in colloidal solution is a "lifting" method, which allows us to quickly fabricate brilliant colloidal crystal films. The thicknesses of the films can be controlled by precisely adjusting the particle concentration and the lifting speed. Moreover, in order to prepare a complicated structure, we have used template methods. Indeed, we have successfully prepared the inverse structure of the wing of a Morpho butterfly with this technique. Initially, however, our structural color films had a whitish appearance due to the scattering of light by defects in the colloidal crystal film. Later, we were able to prepare a non-whitish structural color film by doping an appropriate dye in the colloidal particles to absorb the scattering light. In addition to the structural blue color, the wing of the Morpho butterfly has superhydrophobic properties. According to Wenzel's equation, the hydrophobic and hydrophilic properties are enhanced when the roughness of the hydrophobic and hydrophilic surface is increased, respectively. Based on this mechanism, we have successfully prepared structural color films with superhydrophobic properties, as well as with superhydrophilic properties. Another important property that can be seen in nature is tunable structural color, such as the color change that can be seen on the surface of a damselfish. In order to mimic such color change, we have developed several tunable structural color films. In particular, we have successfully prepared phototunable photonic crystals using photoresponsive azobenzene derivatives. In order to apply these structural color films, we developed a technique for patterning them by taking advantage of the wettability of the substrate surface. These materials can be used in the future for self-cleaning pigments and tunable photonic crystals.     

Morphology-controlled synthesis of ZnO replicas with photonic structures from butterfly (Papilio paris) wing scales for tunable optical properties

ZnO replicas with photonic structures were fabricated from Papilio paris butterfly wing scales and their tunable optical properties were studied. Through modification of the fabrication method, the reticular porous network structure was successfully replicated from dark black (DB) wing scales. The DB wing scale replicas exhibit a photonic band gap (PBG) in the visible region, which overlaps with the visible emission range of ZnO. Both DB and GB (greenish-blue) wing scale replicas can work as one-dimensional diffraction gratings in optical diffraction experiments, whose spot distances can be tuned by different periodic sizes of butterfly wing structure. Moreover, the ZnO DB wing scale replicas exhibit improved photoluminescence (PL) spectra with reduced visible emission and enhanced UV emission, which can both be attributed to the existence of a PBG produced by the reticular porous network structure in DB wing scales. These results can be very helpful in the research of applications of ZnO materials in UV lasing and optical diffraction devices.     

A contribution to the study of color of fabrics

An expansion of the Allen and Goldfinger approach to the prediction of the color of absorbing-scattering substrates is carried out taking into account the effect of optical anisotropy of fibers and definite values of the indices of refraction in the visible spectral range. Using this approach and a computer program the spectral reflectance curves, based on the shape of the spectral transmittance curves of the dye-fiber polymer and Cauchy's dispersion formula, are calculated for an array of dyed nylon fibers. Also, the spectral reflectance curves for the same dyed fiber is calculated using Allen and Goldfinger model. Both spectra are compared with those experimentally measured. Spectra for dyed fibers with different dye concentrations are calculated.     

A novel analysis of color component for top dyed melange yarn with support vector machine

In view of the difficulty of distinguishing the color component in top dyed melange yarn due to the spectral overlap of the component colors, a novel color component analysis method based on support vector machine is presented. With this method, spectra data can be distinguished more accurately and effectively than with the traditional method—human‐eye detection—and therefore, the method will be very helpful for accurate color matching. In our work, the core idea was to convert the overlapped spectra data into linearly separable ones in a high dimension space, followed by recognition and determination of the composition of melange yarn by trained support vector machine classifier. The effects of four kernel functions, i.e., linear, radial basis kernel, sigmoid, and polynomial, as well as five spectral preprocessing methods, including amplification, first derivative, second derivative, principal components analysis, and L*a*b* values were studied. The results demonstrated that with the amplification factor of 100 of reflectance spectra coupled with L*a*b* as input data, and using radial basis kernel as kernel function, the highest recognition rate was achieved, with an average recognition rate of eight colors of 96.5%, indicating that it was a better color component analysis method for top dyed melange yarn. © 2015 Wiley Periodicals, Inc. Col Res Appl, 41, 636–641, 2016     

A multispectral imaging approach to colour measurement and colour matching of single yarns without winding

This paper investigates a multispectral imaging approach to colour measurement and colour matching of single yarns. The small size of a single yarn makes it impossible for spectrophotometers directly to acquire its spectral reflectance. Multispectral imaging systems, on the other hand, have the potential to measure the reflectance of single yarns as they can record both the spectral and the spatial information of a sample. A multispectral imaging system, namely imaging colour measurement, has been developed to conduct colour measurement of single yarns. A single yarn is first detected from backgrounds by a modified K‐means clustering method. The reflectance of the single yarn is then specified by an averaging method. Comparative experiments based on 100 pairs of single yarns and corresponding yarn windings show that the reflectance magnitude of a single yarn acquired by imaging colour measurement is smaller than that of corresponding yarn winding measured by a Datacolor 650 spectrophotometer. Experiments on 16 single yarns show that the repeatability and spatial reproducibility of the imaging colour measurement system in measuring a single yarn colour are 0.1185 and 0.2827 CMC(2:1) units. A colour matching comparison experiment (pass or fail), using 24 pairs of single yarns and corresponding pairs of solid‐colour yarn dyed fabrics, shows that single yarns measured by imaging colour measurement can achieve similar colour matching results to solid‐colour yarn dyed fabrics measured by the Datacolor 650 spectrophotometer, with degrees of similarity of 87.5 and 83.3% when the CMC(2:1) and CIE2000(2:1:1) colour difference formulas are employed.     

Reconstruction of reflectance data by modification of Berns' Gaussian method

Berns' method for the synthesis of spectral reflectance curve from the tristimulus color coordinates is modified. Firstly, the Gaussian bell shape red primary is replaced with a sigmoidal one to solve the dissimilarity between the spectral curves at the end region of spectrum. Secondly, three predetermined Gaussian primaries used in the original Berns' method are replaced by the adaptive ones which their half‐height bandwidths vary with the tristimulus values of the desired color. The mentioned modifications are applied for the recovery of the reflectance curves of 1409 surface colors (including 1269 Munsell color chips and 140 samples of Colorchecker SG) and also 204 textile samples. Results of recovery are evaluated by the mean and the maximum color difference values under other standard light sources. The mean as well as the maximum of root mean squares between the reconstructed and the actual spectra are also calculated. The modifications are compared with the common principal component analysis (PCA) as well as Hawkyard's methods for recovery of reflectance factor. Although the PCA leads to the best results, the modifications significantly improve the recovery outcomes in comparison with the original Berns method. © 2008 Wiley Periodicals, Inc. Col Res Appl, 34, 26–32, 2009.     

Color measurement of single yarn based on hyperspectral imaging system

This article proposed a novel approach to color measurement of a single yarn using hyperspectral imaging system (HIS). Due to the size of a single yarn, it is impossible for spectrophotometers to measure its color directly. The HIS can acquire the spectral reflectance of continuous bands within a region of interest on a yarn sample, which can achieve color measurement of a single yarn compared with traditional spectrophotometers. A single yarn is segmented from the background by a spectral matching method through adaptively setting threshold of Fréchet distance values. The spectral reflectance of single yarn is specified by a method that lightness of pixels used as weight. The experiment based on Pantone Cotton Chip Set shows that the interinstrument agreement between the HIS and a standard spectrophotometer Datacolor SF650 has a significant improvement after using the R-Model, and the average percentage improvement of the color difference is up to 54.99%. The yarn segmentation comparative experimental results show that the proposed method to segment single yarn from background is better in retaining the edge information of the yarn than the modified K-means clustering method, and the color of the yarn segmented by the proposed method is more similar to the actual color of single yarn.     

Application of multi‐flux theory based on Mie scattering to the problem of modeling the optical characteristics of colored pigmented paint films

Multi‐flux theory for multiple scattering calculations in pigmented/protective coating is described. Performance evaluation of the theory is made by comparing theoretically computed reflectance with experimentally measured ones for selected wavelengths for three different paint samples. Diffuse reflectance spectra for hypothetical particulate systems in visible spectral range are generated through computer calculations. Effect of variation in average pigment size and pigment size distribution on reflectance spectra is studied. Overall thrust of morphological characteristics of pigments on the color exhibited by paint dispersion is studied by calculating CIE color and color‐difference parameters of particulate systems. Results show that a very complex relationship exists between the morphological characteristics of pigments and color exhibited by the system. The outcome of the study is important for applications in paint, coating, and plastic industries. © 2001 John Wiley & Sons, Inc. Col Res Appl, 26, 234–245, 2001     

A simplified method for formulating pseudo-object colors

A simplified method is proposed in an attempt to formulate spectral reflectance functions of actual object colors. The method is applied to evaluate the optimal color gamuts and chromaticity-mismatch limits for actual object colors.     

Color Properties of Liquid Crystals and Their Application to Visual Arts*

Selective reflection properties of cholesteric liquid crystals are reviewed. It is then shown that the color of superimposed coatings of these substances is perceived as that of an additive mixture of colors of the coatings. This property permits one to synthesize many spectral reflectance distributions. A greater color gamut than is possible with other colorants can be realized. Applications to visual arts that take advantage of the additive color properties, the highly saturated colors, and the dependence of colors on temperature, angle of illumination, and angle of viewing are discussed.     

Spectral analysis of blacks

The spectral behavior of different black surfaces including papers and fabrics are investigated in this study. Several colored pigments are mixed with the blacks in different concentrations to prepare black surfaces with different shades while a series of black dyestuffs are applied on textile materials to increase the ranges of black objects. The principal component analysis technique is applied to determine the actual spectral size of the reflectance dataset. The technique simply extracts the principal directions of spectral data and organizes them in restricted spectral spaces. Three different spectral spaces, i.e., the reflectance spectra, the Kubelka‐Munk function of reflectance as well as the inverse of reflectance factor are selected to present the samples in the restricted spaces. Based on the results, it is found that, there are no significant differences between the employed spaces and far from the employed spectral domains, black surfaces could be adequately described in a three‐dimensional space. The three extracted statistical colorants are used for reconstruction of reflectance spectra of samples while the root mean square error percentage and the color difference values under the standard observing condition confirm the suitability of such virtual primaries. The work is extended to reconstruction of spectral data from colorimetric information and the adequacy of such three‐dimensional space is reconfirmed. © 2011 Wiley Periodicals, Inc. Col Res Appl, 2012     

A Monte Carlo method for assessing color rendering quality with possible application to color rendering standards

The lighting industry has been increasingly challenged to reduce electrical energy consumption while providing illumination with sufficient color rendering quality. As a result, the problem of accurately assessing color rendering quality has gained increased prominence and the introduction of efficient narrow band light emitting diode (LED) sources has further intensified the debate. This study argues that there is a basic problem with the traditional method of quantifying color quality color rendering index (CRI), one that cannot be solved through minor improvements. The CRI relies on a determination of the degree of color distortion that a test source produces for a small number of test samples of specified spectral reflectance distribution, but there is no clear objective rationale for selecting these few samples. Also, any such arbitrary scoring scheme lacks an objective argument for what constitutes an acceptable score. This study proposes a new method for color rendering assessment that determines the color shift of one thousand, or more, representative reflection spectra that span the full multidimensional range of possible spectral distributions and colors. This broad sampling eliminates the intrinsic selection bias of the CRI calculation and its variants and it is compatible with a more objective standard for a color quality score, one that is statistically based on the fraction of the test spectra that experience color shifts that are less than a just noticeable difference (JND), or an agreed upon multiple of it. Since the concept of JNDs in color has been reproducibly quantified, it is hoped that this approach will be widely acceptable. © 2010 Wiley Periodicals, Inc. Col Res Appl, 2012