Spectral Radiance Factor Research Articles

Fluorescent whiteness measurement of textiles by multispectral imaging system

AbstractAccurate whiteness measurement is critical across various industries including textiles, paper, and detergents. With the advent of fluorescent whitening agents (FWAs), the assessment of whiteness has evolved from simply measuring reflectance to determining the spectral radiance factor (SRF) of materials. While multispectral imaging (MSI) can effectively measure object reflectance and colour, its accuracy is compromised by FWAs that significantly augment spectral reflectance. Thus, reliable whiteness measurement requires the determination of both reflectance and SRF. This article presents the design of a specialised light source equipped with an ultraviolet (UV) filter to provide adjustable illumination within an integrating sphere system. By tuning the position of the UV filter, multispectral images are captured of a fabric under varying UV exposures. These images are subsequently processed and combined to reconstruct full‐range visible spectrum information. This approach is shown to achieve high accuracy and spatial uniformity in quantifying the whiteness of textiles using MSI.

New method of whiteness calibration of spectrophotometers

AbstractA method is described for determining and calibrating the CIE Whiteness Index (WI) of an OBA‐treated specimen without incorporating into the measurement system a mechanically adjustable UV filter. The method requires spectrophotometric measurement of the specimen under two lights: UV‐excluded to infer the reflectance r(λ) and UV to infer the fluorescence profile f(λ). Also required is a stored illuminant spectrum Iin(λ) that covers at least the whole visible wavelengths. Only one scalar value k is then needed to mathematically characterize the total spectral radiance factor (TSRF) of the same specimen under Iin(λ). Calibration uses a reference specimen with known WI, and adjusts k until the computed WI matches the reference WI value. If Iin(λ) is the illuminant chosen from ASTM E313 to define the WI, the k adjustment reduces to solving a single quadratic equation for k that involves the reference WI. In general, relative to directly measuring a specimen's TSRF, it is more instrument‐independent to use the current method to characterize TSRF. Therefore, if a fluorescent specimen is measured using UV and UV‐excluded illuminants, a complete, instrument‐stable characterization of the specimen's TSRF can be derived by adjusting k, which can be done through a closed‐form relation of k to the WI.

Uncertainty budget for PTB’s gonioreflectometers and ways to improve it in the short VIS spectral range

This paper describes a detailed derivation of the uncertainty budget for measurements of the absolute spectral radiance factor with PTB’s gonioreflectometers. The measurement uncertainty is determined according to the ‘Guide to the expression of uncertainty in measurement’ provided by the Bureau International des Poids et Mesures. To give an insight, the uncertainty analysis is applied to measurements of three ceramic samples. The influence of different systematic and statistical sources of uncertainty and their contributions are discussed in detail. For wavelengths larger than 450 nm the total measurement uncertainty is dominated by systematic contributions. Towards UVA the major contribution is caused by statistical effects, which are mainly due to the decreasing power of the radiation source used. We present an approach capable to reduce this contribution for the VIS/UVA-transition spectral range by means of a specially designed LED-based sphere radiation source.

Novel LED-based radiation source and its application in diffuse reflectometry and polarization measurements

An LED sphere radiator (LED-SR) was constructed to improve the accuracy in spectral radiance factor measurements performed with the robot-based gonioreflectometer at PTB. Its properties with respect to the spectral range and coverage, the temporal stability, and the homogeneity of the radiation field are presented. Two types of matte ceramic reflection standards were used for spectral radiance factor validation measurements comparing the standardly used halogen sphere radiator (Halogen-SR) and the LED-SR. Due to its designed spectral range at the border between the visible and the UV-A spectral range, the LED-SR is well suited for many applications in diffuse reflectometry. Its use for absolute radiance factor measurements and investigations of the fluorescence properties of diffuse reflecting samples is shown. Reliable polarization-resolved measurements at wavelengths below 430 nm could be carried out with PTB’s gonioreflectometer for the first time due to the beneficial signal-to-noise ratio of the LED-SR.

Gloss, Light Reflection and Iridescence in Ceramic Tile Enamels Containing ZrO2 and ZnO

Ceramic claddings on building facades not only present functional qualities and good resistance; they also add value to the architecture due to their qualities of light reflection, gloss and iridescence. The colour ranges produced by some enamel application techniques can vary widely. They change depending on one’s angle of vision and movement, colours in the surroundings, sunlight and their angle of incidence. In addition, the iridescent-pearl effect produced by light diffraction can lead to beautiful goniochromatic colours. This study analyses the production of square tiles of stoneware manufactured by extrusion, and their application to the Faculty of Education of the University of Alicante (FEUA) (Spain). Applying an enamel containing zirconium silicate ZrSiO4 and other metals such as Zn and Al produces iridescence-like effects. The physical-chemical properties of enamel and gloss values were characterised. A colorimetric characterisation was conducted by evaluating goniochromatic or iridescent colours, measuring the light’s spectral radiance factor, and comparing these results with other ceramic tiles of marked iridescent effects, with the presence of a final layer of anatase TiO2 enamel.

Iridescent Techniques in Ceramics: Physico-Chemical Analysis and Colorimetric Characterization of the Headquarters of the Botín Foundation in Santander

In recent years, the use of porcelain stoneware in building envelopes has opened new lines of research and innovation favoring energy efficiency and the reduction of environmental impacts. However, there has been little research on its relationship with light vibration and reflectivity, visual perception, and integration in the urban environment and landscape. In this study, an analysis was conducted on the production and application of crafted pieces of porcelain stoneware, shaped in spherical caps, at the headquarters of the Botín Foundation in Santander (Spain). Various enamels with an iridescent-nacreous finish and metallic reflection were applied to the stoneware to generate a vibrant skin that would constantly change with natural light and the environment. A vitrification and metal deposition process were designed through successive applications of enamels and firings. The physico-chemical properties of the enameling and the microcracking factors that produced the iridescent effect were characterized. A colorimetric characterization was performed evaluating the goniochromatic or iridescent colors, measuring the spectral radiance factor of the light, and comparing these results with other ceramic pieces.

Measurement traceability of total radiance factors for white fluorescent materials employing incandescent-lamp Zeiss leucometer photometers and with modern spectrophotometers based on a D65 artificial illuminant

A discussion based on the theory of measurements using an upgraded Zeiss leucometer (0/d) in which the fraction of UV radiation from the incandescent lamp is controllable was used to obtain a photometric equation (appropriate for any modern leucometer-type photometer) relating the total reflectance factors measured using such an instrument to the total spectral radiance factors for white fluorescent materials measured using a modern spectrophotometer (d/0) with a D65 artificial illuminant at the leucometer effective wavelength. A review of the available experimental results using the resulting equation indicated that it was possible to obtain measurement traceability of the total spectral radiance factors for white fluorescent materials when employing Zeiss leucometer photometers with an incandescent lamp and by using modern spectrophotometers with a D65 artificial illuminant.

Color prediction of metallic coatings from measurements at common geometries in portable multiangle spectrophotometers

Illuminating and viewing geometries may strongly affect the color appearance of metallic coatings, which can be characterized accurately by bidirectional reflectance distribution function (BRDF) measurement devices. However, such devices with hundreds or even thousands of different geometries are usually expensive and complex. Accordingly, two modified models were developed in this study, based on the distribution of aluminum-flake pigments in the coatings, to, respectively, estimate the spectral radiance factors and the CIE tristimulus values of metallic coatings utilizing the measurements at 6 common geometries of portable multiangle spectrophotometers. Their performance was examined with 65 achromatic and 20 chromatic metallic coating samples under D65, A, and F11 illuminants. The average CIEDE2000 color differences over all 19 geometries were found to be less than 1.8 for both models, while the average CAM02-SCD and CIELAB color differences can, respectively, reach 1.7 and 2.0, indicating the effectiveness of our methods.

Multi‐wavelength excitation method for measuring FWA‐treated paper

AbstractColorimetric properties of fluorescent materials depend on the SPD of the illumination. That is why most standards for evaluating them specify the illuminations, which are often hard‐to‐realize daylight illuminants. The presented method using commercially available LEDs enables accurate enough colorimetric measurements of FWA‐treated papers or prints on them illuminated by the specified illuminant. The total spectral radiance factor of a fluorescent specimen, from which most colorimetric values are derived, consists of the luminescent spectral radiance factor and the spectral reflectance factor. This method separately estimates those of FWA‐treated paper to add up to the total spectral radiance factor. The luminescent spectral radiance factor is obtained by estimating the SPD of luminescence excited by the specified illuminant as the weighted sum of the multiple SPDs of luminescence excited by the respective narrow band LED emissions at different wavelengths. The LEDs and their weights are determined optimally for generally used papers. The spectral reflectance factor is derived from the estimated SPD of the radiation with fluorescence excluded from the paper illuminated by visible illumination. The method was applied with five different illumination systems each using two or three narrow band LEDs in the excitation range. They were evaluated by measuring the total spectral radiance factors by D50 of seven FWA‐treated papers and CMYK prints on four papers. The derived colorimetric values were compared to the respective references by the ideal D50.

Multilateral spectral radiance factor scale comparison.

The field of spectral radiance factor (SRF) measurements has seen growing interest in recent years. Scale conformity has so far only been established between the national metrology institutes (NMIs) of Germany and the USA. This study aims at a bigger, multilateral scale comparison. For this purpose, a total of six NMIs participated in a scale comparison of goniospectrophotometers based on neutral and colored diffusely reflecting ceramics samples. In addition, two universities, providing a home-built gonioreflectometer and two widely used commercially available color measurement instruments, respectively, were involved. The wavelength range of the scale comparison covers the visible wavelength range from 380 nm to 780 nm. Results indicate systematic issues and that the uncertainty evaluation of the NMIs requires further work; although for the greatest part of the covered spectral range the agreement is good.

Characterization of a versatile reference instrument for traceable fluorescence measurements using different illumination and viewing geometries specified in practical colorimetry—part 2: sphere geometry (8:d)

In the second part of this two-part series on the development of a versatile reference instrument at the National Research Council of Canada (NRC), we have extended the characterization of the NRC Reference Goniospectrofluorimeter to high-accuracy fluorescence measurements in a sphere geometry (8:d) that is specified in standard test methods for many practical applications in colorimetry. This builds upon the work reported in part-one of this series which described in detail the design, characterization and validation of this new instrument for realizing a total spectral radiance factor scale in a bidirectional (45a:0) geometry. To extend the measurement capabilities to a sphere geometry, it was configured with a large diameter integrating sphere accessory. Preliminary results using a substitution-mode operating procedure showed large sphere errors that were characterized and corrected for. To improve this traceability, the sphere was modified to operate in comparison-mode and this effectively eliminated many of the sphere-related errors that typically limit the accuracy of sphere-based fluorescence measurements. The performance of the instrument configured for a sphere geometry (8:d) with this modified sphere design has been validated by means of comparison measurements of both non-fluorescent and fluorescent artifacts. The reflectance component has been validated using non-fluorescent comparison samples that have been calibrated under the same geometric conditions with traceability to the NRC Absolute Reflectometer (d:0 geometry). The fluorescent-only component has been validated using near-Lambertian fluorescent reflecting materials with traceability to the NRC Reference Spectrofluorimeter (45:0 geometry), under the assumption that this component is nearly the same for these two geometries. This work has enabled NRC to provide an uninterrupted link for improved traceability of fluorescence calibrations that specify a sphere geometry. These calibration requests include many important commercial applications, such as whiteness specification of fluorescently whitened paper and textile samples and color specification of fluorescent safety goods.

Detailed analysis of the UV‐adjustment techniques used in paper and graphic industries

AbstractMany commercial materials (papers and boards) contain optical brightening agents also known as fluorescent whitening agents. Adequate adjustment of the UV content of a measurement device (e.g., spectrophotometers) is essential for accurate color measurement. As specified in the ISO standards, the UV content is adjusted against an assigned value of an international reference transfer standard, for example, CIE whiteness (D65/10°) for the CIE illuminant D65 or ISO brightness for the C illuminant. Because of the simplicity, these approaches have gained great popularity in papermaking industry. Yet, there has been little evidence indicating how accurate the total spectral radiance factor corresponding to the single assigned value is reproduced. Hence, we present a method that quantitatively evaluates the accuracy of the UV‐adjustment technique, through comparing the total spectral radiance factors obtained from UV adjustment with the assigned ones. This method has been applied to three second‐level international reference transfer standard illuminated by three standard illuminants, D65, C, and D50. We found that the major differences between the assigned spectra and those obtained from the UV adjustments occur in the blue band where fluorescence is strong. At a few wavelengths, the differences may be up to 4–5%. Nevertheless, their color differences corresponding to the assigned spectra and those obtained from the UV adjustments are still smaller than unity (1 ΔE*) for all of the illumination conditions. Two instruments using the representative UV adjusting techniques, for example, the conventional UV‐adjusting with an adjustable (GG395) UV filter and the numerical UV‐filtering, have been studied. © 2016 Wiley Periodicals, Inc. Col Res Appl, 42, 19–26, 2017

How to efficiently characterize special effect coatings

Coatings and plastics that contain special effect pigments show angular-dependent reflection characteristics. Interference pigments, for instance, exhibit a color impression ("appearance") that changes with the directions of illumination and observation. It is currently an open question how the appearance of such coatings can be best characterized without extensive measurements of the spectral radiance factor. In this paper, a simple and efficient approach is presented on how to characterize the appearance of surfaces composed of metallic and interference pigments. Based on the rotational symmetry of the pigment distribution around the surface normal, it is demonstrated how the appearance at any configuration of illumination and observation can be estimated from a set of in-plane measurements. For practical applications, it is shown that a rather small number of geometries (e.g., 10) are sufficient. The presented methodology is an alternative to a recent principal components analysis-based procedure and offers the advantage of being based directly on an extensible physical model while having at least the same prediction accuracy.

Estimation of bispectral Donaldson matrices of fluorescent objects by using two illuminant projections.

This paper proposes a method for estimating the bispectral Donaldson matrices of fluorescent objects by using only two illuminant projections with continuous spectral power distributions. The Donaldson matrix represents the spectral radiance factor consisting of the sum of two components: a reflected radiance factor and a luminescent radiance factor. First, we describe the spectral characteristics of the observed matrix and model the matrix so that the luminescent radiance factor is separable into the emission and excitation wavelength components. We make no assumption as to the spectral shapes of any components, but derive a physical model that is useful for predicting the excitation spectral component from the reflected radiance component. An algorithm is developed to estimate the entire elements of the Donaldson matrix based on only two sets of spectral sensor outputs under two different illuminants. We suggest that the difference between the observed reflected radiance factors under the two different illuminants is not caused by the reflected radiance component, but only the luminescent radiance component. The algorithm is a sequential estimation of three radiance components of luminescent excitation, luminescent emission, and reflection. The feasibility of the proposed method is confirmed in experiments using a variety of fluorescent samples. The estimation accuracy is evaluated numerically in root-mean squared error and the color difference under the assumption of a viewing illuminant. An optimal selection of the illuminant pair is shown based on a simulation experiment using blackbody radiators with different color temperatures.

Calibration of hyperspectral data aviation mode according with accompanying ground-based measurements of standard surfaces of observed scenes

Using considerable experimental material, we examine whether it is possible to recalculate the initial data of hyperspectral aircraft survey into spectral radiance factors (SRF). The errors of external calibration for various observation conditions and different instruments for data receiving are estimated.

Reconstruction of total radiance spectra of fluorescent samples by means of nonlinear principal component analysis

Nonlinear principal component analysis (NLPCA) was used for compression and reconstruction of the total radiance factors (TRFs) of fluorescent samples. The spectral dataset included a total of 358 fluorescent reflectance spectra in the visible range of the spectrum. Spectral data compression was followed by extracting the parameterized nonlinear manifolds using the NLPCA technique. To compare the performance of NLPCA-based compression with the linear method, the orthonormal feature vectors of the dataset were also extracted by the linear PCA. The spectral performance of NLPCA and PCA-based compression approaches was assessed by the root mean square error and the goodness-fitting coefficient between the real and the reconstructed spectra. The percentages of feasible spectra by each method, i.e., those with nonnegative TRFs, were also reported as other criteria for the evaluation of methods. Furthermore, the colorimetric performance of methods were appraised by the measuring the CIELAB 1976 color difference values between the actual and reconstructed spectra under illuminants D65 and A and the 1964 standard observer. The NLPCA-based compression method performed significantly better than the standard PCA-based technique particularly in the lower dimensional spaces of the spectral radiance factors of fluorescent colors.

Factors affecting the whiteness of optically brightened material

The whiteness of fluorescent white materials is in part due to the absorption of ultraviolet (UV) light and subsequent emission of visible blue light. The UV content of light sources in viewing booths and in spectrophotometers can thus significantly affect the perceived whiteness (PW) and measured sum of reflected and emitted light of fluorescent materials. The effect of UV content on the spectral radiance factor of fluorescent white materials containing different amounts of a fluorescent brightening agent and the subsequent assessment of their PW were evaluated. The UV content of sources in two calibrated viewing booths that simulated D65 and D75 illuminants, separately, was changed by selectively blocking UV emission of the source by approximately 0%, 25%, 50%, 75%, and 100%. The radiance spectra of a series of white fabrics were also obtained using a reflectance spectrophotometer at 0%, 25%, 50%, 75%, and 100% UV transmittance. The CIE and Uchida whiteness indices (WIs) were calculated for white samples and compared to perceptual results under varying illumination and UV conditions. Results indicate relatively modest agreement between perceptual assessments of fluorescent samples and whiteness metrics examined. Results also show that when the UV content of sources used in the viewing booths is adjusted to be similar to that used in measurements, improved correlations between perceptual and calculated results are obtained. The CIE WI was found to outperform the Uchida index under both sources.

Practical method for measuring printed colors on FWA-treated paper

The virtual fluorescent standard (VFS) method is a new approximation method for the practical measurement of the colorimetric properties of an object treated with fluorescent whitening agent (FWA). The essential requirement of the VFS method is that the bi-spectral characteristics of the VFS must be similar in curve shape to those of the object to be measured. In the case of an object printed on an FWA-treated substrate, the bi-spectral characteristics will vary depending on not only the substrate but also the printed ink films. In this study, two simplified VFS methods, one using the bi-spectral characteristics of the substrate and the other using those of typical paper as the VFS instead of those similar to each object, were evaluated. The evaluation was performed using two instrument models for the VFS method with the different illuminations with five sets of 13 samples printed in different colors by five different printer/paper combinations. In this evaluation, the total spectral radiance factor of each sample was obtained by simulated measurement, that is, it was calculated based on the bi-spectral radiance factor of the sample and the spectral power distributions of the light source of the instrument. The total spectral radiance factor of each sample under D50 obtained using both VFS models and CIE L*a*b* values derived therefrom were compared with those by the reference model with ideal D50 illumination. Although the samples are limited, the results shows that both simplified VFS methods remarkably reduce the errors due to fluorescence when compared to the conventional method. © 2011 Wiley Periodicals, Inc. Col Res Appl, 2012

Measurement of the Fluorescent Spectral Radiance Factor to the Fluorescence Whitening Agent in Paper

Measurement of the Fluorescent Spectral Radiance Factor to the Fluorescence Whitening Agent in Paper

An investigation of the SO 2 content of the venusian mesosphere using SPICAV-UV in nadir mode

Using the SPICAV-UV spectrometer aboard Venus Express in nadir mode, we were able to derive spectral radiance factors in the middle atmosphere of Venus in the 170–320 nm range at a spectral resolution of R ≃ 200 during 2006 and 2007 in the northern hemisphere. By comparison with a radiative transfer model of the upper atmosphere of Venus, we could derive column abundance above the visible cloud top for SO 2 using its spectral absorption bands near 280 and 220 nm. SO 2 column densities show large temporal and spatial variations on a horizontal scale of a few hundred kilometers. Typical SO 2 column densities at low latitudes (up to 50°N) were found between 5 and 50 μm-atm, whereas in the northern polar region SO 2 content was usually below 5 μm-atm. The observed latitudinal variations follow closely the cloud top altitude derived by SPICAV-IR and are thought to be of dynamical origin. Also, a sudden increase of SO 2 column density in the whole northern hemisphere has been observed in early 2007, possibly related to a convective episode advecting some deep SO 2 into the upper atmosphere.