Radiance Factor Research Articles

Pre‐Flight and In‐Flight Calibration and Performance of the Terminal Tracking Cameras (TTCams) on the NASA Lucy Mission

AbstractThe Terminal Tracking Camera (TTCam) imaging system on the NASA Lucy Discovery mission consists of a pair of cameras that are being used mainly as a navigation and target acquisition system for the mission's asteroid encounters. However, a secondary science‐focused function of the TTCam system is to provide wide‐angle broadband images over a large range of phase angles around close approach during each asteroid flyby. The scientific data acquired by TTCam can be used for shape modeling and topographic and geologic analyses. This paper describes the pre‐flight and initial in‐flight calibration and characterization of the TTCams, including the development of a radiometric calibration pipeline to convert raw TTCam images into radiance and radiance factor (I/F) images, along with their uncertainties. Details are also provided here on the specific calibration algorithms, the origin and archived location of the required ancillary calibration files, and the archived sources of the raw calibration and flight data used in this analysis.

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.

Revised Radiative Response Coefficients for the Curiosity Rover Mastcam from Direct Solar Images and Analog Tests

We have used solar images taken by the Curiosity rover’s Mastcam to derive new radiometric response coefficients. In addition, we used calibration data from the Perseverance rover’s Mastcam-Z instrument, which used the same detector, to estimate temperature sensitivity of the radiative response. The new coefficients allow calibration of raw data to radiance using the illuminant (the Sun) as the calibration source. Compared to the original pre-flight and early in-flight calibration, the revised calibration is expected to be more consistent across instrument temperature and allow for a self-consistent conversion to radiance factor values when contemporaneous calibration target images are not available or not appropriate. The revised coefficients were incorporated in a recalibration of Mastcam radiance products.

Albedo variegation on Comet 67P/Churyumov–Gerasimenko

ABSTRACT We here study the level of albedo variegation on the nucleus of Comet 67P/Churyumov–Gerasimenko. This is done by fitting the parameters of a standard photometric phase function model to disc–average radiance factor data in images acquired by the Rosetta/OSIRIS Narrow Angle Camera in the orange filter. Local discrepancies between the observed radiance factor and the disc–average solution are interpreted as a proxy $\mathcal {W}$ of the local single–scattering albedo. We find a wide range $0.02 \lesssim \mathcal {W}\lesssim 0.09$ around an average of $\mathcal {W}=0.055$. The observed albedo variegation is strongly correlated with nucleus morphology – smooth terrain is brighter, and consolidated terrain is darker, than average. Furthermore, we find that smooth terrain darken prior to morphological changes, and that stratigraphically low terrain (with respect to the centre of each nucleus lobe) is brighter than stratigraphically high terrain. We propose that the observed albedo variegation is due to differences in porosity and the coherent effect: compaction causes small brighter particles to act collectively as larger optically effective particles that are darker. Accordingly, we consider the dark consolidated terrain materials more compacted than smooth terrain materials, and darkening of the latter is due to subsidence.

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.

Small lobe of comet 67P: Characterization of the Wosret region with ROSETTA-OSIRIS

Aims. We investigated Wosret, a region located on the small lobe of the 67P/Churyumov-Gerasimenko comet subject to strong heating during the perihelion passage. This region includes the two last landing sites of the Philae lander as well as, notably the final one, Abydos, where the lander performed most of its measurements. We study Wosret in order to constrain its compositional properties and its surface evolution. By comparing them with those of other regions, we aim to identify possible differences among the two lobes of the comet. Methods. We analyzed high-resolution images of the Wosret region acquired between 2015 and 2016 by the Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) on board the Rosetta spacecraft, at a resolution ranging from 2 to 10 m px−1 before and close to perihelion, up to 0.07–0.2 m px−1 in the post-perihelion images. The OSIRIS images were processed with the OSIRIS standard pipeline, then converted into I∕F radiance factors and corrected for the viewing and illumination conditions at each pixel using the Lommel–Seeliger disk function. Spectral slopes were computed in the 535–882 nm range. Results. We observed a few morphological changes in Wosret, related to local dust coating removal with an estimated depth of ~1 m, along with the formation of a cavity measuring 30 m in length and 6.5 m in depth, for a total estimated mass loss of 1.2 × 106 kg. The spectrophotometry of the region is typical of medium-red regions of comet 67P, with spectral slope values of 15–16%/(100 nm) in pre-perihelion data acquired at phase angle 60°. As observed globally for the comet, also Wosret shows spectral slope variations during the orbit linked to the seasonal cycle of water, with colors getting relatively bluer at perihelion. Wosret has a spectral phase reddening of 0.0546 × 10−4 nm−1 deg−1, which is about a factor of 2 lower than what was determined for the nucleus northern hemisphere regions, possibly indicating a reduced surface micro-roughness due to the lack of widespread dust coating. A few tiny bright spots are observed and we estimate a local water-ice enrichment up to 60% in one of them. Morphological features such as “goosebumps” or clods are widely present and larger in size than similar features located in the big lobe. Conclusions. Compared to Anhur and Khonsu, two southern hemisphere regions in the big lobe which have been observed under similar conditions and also exposed to high insolation during perihelion, Wosret exhibits fewer exposed volatiles and less morphological variations due to activity events. Considering that the high erosion rate in Wosret unveils part of the inner layers of the small lobe, our analysis indicates that the small lobe has different physical and mechanical properties than the big one and a lower volatile content, at least in its uppermost layers. These results support the hypothesis that comet 67P originated from the merging of two distinct bodies in the early Solar System.

The surface of (4) Vesta in visible light as seen by Dawn/VIR

Aims.We analyzed the surface of Vesta at visible wavelengths, using the data of the Visible and InfraRed mapping spectrometer (VIR) on board the Dawn spacecraft. We mapped the variations of various spectral parameters on the entire surface of the asteroid, and also derived a map of the lithology.Methods.We took advantage of the recent corrected VIR visible data to map the radiance factor at 550 nm, three color composites, two spectral slopes, and a band area parameter relative to the 930 nm crystal field signature in pyroxene. Using the howardite-eucrite-diogenite meteorites data as a reference, we derived the lithology of Vesta using the variations of the 930 and 506 nm (spin-forbidden) band centers observed in the VIR dataset.Results.Our spectral parameters highlight a significant spectral diversity at the surface of Vesta. This diversity is mainly evidenced by impact craters and illustrates the heterogeneous subsurface and upper crust of Vesta. Impact craters also participate directly in this spectral diversity by bringing dark exogenous material to an almost entire hemisphere. Our derived lithology agrees with previous results obtained using a combination of infrared and visible data. We therefore demonstrate that it is possible to obtain crucial mineralogical information from visible wavelengths alone. In addition to the 506 nm band, we identified the 550 nm spin-forbidden one. As reported by a laboratory study for synthetic pyroxenes, we also do not observe any shift of the band center of this feature across the surface of Vesta, and thus across different mineralogies, preventing use of the 550 nm spin-forbidden band for the lithology derivation. Finally, the largest previously identified olivine rich-spot shows a peculiar behavior in two color composites but not in the other spectral parameters.

Methods Calculating the Slab Radiance Factor

One of the most critical problems of realistic visualization of the real-world objects is physically adequate modeling of their reflection of light. Reflection of light by objects occurs both from the surface and the bulk of matter (scattering). Accounting for the light reflection from the surface of objects was solved almost a century ago based on its representation as a Fresnel randomly rough surface. Scattering by a bulk of matter is the subject of radiation transfer theory, which has only recently received its known completion in the form of discrete transfer theory. Strict analytical methods for solving the radiation transport equation (RTE) are often not highly effective for calculating the radiance factor. For a long time, in the absence of effective numerical methods for solving problems and the unavailability of high-speed computers for practical calculations, approximate methods for solving RTE were developed. However, their accuracy and applicability limits were poorly defined. The discrete transfer theory allowed us to evaluate the existing approximate methods for solving the UPI, their accuracy, and the efficiency of application for calculating the radiance factor. It is shown that the most effective method is the method of synthetic iterations. The method is based on the selection of the solution anisotropic part based on a small-angle approximation of the RTE solution. The solution regular part can be calculated by any approximation. Then a simple iteration from the complete solution is performed to refine the angular distribution of the radiance factor.

Phase reddening on asteroid Bennu from visible and near-infrared spectroscopy

Context.The NASA mission OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer) has been observing near-Earth asteroid (101955) Bennu in close proximity since December 2018. In October 2020, the spacecraft collected a sample of surface material from Bennu to return to Earth.Aims.In this work, we investigate spectral phase reddening – that is, the variation of spectral slope with phase angle – on Bennu using spectra acquired by the OSIRIS-REx Visible and InfraRed Spectrometer (OVIRS) covering a phase angle range of 8–130°. We investigate this process at the global scale and for some localized regions of interest (ROIs), including boulders, craters, and the designated sample collection sites of the OSIRIS-REx mission.Methods.Spectra were wavelength- and flux-calibrated, then corrected for the out-of-band contribution and thermal emission, resampled, and finally converted into radiance factor per standard OVIRS processing. Spectral slopes were computed in multiple wavelength ranges from spectra normalized at 0.55μm.Results.Bennu has a globally negative spectra slope, which is typical of B-type asteroids. The spectral slope gently increases in a linear way up to a phase angle of 90°, where it approaches zero. The spectral phase reddening is monotonic and wavelength-dependent with highest values in the visible range. Its coefficient is 0.00044μm−1 deg−1in the 0.55–2.5μm range. For observations of Bennu acquired at high phase angle (130°), phase reddening increases exponentially, and the spectral slope becomes positive. Similar behavior was reported in the literature for the carbonaceous chondrite Mukundpura in spectra acquired at extreme geometries. Some ROIs, including the sample collection site, Nightingale, have a steeper phase reddening coefficient than the global average, potentially indicating a surface covered by fine material with high micro-roughness.Conclusions.The gentle spectral phase reddening effect on Bennu is similar to that observed in ground-based measurements of other B-type asteroids, but much lower than that observed for other low-albedo bodies such as Ceres or comet 67P/Churyumov-Gerasimenko. Monotonic reddening may be associated with the presence of fine particles at micron scales and/or of particles with fractal structure that introduce micro- and sub-micro roughness across the surface of Bennu.

Image-Based Measurement of Structural Color using Spectral Camera

We perceive structural colors by optical phenomena such as light interference and diffraction caused by a fine structure of the object surface. One of the characteristics of structural colors is that a wavelength distribution of light changes depending on an incident angle of a light source and a viewing angle. Generally, for color evaluation and reproduction, it is required to acquire reflection characteristics of objects. Therefore, BRDF (Bidirectional Reflectance Distribution Function) is often used as a function that represents reflection characteristics depending on incident and viewing angles. In this study, we measured BRDF of structural colors based on a method to acquire image-based material reflection characteristics using a spectral camera. The measurement was performed by aligning an optical axis of a spectral camera with a structural color sample and changing an irradiation angle of a light source. Reflection characteristics were represented by using a radiance factor, which was a ratio between a spectral radiance of white material and that of structural color. From measurement results, we confirmed an angle-dependent radiance factor. Finally, based on a measured spectral radiance of a structural color sample, we spectrally reproduced the structural color using a spectral projector based on model fitting of spectral data.

The surface of (1) Ceres in visible light as seen by Dawn/VIR

Aims.We study the surface of Ceres at visible wavelengths, as observed by the Visible and InfraRed mapping spectrometer (VIR) onboard the Dawn spacecraft, and analyze the variations of various spectral parameters across the whole surface. We also focus on several noteworthy areas of the surface of this dwarf planet.Methods.We made use of the newly corrected VIR visible data to build global maps of a calibrated radiance factor at 550 nm, with two color composites and three spectral slopes between 400 and 950 nm. We have made these maps available for the community via the Aladin Desktop software.Results.Ceres’ surface shows diverse spectral behaviors in the visible range. The color composite and the spectral slope between 480 and 800 nm highlight fresh impact craters and young geologic formations of endogenous origin, which appear bluer than the rest of the surface. The steep slope before 465 nm displays very distinct variations and may be a proxy for the absorptions caused by the O2− → Fe3+or the 2Fe3+→ Fe2++ Fe4+charge transfers, if the latter are found to be responsible for the drop in this spectral range. We notice several similarities between the spectral slopes and the abundance of phyllosilicates detected in the infrared by the VIR, whereas no correlation can be clearly established with carbonate species. The region of the Dantu impact crater presents a peculiar spectral behavior – especially through the color and the spectral slope before 465 nm – suggesting a change in composition or in the surface physical properties that is not observed elsewhere on Ceres.

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.

On Variants of the Main Atmospheric Correction Formula

The article studies the accuracy of the main formula of atmospheric correction allowing us to determine albedo of a underlying (earth) surface based on radiance factor of solar light reflected by the system of atmosphere and underlying surface. The problem of atmospheric correction is considered in three-dimensional geometry with spatial non-uniformity of the underlying surface taken into account. It is demonstrated that the accuracy of albedo recovery depends on the used variant of the main formula.

Multiple-constraint inversion of SCOPE. Evaluating the potential of GPP and SIF for the retrieval of plant functional traits

The most recent efforts to provide remote sensing (RS) estimates of plant function rely on the combination of Radiative Transfer Models (RTM) and Soil-Vegetation-Atmosphere Transfer (SVAT) models, such as the Soil-Canopy Observation Photosynthesis and Energy fluxes (SCOPE) model. In this work we used ground spectroradiometric and chamber-based CO2 flux measurements in a nutrient manipulated Mediterranean grassland in order to: 1) develop a multiple-constraint inversion approach of SCOPE able to retrieve vegetation biochemical, structural as well as key functional traits, such as chlorophyll concentration (Cab), leaf area index (LAI), maximum carboxylation rate (Vcmax) and the Ball-Berry sensitivity parameter (m); and 2) compare the potential of the of gross primary production (GPP) and sun-induced fluorescence (SIF), together with up-welling Thermal Infrared (TIR) radiance and optical reflectance factors (RF), to estimate such parameters. The performance of the proposed inversion method as well as of the different sets of constraints was assessed with contemporary measurements of water and heat fluxes and leaf nitrogen content, using pattern-oriented model evaluation.The multiple-constraint inversion approach proposed together with the combination of optical RF and diel GPP and TIR data provided reliable estimates of parameters, and improved predicted water and heat fluxes. The addition of SIF to this scheme slightly improved the estimation of m. Parameter estimates were coherent with the variability imposed by the fertilization and the seasonality of the grassland. Results revealed that fertilization had an impact on Vcmax, while no significant differences were found for m. The combination of RF, SIF and diel TIR data weakly constrained functional traits. Approaches not including GPP failed to estimate LAI; however GPP overestimated Cab in the dry period. These problems might be related to the presence of high fractions of senescent leaves in the grassland. The proposed inversion approach together with pattern-oriented model evaluation open new perspectives for the retrieval of plant functional traits relevant for land surface models, and can be utilized at various research sites where hyperspectral remote sensing imagery and eddy covariance flux measurements are simultaneously taken.

Linking surface morphology, composition, and activity on the nucleus of 67P/Churyumov-Gerasimenko

Aims.The Rosetta space probe accompanied comet 67P/Churyumov-Gerasimenko for more than two years, obtaining an unprecedented amount of unique data of the comet nucleus and inner coma. This has enabled us to study its activity almost continuously from 4 au inbound to 3.6 au outbound, including the perihelion passage at 1.24 au. This work focuses identifying the source regions of faint jets and outbursts and on studying the spectrophotometric properties of some outbursts. We use observations acquired with the OSIRIS/NAC camera during July–October 2015, that is, close to perihelion.Methods.We analyzed more than 2000 images from NAC color sequences acquired with 7–11 filters covering the 250–1000 nm wavelength range. The OSIRIS images were processed with the OSIRIS standard pipeline up to level 3, that is, converted in radiance factor, then corrected for the illumination conditions. For each color sequence, color cubes were produced by stacking registered and illumination-corrected images.Results.More than 200 jets of different intensities were identified directly on the nucleus. Some of the more intense outbursts appear spectrally bluer than the comet dark terrain in the visible-to-near-infrared region. We attribute this spectral behavior to icy grains mixed with the ejected dust. Some of the jets have an extremely short lifetime. They appear on the cometary surface during the color sequence observations, and vanish in less than some few minutes after reaching their peak. We also report a resolved dust plume observed in May 2016 at a resolution of 55 cm pixel−1, which allowed us to estimate an optical depth of ~0.65 and an ejected mass of ~2200 kg, assuming a grain bulk density of ~800 kg m−3. We present the results on the location, duration, and colors of active sources on the nucleus of 67P from the medium-resolution (i.e., 6–10 m pixel−1) images acquired close to perihelion passage. The observed jets are mainly located close to boundaries between different morphological regions. Some of these active areas were observed and investigated at higher resolution (up to a few decimeter per pixel) during the last months of operations of the Rosetta mission.Conclusions.These observations allow us to investigate the link between morphology, composition, and activity of cometary nuclei. Jets depart not only from cliffs, but also from smooth and dust-covered areas, from fractures, pits, or cavities that cast shadows and favor the recondensation of volatiles. This study shows that faint jets or outbursts continuously contribute to the cometary activity close to perihelion passage, and that these events are triggered byillumination conditions. Faint jets or outbursts are not associated with a particular terrain type or morphology.

Surface evolution of the Anhur region on comet 67P/Churyumov-Gerasimenko from high-resolution OSIRIS images

Context. The southern hemisphere of comet 67P/Churyumov-Gerasimenko (67P) became observable by the Rosetta mission in March 2015, a few months before cometary southern vernal equinox. The Anhur region in the southern part of the comet’s larger lobe was found to be highly eroded, enriched in volatiles, and highly active. Aims. We analyze high-resolution images of the Anhur region pre- and post-perihelion acquired by the OSIRIS imaging system on board the Rosetta mission. The Narrow Angle Camera is particularly useful for studying the evolution in Anhur in terms of morphological changes and color variations. Methods. Radiance factor images processed by the OSIRIS pipeline were coregistered, reprojected onto the 3D shape model of the comet, and corrected for the illumination conditions. Results. We find a number of morphological changes in the Anhur region that are related to formation of new scarps; removal of dust coatings; localized resurfacing in some areas, including boulders displacements; and vanishing structures, which implies localized mass loss that we estimate to be higher than 50 million kg. The strongest changes took place in and nearby the Anhur canyon-like structure, where significant dust cover was removed, an entire structure vanished, and many boulders were rearranged. All such changes are potentially associated with one of the most intense outbursts registered by Rosetta during its observations, which occurred one day before perihelion passage. Moreover, in the niche at the foot of a new observed scarp, we also see evidence of water ice exposure that persisted for at least six months. The abundance of water ice, evaluated from a linear mixing model, is relatively high (>20%). Our results confirm that the Anhur region is volatile-rich and probably is the area on 67P with the most pristine exposures near perihelion.

Deviation of white diffuse reflectance standards from perfect reflecting diffuser at visible and near-infrared spectral ranges

The assumption that the reflectance of white diffuse reflectance standards is identical to that of the perfect reflecting diffuser (PRD) allows these standards to be used to characterize reflectance or radiance factors of any surface at any irradiation/collection geometry simply by comparison. However, this assumption is only true within certain limits, and, for some applications, requirements may be out of those limits. PTB and IO-CSIC have studied the variation of the reflectance with respect to the bidirectional geometry for the four most typical white diffuse materials (barium sulfate, opal glass, ceramic and Spectralon), at in- and out-of plane geometries and at spectral range from 380 nm to 1700 nm. We have defined descriptors in order to more clearly quantify the spectral reflectance variation with the bidirectional geometries. The values obtained for these descriptors have been separately presented for the visible and near-infrared spectral ranges. In both spectral ranges, deviations of white diffuse reflectance standards with respect to the PRD were found, regarding both Lambertian behaviour and spectral constancy. The observed deviation from the BRDF is in general very large for high incidence and collection angles (reaching in many cases 20%). Therefore, it is not possible to assume Lambertianity in standards at those geometries when calibrating measuring systems.

Impacts of spectral variations in multiwavelength excitation method on measurements of fluorescently whitened papers

AbstractThe multiwavelength excitation (MWE) method for measuring the colorimetric properties of fluorescent whitening agent (FWA)‐treated specimens illuminated by a standard daylight illuminant computationally approximates not the spectral power distribution (SPD) of the illuminant but the luminescent SPD excited thereby by a weighted sum of the luminescent SPDs excited by a few different narrow‐band illuminations. The weights are optimized for the actual SPDs of those illuminations and the bispectral luminescent radiance factors of typical FWA‐treated paper specimens. Since the latter is invariant among instruments once provided as the common numerical data, the variations of the narrow‐band SPDs give major impacts to the reproducibility of this method. The weights optimized for the varied SPDs, however, mitigate the impacts. For investigating how they impact, one basic illumination system and its 16 simple variation systems were built virtually. The basic system consists of three narrow‐band LEDs and one blue‐excited white LED, whereas the individual simple variation system has either the peak wavelength or spectral width of one of the four LEDs (including the blue LED in the white LED) varied. With those systems, seven FWA‐treated papers with the known bispectral radiance factors were measured computationally by simulating the procedure of the MWE method. The differences in the colorimetric values measured with the simple variation systems from those with the basic system are far below the just noticeable difference, which indicates that the MWE method can be a practical solution for better reproducibility in measuring FWA‐treated papers.