Luminescent Pigments Research Articles

Time-resolved photoluminescence imaging for the mapping of weakly luminescent pigments in paintings

Time-resolved photoluminescence imaging for the mapping of weakly luminescent pigments in paintings

Lighting time analysis of water-based flexographic inks containing phosphorescent pigments

The research investigates the illumination time of four luminescent pigments (green, blue, violet, and orange) dedicated to security printing and other special effect covers. It involves printout preparation on paper bases with and without optical brighteners with nine k-bar applicators and flexographic water-based inks with three different pigment concentrations (5, 15, and 30%). The printouts assessment is in the range of wild ink film thickness concerning the time of illumination. Achieved results led to the modelling of these properties. The Zeiss SEM microscope presents the images of the pigments tested.The innovation in this research is to show the nature of luminescent pigments and inks and to demonstrate its application possibility. The mathematical models presented in this article can help to adjust the appropriate ink layer's thickness and pigment concentration to achieve the desired glow time.The article shows that the green pigment glows the longest, successively blue and violet. The changes in the glow time from the thickness of the layer look similar for green and blue, for violet only at the highest pigment concentration. In contrast, the orange differs significantly from the others and shines approx. 20 times as short as the green one. The article explains how the size of the pigment can affect its glow time and why the samples obtained on a substrate containing optical brighteners glow longer. The optical brighteners intensify the shining time for the longer light wavelengths of pigments, such as green and orange, than for the shorter ones – blue and violet. The influence of the substrate on the glow effect is less visible for higher pigment concentration or greater print layer thickness. Applying a luminescent pigment on a dark-coloured substrate diminishes the lighting effect and eliminates it on a black one.

Special-Effect and Conventional Pigments in Black Light Art: A Multi-Technique Approach to an In-Situ Investigation.

Since their introduction in the early decades of the 20th century, fluorescent pigments have found progressively wider applications in several fields. Their chemical composition has been optimized to obtain the best physical properties, but is not usually disclosed by the manufacturers. Even the other class of luminescent pigments, namely the phosphorescent ones, is now produced industrially. The peculiar optical properties of these pigments have attracted more and more the attention of famous artists since the middle of the last century. The Italian Black Light Art movement exploits the possibility of conveying different aesthetical messages depending on the kind of radiation (UV or visible) with which the artwork is illuminated. In the present work, a non-invasive in-situ investigation based on Raman, fluorescence, and visible-reflectance spectroscopies was performed on a series of Black Light Art paintings exhibited in Milan (Italy) in 2017, succeeding in the identification of the materials used by the artists. In particular, the use of both fluorescent and phosphorescent pigments, alone or combined with conventional synthetic organic pigments, has been recognized.

A 3D modeling workflow to map ultraviolet- and visible-induced luminescent materials on ancient polychrome artifacts

A 2D visualization approach for ensembles of images comprising RGB and luminescence photographs is traditionally used for studying the structure and chemistry of 3D polychrome ancient objects. Here, a method is presented for visualizing material composition of an object in 3D, providing artistic production information, conservation history, and virtual enhancement of faded, concealed and weathered decoration. The approach is based on the fusion of 2D luminescence forensic photographs with RGB images acquired using principles of structure-from-motion photogrammetry to build 3D models. Its potential is demonstrated in a study of a polychrome Hellenistic terracotta funerary head vase, in which three models of the vase were produced: (1) photorealistic; (2) visible-induced luminescence, mapping Egyptian blue and madder lake; and (3) ultraviolet-induced visible luminescence, revealing surface condition and previous conservation interventions. The workflow advances current approaches in modeling chemical signatures of luminescent pigments, extending their traditional 2D viewing platform into 3D.

Study on the formation of solid solution when synthesizing manganese-doped zinc alkaline earth silicate luminescent pigment by impregnation - precipitation method

This work refers to the research results on the ability to form solid solution when synthesizing the manganese-doped zinc alkaline earth silicate luminescent pigment in the form of Zn1,97-xMxMn0,03SiO4 (M is magnesium, calcium, strontium and barium) by the precipitation-impregnation method. The samples were characterized by thermal analysis, X-ray diffraction spectroscopy, luminescence under UV lamp and luminescence spectroscopy at 254 nm excitation wavelength. The results show that when heating the corresponding precipitated precursor at 900 oC for 45 minutes, in general, the ability to form solid solutions decreases when the size difference of alkaline earth metal ions with Zn2+ ions increases. Specifically, Mg2+ ions create single-phase solid solutions with x varying from 0 to 1. Ca2+ ions form solid solutions when x changes from 0 to 0.6. When x = 0.8; in addition to the main phase Zn2SiO4, it also creates Ca2ZnSi2O7 with tetragonal structure. The Ba2+ ions form solid solutions when x varies from 0 to 0.5. When x = 0.6, in addition to Zn2SiO4, there is also BaZn2Si2O7. The Sr2+ ions form a solid solution when x = 0.2. For samples with x ≥ 0.4, in addition to Zn2SiO4, there is also Sr2ZnSi2O7; SrZn2Si2O7/SrO.2ZnO.2SiO2. The single - phase solid solutions all have the rhombo H structure of Zn2SiO4, and all have higher luminescence than the multiphase samples. The sample with the largest x, the sample Zn0,97Mg1,0Mn0,03SiO4 had the highest luminescence, emitting green light corresponding to the wavelength of 525 nm when excited by 254 nm UV light.

Identification of CaCuSi4O10 (Egyptian blue) in the “Birch. Spring” painting by Robert Falk (1907) using photoluminescence

We have detected Egyptian blue pigment in the paint layer of the "Birch. Spring" painting by Robert Falk (1907); we have also found this pigment in the paints of the sketch drawn on the canvas back side. This is probably the first discovery of Egyptian blue in a 20th century work of art. We have analyzed a modern commercial Egyptian blue pigment (Kremer) and found it to be suitable as a standard for photoluminescence spectral analysis. The characteristic photoluminescence band of CaCuSi$_4$O$_{10}$ reaches maximum at the wavelength of about 910 nm. The luminescence is efficiently excited by incoherent green or blue light. The study demonstrates that the photoluminescence spectral micro analysis using excitation by incoherent light can be efficiently used for the identification of luminescent pigments in paint layers of artworks.

Inorganic luminescent pigments

Abstract Inorganic luminescent pigments (luminescent materials, luminophores, phosphors) as synthetically generated crystalline compositions absorb energy followed by emission of light with lower energy, respectively, longer wavelengths. The light emission occurs often in the visible spectral range. External energy is necessary to enable luminescent materials to generate light. Luminescent pigments are divided into fluorescent and phosphorescent pigments. This classification goes back to different energy transitions. Emission based on allowed optical transitions, with decay times in the order of µs or faster is defined as fluorescence. Emission with longer decay times is called phosphorescence. The occurrence of fluorescence or phosphorescence as well as the decay time depend on structure and composition of a specific luminophore. There are four luminescence mechanisms discussed for inorganic luminescent materials: center luminescence, charge-transfer luminescence, donor–acceptor pair luminescence, and long-afterglow phosphorescence. The emission of luminescent light can have its origin in different excitation mechanisms such as optical excitation (UV radiation or even visible light), high-voltage or low-voltage electroluminescence and excitation with high energy particles (X-rays, γ-rays). Inorganic luminescent pigments are used mainly in fluorescent lamps, cathode-ray tubes, projection television tubes, plasma display panels, light-emitting diodes (LEDs) and for X-ray and γ-ray detection. The pigment particles are dispersed for the applications in specific binder systems. They are applied in form of thin layers and by means of luminophore/solvent suspensions, containing adhesive agents, on a substrate.

Colored pigments

Abstract Colored pigments are inorganic or organic colorants, which are insoluble in the application media, where they are incorporated for the purpose of coloration. Their optical action is based on the selective light absorption together with light scattering. Colored pigments do not include pigments that are colored but which are not used because of their colored character but because of properties such as corrosion protection and magnetism. White pigments, black pigments, grey pigments, effect pigments, and luminescent pigments also do not belong to the colored pigments.

Confocal Fluorescence Microscopy as a Tool for Assessment of Photoluminescent Pigments Print on Polyester Fabric

The size and distribution of the photoluminescent pigment particles within the selected binder may affect the quality and appearance of the final print significantly. Yet, the techniques for precise evaluation of size distri¬bution of the pigment particles within a 3D fabric space are rather limited, based on their intrinsic fluorescent properties. The presented work demonstrates a simple screen-printing process for the sustainable application of three different types of commercial fluorescent pigments on polyester (PES) fabric, using polydimethylsiloxane (PDMS) as a binder. A comprehensive toolbox was used to compare and study different commercial photo¬luminescent pigments and their corresponding prints, by means of size distribution and concentration effect of emission intensity, including Confocal Fluorescence Microscopy (CFM) and Scanning Electron Microscopy (SEM) in combination with complementary spectroscopic techniques, i.e. Energy Dispersive X-ray Spectroscopy (EDX) and Ultraviolet-visible (UV-vis) spectroscopy. The focus is on CFM utilised as a non-destructive tool, used for the evaluation of photoluminescent pigments´ spatial distribution within printing pastes, as well as on/within the PES fabrics.

Оптические свойства 2,2’-дигидрокси-1,1’-нафталазина

Люминесцентные пигменты способны поглощать энергию естественных или искусственных источников света и выделять ее в форме видимого свечения в темноте. По химической природе люминофоры подразделяются на неорганические и органические. В отличие от неорганических люминофоров, светящихся только в твердом состоянии, органические люминофоры, в большинстве случаев, люминесцируют в различных агрегатных состояниях. Это различие обусловлено тем, что свечение связано с определенным строением кристаллической решетки, в которую входят активирующие добавки. После разрушения кристаллической решетки люминесценция исчезает. В случае органических люминофоров люминесцируют отдельные молекулы. Хорошо известно, что квантовый выход флуоресценции органических хромофоров обычно уменьшается в твердом состоянии, хотя они имеют высокий квантовый выход свечения в растворе.

Оптические свойства 2,2’-дигидрокси-1,1’-нафталазина

Luminescent pigments are able to absorb the energy of natural or artificial light sources and release it in the form of a visible glow in the dark. By chemical nature, phosphors are divided into inorganic and organic. Unlike inorganic luminophors, which glow only in the solid state, organic luminophors, in most cases, luminesce in various aggregate states. This difference is due to the fact that the glow is associated with a certain structure of the crystal lattice, which includes activating additives. After the destruction of the crystal lattice, luminescence disappears. In the case of organic luminophors, individual molecules luminesce. It is well known that the quantum yield of fluorescence of organic chromophores usually decreases in the solid state, although they have a high quantum yield of luminescence in solution.

Vegetable oils based UV-luminescent ink for screen printed anti-counterfeiting marking

In this work, a new anti-counterfeiting feature relying on invisible fluorescent pattern printed with vegetable-oil-based inks were reported. Soybean oil (SBO) and tung oil (TO) were used as all the vehicle materials with low-cost, VOC-free and renewable characteristics, and grinding with ultraviolet luminescent pigment and specific additives to obtain the synthesized ink. A series of physical properties of the inks with different mixing ratios were investigated. It was found that the formulations of 75 wt.% vehicles, 15 wt.% fluorescent pigments, 3 wt.% fluorescent whitening agents, 2 wt.% ink driers, 2 wt.% defoaming agents, 2 wt.% dispersing agents, and 1 wt.% antioxidants (BHT) showed highly performance on the stability, dryness, acid and alkali resistance. More importantly, the desired-viscosity (3000 ∼ 4000 mPa·s) inks were applied on the coated paper to obtain invisible geometric designs by screen printing technology, which showed bright blue fluorescence under 365 nm UV-light irradiation. Therefore, by applying the vegetable-oil-based ink in fluorescent security marking, the information in images could be protected in such a simple and green way that was convenient to observe and verify. It exhibited the wide potentials on the anti-counterfeiting printing and commodity packaging.

NiWO4 powders prepared via polymeric precursor method for application as ceramic luminescent pigments

NiWO4 was prepared using the polymeric precursor method and studied in terms of physical and chemical properties to verify its stability for industrial applications as pigments. The characterization was accomplished using thermal analyses, X-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence (PL) and UV–Vis spectroscopies, colorimetric coordinates, and Raman spectra. Increasing the temperature, successive exothermic reactions were observed and they are related with thermal decomposition of the organic compound. The stability was reached at ~700 °C. The material is verified to become completely free of second phase at ~800 °C. The end NiWO4 powders showed an intense charge transfer (CT)-related tail centered in the ultraviolet region, resulting in a yellow product. In addition, the powders exhibited broad excitation band and broad deep blue–green emission band, which were enhanced with increasing powders’ crystallinity.

Photoluminescence imaging of modern paintings: there is plenty of information at the microsecond timescale

Luminescent emission from artworks is usually employed by conservators and restorers as a visual and quick method for assessing the presence of protective varnishes and restoration materials, as well to evaluate the painting conservation status. Besides these mere qualitative observations, the analysis of both spectral and temporal features of the optical emission from a painting can provide useful information about the pictorial materials employed by artists. In this work we propose Multi-spectral Time-gated Photoluminescence Imaging to differentiate between luminescent pigments, on the basis of their different emission spectra and lifetimes. We present the application of the approach on two modern paintings from Fondazione Maimeri Private Collection, on which we differently probe the fast-living emission from surface varnish and lake pigment and the long-living trap state emissions from different semiconductor pigments. With the aid of the time gated detection we record the trap state emission of semiconductor pigments, otherwise covered by the intense fluorescence of the organic compound. The method is complemented with Macro-XRF analysis, to confirm the nature of the identified inorganic pigments, as zinc white and cadmium yellow. The protocol represents a rapid, non-invasive and reliable way for assessing the identification of luminescent pigments and their spatial distribution.

Synthesis and optical properties of efficient orange emitting GdB5O9:Sm3+ phosphors

In the present study the single phase polycrystalline GdB5O9:Sm3+ target materials were prepared by aqueous sol–gel method. The powder X-ray diffraction was used to monitor the crystal phase formation. Samples doped with 1 and 2.5% Sm3+ showed bright orange–red emission upon excitation with near-UV radiation. The emission possessed excellent colour saturation and good colour coordinate stability. Emission spectra recorded at different temperatures showed that the thermal quenching activation energy is around 35 meV. Moreover, the temperature dependent photoluminescence (PL) decay measurements revealed that PL lifetime values barely change in the temperature range of 77–500 K showing that the external quantum yield decreases much faster than the internal one. The highest quantum yield of ca. 60% was obtained for the sample doped with 1% Sm3+ ions. Further increase of Sm3+ content resulted in concentration quenching as a consequence of increased probability of cross-relaxation. Rather high quantum yield and well resolved sharp emission lines makes these phosphors suitable for application as an orange–red component in near-UV LEDs, on the one hand, and the luminescent security pigments, on the other hand.

Characterization of commercial luminescent powders

Luminescent powders have attracted the attention of scientists, boosting properties as specific as mechanoluminescence, thermoluminescence, and photoluminescence among other characteristics. The uses of these properties have grown exponentially from recreational uses in luminescent paints, toys, plasticines, and other industry-specific uses, such as crack sensors, flaw detectors, and radiation meters in medicine. These uses have grown proportionately with their more economical and efficient manufacturing methods and processes, generating more economical fluorescent pigments. This paper characterizes and compares a low-cost luminescent pigment with those reported by different authors, its chemical composition was characterized by X-ray energy dispersion spectrometry, X-ray fluorescence, and Raman spectroscopy. Its morphology was analyzed by scanning electron microscopy, and its particle size with a laser meter its structural condition with X-ray diffraction, the powder obtained was presented as a strontium alumínate doped with divalent europium and trivalent dysprosium (Al2O4: Eu2+, Dy3+) one of the most efficient powders around persistence and luminance intensity at present. The structure of the strontium aluminate was determined by the main peaks of the diffractogram showing a monoclinic system. The elemental composition helped to determine the doping of the strontium aluminate corroborating them with the obtained with Raman spectroscopy and the scanning electron microscopy images.

In-situ technical study of modern paintings - Part 2: Imaging and spectroscopic analysis of zinc white in paintings from 1889 to 1940 by Alessandro Milesi (1856-1945).

We present a multi-analytical in situ non-invasive study of a series of emblematic paintings by Alessandro Milesi (1856-1945) from the collection of the International Gallery of Modern Art Ca' Pesaro in Venice. Eight paintings dated from 1897 to 1910 were studied with imaging and spectroscopic techniques. White pigments were characterized by a combination of X-ray fluorescence spectroscopy which traced the presence of zinc-based pigments in Milesi's paintings, Raman Spectroscopy, Laser Induced Fluorescence (LIF) Spectroscopy and Time-resolved Luminescence Imaging. Time-resolved analysis of luminescence emissions revealed the nanosecond emission from organic compounds and the slower emission from the luminescent inorganic pigment Zinc Oxide that varied between 1.1 and 1.6 microseconds. In this work, data regarding the distribution of luminescent pigments was acquired with a time-gated imaging detector. Furthermore, differences in emission decay kinetics recorded from different paintings can be ascribed to different paint formulations or origins of the Zinc white in paint.

Combined photoluminescence and Raman microscopy for the identification of modern pigments: explanatory examples on cross-sections from Russian avant-garde paintings

In conservation science, the identification of painting materials is fundamental for the study of artists’ palettes, for dating and for understanding on-going degradation phenomena. For these purposes, the study of stratigraphic micro-samples provides unique information on the complex heterogeneity of the pictorial artworks. In this context, we propose a combined-microscopy approach based on the application of time-resolved photoluminescence (TRPL) micro-imaging and micro-Raman spectroscopy. The TRPL device is based on pulsed laser excitation (excitation wavelength = 355 nm, 1 ns pulse width) and time-gated detection, and it is suitable for the detection of photoluminescent emissions with lifetime from few nanoseconds to hundreds of microseconds. In this work, the technique is beneficially applied for identifying different luminescent semiconductor and mineral pigments, on the basis of their spectral and decay kinetic emission properties. The spatial heterogeneities, detected in the micro-sample, are investigated with Raman spectroscopy (785-nm in CW mode) for a further identification of the paint composition on basis of the molecular vibrations associated with the crystal structure. The effectiveness and limits of the proposed combined method is discussed through analysis of a corpus of stratigraphic micro-samples from Russian Avant-garde modern paintings. In the selected samples, the method allows the identification of modern inorganic pigments such as cadmium-based pigments, zinc white, titanium white, chrome yellow, ultramarine and cinnabar.

New insight into printable europium-doped yttrium borate luminescent pigment for security ink applications

Counterfeiting is a major current concern throughout the world, and recent technological advances have made counterfeiting of sophisticated products easy. It is therefore essential to develop further means to combat counterfeiting, including the use of new luminescent materials for security inks. Here, we report the synthesis by a sol–gel technique of a highly luminescent YBO3:Eu3+ phosphor, which can be scaled up to 1 kg amounts in single batches. This phosphor is both structurally and chemically stable and has a particle size of 110 ± 10 nm. It emits strongly at a wavelength of 591 nm (orange–red) upon excitation at a wavelength of 245 nm and is also excitable by wavelengths in the 280–480 nm range. The structural and microstructural characteristics and the photoluminescence behavior of the phosphor were characterized by SEM, TEM/HRTEM, and fluorescent spectroscopy, respectively. The phosphor was used with a commercially available PVC gold medium to produce a luminescent security ink for printing security codes, and the results were investigated by photoluminescence mapping instruments. TheYBO3: Eu3+ phosphor-based security ink described here provides a low-cost material for use in printing security codes that are easy to detect but difficult to counterfeit.

Single excitable dual emissive novel luminescent pigment to generate advanced security features for anti-counterfeiting applications

A novel concept for a single excitable dual emissive luminescent pigment has been demonstrated, which emit red and green colors with the single excitation wavelength of a 254 nm UV lamp switched ON and after switched OFF, respectively.