This erratum corrects a mistake in Opt. Express32, 9042 (2024)10.1364/OE.510888.

In this paper, we attempt to make ink talk in historical documents by extracting visual information materialized by the trace left on the support, thus addressing two tasks: anomaly localization in hand-pressed ornaments in the framework of Rey’s publishing and source separation in Queen Marie-Antoinette’s correspondence where the letters are hidden with loops. Anomalies are defined as changes in shape with respect to the original skeleton. A logarithmic image processing is applied for both anomaly visualization and image generation by mixing letters with loops to train a source separation model, respectively. Such an image generation is tackled from spectral modeling in halftoning.

5-ALA-induced PpIX fluorescence has been proposed to improve the identification of benign from malignant brain tumors during surgery. Based on simulation, this work helps at determining the best operating measurement conditions for estimating PpIX concentration. (c) 2025 The Author(s)

Laser-induced printing is an affordable, fast, and non-contact method for creating high-resolution images. Using plasmonic nanocomposite thin films, it enables the printing of color images with visual effects. However, the color gamut of these images is limited compared to inkjet printing. Therefore, it is necessary to optimize this gamut in order to print images that contain the widest range of colors and are closest to the original. There is currently no model to directly infer the color from the laser parameters used. Instead, a lookup table is required to associate these parameters with actual colors. Since colors vary depending on the type of sample used, it is essential to have a reliable method to quickly determine the laser parameters that produce the best colors. Two methods have been implemented to optimize the color gamut: a genetic algorithm approach to find colors that both increase both hue diversity and saturation, and a Bayesian approach to increase the size of the color gamut. Gamut mapping is then used to print the image, and the quality of the final printed image is assessed using metrics obtained from a psychophysical study.

Le traitement par laser de films photosensibles contenant des nanoparticules métalliques permet de produire de manière contrôlée des centaines de milliers de nanostructures, affichant chacune une palette de couleurs variées suivant l’angle de vue et d’éclairage. Un choix judicieux parmi ces nanostructures permet d’afficher différentes images au même endroit d’un support, une révolution dans le domaine de l’impression qui intéresse l’industrie des documents sécurisés.

Spectroscopy is a popular technique for identifying and quantifying fluorophores in fluorescent materials. However, quantifying the fluorophore of interest can be challenging when the material also contains other fluorophores (baseline), particularly if the emission spectrum of the baseline is not well-defined and overlaps with that of the fluorophore of interest. In this work, we propose a method that is free from any prior assumptions about the baseline by utilizing fluorescence signals at multiple excitation wavelengths. Despite the nonlinearity of the model, a closed-form expression of the least squares estimator is also derived. To evaluate our method, we consider the practical case of estimating the contributions of two forms of protoporphyrin IX (PpIX) in a fluorescence signal. This fluorophore of interest is commonly utilized in neuro-oncology operating rooms to distinguish the boundary between healthy and tumor tissue in a type of brain tumor known as glioma. Using a digital phantom calibrated with clinical and experimental data, we demonstrate that our method is more robust than current state-of-the-art methods for classifying pathological status, particularly when applied to images of simulated clinical gliomas. To account for the high variability in the baseline, we are examining various scenarios and their corresponding outcomes. In particular, it maintains the ability to distinguish between healthy and tumor tissue with an accuracy of up to 87%, while the ability of existing methods drops near 0%.

Optical characterization and appearance prediction of translucent materials are required in many fields of engineering such as computer graphics, dental restorations or 3D printing technologies. In the case of strongly scattering materials, flux transfer models like the Kubelka-Munk model (2-flux) or the Maheu’s 4-flux model have been successfully used to this aim for decades. However, they lead to inaccurate prediction of the color variations of translucent objects of different thicknesses. Indeed, as they rely on the assumption of lambertian fluxes at any depth within the material, they fail to model the internal reflectance at the interfaces, penalizing the accuracy of the optical parameter extraction. The aim of this paper is to investigate the impact of translucency on light angular distribution and corresponding internal reflectances by the mean of the radiative transfer equation, which describes more rigorously the impact of scattering on light propagation. It turns out that the light angular distribution at the bordering interfaces is often far from being lambertian, and that the internal reflectance may vary significantly according to the layer’s thickness, refractive index, scattering and absorption coefficients and scattering anisotropy. This work enables to better understand the impact of scattering within a translucent layer and also invites to revisit the well-known Saunderson correction used in 2- or 4-flux models.

Objective: To assess the prediction accuracy of recent optical and numerical models for the spectral reflectance and color of monolithic samples of dental materials with different thicknesses. Methods: Samples of dental resin composites of Aura Easy Flow (Ae1, Ae3 and Ae4 shades) and Estelite Universal Flow Super Low (A1, A2, A3, A3.5, A4 and A5 shades) with thicknesses between 0.3 and 1.8 mm, as well as Estelite Universal Flow Medium (A2, A3, OA2 and OA3 shades) with thicknesses between 0.4 and 2.0 mm, were used. Spectral reflectance and transmittance factors of all samples were measured using a X-Rite Color i7 spectrophotometer. Four analytical optical models (2 two-flux models and 2 four-flux models) and two numerical models (PCA-based and L*a*b*-based) were implemented to predict spectral reflectance of all samples and then convert them into CIE-L*a*b* color coordinates (D65 illuminant, 2°Observer). The CIEDE2000 total color difference formula (ΔE00) between predicted and measured colors, and the corresponding 50:50% acceptability and perceptibility thresholds (AT00 and PT00) were used for performance assessment. Results: The best performing optical model was the four-flux model RTE-4F-RT, with an average ΔE00 = 0.72 over all samples, 94.87% of the differences below AT00 and 65.38% below PT00. The best performing numerical model was L*a*b*-PCHIP (interpolation mode), with an average ΔE00 = 0.48, and 100% and 79.69% of the differences below AT00 and PT00, respectively. Significance: Both optical and numerical models offer comparable color prediction accuracy, offering flexibility in model choice. These results help guide decision-making on prediction methods by clarifying their strengths and limitations.

Enregistrer l’apparence des oeuvres d’art moderne est un enjeu majeur car celle-ci évolue dans le temps, et un défi qui exige le développement de nouveaux principes d’imagerie ou d’adapter les principes existants car la couleur n’est pas le seul attribut visuel exploité par les peintres : le brillant, le relief, la texture jouent aussi un rôle essentiel, en témoigne l’analyse en mars 2023 d’une toile de Pierre Soulages.

The fluorescence property of human teeth under UV light has long been studied in dentistry and is now used in the diagnosis of anomalies, such as dental decays.Its role in the appearance of teeth and dental restorations has also been demonstrated, and fluorescence, even under daylight, may sensibly modify the appearance of dental restorations.As such, dental resin composites used in aesthetic restorative dentistry include fluorescent agents which aim to imitate the natural fluorescence of teeth.While several studies have measured the fluorescence properties of dental biomaterials and a few other studies have focused on predicting the color of samples, the influence of fluorescence on color prediction models remains to be assessed.In this paper, we propose a prediction model for the spectral emission of slices of a dental biomaterial as a function of their thickness, in reflection and in transmission modes, with the aim of improving color prediction models for these materials.

Coating a printed surface with a smooth transparent layer can modify its color. This is due to light interreflections within the coating layer which produce a halo-shaped point spread function. The change of color is related to the coating thickness and the halftone screening used for printing. Thanks to an optical model able to predict the spectral reflectance of the coated print from the one of the non-coated print, we propose to study the impact of the halftone pattern (shape and profile) on the color change caused by the coating layer. It was found that line halftone patterns with a crenel profile induces the strongest changes of color. This is therefore the pattern that we use for an innovative application of this phenomenon: revealing a binary image by adding or removing a coating layer on the print that is originally uniform.

This paper investigates the optical phenomenon responsible for the colored shine that sometimes appears at the surface of ink layers in the specular direction, often called "bronzing" or "gloss differential." The prediction of this shine effect relies on the Fresnel formulas of the air/ink interface. The complex refractive index of the ink must therefore be determined, which is made difficult because of the roughness of inked printing supports. We propose a generic method that can be applied to any ink, without any prior knowledge of its composition or the printing substrate. In order to reduce light scattering, a solid colored area is printed with the studied ink on a glossy paper previously printed with black ink. By ellipsometry, we determine the effective refractive index of the sample. The intrinsic complex refractive index of the ink can then be extracted by modeling the optical response of the inked surface with a set of Gaussian oscillators, among which one of them approaches residual scattering. With this data, we could proceed to a fine colorimetric analysis of the bronzing color of some cyan, magenta, and yellow inks. In particular, we show that this gloss color is slightly shifted from the complementary of the ink's usual color in diffuse reflection.

Stacked glass plates have discreetly accompanied the understanding of light since the origins of modern optics. They were studied by Bouguer, Lambert, Brewster, Arago, Stokes, Rayleigh, and many others, whose successive works progressively refined the predictive formulas of the reflectance and transmittance of piles of glass plates as a function of the number of plates and the angle of incidence by considering the decay of light flux by absorption, the multiple reflections between plates, the change in the degrees of polarization, and the possible interferential effects. Through this history of ideas about the optical properties of piles of glass plates, up to the mathematical formalisms from only a few years ago, we show that these successive works, and their subsequent errors and corrections, are inseparable from the evolution of the quality of the glass available each time, in particular its absorptance and its transparency, which strongly influence the quantities and the degree of polarization of the reflected and transmitted beams.

Protoporphyrin IX (PpIX) is a fluorophore being currently used to localize tumoral tissues. The tissue is usually excited at one wavelength, e.g., 405 nm, and the fluorescence signal is used to estimate the amount of PpIX during surgery. However, other fluorophores (baseline) whose emission spectra are close to the one of PpIX impair the quantification of PpIX and consequently the tissue pathological status classification. An efficient multi-excitation wavelengths method, free from any a priori on the baseline shape, has been proposed to cope with this issue. This method requires decorrelated measurements in the range of PpIX emission at multiple excitation wavelengths. We investigated the influence of the source bandwith on this decorelation by comparing two experimental setups using either LED or laser diode sources. The experimental setup using laser diodes for excitation increases the decorrelation by 35.3 % compared to the one using LEDs in the spectral range of PpIX emission.

Objective: To evaluate the prediction accuracy of the Kubelka-Munk Reflectance Theory and other more innovative two-flux and four-flux models for predicting the reflectance and transmittance factors of two flowable dental resin composites of various thicknesses within clinically acceptable color difference.Methods: Cylindrical samples of Aura Easy Flow resin composite (Ae1, Ae2, Ae3, Ae4 shades) and Estelite Universal Flow SuperLow resin composite (A1, A2, A3, A3.5, A4, A5 shades) were prepared with thicknesses ranging from 0.3 mm to 1.8 mm. Their reflectance and transmittance factors were measured with a spectrophotometer based on an integrating sphere, and were also predicted by 3 different two-flux models and 2 different four-flux models. The accuracy of reflectance and transmittance factor predictions was assessed using the CIEDE2000 color distance metric and 50:50% acceptability and perceptibility threshold criteria. Results: Eymard's four-flux model is found to be the most accurate for predicting the spectral reflectance and transmittance factors, with 85% (resp. 100%) of all color deviations below the acceptability threshold, and below the perceptibility threshold for 40% (resp. 57%) of the samples with thickness ranging from 0.3 to 1.8 mm in reflectance (resp. transmittance) mode. The Kubelka-Munk Reflectance Theory is found to be the least accurate model for predicting the spectral reflectance and transmittance factors of dental resin of thickness ranging from 0.3 to 1.8 mm. Significance: Eymard's four-flux model enables to predict the color of slices of dental materials within acceptable color differences. Eymard's four-flux model's optical parameters thus describe light-matter interactions in dental materials more accurately than state of the art Kubelka-Munk Reflectance Theory.

When a clear layer is coated on a diffusing background, light is reflected multiple times within the transparent layer between the background and the air-layer interface. If the background is lit in one point, the angular distribution of the scattered light and Fresnel's angular reflectance of the interface induce a specific irradiance pattern on the diffuser: a ring-like halo. In the case where the background is not homogenously colored, e.g. a half-tone print, the multiple reflection process induces multiple con-volutions between the ring-like halo and the halftone pattern, which increases the probability for light to meet differently col-ored areas of the background and thus induces a color change of the print. This phenomenon, recently studied in the case of a smooth layer surface (glossy finishing) is extended here to rough surface layer (matte finishing) in order to see the impact of the surface roughness on the ring-like halo, and thereby on the print color change. A microfacet-based bi-directional reflectance dis-tribution function (BRDF) model is used to predict the irradi-ance pattern on the background, and physical experiments have been carried out for verification. They show that the irradiance pattern in the case of a rough surface is still a ring-like halo, and that the print color change is similar to the one observed with a smooth interface, by discarding the in-surface reflections which can induce additional color change.

When one seeks to characterize the appearance of art paint-ings, color is the visual attribute that usually focuses most atten-tion: not only does color predominate in the reading of the pic-torial work, but it is also the attribute that we best know how to evaluate scientifically, thanks to spectrophotometers or imaging systems that have become portable and affordable, and thanks to the CIE color appearance models that allow us to convert the measured physical data into quantified visual values. However, for some modern paintings, the expression of the painter relies at least as much on gloss as on color; Pierre Soulages (1919-2022) is an exemplary case. This complicates considerably the characterization of the appearance of the paintings because the scientific definition of gloss, its link with measurable light quan-tities and the measurement of these light quantities over a whole painting are much less established than for color. This paper re-ports on the knowledge, challenges and difficulties of character-izing the gloss of painted works, by outlining the track of an im-aging system to achieve this.

This paper presents MARMIT-2, a radiative transfer model that predicts the spectral reflectance of soils in the solar domain (0.4-2.5 mu m) as a function of their surface moisture. This is an improved version of MARMIT (multilayer radiative transfer model of soil reflectance) that represents a wet soil as a dry soil topped with a thin layer of liquid water. The changes brought in this article concern the mixing of the spectral reflectance of the dry and wet soil areas, the transmission of diffuse light in the water layer, and the inclusion of soil particles in the water layer. Wet soil reflectance is now expressed in terms of dry soil reflectance and three free parameters: the thickness of the water layer, the surface fraction of the wet soil, and a new parameter, the volume fraction of soil particles in the water layer. With more accurate physical modeling, MARMIT-2 simulates soil spectral reflectance with better accuracy than MARMIT. In particular, the fit of the soil reflectance spectra is much better for high water contents, both in the visible range (0.4-0.7 mu m) and in the water absorption bands around 1.45 mu m and 1.95 mu m. The average root mean square error between measured and predicted reflectance obtained on a set of 225 soil samples is about 0.8% with MARMIT-2 versus 1.8% with MARMIT.