The spectral composition of the light that reaches any indoor work plane depends on the characteristics of the light sources and the spectral reflectances of the surrounding surfaces due to the multiple reflections experienced by the light rays along their paths from the source to the observation point. We show that in indoor spaces, the source and surface radiances must obey a definite self-consistent relationship derived from the fact that each illuminated surface point acts as a secondary source of light. It is then established that the spectral irradiance on any plane is linearly dependent on the spectral radiance of the light source. The explicit
integral form of this relationship provides a theoretical framework for a quantitative description of the surface effects. Additionally, under very general assump-tions, we show that the spectral irradiance can be computed from the spectral flux of the source through a simple multiplication by a wavelength-dependent function. This function, with units of inverse surface (1/m2), provides a convenient way for evaluating the effects that arbitrary changes in the source spectrum will produce on the spectral irradiance at the indoor point under study.
The widespread use of self-luminous devices at nighttime (cell-phones, computers, and tablets) raises some reasonable concerns regarding their effects on human physiology. Light at night is a known circadian disruptor, particularly at short visible wavelengths, and it seems advisable to have practical tools for tailoring the spectral radiance of these displays. We analyse two possible strategies to achieve this goal, using hardware filters or software applications. Overall, software applications seem to offer, at the present time, the best trade-offs for controlling the light spectra emitted by existing devices. We submit that such tools should be included as a standard feature on any self-luminous device and that their default settings should be established according to the best available knowledge on the circadian effects of light.
Se ha medido la radiancia espectral de una pantalla de visualización, tipo tableta, con un conjunto filtros comerciales de protección frente a la luz azul y también sin filtros. Se ha determinado el efecto de los filtros en cuanto a su transmitancia en la banda azul del espectro. De los resultados obtenidos se infieren valores de transmitancia superiores al 70% en esa banda.
Fabric openness factor (OF) is the fraction of the web area that is uncovered by yarns. OF is a critical feature regarding the end-use performance of the fabric and should be accurately assessed. However, digital OF estimates yielded by image binarization algorithms differ among them depending on the criteria used, mainly due to ill-defined boundaries, thus precluding a straightforward assessment of the actual fabric OF value. Lacking any standard to compare actual OF values with measured OF values, we addressed the validation procedure of the digital assessment method from visual OF estimates. OF of 81 distinct fabric samples was evaluated from digital images by a panel of 18 observers using visual binarization technique. Following the psychophysical models of Fechner and Stevens, these visual estimates were correlated with digital estimates yielded by several binarization algorithms. Stevens’ psychophysical model and an automatic binarization algorithm developed by us scored the highest correlation.
Tachó, A.; Mitja, C.; Martínez-Navarro, B.; Escofet, J.; Rallo, M. Iberoamerican Optics Meeting and Latin American Meeting on Optics, Lasers, and Applications. p. 87858V-1-87858V-8 DOI: 10.1117/12.2027001 Presentation's date: 2013-07-25 Presentation of work at congresses
Many digital image applications like digitization of cultural heritage for preservation purposes operate with compressed
files in one or more image observing steps. For this kind of applications JPEG compression is one of the most widely
used. Compression level, final file size and quality loss are parameters that must be managed optimally. Although this
loss can be monitored by means of objective image quality measurements, the real challenge is to know how it can be
related with the perceived image quality by observers. A pictorial image has been degraded by two different procedures.
The first, applying different levels of low pass filtering by convolving the image with progressively broad Gauss kernels.
The second, saving the original file to a series of JPEG compression levels. In both cases, the objective image quality
measurement is done by analysis of the image power spectrum. In order to obtain a measure of the perceived image
quality, both series of degraded images are displayed on a computer screen organized in random pairs. The observers are
compelled to choose the best image of each pair. Finally, a ranking is established applying Thurstone scaling method.
Results obtained by both measurements are compared between them and with other objective measurement method as
the Slanted Edge Test.
Martínez-Navarro, B.; Mitja, C.; Escofet, J. Iberoamerican Optics Meeting and Latin American Meeting on Optics, Lasers, and Applications. p. 8785BD-1-8785BD-11 DOI: 10.1117/12.2026999 Presentation's date: 2013-07-20 Presentation of work at congresses
The digitization of both volumetric and flat objects is the nowadays-preferred method in order to preserve cultural heritage items. High quality digital files obtained from photographic plates, films and prints, paintings, drawings, gravures, fabrics and sculptures, allows not only for a wider diffusion and on line transmission, but also for the preservation of the original items from future handling. Early digitization procedures used scanners for flat opaque or translucent objects and camera only for volumetric or flat highly texturized materials. The technical obsolescence of the high-end scanners and the improvement achieved by professional cameras has result in a wide use of cameras with digital back to digitize any kind of cultural heritage item. Since the lens, the digital back, the software controlling the camera and the digital image processing provide a wide range of possibilities, there is necessary to standardize the methods used in the reproduction work leading to preserve as high as possible the original item properties. This work presents an overview about methods used for camera system characterization, as well as the best procedures in order to identify and counteract the effect of the lens residual aberrations, sensor aliasing, image illumination, color management and image optimization by means of parametric image processing. As a corollary, the work shows some examples of reproduction workflow applied to the digitization of valuable art pieces and glass plate photographic black and white negatives.
Los tejidos textiles transparentes permiten el paso de luz e información de baja frecuencia a través de ellos. A plena luz del día un observador situado en el interior de una habitación, al mirar a través de un tejido transparente los objetos situados en el exterior, percibe una imagen del objeto original mucho menos nítida y, en algunos casos, con ruido añadido. En este trabajo se presenta un método objetivo de medida de la nitidez de la imagen cuando es vista a través de un tejido transparente. Se reemplaza el sistema visual humano por una cámara fotográfica y se fotografía la imagen de un Test de Borde Inclinado a través del tejido. La función de modulación de la transferencia del sistema (MTF) caracteriza la imagen observable a través del tejido textil.
La digitalización de documentos de imagen es un importante campo de trabajo que afecta tanto a individuales como instituciones. Los métodos y formatos de archivo utilizados en la digitalización de colecciones de imágenes se deciden como un equilibrio entre presupuesto, volumen de archivo y calidad de imagen, aunque no necesariamente en este orden. Los formatos estandarizados más utilizados en estos ámbitos son el TIFF y el JPEG, ambos con diversas opciones de compresión. El presente trabajo evalúa la calidad resultante en versiones de una misma imagen con diversas relaciones de compresión mediante la medida de la MTF por los métodos del borde inclinado y de las hojas muertas (Dead Leaves), relacionados respectivamente con los bordes y las texturas.
In this thesis we applied, in a complementary way, techniques of digital image processing and reflectance spectroscopy to perform cover factor (CF) measurements in monochrome openwork fabrics. To evaluate the results provided by both techniques, we used the average CF obtained by a group of 18 expert observers as a reference.
IMAGE PROCESSING PART: We developed a new, automatic and objective method for measuring both total and partial CF, and the width of the threads, from the digital image of the fabric. Two algorithms were developed for total and partial CF determinations. The image processing technique used to measure the total CF was the segmentation of the holes from the fabric threads. Conventional methods of segmentation did not lead to similar results than those of the total visual CF. Therefore, we designed a new algorithm (MSSQ) to measure the total CF of fabrics, independently of their degree of coverage. All the CF obtained in 81 typified textile samples were correlated with the CF obtained visually. The best fit to the Stevens' psychophysical law was achieved with the method MSSQ. These results were also consistent with the construction parameters of fabrics. The determination of partial CF in the warp and weft directions was performed by filtering in the Fourier space the warp and weft threads of the sample. For the design of these filters we applied the Radon transform to the power spectrum of the image of the fabric, in order to determine the direction of the warp and weft harmonics. The values of partial CF were consistent with the parameters that characterize the threads and were logical, expected and confirmed the robustness of the method. The values of the diameters of the threads were also consistent with the parameters of the yarn manufacturing and with the manual measurement performed on the image of the same fabric. This method is easily implementable in a laboratory of textile analysis. REFLECTANCE PART: We modelled the reflectance of a flat layer of fabric arranged on a support of known reflectance using the Kubelka-Munk theory. For this purpose we assumed that the optical behaviour of a layer of fabric could be determined by the superposition of two ideal elements: a neutral density filter with no thickness and with different values of transmittance depending on both the direction of the beam (input or output) and whether the beam passes through the covered part of the yarns or through the holes, plus a uniform and homogeneous layer of a material that meets the assumptions of the theory of Kubelka-Munk. The final model contained CF as a parameter. For a set of different samples of fabrics, the integrated spectral reflectance of a stack of increasing number of layers was experimentally measured with a spectroradiometer, up to stabilize the measure. The experimental reflectances were introduced in the generalized model to calculate the values of the parameters involved, particularly the CF. We solved the resulting system of 101 ¿ (m - 1) equations using the least squares method, where m is the total number of stacked layers of fabric, with 103 unknowns. This yielded a couple of estimates of CF. We established correlations between the measured CF (obtained as the solution of these equations) and the visual CF and analyzed the effect of perturbations in the reflectance measurements to explain the discrepancies between these two estimates of CF. Even if we observed problems of precision in one of the estimates and inaccuracy in the other, arising from an unknown parameter in the model, the intervention of the CF in the described model has been proven.
The amount of images produced to be viewed as soft copies on output displays are significantly increasing. This growing occurs at the expense of the images targeted to hard copy versions on paper or any other physical support. Even in the case of high quality hard copy production, people working in professional imaging uses different displays in selecting, editing, processing and showing images, from laptop screen to specialized high end displays. Then, the quality performance of these devices is crucial in the chain of decisions to be taken in image production. Metrics of this quality performance can help in the equipment acquisition. Different metrics and methods have been described to determine the quality performance of CRT and LCD computer displays in clinical area. One of most important metrics in this field is the device spatial frequency response obtained measuring the modulation transfer function (MTF). This work presents a comparison between the MTF of three different LCD displays, Apple MacBook Pro 15", Apple LED Cinema Display 24" and Apple iPhone4, measured by the white noise stimulus method, over vertical and horizontal directions. Additionally, different displays show particular pixels structure pattern. In order to identify this pixel structure, a set of high magnification images is taken from each display to be related with the respective vertical and horizontal MTF.
Mitja, C.; Bover, T.; Bigas, M.; Escofet, J. Proceedings of SPIE, the International Society for Optical Engineering Vol. 7723, p. 77230D-77230D-12 DOI: 10.1117/12.850051 Date of publication: 2010 Journal article
Current photographic still cameras, in the professional SLR step, are available in two basic sensor sizes, 16x24mm and 24x36mm and both of them can be used with the same or similar range of focal length lenses. Lens aperture determines resolving power and diffraction effects and indeed, MTF function. In order to preserve an acceptable image quality level, it must be taken into account that a high lens resolving power at larger apertures can be replicated by the sensor as a false response or aliasing, while the size of the Airy disc must be related with photo receptors pitch. Provided that a standardized metric of image quality is the system MTF, this work compares different lenses resolving power as a function of aperture with the lens and system MTF; both aliasing and resolution affectations can be observed in the system MTF. Lens resolving power has been measured by visual inspection of the aerial image of an USAF1951 test target through a suitable microscope. The lens PSF and MTF has been measured by means of a Shack-Hartmann optical wave front sensor. The system MTF is measured by the slanted edge method. The different experimental procedures have been applied to two professional SLR cameras equipped with the same general use lens.
Optimal diameter based in PSF and MTF system measurement is determined for a pinhole camera. Optimal
pinhole images are improved by deconvolution with previously estimated PSF and the result is compared
with unsharp mask enhancement.
In the approach of geometrical optics, ray tracing is an illustrative tool to determine the light path through an optical system. We show different educational resources used to help students to develop their skills in
graphical ray tracing.
Es defineix el factor de cobertura d’un teixit com la relació entre l’àrea coberta pels fils respecte de l’àrea total del teixit. En aquest treball es presenten dos nous mètodes de mesura del factor de cobertura basats en la llindarització automàtica i manual d’una imatge digital d’un teixit. Els resultats obtinguts pels dos mètodes coincideixen amb un alt grau de fiabilitat.