Objectif photographique | EPFL Graph Search

vignette|Gauche : 1:2.8/50 MacroCentre : 1:4-5.6/70-300 Droite : 1:4-5.6/10-20 Un objectif photographique est un système optique convergent formé de plusieurs lentilles, et éventuellement de miroirs, donnant des images réelles sur la surface sensible de l'appareil photographique. Un objectif se caractérise en premier lieu par sa distance focale (ou sa plage de focale pour un zoom), son ouverture maximale, et le format maximum de la surface sensible (film ou capteur) avec lequel il est utilisable. Un objectif photographique ou la cornée de l’œil n'ont à l'origine qu'une seule fonction : concentrer la lumière (pour chaque point de l'image) pour sensibiliser la rétine ou le capteur photo. C'est un peu ce que fait la parabole pour des signaux lointain donc pratiquement parallèles. Tout le reste : distorsion, profondeur de champ ne sont que les défauts du système. Par exemple pour une source très lumineuse à une grande distance un simple petit trou suffit. Une immense variété

A high sensitivity, low noise and high spatial resolution multi-band infrared reflectography camera for the study of paintings and works on paper

Marie Eve Pascale Didier

Background: Infrared reflectography (IRR) remains an important method to visualize underdrawing and compositional changes in paintings. Older IRR camera systems are being replaced with near-infrared cameras consisting of room temperature infrared detector arrays made out of indium gallium arsenide (InGaAs) that operate over the spectral range of similar to 900 to 1700 nm. Two camera types are becoming prevalent. The first is staring array infrared cameras having 0.25-1 Megapixels where the camera or painting is moved to acquire tens of individual images that are later mosaicked together to create the infrared reflectogram. The second camera type is scanning back cameras in which a small InGaAs array (linear or area array) is mechanically scanned over a large image formed by the camera lens to create the reflectogram, typically 16 Megapixels. Both systems have advantages and disadvantages. The staring IR array cameras offer more flexible collection formats, provide live images, and allow for the use of spectral bandpass filters that can provide reflectograms with better contrast in some cases. They do require a mechanical system for moving the camera or the artwork and post-capture image mosaicking. Scanning back cameras eliminate or reduce the amount of mosaicking and movement of the camera, however the need to minimize light exposure to the artwork requires short integration times, and thus limits the use of spectral bandpass filters. In general, InGaAs cameras are not sensitive in the 1700 to similar to 2300 nm spectral region, which has been identified in prior studies as useful for examining paintings with copper green pigments or thick lead white paints. Prior studies using cameras with sensitivity from 1000 to 2500 nm have found in general the performance at wavelengths longer than 1700 nm degraded relative to the performance at shorter wavelengths. Thus, there is interest in a camera system having improved performance out to 2500 nm that can utilize spectral bandpass filters. Methods: Design requirements for such an improved IRR camera system were determined by first re-examining the optimal spectral window for detecting underdrawing. Thus the wavelength dependence of the clarity of carbon black underdrawings on a chalk ground covered by various paint swatches was measured from 750 to 2500 nm. Second, analysis of the loss of light transmission (1000-2500 nm) and the impact of thermal radiation (3000-5000 nm) on the performance of IR arrays sensitive to the 1000-5000 nm region was analyzed. From the results of these studies, a high sensitivity, near-infrared (1000-2450 nm) indium antimonide (InSb) staring array camera with a cold filter that blocked light >2450 nm was constructed with a color-corrected macro lens capable of holding interference filters. The camera system was characterized in three spectral bands (1100-1400, 1500-1800, and 2100-2400 nm) using test targets and art objects. Results: The experimental results of the contrast difference between the model underdrawing on the chalk ground showed the optimal spectral window for a given pigment varied over the range from 700 to 2300 nm. The contrast in all cases was found to be low from 2300 to 2600 nm and even lower from 2600 to 5000 nm. This is attributable in part to broad absorption by the drying oil paint binder. Performance testing of the IRR camera found high signal-to-noise was observed in all three spectral bands due to the optimized macro lens having high transmission from 1000 to 2600 nm, a 100% efficiency cold stop, and a cold filter that blocks light >2450 nm. The camera was found to have high light sensitivity, requiring only similar to 30 to 50 lx from incandescent lamps having a color temperature of similar to 2060 K. The images produced in each spectral band were sharp, and the modulation at Nyquist (1/2 the sampling frequency) was 20%. IRR images of Old Master paintings, works on paper, and a warped panel painting were collected to test the practical utility of the camera system. Conclusions: The IR camera system presented here was found to produce a variety of high-resolution image products. The ability to extend image collection over the 1700-2450 nm spectral range was found in some examples to provide improved visibility of underdrawing. The ability to register the resulting multi-band IRR images with the color image offers opportunities to produce new image products such as false-color images and principal component images. These image products were found to give new insights into the construction of works of art. A collection scheme based on a 'Z-stack' method was found to solve the problem of producing high-resolution IRR images of highly warped panel paintings.

Biomed Central Ltd2017

Camera Calibration Models and Methods for Corridor Mapping with UAVs

Emmanuel Pierre Quentin Clédat, Davide Antonio Cucci, Jan Skaloud

Camera calibration refers to the modeling of the relationship between the coordinates of object points and their projections on the image plane. This is usually done by parametric models that describe the physical properties of the lens systems and camera assemblies, such as the camera principal distance, the principal point, and various types of optical distortions. In photogrammetry, accurate knowledge of the parameters of such models, often referred to as Interior Orientation(IO), is of ultimate importance. In this work, we target advanced corridor mapping applications with UAVs. In this scenario, the camera calibration is not completely observable due to the unfavorable geometry of the flight trajectory (e.g., no cross-flight lines available and a single altitude) and needs to be determined beforehand. Further challenges are introduced by the limited mechanical stability of UAV-grade cameras. This may cause slight variations in the IO that need to be recovered while processing production flights. We review and compare two well-known camera models, the Brown-Conrady and the Ebner's self-calibration functions, in 36 calibration setups and provide a discussion of the results, where sub ground sampling distance accuracy in the checkpoints was achieved for some, but not all, configurations.

2020

A high sensitivity, low noise and high spatial resolution multi-band infrared reflectography camera for the study of paintings and works on paper

Marie Eve Pascale Didier

Biomed Central Ltd2017