Rudolf Gschwind

Persistent spectral hole burning used for spectrally high-resolved imaging of the sun

Persistent spectral hole burning (PSHB) makes it possible to store images of the sun spectrally and spatially in a single exposure step at very high resolution. The current system consists of a chlorin-doped polymer film (polyvinylbutyral), cooled to 2 K. It has a spectral resolution of 300 MHz (0.0004 nm) and may be used in the range of about 628 to 638 nm. Theoretically the spatial resolution is confined to molecular dimensions. In solar observations, however, it is determined by the optical setup and atmospheric conditions. The exposure is done by imaging the sun onto the sample (exposure energy: 6 mJ/cm2 GHz). Afterwards the stored information is read out by scanning a tunable dye-laser across the spectal range of interest. The laser light is used to image the sample at each frequency point onto a cooled 12 bit CCD- camera. For acquisition, archiving, processing, and visualization of the huge amount of data (up to 10 GByte per experiment), a parallel processor system is used.

Automatic detection of dust and scratches in silver halide film using polarized dark-field illumination

We present a method to automatically detect dust and scratches on photographic material, in particular silver halide film, where traditional methods for detecting and removing defects fail. The film is digitized using a novel setup involving cross-polarization and dark-field illumination in a cardinal light configuration, which compresses the signal and highlights the parts that are due to defects in the film. Applying a principal component analysis (PCA) on the four cardinal images allows us to further separate the signal part of the film from the defects. Information from all four principal components is combined to produce a surface defect mask, which can be used as input to inpainting methods to remove the defects. Our method is able to detect most of the dust and scratches while keeping false-detections low.

Electronic imaging: a tool for the reconstruction of faded color photographs and color movies

A method to restore faded color materials by digital image processing is presented. The algorithms used for the reconstruction are based on photographic experiments, i.e., on accelerated fading tests of various photographic materials. The densities of the original and the faded materials were measured. Based on this data, a mathematical model for fading can be described by a linear equation. The faded image is digitized using a scanner of high spatial and photometric resolution. For good spectral resolution, channel separations are done with narrow- band interference filters. The original colors are reconstructed by applying the inverse of the facing equation. The corrected image is exposed with a high-resolution film recorder on color film. The method shows good results for color slides, prints, and 16 mm movies.

Automated digital camera sensor characterization

The color characterization of professional imaging devices typically involves the capture of a reference color target under the scene-specific lighting conditions and the use of dedicated profiling software. However, the limited set of color patches on the target may not adequately represent the reflection spectra found in the scene. We present a solution developed in collaboration with a camera manufacturer for the automatic color characterization of the sensors without the need of a physical color target. The optimal color transforms are computed based on the individually measured sensor spectral sensitivities, computer generated sets of color spectra forming a virtual characterization target and a mathematical model of the camera. The use of a virtual target enables the optimization of the color transform for specific image capturing situations by selective generation of the reflection spectra.

Fluctuations of light absorption of healthy papillae repeatedly photographed over a long period of time.

In order to distinguish normal, age-related changes of the papilla from changes due to glaucoma, we investigated 7 normal papillae that had been photographed several times over a period of 11 years. By means of digital-image processing techniques, the red- and green-filtered, digitized pictures were compared directly using a subtraction method. The results seem to indicate that normal papillae do not blanch over a long period of time. The difficulties of comparison are discussed.

Optical Detection of Dust and Scratches on Photographic Film

Today’s information society needs efficient and economic solutions for the digital restoration of the photographic heritage. Different methods have been adopted up to now for the automatic detection of dust and scratches; each method has pros and cons, and a limited field of effectiveness. The use of infrared radiation and the spatiotemporal image analysis are among the most effective methods, although they have their limits. The infrared radiation only works for dye-based material, while the spatiotemporal image analysis is not applicable for still images and is limited due to motion in the scene. The present work defines in detail a set of methods for optical dust and scratches detection applicable on any type of transparent photographic material (silver-based as well as dye-based material, still images as well as moving images). The term “optical” refers to the fact that the considered methods seek physical evidence of the presence of foreign bodies or irregularities on the film; this allows avoiding the typical digital artifacts produced by “nonoptical” methods, for which certain elements of the scenes are erroneously obliterated because they resemble dust grains or scratches. “PDD” (Polarized Dark-field Detection) detects the flaws using an image acquired in a polarized dark-field setup; “DCD” (Dual Collimation Detection) takes advantage of the Callier effect to locate the flaws; “n-MDD” (Multiple Direction Detection) entails the acquisition of n images in dark-field setups with different directions of illumination, and the extraction of the differences between the images through multivariate analysis. A numerical evaluation of the performances of the MDD method with an eightfold acquisition (8-MDD) is carried out by comparing its flaw detection with the flaw detection provided by commercial software based on spatiotemporal image analysis.

PEVIAR: Digital originals

Even decades after the advent of computer technology, preserving information in digital archives remains a challenging task. Technological progress on all levels calls for migration as an essential component of any digital archive. Migration, however, is not only expensive, but also makes it difficult to ensure the authenticity of digital documents. The University of Basels Imaging and Media Lab has developed PEVIAR (Permanent Visual Archive), an archiving solution addressing one of the fundamental challenges of digital archiving, migration, and introducing the notion of digital originals. Essentially, digital documents are inseparably bound to a migration-free medium, thereby overcoming the transient nature of todays state-of-the-art storage systems.

Conservation of a room: A treatment proposal for Mark Rothko's Harvard Murals

A treatment with projected light for Mark Rothkos Harvard Murals (1962) is proposed. The group of five paintings on canvas has changed color due to the presence of a fugitive red pigment and excessive exposure to natural light in a room with large windows. For the conservation of Rothkos Harvard room, it is brought into context within his other commissions and environments. The original color of the works is determined by the digital restoration of contemporary photographs. With a camera projector system a compensation image is calculated that is projected onto the original canvas resulting in a restored color appearance. This approach of inpainting with light is compared with considerations of cleaning and inpainting in conventional conservation treatments. Overall lighting and architecture including the unusual wall color carefully chosen by Rothko play a key role in the treatment of the Mural cycle as an environment.

Digital images for eternity : color microfilm as archival medium

In the archiving and museum communities, the long-term preservation of artworks has traditionally been guaranteed by making duplicates of the original. For photographic reproductions, digital imaging devices have now become standard, providing better quality control and lower costs than film photography. However, due to the very short life cycle of digital data, losses are unavoidable without repetitive data migrations to new file formats and storage media. We present a solution for the long-term archiving of digital images on color microfilm (Ilfochrome® Micrographic). This extremely stable and high-resolution medium, combined with the use of a novel laser film recorder is particularly well suited for this task. Due to intrinsic limitations of the film, colorimetric reproductions of the originals are not always achievable. The microfilm must be first considered as an information carrier and not primarily as an imaging medium. Color transformations taking into account the film characteristics and possible degradations of the medium due to aging are investigated. An approach making use of readily available color management tools is presented which assures the recovery of the original colors after re-digitization. An extension of this project considering the direct recording of digital information as color bit-code on the film is also introduced.

Digital slide reproduction using densitometry

Many contemporary art collections contain important art installations where artists have used 35 mm slides as the primary medium. The number of ours these works are on show makes it necessary to regularly change the slides due to light fading. With funding from the Henry Moore Foundation. The conservation department at Tate initiated a project to examine ways in which digital technology could be used to aid the conservation of these works. The aim of the project was to place the original slides in cold storage and explored the possibility of using digital technology to make duplicate sets for display in the gallery. The reproductions needed to be of very high quality both in terms of resolution and color management. This paper discusses the use of densitometry to calibrate both device dependent and device independent systems for digitally reproducing 35 mm slides using a scanner and a film recorder and the effect of metamery when using slide films which employ different dyes.