Design and Construction of a Multispectral Camera for Spectral and Colorimetric Reproduction

Multi-spectral imaging and spectral reflectance reconstruction can be used in cultural-heritage institutes to digitalize their collections for documentation purposes. It can be used to simulate artwork under any lighting condition, and to analyze colorants that were used. The basic idea of a multi-spectral imaging system is to sub-sample spectral reflectance factor, producing results similar to a spectrophotometer. The sampled data are used to reconstruct reflectance for the visible spectrum. In this thesis, a wide band multispectral camera was designed and constructed to achieve high spectral and color accuracy as well as high image quality. Noise propagation theory was introduced and tested. A seven channel band- pass filter set was modeled using Gaussian functions and optimized to yield high spectral and colorimetric reproduction accuracy as well as low colori- metric noise. Single and sandwich filters were selected from o!-the-shelf absorption filters using the Gaussian bandpass filter model. Experiments were conducted to test the spectral, color and noise performance of the novel sandwich filters and compared with interference filters. The novel sandwich fil- ters led to increased colorimetric accuracy along with a reduction colorimetric noise. This imaging system will be used as part of a recommended workflow for museum archiving, and will be an important addition to the spectral imaging capabilities at MCSL

Prototype software for colorant formulation using Gamblin conservation colors

When selecting pigments from a large set for restorative inpainting, it can often be challenging to create a mixture that will provide an exact match to the original artwork under a range of viewing and illumination conditions. In this research, a prototype computer program was developed that will aid the user by providing a color match and paint recipe that exhibits minimal metamerism when compared to the original artwork. The Gamblin Conservation Colors, a set of 43 colorants specially formulated for inpainting, were characterized in terms of their optical properties, absorption and scattering, according to Kubelka-Munk turbid media theory. Formulations were made using traditional spectrophotometric measurements and image-based measurements. The multispectral imaging system consisted of a trichromatic CFA camera coupled with two absorption filters; spectral reflectance data for each pixel location was estimated with a transformation based on calibration target images. Three targets were used for testing formulation accuracy: a target consisting of mixtures of Gamblin Conservation Colors, and two oil paintings. Pigment selection was reasonably successful, and good predictions resulted from both measurement techniques, but for more complex tasks such as pigment identification, a more rigorous colorant characterization approach may be needed. Predictions from image-based measurements were generally less accurate, and improvements in the camera model would likely remedy this. It is expected that this software will be of assistance to conservators by simplifying the process of selecting from a large set of available pigments, as well as reducing the possibility of damage to painted surfaces in cases where direct measurements are impractical. The open source nature of the software provides the opportunity for changes and addition of features in the future

Filter Selection for Spectral Estimation Using a Trichrmatic Camera

Current imaging practices are based on exploiting metamerism to record and reproduce images. As a result, the data obtained in these images are dependent on the viewing conditions and the observer. While these methods produce acceptable results for day to day use, they often do not exhibit the type of accuracy and control required for scientific purposes such as art conservation. As a solution, many research institutions are now advocating the use of multispectral imaging to record the objects fundamental spectral properties to remove the data\u27s dependency on the observer and viewing environment. The research described in this thesis involved determining if a trichromatic camera and readily available filters can be used for spectral estimation purposes. The Pixel Physics TerraPix camera system was characterized, its response to a target and 105 Kodak Wratten Filters under tungsten illumination was simulated, and spectral reflectance estimations were generated. The top filter candidates were chosen based on their simulated performance. These filters were then used in an imaging experiment designed to approximate conditions that would be found in an art gallery or other place where copy work is performed. The results of the imaging experiment were compared with the simulation, and shortcomings of the model were identified. The results of the experiment show that a camera model can be used as a guiding tool to make filter selections for spectral estimation

Bio-Inspired Multi-Spectral and Polarization Imaging Sensors for Image-Guided Surgery

Image-guided surgery (IGS) can enhance cancer treatment by decreasing, and ideally eliminating, positive tumor margins and iatrogenic damage to healthy tissue. Current state-of-the-art near-infrared fluorescence imaging systems are bulky, costly, lack sensitivity under surgical illumination, and lack co-registration accuracy between multimodal images. As a result, an overwhelming majority of physicians still rely on their unaided eyes and palpation as the primary sensing modalities to distinguish cancerous from healthy tissue. In my thesis, I have addressed these challenges in IGC by mimicking the visual systems of several animals to construct low power, compact and highly sensitive multi-spectral and color-polarization sensors. I have realized single-chip multi-spectral imagers with 1000-fold higher sensitivity and 7-fold better spatial co-registration accuracy compared to clinical imaging systems in current use by monolithically integrating spectral tapetal and polarization filters with an array of vertically stacked photodetectors. These imaging sensors yield the unique capabilities of imaging simultaneously color, polarization, and multiple fluorophores for near-infrared fluorescence imaging. Preclinical and clinical data demonstrate seamless integration of this technologies in the surgical work flow while providing surgeons with real-time information on the location of cancerous tissue and sentinel lymph nodes, respectively. Due to its low cost, the bio-inspired sensors will provide resource-limited hospitals with much-needed technology to enable more accurate value-based health care

Comparison of the accuracy of various transformations from multi-band images to reflectance spectra

This report provides a comparative study of the spectral and colorimetric accuracy of various transformations from multi-band digital signals to spectral reflectance. The multiband channels were obtained by multi-channel visible-spectral imaging (MVSI) using a monochrome CCD and two different filtering systems. In the first system we used a liquid-crystal tunable filter (LCTF) capturing 31 narrow-band channels. We also used a filter wheel with a set of 6 glass filters imaging with and without an extra Wratten absorption filter giving a total of 12 channels. Four different mathematical methods were tested to derive reflectance spectra from digital signals: pseudo-inverse, eigenvector analysis, modified-discrete sine transformation (MDST) and non-negative least squares (NNLS). We also considered two different approaches to sampling the digital signals; in one approach we averaged the digital counts

A new multi-spectral imaging system for examining paintings

A new multispectral system developed at the National Gallery is presented. The system is capable of measuring the spectral reflectance per pixel of a painting. These spectra are found to be almost as accurate as those recorded with a spectrophotometer; there is no need for any spectral reconstruction apart from a simple cubic interpolation between measured points. The procedure for recording spectra is described and the accuracy of the system is quantified. An example is presented of the use of the system to scan a painting of St. Mary Magdalene by Crivelli. The multispectral data are used in an attempt to identify some of the pigments found in the painting by comparison with a library of spectra obtained from reference pigments using the same system. In addition, it is shown that the multispectral data can be used to render a color image of the original under a chosen illuminant and that interband comparison can help to elucidate features of the painting, such as retouchings and underdrawing, that are not visible in trichromatic images

Expanding Dimensionality in Cinema Color: Impacting Observer Metamerism through Multiprimary Display

Television and cinema display are both trending towards greater ranges and saturation of reproduced colors made possible by near-monochromatic RGB illumination technologies. Through current broadcast and digital cinema standards work, system designs employing laser light sources, narrow-band LED, quantum dots and others are being actively endorsed in promotion of Wide Color Gamut (WCG). Despite artistic benefits brought to creative content producers, spectrally selective excitations of naturally different human color response functions exacerbate variability of observer experience. An exaggerated variation in color-sensing is explicitly counter to the exhaustive controls and calibrations employed in modern motion picture pipelines. Further, singular standard observer summaries of human color vision such as found in the CIE’s 1931 and 1964 color matching functions and used extensively in motion picture color management are deficient in recognizing expected human vision variability. Many researchers have confirmed the magnitude of observer metamerism in color matching in both uniform colors and imagery but few have shown explicit color management with an aim of minimized difference in observer perception variability. This research shows that not only can observer metamerism influences be quantitatively predicted and confirmed psychophysically but that intentionally engineered multiprimary displays employing more than three primaries can offer increased color gamut with drastically improved consistency of experience. To this end, a seven-channel prototype display has been constructed based on observer metamerism models and color difference indices derived from the latest color vision demographic research. This display has been further proven in forced-choice paired comparison tests to deliver superior color matching to reference stimuli versus both contemporary standard RGB cinema projection and recently ratified standard laser projection across a large population of color-normal observers

Calibrating the elements of a multispectral imaging system

We describe a method to calibrate the elements of a multispectral system aimed at skylight imaging, which consists of a monochrome charge-coupled device (CCD) camera and a liquid-crystal tunable filter (LCTF). We demonstrate how to calibrate these two devices in order to build a multispectral camera that can obtain spectroradiometric measurements of skylight. Spectral characterizations of the tunable filter and the camera are presented together with a complete study into correcting temporal and spatial noise, which is of key importance in CCDs. We describe all the necessary steps to undertake this work and all the additional instrumentation that must be used to calibrate the radiometric devices correctly. We show how this complete study of our multispectral system allows us to use it as an accurate, high resolution spectroradiometer.This work was financed by the Spanish Red Temática “CIENCIA Y TECNOLOGÍA DEL COLOR” (FIS2005-25312-E), the Spanish Ministry of Education and Science, and the European Fund for Regional Development (FEDER) through grant FIS2007-60736. We thank our English colleague A. L. Tate for revising our English text.Peer reviewe

Quality metrics for spectral estimation

Tesina realitzada en col.laboració amb Centre de Desenvolupament de Sensors, Instrumentació i SistemesThe quantitative assessment of the spectral estimation quality in multispectral imaging systems is an active field of research. The design and optimization of multispectral imaging systems are very dependent on how the cost function is selected. Several spectral estimation metrics have been used depending on the attribute it is intended to measure: visual matching, correlation of spectral curves or reduction of metamerism. The purpose of this project is to analyze various metrics that have been used for spectral matches and to show the appropriateness and weakness of each metric