Hyperspectral colour imaging and spectrophotometric instrumentation

Abstract

The trichromatic nature of commercial photography is strictly connected with the nature of human colour vision, although the characteristics of usual colour imaging devices are quite different from the human visual system. The increase in the number of colour channels for spectral (either multispectral or hyperspectral) imaging is an active field of research with many potential applications in different fields. Each element of the captured scene is specified in the spectral image by the spectral reflectance factor. This measurement is independent of the particular illumination of the scene and allows the colorimetric computation in a device-independent colour space for any chosen illuminant and any observer. This thesis describes the project and construction of a compact spectrophotometric camera, which can be used in both portable and in-situ applications. The compactness is made possible by a suitable image spectral scanning based on an Induced Transmission Filter (ITF). This filter is made by a set of thin-film coatings of dielectric materials with high and low refraction index, whose shape like a wedge induces a wavelength selective transmittance, continuously variable along one direction and uniform in the perpendicular direction. Such a filter, classified as Linearly Variable Filter (LVF), operates continuously from 430nm to 940nm and allows hyperspectral imaging. In traditional scanners the whole apparatus is moved along a path as long as the scene, whereas in this instrument the camera body is still and the LVF is the only moving part. The sequence of operations for wavelength and radiometric calibrations are discussed. The expected acquisition times and number of images as a function of the spectral sampling step are considered. The resulting properties make the instrument easy to use and with short acquisition times. Moreover, overviews of the historic evolution of colour vision fundamentals, colour spaces and spectral imaging technology are given for introducing the reader to the essential concepts useful for the understanding of the text.