Near-infrared hyperspectral imaging is becoming a popular tool in the biomedical field, especially for detection and
analysis of different types of cancers, analysis of skin burns and bruises, imaging of blood vessels and for many other
applications. As in all imaging systems, proper illumination is crucial to attain optimal image quality that is needed for
best performance of image analysis algorithms. In hyperspectral imaging based on filters (AOTF, LCTF and filter wheel)
the acquired spectral signature has to be representative in all parts of the imaged object. Therefore, the whole object must
be equally well illuminated - without shadows and specular reflections. As there are no restrictions imposed on the
material and geometry of the object, the desired object illumination can only be achieved with completely diffuse
illumination. In order to minimize shadows and specular reflections in diffuse illumination the light illuminating the
object must be spatially, angularly and spectrally uniform. We present and test two diffuse illumination system designs
that try to achieve optimal uniformity of the above mentioned properties. The illumination uniformity properties were
measured with an AOTF based hyperspectral imaging system utilizing a standard white diffuse reflectance target and a
specially designed calibration target for estimating the spatial and angular illumination uniformity.
The goal of this article is to present a novel method for spectral characterization and calibration of spectrometers and
hyper-spectral imaging systems based on non-collinear acousto-optical tunable filters. The method characterizes the
spectral tuning curve (frequency-wavelength characteristic) of the AOTF (Acousto-Optic Tunable Filter) filter by
matching the acquired and modeled spectra of the HgAr calibration lamp, which emits line spectrum that can be well
modeled via AOTF transfer function. In this way, not only tuning curve characterization and corresponding spectral
calibration but also spectral resolution assessment is performed. The obtained results indicated that the proposed method
is efficient, accurate and feasible for routine calibration of AOTF spectrometers and hyper-spectral imaging systems and
thereby a highly competitive alternative to the existing calibration methods.
Optical aberrations present an important problem in optical measurements. Geometrical calibration of an imaging system
is therefore of the utmost importance for achieving accurate optical measurements. In hyper-spectral imaging systems,
the problem of optical aberrations is even more pronounced because optical aberrations are wavelength dependent. Geometrical
calibration must therefore be performed over the entire spectral range of the hyper-spectral imaging system,
which is usually far greater than that of the visible light spectrum. This problem is especially adverse in AOTF (Acousto-
Optic Tunable Filter) hyper-spectral imaging systems, as the diffraction of light in AOTF filters is dependent on both
wavelength and angle of incidence. Geometrical calibration of hyper-spectral imaging system was performed by stable
caliber of known dimensions, which was imaged at different wavelengths over the entire spectral range. The acquired
images were then automatically registered to the caliber model by both parametric and nonparametric transformation
based on B-splines and by minimizing normalized correlation coefficient. The calibration method was tested on an
AOTF hyper-spectral imaging system in the near infrared spectral range. The results indicated substantial wavelength
dependent optical aberration that is especially pronounced in the spectral range closer to the infrared part of the spectrum.
The calibration method was able to accurately characterize the aberrations and produce transformations for efficient
sub-pixel geometrical calibration over the entire spectral range, finally yielding better spatial resolution of hyperspectral
imaging system.
Visualization of subcutaneous veins is very difficult with the naked eye, but important for diagnosis of medical
conditions and different medical procedures such as catheter insertion and blood withdrawal. Moreover, recent studies
showed that the images of subcutaneous veins could be used for biometric identification. The majority of methods used
for enhancing the contrast between the subcutaneous veins and surrounding tissue are based on simple imaging systems
utilizing CMOS or CCD cameras with LED illumination capable of acquiring images from the near infrared spectral
region, usually near 900 nm. However, such simplified imaging methods cannot exploit the full potential of the spectral
information. In this paper, a new highly versatile method for enhancing the contrast of subcutaneous veins based on
state-of-the-art high-resolution hyper-spectral imaging system utilizing the spectral region from 550 to 1700 nm is
presented. First, a detailed analysis of the contrast between the subcutaneous veins and the surrounding tissue as a
function of wavelength, for several different positions on the human arm, was performed in order to extract the spectral
regions with the highest contrast. The highest contrast images were acquired at 1100 nm, however, combining the
individual images from the extracted spectral regions by the proposed contrast enhancement method resulted in a single
image with up to ten-fold better contrast. Therefore, the proposed method has proved to be a useful tool for visualization
of subcutaneous veins.
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