RGBD cameras provide images of depth and 3D information of physical objects. These cameras, which are usually based on Time of Flight techniques, have become an alternative for detecting superficial defects in industrial production. Traditionally, the measurement of changes in thickness of a transparent flat glass has been carried out by means of point like techniques. Thus, for the measurement of an extended surface of the glass a scanning process is necessary. This drawback can be improved using cameras RGBD like the Kinect V2. In this article, a novel method for detecting the change in thickness of an area of interest of a glass is described. This proposal is based on the refraction of the light when it goes from air to glass, i.e., it is based on the change of direction and velocity of the light due to the change of refraction index between the media. This phenomenon is evident in the RGBD images of a known scene when a transparent flat glass is put before its background. The changes in the RGBD images of the scene with and without glass are caused by the increment in time of flight by the refraction in the glass. We obtained a correlation between the thicknesses of different glasses and the depth measurements for a controlled scene, by using an experimental setup with a Kinect V2 as RGBD camera. We processed the data using a K-means algorithm to determine the change of thickness in transparent flat glasses.
In Digital Holography (DH), the size of the bidimensional image sensor to record the digital hologram, plays a key role on the performance of this imaging technique; the larger the size of the camera sensor, the better the quality of the final reconstructed image. Scientific cameras with large formats are offered in the market, but their cost and availability limit their use as a first option when implementing DH. Nowadays, DSLR cameras provide an easy-access alternative that is worthwhile to be explored. The DSLR cameras are a wide, commercial, and available option that in comparison with traditional scientific cameras, offer a much lower cost per effective pixel over a large sensing area. However, in the DSLR cameras, with their RGB pixel distribution, the sampling of information is different to the sampling in monochrome cameras usually employed in DH. This fact has implications in their performance. In this work, we discuss why DSLR cameras are not extensively used for DH, taking into account the problem reported by different authors of object replication. Simulations of DH using monochromatic and DSLR cameras are presented and a theoretical deduction for the replication problem using the Fourier theory is also shown. Experimental results of DH implementation using a DSLR camera show the replication problem.
Retinal image quality measurements (double-pass and Hartmann-Shack) using spatially coherent light sources like lasers or super-luminescent diodes suffer from the presence of speckle in the final images. This well-known phenomenon diminishes the performance of those systems. Although solutions to this problem have been proposed, there still exist room to implement effective methods to face this challenge. We evaluate the influence of changing the polarization states of a laser beam in a double-pass system in order to reduce the speckle noise. By rotating the linear polarization state during the exposure time of the camera the speckle changes and partially averages out. We use the speckle contrast metric to evaluate the performance of the proposed method over experimental results
This work shows the realization of speckle reduction in the numerical reconstruction of digitally recorded holograms by the superposition of multiple slightly rotated digital holographic images of the object. The superposition of T uncorrelated holographic images reduces the contrast of the speckle noise of the image following the expected 1/T law. The effect of the method on the borders of the resulting image is evaluated by quantifying the utilization of the dynamic range or the contrast between the white and black areas of a regular die. Experimental results validate the feasibility of the proposed method.
This paper shows the simulations of the usage of a LED cluster as the illumination source for a multispectral imaging
system covering the range of wavelengths from 350 to 1650 nm. The system can be described as being composed of two
modules determined by the spectral range of the imaging sensors responses, one of them covering the range from 350-
950nm (CCD camera) and the other one covering the wavelengths from 900-1650nm (InGaAs camera). A well known
method of reflectance estimation, the pseudo-inverse method, jointly with the experimentally measured data of the
spectral responses of the cameras and the spectral emission of the LED elements are used for the simulations. The
performance of the system for spectral estimation under ideal conditions and realistic noise influence is evaluated
through different spectral and colorimetric metrics like the GFC, RMS error and CIEDE2000 color difference formula.
The results show that is expectable a rather good performance of the real setup. However, they also reveal a difference in
the performances of the modules. The second module has poorer performance due to the less narrow spectral emission
and less number of LED elements that covers the near-infrared spectral range.
For the first time, transmission digital holography microscopy is applied to observe coal palynofacies, which are organic
fossil microcomponents contained in the coal grains. The recorded holograms were produced by using microscope lenses
with 20x and 40x of lateral magnification respectively, and He-Ne laser of wavelength 594.5 nm. The results show that
reflection digital holography microscopy is required for observing relative opaque particles, because the phase recovery
is strong diminished by light transmission in those cases. On the other hand, the phase distribution is related to the relief
of the particles and the variations of their refraction index. Therefore, a priori information should be necessary to
properly relate the phase information to physical features of the particles. Numerical unwrapping procedures are also
crucial. Procedures with special requirements can be needed for analysing fast varying phase distributions. However,
digital holography microscopy becomes a high performance tool for 3D modelling of fossil particles if the above
requirements are enough fulfilled.
In numerical hologram reconstruction like in optical holography, there exist different diffraction orders, which many times produce difficulties for the image visualizations. In numerical reconstruction, the filtering of the DC term, or zero order diffraction, provides an improved utilization of the dynamic range of the visualization systems, which is needed for the right displaying purposes of reconstructed images. In the case of the in-line set-ups, the zero diffraction order has even more devastating effects because it does not allow reconstructing the images itself as in Gabor's holograms, so it is necessary to attempt a filtering of this DC term. In this work is shown a qualitative assessment of different techniques of DC term filtering applied to numerical reconstruction of Fresnel's holograms recorded by means of off-line set-ups.
A fundamental problem in holography, as well optical as digital, is the presence of speckle noise in the reconstruction process. Many approaches have been carried out in order overcome such a problem, ranging from altering the spatial coherence (optical techniques) of the illumination to imaging processing techniques (digital techniques). This work shows the merged use of digital imaging techniques in order to reduce the speckle noise in digital reconstruction of optically recorded Fresnel's holograms. The proposed filtering techniques are illustrated with experimental results.