Dynamic speckle analysis is a promising tool for inspection of speed of processes. It relies on highly sensitive of speckle phenomenon to micro-changes in the optical paths. The same phenomenon is a source of severe noise, which contaminates substantially the outcome of analysis when a pointwise processing is applied to correlated in time speckle patterns. We obtained improvement of quality of the two-dimensional activity map produced by the pointwise processing by illumination at two wavelengths in the red and green part of the spectrum and separate processing of the raw data acquired in the red and the green channels of a color camera used to capture the speckle patterns. Summation of the activity maps corresponding to the two channels results in narrower spread of fluctuations of the estimate chosen to describe the activity.
Dynamic speckle analysis is an effective approach for industrial inspection of speed of processes. The relevant information is retrieved by statistical processing of temporal sequences of speckle patterns formed on the surface of diffusely reflecting objects under laser illumination. The measurement output is a 2D activity map, which shows regions of higher or lower activity in the object. Following a process in time requires a great number of images, in which the recorded values of intensity are of minor importance. In view of the dynamic speckle measurement specifics, we propose in the paper coarser quantization of the raw speckle data as an approach for data compression. Its efficacy is proved by processing numerical and experimental speckle patterns. Decreasing the number of the quantization levels down to 32 causes no distortions in the activity map. Further decrease of the quantization level number is applicable only to data acquired at uniform illumination and equal reflectivity across the object surface.
Non-destructive detection of physical or biological activity through statistical processing of speckle patterns on the
surface of diffusely reflecting objects is an area of active research. A lot of pointwise intensity-based algorithms have
been proposed over the recent years. Efficiency of these algorithms is deteriorated by the signal-dependent speckle data,
non-uniform illumination or varying reflectivity across the object, especially when the number of the acquired speckle
patterns is limited. Pointwise processing of a sequence of 2D images is also time-consuming. In this paper, we propose to
transform the acquired speckle images into binary patterns by using for a sign threshold the mean intensity value
estimated at each spatial point from the temporal sequence of intensities at this point. Activity is characterized by the 2D
distribution of a temporal polar correlation function estimated at a given time lag from the binary patterns. Processing of
synthetic and experimental data confirmed that the algorithm provided correct activity determination with the same
accuracy as the temporal normalized correlation function. It is efficient without the necessity to apply normalization at
non-uniform distribution of intensity in the illuminating laser beam and offers acceleration of computation.
Holography is defined as a two-steps process of capture and reconstruction of the light wavefront scattered from three-dimensional (3D) objects. Capture of the wavefront is possible due to encoding of both amplitude and phase in the hologram as a result of interference of the light beam coming from the object and mutually coherent reference beam. Three-dimensional imaging provided by holography motivates development of digital holographic imaging methods based on computer generation of holograms as a holographic display or a holographic printer. The holographic printing technique relies on combining digital 3D object representation and encoding of the holographic data with recording of analog white light viewable reflection holograms. The paper considers 3D contents generation for a holographic stereogram printer and a wavefront printer as a means of analogue recording of specific artifacts which are complicated objects with regards to conventional analog holography restrictions.
Dynamic laser speckle analysis is non-destructive detection of physical or biological activity through statistical processing of speckle patterns on the surface of diffusely reflecting objects. This method is sensitive to microscopic changes of the surface over time and needs simple optical means. Advances in computers and 2D optical sensors forced development of pointwise algorithms. They rely on acquisition of a temporal sequence of correlated speckle images and generate activity data as a 2D spatial contour map of the estimate of a given statistical parameter. The most widely used pointwise estimates are the intensity-based estimates which compose each map entry from a time sequence of intensity values taken at one and the same pixel in the acquired speckle images. Accuracy of the pointwise approach is strongly affected by the signal-dependent nature of the speckle data when the spread of intensity fluctuations depends on the intensity itself. The latter leads to erroneous activity determination at non-uniform distribution of intensity in the laser beam for the non-normalized estimates. Normalization of the estimates, introduces errors. We propose to apply binarization to the acquired speckle images by comparing the intensity values in the temporal sequence for a given spatial point to the mean intensity value estimated for this point and to evaluate a polar correlation function. Efficiency of this new processing algorithm is checked both by simulation and experiment.
The phenomenon of dynamic speckle allows for non-invasive whole-field detection of physical or biological activity in objects through statistical description of laser speckle dynamics. Effective way to improve the statistical analysis is generation of controlled speckle patterns. SLM implementation of an optical simulator of dynamic speckle patterns is proposed by feeding a correlated sequence of 2D random phase distributions to the phase-only SLM. Atthevarying in space correlation radius of the phase fluctuations in the successive frames, the SLM produces regions of different activity on a screen under laser illumination. Feasibility of the proposed approach is proved both by simulation and experiment.
The phenomenon of laser speckle can be used for detection and visualization of physical or biological activity in various
objects (e.g. fruits, seeds, coatings) through statistical description of speckle dynamics. The paper presents the results of
non-destructive monitoring of bread cooling by co-occurrence matrix and temporal structure function analysis of speckle
patterns which have been recorded continuously within a few days. In total, 72960 and 39680 images were recorded and
processed for two similar bread samples respectively. The experiments proved the expected steep decrease of activity
related to the processes in the bread samples during the first several hours and revealed its oscillating character within
the next few days. Characterization of activity over the bread sample surface was also obtained.
The paper presents a holographic optical element (HOE) for a phase-stepping digital electronic speckle pattern
interferometry with double symmetrical illumination of the object both in vertical and horizontal planes for precision
full-field displacement measurement of objects under loading . More specifically, the proposed HOE allows for
simultaneous reconstruction of four virtual and parallel to the HOE surface reference planes for off-axis illumination
with two pairs of diode lasers emitting at two different wavelengths. In this way we transform a two-beam interferometer
into a multiple beam interferometer with four separate channels. The HOE is constructed as a sandwich structure of two
reflection (Denisuyk type) holograms of a diffuse metal screen illuminated at 30 degrees to the normal. Each of the
holograms comprises two holographic reflection records made successively on a single holographic plate. Recording is
performed in the visible part of the spectrum, and through additional chemical treatment the spectral maximum of the
developed holograms is shifted into the IR region to achieve correct reconstruction for illumination at 790 nm and 830
nm. Super high resolution silver halide emulsion with resolution over 6000 line/mm is used for recording of holograms.
The reconstructed four reference beams interfere with the beam reflected from the object, thus forming four independent
optical channels for two laser beams at 790 nm with S and P polarizations, as well as for two S and P polarized beams at
Coherent illumination of a diffuse object yields a randomly varying interference pattern, which changes over time at any
modification of the object. This phenomenon can be used for detection and visualization of physical or biological activity
in various objects (e.g. fruits, seeds, coatings) through statistical description of laser speckle dynamics. The present
report aims at non-destructive full-field evaluation of bread by spatial-temporal characterization of laser speckle. The
main purpose of the conducted experiments was to prove the ability of the dynamic speckle method to indicate activity
within the studied bread samples. In the set-up for acquisition and storage of dynamic speckle patterns an expanded
beam from a DPSS laser (532 nm and 100mW) illuminated the sample through a ground glass diffuser. A CCD camera,
adjusted to focus the sample, recorded regularly a sequence of images (8 bits and 780 x 582 squared pixels, sized 8.1 ×
8.1 μm) at sampling frequency 0.25 Hz. A temporal structure function was calculated to evaluate activity of the bread
samples in time using the full images in the sequence. In total, 7 samples of two types of bread were monitored during a
chemical and physical process of bread's staling. Segmentation of images into matrixes of isometric fragments was also
utilized. The results proved the potential of dynamic speckle as effective means for monitoring the process of bread
staling and ability of this approach to differentiate between different types of bread.
This work reports on investigation of the sensitivity of a Fourier-transform spectrometer to noise sources based on
Monte-Carlo simulation of measurement of a single spectrum. Flexibility of this approach permits easily to imitate
various noise contaminations of the interferograms and to obtain statistically reliable results for widely varying noise
characteristics. More specifically, we evaluate the accuracy of restoration of a single absorption peak for the cases of an
additive detection noise and the noise which adds a fluctuating component to the carrier frequency in the source and the
measurement channel of the interferometer. Comparison of spectra of an etalon He-Ne source calculated from more than
200 measured interferograms with the true spectrum supports a hypothesis that the latter fluctuations have characteristics
of a coloured noise. Taking into account that the signal-to-noise ratio in the Fourier spectroscopy is constantly
increasing, we focus on limitations on the achievable accuracy of spectrum restoration that are set by this type of noise
which modifies the shape of the recorded interferograms. We present also results of the test measurements of the
spectrum of a laser diode chosen as a test source using a three-channel Fourier spectroscopic system based on a white-sourced
Michelson interferometer realized with the Twyman-Green scheme.
The obtained results exhibit that fluctuations in the current displacement of the movable mirror of the interferometer
should remain below 20 nm to restore the absorption spectrum with acceptable accuracy, especially at higher frequency
bandwidth of the fluctuations.