A system using laser speckle effect is proposed to segment images reflecting vibration movements of di use targets. Longitudinal movements are difficult to identify when simple imaging systems are used. The proposed system produces a two dimensional segmentation of the target and it is sensitive to longitudinal movements. The speckle effect, produced when coherent light is reflected and interferes when hitting rough surfaces, can be used in order to accomplish this purpose. A pattern with high and low intensity spots is observed depending on the illuminated scene. In our optical system, two silicone membranes are illuminated using a beam expanded laser source and their patterns are recorded using a video camera. One of the membranes experiences a longitudinal controlled movement while the remaining scene is still. Speckle data is processed using a temporal gradient and a regional entropy computation. This method produces a binary individual pixel classification. Four sets of parameters have been tested for the entropy computation and the area under the receiver operating characteristic (ROC) curve was used to select the best one. The selected set-up achieved a ROC value of 0.9879. A data set with 12 different membrane velocities was used to define the threshold that maximizes the classifier accuracy. This threshold was applied to a validation data-set composed by 4 sinusoidal movements with distinct velocities. The accuracy of this technique has achieved values between 92% and 97%. The results show that the target was accurately identified with the optical non-contact apparatus and the developed algorithm.
The laser diode self-mixing technique is a well-known and powerful interferometric technique that has been used in biomedical applications, namely for the extraction of cardiovascular parameters. However, to construct an optical probe using the self-mixing principle which is able to acquire signals in the human carotid artery, some problems are expected. The laser diode has a small aperture area, which means that, for physiological sensing purposes, it can be considered as a point-like detector. This feature imparts difficulties to quality recording of physiological signals since the number of photons collected and mixed in the cavity of the photodiode is very small. In order to overcome this problem, a new mixing geometry based on an external large area planar photodiode (PD) is used in the probe, enabling a much larger number of photons to be collected, hence improving the quality of the signal. In this work, the possibility to obtain the mixing effect outside the laser cavity using an external photodetector, such as a planar photodiode, is demonstrated. Two test benches were designed, both with of two reflectors. The first one, which reflects the light beam with the same frequency of the original one is fixed, and the second one, is movable, reflecting the Doppler shifted light to the photodetector. The first test bench has a fixed mirror in front of the movable mirror, creating an umbra and penumbra shadow above the movable mirror. To avoid this problem, another test bench was constructed using a wedged beam splitter (WSB) instead of a fixed mirror. This new assembly ensures the separation of a single input beam into multiple copies that undergo successive reflections and refractions. Some light waves are reflected by the planar surface of WSB, while other light beams are transmitted through the WSB, reaching the movable mirror. Also in this case, the movable mirror reflects the light with a Doppler frequency shift, and the PD receives both beams. The two test benches were designed to demonstrate that it is possible to obtain mixing effect outside the laser cavity, using a planar photodiode. The Doppler spectrograms from the signals acquired in the test benches show that the signal frequency changes along time which correspond to the modulus of the derivative of the mirror movement, as expected in the self-mixing signals. Nevertheless, the test bench A showed better results than the test bench B. This fact probably results from the attenuation that the original beam suffers in each reflection and refraction in the WBS. Tests developed in the test benches opened the possibility to construct a probe that uses a planar photodiode with a large area to collect medical signals, and improve the quality of the acquisition with a better SNR.