Due to their manufacturing process, plastic injection molded lenses do not achieve a constant density throughout their volume. This change of density introduces tensions in the material, inducing local birefringence, which in turn is translated into a variation of the ordinary and extraordinary refractive indices that can be expressed as a retardation phase plane using the Jones matrix notation. The detection and measurement of the value of the retardation of the phase plane are therefore very useful ways to evaluate the quality of plastic lenses. We introduce a polariscopic device to obtain two-dimensional maps of the tension distribution in the bulk of a lens, based on detection of the local birefringence. In addition to a description of the device and the mathematical approach used, a set of initial measurements is presented that confirms the validity of the developed system for the testing of the uniformity of plastic lenses.
The performances of satellite-to-ground downlink optical communications over Gamma–Gamma distributed turbulence are studied for a multiple-aperture receiver system. Equal gain-combining (EGC) and selection-combining (SC) techniques are considered as practical schemes to mitigate the atmospheric turbulence under thermal-noise-limited conditions. Bit-error rate (BER) performances for on-off keying-modulated direct detection and outage probabilities are analyzed and compared for SC diversity receptions using analytical results and for EGC diversity receptions through an approximation method. To show the net diversity gain of a multiple-aperture receiver system, BER performances and outage probabilities of EGC and SC receiver systems are compared with a single monolithic-aperture receiver system with the same total aperture area (same average total incident optical power) for satellite-to-ground downlink optical communications. All the numerical results are also verified by Monte-Carlo simulations.
Peripheral refraction, the refractive error present outside the main direction of gaze, has lately attracted interest due to its alleged relationship with the progression of myopia. The ray tracing procedures involved in its calculation need to follow an approach different from those used in conventional ophthalmic lens design, where refractive errors are compensated only in the main direction of gaze. We present a methodology for the evaluation of the peripheral refractive error in ophthalmic lenses, adapting the conventional generalized ray tracing approach to the requirements of the evaluation of peripheral refraction. The nodal point of the eye and a retinal conjugate surface will be used to evaluate the three-dimensional distribution of refractive error around the fovea. The proposed approach enables us to calculate the three-dimensional peripheral refraction induced by any ophthalmic lens at any direction of gaze and to personalize the lens design to the requirements of the user. The complete evaluation process for a given user prescribed with a-5.76D ophthalmic lens for foveal vision is detailed, and comparative results obtained when the geometry of the lens is modified and when the central refractive error is over-or undercorrected. The methodology is also applied for an emmetropic eye to show its application for refractive errors other than myopia.
Sanabria, F.; Aldaba, M.; Diaz-Douton, F.; Garcia, C.E.; Pujol, J. Optical engineering Vol. 53, num. 6, p. 061710-1-061710-8 DOI: 10.1117/1.OE.53.6.061710 Data de publicació: 2014-06-01 Article en revista
We have studied the possibility of improving the performance, simplifying, and reducing the cost of a double-pass system by the use of alternative technologies. The system for correcting the spherical correction has been based on a focusable electro-optical lens, and a recording device based on CMOS technology and a superluminescent diode (SLED) light source have been evaluated separately. The suitability of the CMOS camera has been demonstrated, while the SLED could not break the speckle by itself. The final experimental setup, consisting of a CMOS camera and a laser diode, has been compared with a commercial double-pass system, proving its usefulness for ocular optical quality and scattering measurements. (C) 2014 Society of Photo-Optical Instrumentation Engineers (SPIE)
A robust and efficient method for overcoming the negative effects of long-time occlusion in the tracking process is presented. The proposed approach is based on the matching of multiple trajectories in time. Trajectories are sets of 2-D points in time and in a joint ground plane of the world coordinate system. In order to avoid mismatches due to possible measurement outliers, we introduce an integral distance between compared trajectories. The proposed method can also be considered as an interpolation algorithm for a disconnected trajectory during the blackout. Thus this technique solves one of the most difficult problems of occlusion handling: the matching of two unconnected parts of the same trajectory.
Though it is the first step of a real video surveillance application, detection has received less attention than tracking in research on video surveillance. We show, however, that the majority of errors in the
tracking task are due to wrong detection. We show this by experimenting with a multi object tracking algorithm based on a Bayesian framework and a particle filter. This algorithm, which we have named iTrack, is specifically designed to work in practical applications by defining a statistical model of the object appearance to build a robust likelihood function.
Likewise, we present an extension of a background subtraction algorithm to deal with active cameras. This algorithm is used in the detection task to initialize the tracker by means of a prior density. By
defining appropriate performance metrics, the overall system is evaluated to elucidate the importance of detection for video surveillance applications.
The minimum number of samples that must be taken from a
sinusoidal signal affected by white Gaussian noise, in order to find its
frequency with a predetermined maximum error, is derived. This analysis
is of interest in evaluating the performance of velocity-measurement systems
based on the Doppler effect. Specifically, in laser Doppler anemometry
(LDA) it is usual to receive bursts with a poor signal-to-noise
ratio, yet high accuracy is required for the measurement. In recent years
special attention has been paid to the problem of monitoring the temporal
evolution of turbulent flows. In this kind of situation averaging or filtering
the data sequences cannot be allowed: in a rapidly changing environment
each one of the measurements should rather be performed
within a maximum permissible error and the bursts strongly affected by
noise removed. The method for velocity extraction that will be considered
here is the spectral analysis through the squared discrete Fourier transform,
or periodogram, of the received bursts. This paper has two parts. In
the first an approximate expression for the error committed in LDA is
derived and discussed. In the second a mathematical formalism for the
exact calculation of the error as a function of the signal-to-noise ratio is
obtained, and some universal curves for the expected error are provided.
The results presented here appear to represent a fundamental limitation
on the accuracy of LDA measurements, yet, to our knowledge, they have
not been reported in the literature so far.