Object tracking with subpixel accuracy often needs targets with special shapes, which include random dot patterns, circular objects or targets with irregular contours. Unfortunately, in majority of real applications no such objects are found or cannot be attached to the region of interest and non-optimal objects must be used. Among those, we will analyze here the performance of rectangular objects that may be common in natural or artificial targets. Object tracking has been performed through the two most common methods that appear in the literature: centroid calculation and cross-correlation with peak interpolation. Numerical simulations show that tracking results for such objects are highly dependent of the object orientation with respect to the direction of movement. This is due to the interference of the object borders and the sensor and how is the subpixel information obtained. Best results are obtained for object orientations from 5 to 30 deg with respect to the normal to the displacement directions. Experimental results confirm the simulations and allow us to establish that, although object alignment provides a better image on the sensor with sharper borders, this situation is not desirable for accurate subpixel tracking, with accuracies varying from accuracies of 0.08 px (RMS error) with aligned objects to 0.02px with a misaligned target.
The study of the blink can provide a lot of information regarding ocular surface health, cognitive status, psychological health, human neurological disorders, etc. Blinking can be classified as spontaneous, reflex and voluntary. Between the reflex blinks, direct and consensual reflex can be categorized, being direct that from the eye that is being stimulated and by consensual the other one. A system to evaluate of spontaneous and reflex to light blinking is proposed using a led flash and a high-speed camera that records the blinking of each eye. The LED flash only illuminated one eye and alternated arbitrarily. The light intensity diffused by the eye is almost constant when the eyelid is open but changes when the eyelid closes until reaching a maximum peak, and so does the intensity that was registered by the camera. This variation is directly related to the variation of the eyelid position. Several parameters characterizing the blinks have been defined from the fitting of the blink curve to an exponentially modified Gaussian function. Sequences of 60 seconds of 9 subjects blinking were recorded. Results show that there are no significant differences between reflex and spontaneous blinking, however, significant differences between direct and consensual reflex blinking have been found.
We present a purposeful initiative to open new grounds for teaching Geometrical Optics. It is based on the creation of an innovative education networking involving academic staff from three Spanish universities linked together around Optics. Nowadays, students demand online resources such as innovative multimedia tools for complementing the understanding of their studies. Geometrical Optics relies on basics of light phenomena like reflection and refraction and the use of simple optical elements such as mirrors, prisms, lenses, and fibers. The mathematical treatment is simple and the equations are not too complicated. But from our long time experience in teaching to undergraduate students, we realize that important concepts are missed by these students because they do not work ray tracing as they should do. Moreover, Geometrical Optics laboratory is crucial by providing many short Optics experiments and thus stimulating students interest in the study of such a topic. Multimedia applications help teachers to cover those student demands. In that sense, our educational networking shares and develops online materials based on 1) video-tutorials of laboratory experiences and of ray tracing exercises, 2) different online platforms for student self-examinations and 3) computer assisted geometrical optics exercises. That will result in interesting educational synergies and promote student autonomy for learning Optics.
Retinal image quality is usually analysed through different parameters typical from instrumental optics, i.e, PSF, MTF
and wavefront aberrations. Although these parameters are important, they are hard to translate to visual quality
parameters since human vision exhibits some tolerance to certain aberrations. This is particularly important in postsurgery
eyes, where non-common aberration are induced and their effects on the final image quality is not clear.
Natural images usually show a strong dependency between one point and its neighbourhood. This fact helps to the image
interpretation and should be considered when determining the final image quality. The aim of this work is to propose an
objective index which allows comparing natural images on the retina and, from them, to obtain relevant information abut
the visual quality of a particular subject.
To this end, we propose a individual eye modelling. The morphological data of the subject's eye are considered and the
light propagation through the ocular media is calculated by means of a Fourier-transform-based method. The retinal PSF
so obtained is convolved with the natural scene under consideration and the obtained image is compared with the ideal
one by using the structural similarity index. The technique is applied on 2 eyes with a multifocal corneal profile
(PresbyLasik) and can be used to determine the real extension of the achieved pseudoaccomodation.
We present a new algorithm to process captured images of reflected Placido rings. Up to our knowledge, conventional
topographers transform from Cartesian to polar coordinates and vice-versa, thus extrapolating corneal data and
introducing noise and image artefacts. Moreover, captured data are processed by the device according to proprietary
algorithms and offering a final map of corneal curvature. Corneal topography images consists of concentric rings of
approximately elliptical shape. Our proposal consists of considering the information that provides each separate ring. A
snake-annealing-like method permits identifying the ring even with discontinuities due to eye-lashes and reflections. By
analysing the geometrical parameters of rings (centre, semi-axis and orientation), one can obtain information about small
morphological micro-fluctuations and local astigmatisms. These parameters can be obtained with sub-pixel accuracy so
the method results of high precision. The method can be easily adapted to work on any topographer, so that it can
provide additional information about the cornea at no additional cost.
The behaviour of a construction safety net and its supporting structure was monitored with a high speed camera and
image processing techniques. A 75 kg cylinder was used to simulate a falling human body from a higher location in a
sloped surface of a building under construction. The cylinder rolled down over a ramp until it reaches the net. The
behaviour of the net and its supporting structure was analysed through the movement of the cylinder once it reaches the
net. The impact was captured from a lateral side with a high speed camera working at 512 frames per second. In order to
obtain the cylinder position each frame of the sequence was binarized. Through morphological image processing the
contour of the cylinder was isolated from the background and with a Hough transform the presence of the circle was
detected. With this, forces and accelerations applying on the net and the supporting structure have been described,
together with the trajectory of the cylinder. All the experiment has been done in a real structure in outdoors location.
Difficulties found in the preparation on the experiment and in extracting the final cylinder contour are described and
some recommendations are giving for future implementations.
Analysis of vibrations and displacements is a hot topic in structural engineering. Video cameras can provide good
accuracy at reasonable cost. Proper system configuration and adequate image processing algorithms provide a reliable
method for measuring vibrations and displacements in structures. In this communication we propose using a pocket
camera (Casio) for measuring small vibrations and displacements. Low end cameras can acquire high speed video
sequences at very low resolutions. Nevertheless, many applications do not need precise replication of the scene, but
detecting its relative position. By using targets with known geometrical shapes we are able to mathematically obtain
subpixel information about its position and thus increase the system resolution. The proposal is demonstrated by using
circular and elliptic targets on moving bodies The used method combines image processing and least squares fitting and
the obtained accuracy multiplies by 10 the original resolution. Results form the low-end camera (400 euros) working at
224×168 px are compared with those obtained with a high-end camera (10000 euros) with a spatial resolution of 800×560 px.
Although the low-end camera introduces a lot of noise in the detected trajectory, we obtained that results are comparable.
Thus for particular applications, low-end pocket cameras can be a real alternative to more sophisticated and expensive
Videokeratometers and Scheimpflug cameras permit accurate estimation of corneal surfaces. From height data it is possible to adjust analytical surfaces that will be later used for aberration calculation. Zernike polynomials are often used as adjusting polynomials, but they have shown to be not precise when describing highly irregular surfaces. We propose a combined zonal and modal method that allows an accurate reconstruction of corneal surfaces from height data, diminishing the influence of smooth areas over irregular zones and vice versa. The surface fitting error is decreased in the considered cases, mainly in the central region, which is more important optically. Therefore, the method can be established as an accurate resampling technique.
The eye is not a centered system. The line of sight connects the fovea with the center of the pupil and is usually tilted in the temporal direction. Thus, off-axis optical aberrations, mainly coma and oblique astigmatism, are introduced at the fovea. Tabernero et al. [J. Opt. Soc. Am. A 24(10), 3274-3283 (2007)] showed that a horizontal tilt of the crystalline lens generates a horizontal coma aberration that is compensated by the oblique light incidence on the eye. Here we suggest that corneal astigmatism may also play a role in compensation of oblique aberrations, and we propose a simple model to analyze such a possibility. A theoretical Kooijman eye model with a slight (~0.6 D) with-the-rule astigmatism is analyzed. Light rays at different incidence angles to the optical axis are considered, and the corresponding point spread functions (PSFs) at the retina are calculated. A quality criterion is used to determine the incidence angle that provides the narrowest and highest PSF energy peak. We show that the best image is obtained for a tilted incidence angle compatible with mean values of the angle kappa. This suggests that angle kappa, lens tilt, and corneal astigmatism may combine to provide a passive compensation mechanism to minimize aberrations on the fovea.
In this communication, the authors have determined the longitudinal chromatic aberrations in real eyes. The method that has been used combines real data of corneal morphology, central thickness of crystalline lens and biometric measures of axial length together with numerical calculation of the propagation process. The curvature of the crystalline lens has been adjusted to different curvature models and refractive index distributions. The wavelength dependence of all ocular media has been modelled through the Cauchy formula. Propagation through anterior and posterior chambers has been accomplished through numerical calculation of diffraction integral instead of classical ray-tracing approach. This imposes serous restrictions on the number of samples that are needed for a full propagation process. If we are only interested in amplitude calculations the method consists of evaluating propagation from cornea to crystalline lens with a spectrum propagation method. Propagation from the lens to the best image plane is accomplished by a direct calculation of Fresnel integral. With this model, we have obtained the refraction chromatic difference in diopters for several eyes. Results are compared with real measures of the chromatic aberration, showing a good agreement with numerical calculations. The capabilities of the technique have been demonstrated by applying the method to the study of the chromatic aberration of a keratoconus.