Search results for "Spatial frequency"
showing 10 items of 92 documents
Geometrical Transformations In The Fraunhofer Plane
1987
A virtual display of the Fraunhofer diffraction pattern is generated solely by illuminating the object with a point source. If this pattern is imaged with an anamorphic system, several linear geometrical transformations can be achieved. Furthermore, a nonsymmetrical Fourier transformer with a variable degree of anamorphic magnification on the Fraunhofer pattern can be implemented.
Cue combination in a combined feature contrast detection and figure identification task
2006
AbstractTarget figures defined by feature contrast in spatial frequency, orientation or both cues had to be detected in Gabor random fields and their shape had to be identified in a dual task paradigm. Performance improved with increasing feature contrast and was strongly correlated among both tasks. Subjects performed significantly better with combined cues than with single cues. The improvement due to cue summation was stronger than predicted by the assumption of independent feature specific mechanisms, and increased with the performance level achieved with single cues until it was limited by ceiling effects. Further, cue summation was also strongly correlated among tasks: when there was …
On the function of cell systems in area 18. Part I
1981
In addition to the asymmetry of the spatial coupling and of the specific temporal combination of excitation and inhibition, the non-linearity is very pronounced in area 18. Taking the sequence of a linear operation and a stationary nonlinear characteristic as a model, the experimental findings can be systematized and a cell classification specified which departs from the customary ones. The hypercomplex cell system probably originates in recurrent inhibition and leads to differentiation of the patterns along their contour line. Problems of cell classification and of the type of parallelism in the visual cortex are discussed.
Modelling spatial and spectral systematic noise patterns on CHRIS/PROBA hyperspectral data
2006
In addition to typical random noise, remote sensing hyperspectral images are generally affected by non-periodic partially deterministic disturbance patterns due to the image formation process and characterized by a high degree of spatial and spectral coherence. This paper presents a new technique that faces the problem of removing the spatial coherent noise known as vertical stripping (VS) usually found in images acquired by push-broom sensors, in particular for the Compact High Resolution Imaging Spectrometer (CHRIS). The correction is based on the hypothesis that the vertical disturbance presents higher spatial frequencies than the surface radiance. The proposed method introduces a way to…
Correction of systematic spatial noise in push-broom hyperspectral sensors: application to CHRIS/PROBA images
2008
Hyperspectral remote sensing images are affected by different types of noise. In addition to typical random noise, nonperiodic partially deterministic disturbance patterns generally appear in the data. These patterns, which are intrinsic to the image formation process, are characterized by a high degree of spatial and spectral coherence. We present a new technique that faces the problem of removing the spatially coherent noise known as vertical striping, usually found in images acquired by push-broom sensors. The developed methodology is tested on data acquired by the Compact High Resolution Imaging Spectrometer (CHRIS) onboard the Project for On-board Autonomy (PROBA) orbital platform, whi…
Rotation-invariant optical recognition of three-dimensional objects.
2008
An automatic method for rotation-invariant three-dimensional (3-D) object recognition is proposed. The method is based on the use of 3-D information contained in the deformed fringe pattern obtained when a grating is projected onto an object’s surface. The proposed method was optically implemented by means of a two-cycle joint transform correlator. The rotation invariance is achieved by means of encoding with the fringe pattern a single component of the circular-harmonic expansion derived from the target. Thus the method is invariant for rotations around the line of sight. The whole experimental setup can be constructed with simple equipment. Experimental results show the utility of the pro…
Synergy of features enables detection of texture defined figures
2006
Traditional theories of early visual processing suggest that elementary visual features are handled in parallel by independent neural pathways. We studied the interaction of orientation and spatial frequency in the discrimination of Gabor random fields. Target textures differed from reference textures either in mean feature value, showing an edge-like transition between both textures (edge defined), or in the degree of feature homogeneity with smooth transitions (region defined). Irrespective of the kind of texture definition, we found strong cue summation for targets defined by both cues simultaneously, provided two conditions were fulfilled. First, they were barely discriminable when defi…
Contrast sensitivity of the visual system in speckle imagery
1994
The contrast sensitivity function (CSF) of the whole visual system is determined with the use of coherent diffuse illumination. This function provides supplementary data about the effect of speckle on the ability of the visual system to perceive the spatial information contained in an image. The results show that speckle not only prevents perception of the finest details (highest frequencies) but also reduces the visibility of lower frequencies (especially where contrast is low). The difference between the CSF's determined with and without speckle is quantitatively very important. And the ratio between the two CSF's is a measure of the retinal ability to perceive contrast in the presence of…
Influence of the contrast sensitivity function on the reaction time
1993
The reaction time (RT) vs spatial frequency (SF) curve is determined, using gratings from 1 to 40 c/deg, at seven different contrast levels between 0.95 and 0.02. The form of the RT/SF function: (a) replicated the inverse of the contrast sensitivity function (CSF) at near threshold contrast levels; (b) behaved differently at higher contrasts, exhibiting two branches at contrast close to 1. The interpretation is that there are two factors determining this function: (1) the transition from the operation of fast transient channels at low SF to the operation of slow sustained channels at high SF, the transition taking place within a narrow SF band close to 6 or 8 c/deg (depending on the subject…
Correlation of pupil size with visual acuity and contrast sensitivity after implantation of an apodized diffractive intraocular lens
2007
To determine whether pupil size is correlated with visual acuity and contrast sensitivity at all distances in eyes with an apodized diffractive intraocular lens (IOL).Private Clinic, Oviedo, Spain.Six months after surgery, the best corrected distance visual acuity, best distance-corrected near visual acuity, intermediate visual acuity, and distance contrast sensitivity under photopic (85 cd/m2) and mesopic (5 cd/m2) conditions were measured in 670 eyes of 335 consecutive patients who had implantation of the AcrySof ReSTOR Natural IOL (SN60D3, Alcon). Pupil diameters in distance vision were measured using a pupillometer.The logMAR best corrected distance acuity was significantly better with …