Search

This study focuses on the changes induced in both the asphericity and homogeneity of the cornea for a group of myopic eyes undergoing LASIK surgery. Eyes were characterized by a Kooijman-based customized eye model in which changes were introduced in the form of Gaussian-distributed refractive-index variations of given correlation length for the inhomogeneities and in the form of an expression, based on the modified Munnerlyn's paraxial formula, for the post-LASIK asphericity. Visual quality was evaluated in terms of the Modulation Transfer Function and the Point-Spread Function. The results show that, on average, the evolution of visual acuity is consistent with the change in corneal asphericity, while the evolution of contrast sensitivity requires a loss in corneal homogeneity in order to be explained. By including both effects in the model, the overall model performance in predicting visual quality is improved.

An expression has been deduced theoretically from a schematic-eye model, for the transverse or lateral chromatic aberration (TCA) after refractive surgery. The aim was to investigate analytically how chromatic aberration varies after the emmetropization process. These changes in the TCA have been characterized from changes in corneal asphericity. The results indicate that TCA after refractive surgery diminishes as the degree of myopia increases, a trend contrary to that occurring with monochromatic aberrations, such as spherical or coma. These results can explain the fact that the real deterioration of the visual function under photopic conditions detected in those operated on for myopia is less than expected when only monochromatic aberrations are taken into account.

In this work, an improvement to the Munnerlyn formula used in refractive surgery is proposed by selecting the final corneal asphericity. A theoretical optical analysis has been performed by adopting the RMS (root mean square) spot size as a merit function. Two different sets of myopes having refractive errors ranging up to —7D have been tested. The results show that a corneal asphericity of around Q=—0.61, with the radius provided by the emmetropization formula based on thin lens theory, may improve image quality after surgery. This value could be used in practical surgery because it is easy to implement in the ablation algorithm and it requires no additional data from other clinical surgery instrumentation.

Purpose. To determine the influence of retinal illuminance on monocular or binocular visual reaction time (VRT). Methods. On two normal subjects, uniform circular stimuli were presented with respect to a reference stimulus at the fovea under suprathreshold conditions, and the detection of positive and negative luminance variations was recorded. Binocular and monocular reaction times were measured (by the index finger pressing on a mouse key) for viewing with both natural and artificial pupils. Results. Binocular reaction times were shorter than monocular reaction times; nevertheless, this binocular-summation effect was less marked in trials with the artificial pupil. Analyses of binocular-summation ratios for contrast changes for both pupil types indicated maximum and minimum binocular-saturation values depending on contrast variations in both positive and negative luminance changes. Conclusions. Binocular summation can be influenced by pupil size under suprathreshold conditions. Results are discussed in terms of retinal illuminance and cortical pupil response mechanisms.

The effect of spatial summation on chromatic detection at suprathreshold levels for the red-green and yellow-blue chromatic opponent mechanisms has been studied using simple reaction time (RT) as a measurement of the response. We varied the target size in the experiments from 8 min of arc in diameter to 10 deg, and the hue-substitution paradigm was adopted to generate only chromatic changes in the cone input. Equiluminant stimuli were distributed along three tritan and three red-green confusion lines. Three reference stimuli were chosen as both reference and adapting stimuli, in this case to determine whether the chromatic adaptation affected the spatial summation. The results showed that the mean value of RT at a constant cone-input variation, either of the L #x2212; 2M channel or of the S #x2212; (L #x002B; M) channel, against the target size decreased until a stimulus size was reached, the RT being constant for greater sizes. The spatial summation was fulfilled for a maximum target size which was less for the L #x2212; 2M channel than for the S #x2212; (L #x002B; M) channel, this size decreasing as the cone-input variation rose. Furthermore, there was no spatial summation for high-enough variations in the cone input. In addition, the chromatic mechanisms appear to organize their spatial integration with the adapting level.

The present study uses simple reaction time (RT) to examine the temporal response to chromatic changes of the red-green (L-2M) and yellow-blue (S-(L 1M)) chromatic opponent channels. The hue-substitution method was adopted to negate the luminance channel and generate changes in the chromatic input. The equiluminant stimuli were distributed along three tritanopic and three red-green confusion lines. RTs were measured at equiluminance from three reference stimuli with chromatic changes occurring in either the tritan or red-green directions. These reference stimuli also allowed evaluation of the influence of chromatic adaptation level on RT. The results showed that the mean value of RT, for the cone-input variation either of the L-2M channel (S-constant cone axis) or of the S-(L 1M) channel (L&M-constant cone axis), was described by a similar law as for luminance variations (Pieron's law). Then, the sen- sitivity of the two channels for chromatic changes was equalized to study their relative response speed. In addition, for both axes, an increase in the chromatic adaptation level lengthens response delay.© 2001 John Wiley & Sons, Inc. Col Res

On the basis of signal theory, we analyse the different information content of colour stimuli that correspond to variations along the red-green and yellow-blue channels (or chromatic signals). The experimental results for stimuli generated by two CRT colour monitors and extended to handmade samples show a higher chromatic contrast when pairs of stimuli are generated by red-green variations than by yellow-blue variations. This information should be taken into account in such applied areas as colour reproduction, image processing, colour displays, and computer graphic design, which use chromatic information based on human chromatic perception in order to achieve a more effective presentation of colour information.

In the present work, we offer a theoretical analysis of retinal image quality for different groups of myopes (-2D, -4D, -8D, -12D) subjected to refractive surgery. For this, the polychromatic optical MTF (modulation transfer function) corresponding to the far point of a myopic eye was compared with the image for the infinity point after refractive surgery. The optical quality of the retinal image was studied, considering the defocusing corresponding to chromatic aberration and the spherical aberration. The results for retinal image quality show a general deterioration, which was accentuated with the degree of myopia. The results suggest that refractive surgery could be improved by modifying the asphericity of the anterior surface of the cornea, which, in addition to making the eye emetropic, would optimize the MTF of the subjects. We also present a theoretical analysis involving non-spherical surfaces before and after operation, testing the influence of corneal asphericity in this case, as well a...

In the present work, we use random-dot stereograms to test a possible relationship between associated phoria and stereopsis. We determined, using a modified constant-stimulus method, the disparity range that indicates the maximum range at which stereoscopic correspondence can be achieved. A total of 27 observers took part in the experiment. The value of the disparity range as a function of the associated phoria (measured with a Mallet unit) seems to indicate that greater associated phoria (fixation disparity) correlates with a deterioration in stereoscopic vision, reducing the disparity range and therefore the space region in which stereoscopic vision can be attained.

The color-matching functions of six observers have been measured in the range of 400–700 nm in intervals of 10 nm using two modified Donaldson-type colorimeters at a luminance of 4.5 cd/m. A statistical study has been carried out to determine the interobserver variability (in our results), and the results of other authors using various representational color systems have been compared with ours. The most important findings are that, although a common mathematical mean cannot be found, the shape of the curves for the color-matching functions obtained in our laboratory for all the observers is very close to that of other authors, thereby implying that the overall color response of the human visual system is similar in all observers. In addition, the results show that, for the long wavelengths (around 600 nm), a great amount of desaturant primary (460 nm) is necessary, indicating possible rod intrusion under our experimental conditions.© 1999 John Wiley & Sons, Inc. Col Res Appl, 24, 377–388, 1999

In the present experimental study, we quantify the influence of the brightness and contrast levels of a CRT- color monitor in the color reproduction of 60 Munsell chips distributed throughout the chromatic diagram. The images were captured by two CCD cameras, and the color differ- ences were evaluated after reproducing the chips on a color monitor (the experiment was performed with 3 different monitors) for 9 combinations of brightness-contrast levels. We evaluated the color differences with 3 different formu- las: CIELAB, CIELUV, and CIE94. The results indicate that the optimal settings of a monitor, to minimize the color differences, is a medium or minimum brightness level in combination with a maximum contrast level. This combina- tion ensures a more faithful color reproduction with respect to the original image.© 1999 John Wiley & Sons, Inc. Col Res Appl

Many works have shown colorimetric additivity failures in colour-matching experiments. Thus, in the present work, using the colour-matching functions (CMF) of three real observers, we have conducted five experiments in isomeric and metameric matching. The stimuli matched, using two modified Donaldson colorimeters, were two achromatic, one blue, one orange and one green. Afterwards, we determined both the isomeric and the metameric discrimination ellipsoids for all the matches and for each observer. The results reflected differences in chromaticity and luminance with respect to the results of the CIE-1931 Standard Observer. When we compared our results (of each observer with his CMF) for the isomeric and metameric matches, we found that, according to the differential colour thresholds, the prediction was acceptable for chromaticity but not for luminance. The experimental results revealed the need to revise some aspects of the CIE-1931 Standard Observer.

The present work examines the relationship between random-dot stereograms (via the disparity range parameter) and color-vision mechanisms (via the luminance channel and red-green and tritan directions at isoluminance). The results clearly indicate that the variations in the stereograms along red-green confusion lines contribute to stereopsis. Stereoscopic perception depends on spatial information for stereograms generated with variations along tritan confusion lines. For observers who perceive stereopsis via tritan directions, the results show a gradation in the disparity range, with the disparity range for stereograms generated by luminance variations being greater than for stereograms generated in red-green directions; the latter range is, in turn, greater than for stereograms generated along tritan directions. © 1997 Elsevier Science Ltd. All rights reserved

In this work, we analyze the way in which pupil size, optical zone, and initial hyperopic level influence optical quality for hyperopic Q-optimized corneal refractive surgery. Different Q-optimized algorithms and the Munnerlyn formula were tested to analyze the optical quality of the final retinal image for initial hyperopic errors from 1D to 5D. Three optical zones (5.5, 6, and 6.5 mm) and two pupil diameters (5 and 7 mm) were considered. To evaluate optical quality, we computed the modulation transfer function (MTF) and the area under MTF (MTFa). Q-optimized values at around Q = −0.18 were found to provide the best optical quality for most of the conditions tested. This optimum final asphericity for hyperopic ablation was not depending on the degree of hyperopia corrected, the optical zone or the pupil size being this information important for clinical practice.

The influence of color signals on stereopsis has been studied using figural-stimuli stereograms with variations introduced according to the opponent chromatic channels (red-green and yellow-blue), derived from Boynton's color-vision model. We used wallpaper stereograms, which enable the rank-order disparity ranges of the chromatic and luminance signals to be compared with the rank-order disparity range of proximity, a particular spatial configuration of the stereogram in which there are no variations in chromaticity and/or luminance. The results indicate that both chromatic signals contribute to stereopsis as does the luminance signal, contradicting the model of Hubel and Livingstone. The results also show there are no clear dependencies upon the kind of signal processed, as luminance and chromatic variations are processed with the same efficiency.

The experimental determination of differential chromaticity thresholds from metameric matches show discrepancies in colorimetric prediction using the standard observers established by the CIE (Hita et al., J Opt Soc Am A 1986; 3: 1203-09; Jimenez del Barco et al., J Optics 17, 1986; 247-54). In this work we have determined whether the discrepancies of colorimetric prediction are contained in the differential color thresholds. To study the possibility of generalizing prior results to a wider range of experimental conditions we have carried out new experiments determining the differential color thresholds of an achromatic stimulus from metameric matches, using direct vision and a foveal field of 1.5 degrees . The results are expressed in terms of the color-matching functions of CIE-2 degrees standard observer, as well as those proposed by 2 degrees -Stiles and Burch and Nayatani et al. The discussion of the results is made taking into account different causes which may explain the failures of colorimetric prediction detected: rod intrusion, interactions of the responses of cones sensitive to short-wavelengths, errors and matching imprecision. Results show that the failures of colorimetric prediction obtained under our experimental conditions exceed the color differential threshold. These discrepancies increase with the relative difference of the S-cone excitation level for the matched stimuli.

In this paper, we provide an analytic expression for an adjustment factor in ablation algorithms for photorefractive laser surgery which take into account reflection losses and the nonnormal incidence on the cornea. We evaluate the influence of this factor on certain ocular parameters, calculating alterations in the radius of anterior cornea and corneal asphericity when we use the algorithm given by Munnerlyn’s paraxial formula. Our data indicate that this adjustment factor should be considered in the ablation algorithms that are currently being proposed in customized corneal ablation and that need great accuracy for the correction of eye aberrations.

The solution of the differential Eq. Delta g omega =rho 2 (where Delta g is the Laplace operator with respect to the metric g and rho is the scalar curvature) can provide some information in order to obtain an isotropic color space, i.e. a system in which the differential chromaticity thresholds are circles of different sizes. In three dimensions Weyls' theorem offers a geometric criterion to analyze new line elements from a practical viewpoint.

In order to study the mathematical reconstruction of diffuse spectral reflectance functions, the authors have generated an eigenvectors basis obtained after realizing a singular value analysis over Munsell chips, clustered by a hue classification.

Visual reaction time (VRT) has been measured for the detection of an achromatic stimulus on a CRT colour monitor. The adapting backgrounds were blue, cyan, green, yellow and red and the spatial and temporal presentation of the stimulus was computer-controlled. Comparisons of VRT have also been made between various retinal zones. The results are compared with those in the literature, obtained through conventional optical methods and are explained in terms of the opponent-colour theory.

New results of chromatic discrimination have been obtained for light transmitted by coloured filters and with a method which guarantees their reproducibility. These results are compared with others obtained for surface-colour presentation through the predictions of several colour-difference formulae: those recommended by CIE (CIELUV and CIELAB), the LABNHU formula, and the cdf-G, cdf-G* formulae, recently proposed by the authors' laboratory.

We determined visual reaction times to monocular and binocular changes in the luminance of isochromatic stimuli and to monocular and binocular changes in the color of isoluminant stimuli. Two isoluminant color changes were tested: chromatic variations along the red—green axis of Boynton’s (1986) two-stage color vision model and chromatic variations along the yellow—blue axis of the same model. The results indicate a greater degree of binocular summation for luminance change than for color change. This result was largely independent of the motor component of reaction time.

In this work, we evaluate the impact of induced aniseikonia on stereopsis. For this, we determined the disparity range (maximum disparity), a parameter related to the size of the physical region that can be perceived stereoscopically. A significant decline in the disparity range was detected with aniseikonia induced by size lenses of 3% for five of the seven observers tested; 5% was necessary for the other two observers. The data indicate the influence of aniseikonia in stereopsis and the need to minimize such impact. These results may be useful in surgical processes such as the correction of pseudophakic patients and refractive surgery in which aniseikonia can be induced to alter the binocular function of the patient.

The effect that decentered spectacle (ophthalmic) lenses exert on depth perception has been studied, evaluating stereopsis through the disparity range (maximum horizontal disparity) for random-dot stereograms (RDS). The results show that variations in fusional convergence due to increments of decentration can diminish the stereopsis in observers, reducing the region within which stereoscopic correspondence can take place. Decreases in disparity ranges were found for vertical and horizontal prismatic effects, although the prismatic effect necessary for this was less in the vertical case. A decreased disparity range has also been confirmed with figural-stimuli stereograms and using prisms for generating the prismatic effects.

The influence of color signals on stereopsis has been studied using figural-stimuli stereograms with variations introduced according to the opponent chromatic channels (red-green and yellow-blue), derived from Boynton's color-vision model. We used wallpaper stereograms, which enable the rank-order disparity ranges of the chromatic and luminance signals to be compared with the rank-order disparity range of proximity, a particular spatial configuration of the stereogram in which there are no variations in chromaticity and/or luminance. The results indicate that both chromatic signals contribute to stereopsis as does the luminance signal, contradicting the model of Hubel and Livingstone. The results also show there are no clear dependencies upon the kind of signal processed, as luminance and chromatic variations are processed with the same efficiency.

Is the response of color vision mechanisms linear and additive for stimuli with different spectral radiant power distributions? We studied this possibility by means of results obtained when differential chromaticity thresholds are determined in metameric conditions for an achromatic stimulus. Colors corresponding to stimuli with smooth spectral radiant power distributions and a wide bandwidth are matched with colors corresponding to stimuli and narrow bandwidth (very different degrees of metamerism).

An evaluation of chromatic and color-threshold differences was carried out by using the CMC(l:c) and the BFD(l:c) color-difference formulas [ J. Soc. Dyers Colour.100, 128 ( 1984); J. Soc. Dyers Colour.103, 86 ( 1987)] from experimental threshold results obtained with visual colorimeters [ J. Opt. Soc. Am.32, 247 ( 1942); J. Opt. Soc. Am.39, 808 ( 1949); J. Opt. Soc. Am.47, 137 ( 1957); J. Opt. Soc. Am.61, 1135 ( 1971)]. The good results previously reported on the use of these two new formulas with surface colors are also found for the chromatic differences calculated, especially with the BFD(1:1) formula. In the case of color differences BFD(0.6:1) improves the results achieved with BFD(1:1) and gives results similar to those of CMC(1:1). The claim of CMC(l:c) and BFD(l:c) as sometimes superior alternatives to the currently recommended CIE formulas CIELUV and CIELAB can be supported by the present results.

An experimental study of Weber's law is proposed using two different methods: by means of a bipartite field or by using the Masson rotating disc. Simple devices are shown with which university students can perform the experiments. Differences between the two methods are analysed in order to verify the above mentioned law.

The relationship between reaction time and visual latency relative to a fixed stimulus has been measured in haploscopic vision by means of the flicker effect produced by two stimuli of different luminance levels. The fields of foveal vision were of 50', the retinal illuminance was from 8 to 100 td and three observers were used. The results show in all cases that the motor component in the nervous system is not constant, and that the evolution of this factor according to the luminance of the stimulus conforms to a function of the same kind as Pieron's law.

Chromaticity differential thresholds for five colour stimuli and three observers have been measured from metameric matching. For each of the stimuli four differential thresholds were obtained, each of them corresponding to a different value of the degree of metamerism of the matching. Results obtained show failures in chromaticity predictions of the stimuli evaluated in the metameric matchings which are discussed with regard to the degree of metamerism, evaluated by different indexes proposed in the literature. These metamerism indexes lead to different results which are also discussed.

The visual reaction time has been analyzed as a function of two experimental parameters : size of the field and chromaticity of the stimulus. With regard to the first one, field sizes of 0.72, 1.25, 2, 3.5, 5 and 7.5 degrees were studied. The results show that reaction time decreases when the field size increases, also for values above the limits of Ricco's law. On the other hand, the reaction time was measured for chromatic stimuli with dominant wavelengths at 472, 500, 547, 571, 602 and 634 nm. In this case, no correlation between reaction time and chromaticity was found. The fact that these results were in agreement with those obtained for achromatic stimuli at the same luminance was analyzed in terms of color vision models, especially the opponent-colors theory.

Simultaneous and successive methods of comparison of stimuli are studied by comparing experimental results of colour discrimination experiments. In this way, colour differential thresholds for two normal observers and four different stimuli were measured by the two methods. In most cases, the capacity to discriminate colour decreased when the successive method was used, although no differences were found in qualitative aspects of discrimination. These results differ somewhat from previous reports in this field, probably because of experimental differences in the method of obtaining the thresholds.

Colour differential thresholds have been measured for three reference colours (red, green and blue) under five adaptation conditions (red, green, blue, achromatic and darkness) by a process of successive presentation. The resulting discrimination ellipsoids have been analysed both in terms of chromaticity and luminance. An overall worsening of discrimination capacity occurs under red-adaptation conditions, while all the adaptation conditions produce alterations in discrimination capacity along specific lines in the CIE1931 chromaticity diagram. The results conform to an opponent-colour model for colour vision.

Experimental results obtained for different observers and stimuli are analyzed to study the influence of the exposure time of the test on the differential thresholds of color. Exposure times from 0.2 to 9 sec are studied. A minimum for the differential thresholds of luminance and chromaticity and for the global differential threshold is found around 1 sec.

The differential chromaticity thresholds of three color stimuli have been determined from metameric matches carried out during the cone-plateau period after photopigment bleaching. The results are compared with some recently published ones [E. Hita, L. Jiménez del Barco, and J. Romero, J. Opt. Soc. Am. A 3, 1203 (1986)] obtained under similar experimental conditions but without prior photopigment bleaching. The possible effects of rod intrusion on color-prediction discrepancies are discussed in the light of both sets of results. Under our experimental conditions, with foveal fields of 3 deg, luminance within the low photopic level, and sets of primary colors produced by filters of differing spectral bandwidth, the rod intrusion effect can be ruled out as a possible cause of color-prediction discrepancies, although it does appear to produce variations in the sizes of the discrimination ellipses and in their orientation.

The differential chromaticity thresholds obtained by varying degrees of metamerism have been evaluated for five target chromaticities. Differences have been found between the chromaticities predicted by the CIE-1931 standard observer and those actually observed under our experimental conditions, with foveal fields of 3 degrees, luminance within the low photopic level, and sets of primary colors produced by filters of differing spectral bandwidths. Failures of colorimetric additivity under these experimental conditions are discussed. The results have also been analyzed by using other sets of color-matching functions, i.e., Judd's proposed modification at short wavelengths and also the supplementary standard observer, CIE-1964.