J. M. Artigas

Influence of the tear film on optical quality of the eye

The optical quality of the tear film of the eye was tested by measuring the retinal image before and after the break-up time (BUT). An objective method was used based on the evaluation of the retinal image of a point test projected onto the fovea. The loss of an entire tear film would result in a decrease in the optical quality because of corneal irregularities and the formation of an irregular tear lens after the BUT. Our results confirm the expected loss both of non-contact lens wearers and contact lens wearers. Also, the fact that the optical deterioration found after the BUT is greater for contact lens wearers confirms that soft contact lens wear produces a disruption of the tears.

Spectral Transmission of the Human Crystalline Lens in Adult and Elderly Persons: Color and Total Transmission of Visible Light

PURPOSE To experimentally measure the spectral transmission of human crystalline lenses belonging to adult and elderly persons, and to determine the color and total transmission of visible light of such crystalline lenses. METHODS The spectral transmission curve of 32 human crystalline lenses was measured using a PerkinElmer 800UV/VIS spectrometer. Total transmission of visible light and the chromatic coordinates of these crystalline lenses were determined from these curves for solar illumination. RESULTS The crystalline lens that filters UV and its transmission in the visible spectrum decreases with age; such a decrease is greater for short wavelengths. The total transmission of visible light decreases, especially after the age of 70 years, and the crystalline color becomes yellower and saturated. CONCLUSIONS The great variability existing in the spectral transmission of the human crystalline lens is lesser between the ages of 40 and 59 years, but greater from the age of 60 and older. The decrement in transmittance between these two age groups varies from 40% for 420 nm to 18% for 580 nm. Nevertheless, it is proven that age is not the only parameter affecting crystalline transmission. In the range of 40 to 59 years, age does not bear an influence on total transmission of light, but from 60 years and older it does. Moreover, the light transmitted decreases with age. This total transmission of light is similar to or lower than the amount that the different intraocular lenses transmit, even with a yellow or orange filter. The color of the human lens becomes yellowish and saturated with age.

Influence of the contrast sensitivity function on the reaction time

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) and (2) the contrast attenuation by the optical and neural transfer function, operating throughout the SF range. At high contrasts, the effect of the first factor can be clearly observed, because the effect of the second factor does not change with spatial frequency except in a region where the RT/SF function changes rapidly. At lower contrasts, however, the second factor becomes increasingly relevant while the first becomes less and less observable.

Subjective image fidelity metric based on bit allocation of the human visual system in the DCT domain

Until now, subjective image distortion measures have partially used diverse empirical facts concerning human perception: non-linear perception of luminance, masking of the impairments by a highly textured surround, linear filtering by the threshold contrast frequency response of the visual system, and non-linear post-filtering amplitude corrections in the frequency domain. In this work, we develop a frequency and contrast dependent metric in the DCT domain using a fully non-linear and suprathreshold contrast perception model: the Information Allocation Function (IAF) of the visual system. It is derived from experimental data about frequency and contrast incremental thresholds and it is consistent with the reported noise adaptation of the visual system frequency response. Exhaustive psychophysical comparison with the results of other subjective metrics confirms that our model deals with a wider range of distortions more accurately than previously reported metrics. The developed metric can, therefore, be incorporated in the design of compression algorithms as a closer approximation of human assessment of image quality.

Image quality metric based on multidimensional contrast perception models

Abstract The procedure to compute the subjective difference between two input images should be equivalent to a straightforward difference between their perceived versions, hence reliable subjective difference metrics must be founded on a proper perception model. For image distortion evaluation purposes, perception can be considered as a set of signal transforms that successively map the original image in the spatial domain into a feature and a response space. The properties of the spatial pattern analyzers involved in these transforms determine the geometry of these different signal representation domains. In this work the general relations between the sensitivity of the human visual system and the perceptual geometry of the different representation spaces are presented. This general formalism is particularized through a novel physiological model of response summation of cortical cells that reproduce the psychophysical data of contrast incremental thresholds. In this way, a procedure to compute subjective distances between images in any representation domain is obtained. The reliability of the proposed scheme is tested in two different contexts. On the one hand, it reproduces the results of suprathreshold contrast matching experiences and subjective contrast scales (Georgeson and Shackleton, Vision Res. 34 (1994) 1061–1075; Swanson et al., Vision Res. 24 (1985) 63–75; Cannon, Vision Res. 19 (1979) 1045–1052; Biondini and Mattiello, Vision Res. 25 (1985) 1–9), and on the other hand, it provides a theoretical background that generalizes our previous perceptual difference model (Malo et al., Im. Vis. Comp. 15 (1997) 535–548) whose outputs are linearly related to experimental subjective assessment of distortion.

Residual astigmatism produced by toric intraocular lens rotation

PURPOSE: To analyze changes in the eye’s refractive properties when a toric intraocular lens (IOL) rotates. SETTING: Fundación Oftalmológica del Mediterráneo, Valencia, Spain. DESIGN: Experimental study. METHODS: The matrix definition of astigmatism was used in this theoretical study and compared with another vector representation. Two methods were compared: (1) The cylinder, C, resulting from the addition of 2 cylinders C1 and C2 whose axes form an angle a, is obtained by the addition of 2 vectors of values C1 and C2 forming an angle 2a; (2) the power matrix, F, of a thin astigmatic dioptric system that decomposes naturally into 3 orthogonal components: the purely spherical part Fnes, the ortho‐astigmatism For, and oblique astigmatism Fob. RESULTS: The residual cylinder was one third of the corneal astigmatism when a toric IOL rotated ±10 degrees when the cylinder values for the cornea (C1) and IOL (C2) were equal. Nevertheless, in most cases C1 is greater than C2; therefore, the residual astigmatism did not change noticeably with small rotations. The angle of rotation, b, which annuls the astigmatism correction, could be obtained from the following: cos(π + 2b) = −r/2, with r being the ratio between the IOL and cornea cylinders. CONCLUSIONS: The 2 methods gave equivalent results. When the IOL cylinder had a value different from that of the corneal astigmatism, a better choice would be a lower, rather than higher, cylinder value to reduce residual astigmatism. In general, toric IOL rotations less than 10 degrees changed the eye’s refraction less than 0.50 diopter. Thus, small axis rotations are not an obstacle for satisfactory astigmatism correction with toric IOLs. Financial Disclosure: No author has a financial or proprietary interest in any material or method mentioned.

Spectral Transmittance of Intraocular Lenses under Natural and Artificial Illumination: Criteria Analysis for Choosing a Suitable Filter

PURPOSE To compare the spectral transmission of different intraocular lenses (IOLs) with either ultraviolet (UV) or blue-light filters, and to analyze the performance of these filters with artificial light sources as well as sunlight. DESIGN Experimental study. METHODS The spectral transmission curve of 10 IOLs was measured using a PerkinElmer Lambda 800 UV/VIS spectrometer (Waltham, MA). Different filtering simulations were performed using the D65 standard illuminant as daylight and standard incandescent lamp and fluorescent bulb illuminants. MAIN OUTCOMES MEASURES Spectral transmittance of the IOLs. RESULTS All the IOLs studied provide good UVC (200-280 nm) and UVB (280-315 nm) protection, except for one that presented an appreciable window at 270 nm. Nevertheless, both natural and artificial sources have practically no emission under 300 nm. In the UVA (315-380 nm) range the curves of the different IOLs manifested different degrees of absorption. CONCLUSIONS Not all the UV filters incorporated in different IOLs protect equally. The filters that provide greater photoprotection against UV radiation, even blue light, are yellow and orange. Then, yellow and orange IOL filters may be best suited for cases requiring special retinal protection. The filters that favor better photoreception of visible light (380-780 nm) are those that transmit this radiation close to 100%. Artificial illumination practically does not emit in the UV range, but its levels of illumination are very low when compared with solar light. A possible balance between photoprotection and photoreception could be a sharp cutoff filter with the cutoff wavelength near 400 nm and a maximum transmittance around 100%.

Correlation between optics quality of multifocal intraocular lenses and visual acuity Tolerance to modulation transfer function decay

PURPOSE: To study the relationship between the optics quality of multifocal intraocular lenses (IOLs) and distance‐corrected near, intermediate, and distance visual acuity to determine the degree to which optics quality must change to produce a significant difference in visual acuity. SETTING: Fundación Oftalmológica del Mediterráneo, Valencia, Spain. METHODS: Monocular distance‐corrected visual acuity (decimal and logMAR) was measured at 4 m, 70 cm, and 30 cm under photopic and mesopic conditions in eyes with 1 of 3 multifocal IOL models. Visual acuity versus the “average modulation” of the IOL was assessed. RESULTS: The pupil diameter and patient age were similar between the 3 IOL groups (20 eyes each). A difference in average modulation (ie, optics quality) up to 15% did not produce significant differences in mean visual acuity between 2 groups; however, there was a significant difference in mean visual acuity between 2 groups when the average modulation values differed by at least 25%. The slope of the linear correlation between visual acuity and average modulation was 0.018 (r2 = 0.91) under photopic conditions and 0.024 (r2 = 0.089) under mesopic conditions. CONCLUSIONS: There was a strong linear correlation between IOL optics quality and visual acuity with distance correction under photopic conditions with a pupil of approximately 3.5 mm in a cohort in which the patient age was controlled. The correlation was similar under mesopic conditions. The eyes tolerance to modulation transfer function decay was approximately 15% of the average modulation value. Financial Disclosure: No author has a financial or proprietary interest in any material or method mentioned.

Modulation transfer function: Rigid versus foldable phakic intraocular lenses

PURPOSE: To study in a standard eye model the changes in modulation transfer function (MTF) of a monofocal intraocular lens (IOL) when a phakic IOL (pIOL) is placed in the anterior chamber, compare the MTFs of the rigid Artisan pIOL and foldable Artiflex pIOL, and evaluate the temporal evolution of the MTF of the foldable pIOL after the mechanical stress the pIOL undergoes when injected. SETTING: Fundación Oftalmológica del Mediterráneo, Valencia, Spain. METHODS: The MTF values of the IOLs were calculated from the cross‐line spread function recorded with the Opal Vector System. The measurements were taken using an eye model following the British and EN‐ISO standards with 2.0 mm, 3.0 mm, 4.0 mm, and 5.0 mm pupils. A 28.00 diopter (D) Ophtec monofocal IOL was used as the crystalline lens. The 2 pIOLs were −9.00 D. RESULTS: The MTF of the rigid pIOL was slightly better than the MTF of the foldable pIOL with all pupil sizes. Both pIOLs provided good optics quality when compared with the monofocal IOL. The injection effect of the foldable IOL disappeared after 2 hours. CONCLUSIONS: The MTF of the monofocal IOL was slightly reduced with implantation of a negative pIOL in the anterior chamber. The rigid pIOL provided better optical performance than the foldable pIOL with all pupil sizes, as shown by the MTF values. The decrease in MTF caused by the mechanical stress on the foldable pIOL was nullified after 2 hours with no effect on optical quality.

Contrast sensitivity of the visual system in speckle imagery

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 CSFs determined with and without speckle is quantitatively very important. And the ratio between the two CSFs is a measure of the retinal ability to perceive contrast in the presence of speckle noise. The influence of the pupil size and luminance level on the CSF with speckle is studied and compared with the influence of the same parameters on the classical CSF.