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A formula for calculating the focal IOL power based on SRK-II is suggested and clinically tested. Quadratic non-linear allowances in small (below 23 mm) and big values of the anterior-posterior eye axis are introduced into it, instead of linear allowances, to enhance the calculation accuracy.
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We have investigated the effects of spherical aberration (SA) on visual performance by fitting subjects with rigid lenses of varying front surface asphericity. Three pairs of lenses were worn by each subject with front surface asphericities of Q = 0 (conventional spherical surface), Q = -0.26 (similar to the average corneal asphericity) and Q = -0.51. The calculated average change in spherical aberration induced by each pair of contact lenses in photopic conditions was +0.02 D (Q = 0), -0.49 D (Q = -0.26) and -0.94 D (Q = -0.51). In mesopic conditions these same lenses induced an average change in spherical aberration of +0.01 D (Q = 0), -1.18 D (Q = -0.26) and -2.16 D (Q = -0.51). In this double-blind study, 12 adapted, low to moderate myopic rigid lens wearers wore each pair of lenses for one week and noted visual performance. With each pair of lenses we also measured the subject's visual acuity with high and low contrast logMAR charts. All measurements were conducted using the right eye at distance in photopic and mesopic conditions. When forced to select the lenses they would accept as compensation for participation in the study, most subjects (9 out of 12) chose the Q = 0 lenses, 3 subjects those the Q = -0.26 lenses and none of the subjects chose the Q = -0.51 lenses. Visual performance measures showed little difference between the Q = 0 and Q = -0.26 lenses. However, the Q = -0.51 lenses significantly reduced visual acuity in mesopic conditions with the low contrast acuity chart.
This study presents results obtained after implantation of 25 diffractive multifocal intraocular lenses. The follow-up ranged from six to 12 months. Visual acuity for distance without correction was 0.5 or better in 72% of cases; for near, it was J1 to J3 in 84% of cases. Slight decentration of the lens and pupil size did not affect visual acuity.
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Optical zone diameters in photorefractive keratectomy are small in order to minimize the depth of stromal tissue excision and, possible, the resultant stromal scarring. A small optical zone demands accurate placement on the corneal surface. This article reviews the principles that are important to location and measurement of the optical zone in photorefractive keratectomy. The ablated area should be centered on the cornea overlying the entrance pupil while the patient is fixating coaxially with the surgeon. An optical zone that is too small or decentered may decrease acuity, lessen contrast sensitivity, or produce glare, either on the fovea or on the perifoveal retina. The relationship between optical zone size and location and quality of vision is not yet known. Studies of this relationship will require videokeratography to locate the optical zone and its relationship to the entrance pupil, a capability that current videokeratoscopes do not possess.
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The articular cartilage of the hip joint and the intervertebral disks (LIV and LV), obtained from 42 corpses without any signs of pathology in these tissues, have been investigated. Six age groups have been distinguished: 21-30, 31-40, 41-50, 51-60, 61-70, 71 years of age and older. After the topo-optic reactions have been get, double refraction of glycosamineglycans (GAG) and collagen is studied in the structure and orientational regularity is estimated in these macromolecules in the matrix. The peculiarities on distribution of the collagenous fiber fasciculi in the intercellular substance in the hyalin and collagen-fibrillar cartilaginous tissue are demonstrated at the areas and depending on the age. Correlations in qualitative and quantitative changes of the GAG and collagen with age are analysed; this gives certain possibilities to reveal signs of similarity and difference in morphogenesis of hyalin and collagen-fibrillar cartilage tissue. Basing on the comparative analysis performed concerning the GAG and collagen state in the age aspect, as well as taking into account the data of previously performed investigations, the collagenous carcass can be characterized as the most stable component of the intercellular substance of cartilages, possessing a great reserve of strength. Homokinesis of the cartilaginous tissue is performed at the expense of the GAG; this ensures a high ability of the cartilaginous tissue to adaptation. A hypothesis on the GAG role in realization of adaptive rearrangements of the cartilaginous matrix at the macromolecular level is put forward.
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This article reviews published formulas for intraocular lens power determination. In the author's opinion, lack of agreement as to which formula is the most accurate is because none of them reduce the fundamental (biological) problem of variation (scatter, or standard deviation) of results. Suggestions are made for wise use.
A special set of spectacles with the lenses reversed (concave anteriorly) were fabricated for use with microscopic and/or telescopic eyepieces in order to reduce the vertex distance and improve the useful field of view. Although these spectacles were found to be of limited use to microscopists, they were essential to the astronomer whose astigmatism could not be corrected with contact lenses.
Angular magnification, resolving power, astigmatism, field of view, eye relief, and light transmittance of 157 low power telescopes comprising 25 models from seven manufacturers were measured. Roof-prism Keplerian telescopes provided about one-half the resolution, 30% lower transmittance, and more objectionable image flare than Galilean designs. The roof-prism was responsible for producing overlapping doubled images that appeared astigmatic-like. The prism further compromised rotational symmetry by deviating the image in a direction along the roof edge. The Keplerian telescopes, however, had about twice the field of view of the Galilean telescopes. Notably, several Keplerian telescope models from different suppliers were found to be optically identical, although they varied 50-100% in price.
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Explore the source record for details and available documents.
Explore the source record for details and available documents.