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A conic tube may be adjusted by varying the distal diameter and by using binocular vision for measuring distances. This device appears to be the earliest telemeter in the world. It is mentioned in the Talmud as being used in the first century A.D.
In optical microscopic measurement of internal blood-vessel diameters, the effect of refraction must be taken into account to ensure accuracy of the result. This effect is discussed and an analytical correction formula derived. Phantom blood vessels with known internal and external diameters were used to test the validity of the correction formula. The errors obtained prior to correction were reduced significantly after correction.
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In 17 ocularly healthy persons the IOP was artificially raised by suction cup oculopression (negative pressure: 65 mmHg [8.67 kPa]) from initially 15.5 +/- 2.2 mmHg (2.07 +/- 0.29 kPa) to 25.2 +/- 4.0 mm Hg (3.36 +/- 0.53 kPa) (mean +/- SD). Continuing the initial refractive correction, visual acuity (Landolt's rings) decreased from initially 1.06 +/- 0.13 to 0.39 +/- 0.22 (P less than 0.001* [*t-test, paired ties each]). A new optical refraction with suction cup in position significantly raised the visual acuity to 0.64 +/- 0.24 (P less than 0.002*). The suction cup itself induced astigmatism of 2.15 +/- 1.46 dpt (Canon Auto-Refractometer RK-1), which significantly differed from the initial astigmatism (0.32 +/- 0.23 dpt) (P less than 0.001*). The minus cylinder axis initially showed no preferential position. During artificial IOP elevation it shifted to 90 degrees...125 degrees (referred to the right eye) and thus ran about perpendicular to the meridian the suction cup was positioned on the eyeball. Effects of such refractive changes on differential light threshold and VEP amplitude are demonstrated. The results presented allow critical interpretation of previous IOP tolerance tests. The influence of suction cup oculopression on the perception of the white-noise field is shown in some glaucoma patients: the speed of the white-noise campimetry allows the increase in field defects during artificial IOP elevation to be followed up directly.
The optical theory of keratophakia employing implant lenticules with a refractive index different from that of the cornea is presented. This generalized theory also embraces keratophakia employing donated stromal lenticules and keratomileusis, with equal accuracy. The method of solution of the formulas is indicated. The effects of the various parameters upon the radius of curvature to be applied to the cornea are illustrated and discussed.
In this paper we discuss whether the presence of an optic nerve crescent might affect the way in which axial length and corneal curvature interact to determine refractive error. Subjective refraction, keratometry, measurement of body height, axial length of the eye, and stereophotography of the optic nerves were performed on 224 subjects, 8- to 25-years-old. Photographs were examined under magnification; optic nerve crescents, if present, were measured in the horizontal dimension. Those measurements were then corrected for magnification due to the eye and the camera. Logistic regression analysis suggested that male gender and myopic refractive error were most directly associated with the presence of a large crescent, whereas axial length, age, and horizontal keratometry reading were less directly associated with the presence of a crescent. The relation between axial length and refractive error differed among those with large crescents compared to those with small or no crescents. Simple regression showed that, for those with a crescent at least 0.2-mm wide, a 1-mm greater axial length was associated with, on the average, 1.26 D of myopia. For those with smaller or no crescents, a 1-mm greater axial length was associated with only 0.66 D of myopia. This difference was statistically significant at the 0.02 level of confidence.
On the basis of examination of 100 persons aged 3-18 years registered in the Olsztyn Division of the Union of the Blind the authors determined the causes which led to the loss of vision or of its impairment. The most frequent cause of the invalidism of the visual system were: errors of refraction (32%), optic atrophy (24%), congenital cataract (16%), retinal and choroidal degeneration (9%), retrolental fibroplasia (6%), anophthalmia or malformation of the globe (4%), coloboma of the iris or choroid (3%), retinoblastoma (3%), injuries (2%) and congenital glaucoma (1%).
Errors of refraction of the eye can be corrected not only by means of spectacles, contact lenses or intra-ocular lenses, but also by operative procedures on the cornea. At present, correction of myopia is accomplished with radial keratotomy (RK), a procedure that achieves the best results in the range up to - 6.0 diopters. However, on account of its side effects, in particular poor predictability, the method is no true alternative to the use of optical aids. Epikeratoplasty done to correct hyperopia and aphakia, is even more imprecise, and should only be employed in exceptional cases. Laser surgery of the cornea is still undergoing clinical testing, but has already produced promising results.
A new model of the eye is presented, that can demonstrate quantitatively the principal optical concepts such as focal distance, refractive power, principal point, nodal point etc. Ametropias and their correction can be shown. This model can be used to illustrate the dependence of the vertex distance on the effective refractive power of a spectacle lens and the influence of a spectacle lens on the image size on the retina. The phenomena of retinoscopy can also be demonstrated very clearly. An instruction manual for the model exists that contains theory, control tests and suggested experiments.
Three different components contribute to the modulation transfer function of the visual system: (1) formation of the optical image (refractive media, pupil); (2) scattering of light in the prereceptoral layers of the retina; (3) neuronal processing in the retina und superior visual centers. In the presence of media opacities or non-correctable refractive errors, the clinical question often arises as to which macular function can be expected under the assumption of normal optical image formation (e.g. prior to cataract extraction, corneal transplantation, or vitrectomy). Simple tests such as light projection, color discrimination, and two-point discrimination cannot provide adequate information about macular function. The same holds true for the global luminance ERG. The X-ray phosphene is obsolete. The Maddox rod (with limitations), transilluminated Amsler grid, and various entoptic phenomena (Purkinje vascular phenomenon, foveal chagrin, Haidinger's brushes, blue field phenomenon) are available as qualitative subjective tests. Maxwellian view systems with pinhole aperture (potential acuity meter PAM) and the interferometers (retinometer, visometer, SITE-IRAS interferometer) provide quantitative subjective methods. The flash VECP is primarily a qualitative objective test that allows semiquantitative acuity prediction under special conditions (unilateral opacities). Psychophysical criteria that are less affected by the quality of the retinal image show promising developments in future subjective tests, e.g. optotypes in positive contrast, optotypes or targets superimposed on a background of optical noise, or hyperacuity. Future objective test developments are pattern VECP or even pattern ERG elicited by interferometric stimulation, speckle VECP and focal ERG.
Because the optic disc in myopic eyes is different from a normal optic disc, there are many difficulties in examining the optic discs of myopic eyes. To study optic disc change due to myopia, we performed a morphometrical study of stereophotographs of 61 men, 109 eyes, who had no glaucoma history. The range of refractive error was from +0.75 diopter to -12.75 diopter, and all subjects had intraocular pressure below or equal to 21 mmHg. According to the increase in the myopic degree, the temporal slope of the disc cup was significantly decreased, but the ratio of the vertical disc diameter (VDD) to the horizontal disc diameter and the ratio of the width of peripapillary atrophy (PPA) to the VDD were significantly increased. The above results suggests that in high myopia the optic disc was tilted and the rim-cup border was indistinct and there are some problems in the estimation of the morphometric parameters. Also in evaluation of the PPA of myopic glaucoma patients, there may be some difficulty in deciding whether it is due to myopic change or glaucomatous damage.
Diakor aluminium oxide implants of different forms and sizes have been investigated and compared by photoelastic stress analysis. On forming the conditions of investigations the authors aimed at the modelization based upon the physiological direct implant-bone contact, installing implants having resin that is optically double refractive. In the sample results show a move advantageous stress relation at multi grooved implants of extended dimensions.
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In a retrospective study, data from 116 patients suffering from different forms of retinitis pigmentosa were analysed, and 15 categories comprising altogether 34 symptoms or clinical signs were tabulated from each patient's record. The 15 categories evaluated were: visual acuity, visual field diameter, ring or central scotoma, nyctalopia, susceptibility to glare, refraction, cataract, electroretinography, colour of the optic disc, bone-spicule pigmentation of the retina, retinal vessel diameters, tapetoretinal reflex, sex, heredity, and age. Correlations between the tabulated 34 subcategories or symptoms were calculated. The results of a factorial analysis of the data showed a high number of highly significant correlations between the different categories. It seemed possible to discriminate between two groups of categories, with the members of each group being closely correlated but correlations with members of the other group being much smaller, if not nonexistent. We tentatively associated the first group with the functional state of the central retina and the second group with the state of the peripheral retina.
The chemical identification of collagens and proteoglykans adds to the knowledge of the fine structure of the cornea as seen in histological sections and with the electron microscope. Chemical properties of collagens and proteoglykans help to understand the microscopical appearance of the tissue and some special functions of the cornea, such as transparency, stromal hydration, cohesion, elasticity and shear force resistance. Understanding mechanical properties of the cornea may be valuable for corneal and especially refractive surgery. A review of recent literature on intrastromal implants to be used for optical correction of refractive errors showed, that the resulting problems of corneal nutrition and also those of hydration may be solved by the time. But presumably proteolytic processes around intrastromal implants still prevent longterm tolerance of such implants.
The profession of optometry has been very successful in providing optical corrections for spherocylindrical refractive errors. In this paper, I examine one attempt to improve retinal image quality beyond that afforded by a standard refractive correction. Ocular chromatic aberration is one of the factors that prevent retinal image quality from reaching the upper limit set by the wave nature of light. It can be subdivided into three primary aberrations (wavelength-dependent differences in imaging plane, image position, and image size). We have been able to measure all three of these using psychophysical techniques. Although attempts to provide an optical correction for wavelength-dependent refractive errors have been optically successful, they have failed to improve vision. Several possible explanations are given for this failure.
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BACKGROUND: Several recent papers have discussed the use of engineering-based computer methods for the analysis of keratorefractive surgical procedures. What has been lacking is a broader view of the role of engineering analysis in keratorefractive surgery. This article demonstrates how these various analysis methods can be coupled to provide a comprehensive methodology for the design of refractive surgical procedures. METHODS: A structural model of the eye, based on a linearly elastic, transversely isotropic finite element formulation is coupled to a full-eye optical model. The optical errors due to refractive keratotomy are estimated by ray tracing through the optical model and measuring the position of the resulting focal plane relative to the retina. Computer-based optimization methods are employed to determine the surgical parameters necessary to correct myopia for a given set of surgical design goals. RESULTS: Results based on a hypothetical eye demonstrate agreement with clinical trends. Radial keratotomies are designed that eliminate refractive error while minimizing invasiveness in one case and maximizing the optical zone size in another. It is also shown that there is significant potential to customize this process on a patient-by-patient basis using clinically measured data. CONCLUSIONS: We present an overview of the research necessary to bring this approach to fruition. While only a first step, the methodology presented in this article has the potential to increase the predictability of keratorefractive surgery by substantially increasing both the quality and the quantity of the information available to the refractive surgeon preoperatively.