Third order (primary) aberration theory has had little application to the design of aspheric spectacle lenses. Such an application would be useful because: 1. Third order theory is useful in designing simple optical systems, as relatively simple equations can be used to obtain approximate magnitudes of aberrations and to show how these aberrations change with variations in design parameters without recourse to a large mass of data. 2. Aspherising one or both surfaces of spectacle lenses allows the correction of off-axis power errors in high positive power lenses where this would be otherwise impossible, and enables other factors, such as distortion correction, to be considered simultaneously with off-axis power error correction over the total range of lens powers. Third order formulae are developed for calculation of distortion in thin spectacle lenses, when one or both surfaces are conicoid aspherics. Results are presented which show the validity of using third order theory. Solutions which allow correction of rotatory or peripheral distortion, when one lens surface is a conicoid aspheric, are illustrated. A study of these solutions shows that one of the off-axis power errors (eg. oblique astigmatism) and one of the distortions can be simultaneously eliminated, but the lens forms required are too curved to be cosmetically feasible.
Spline surfaces can be formed at will. They are always twice continuously differentiable, i, e. they have no jumps, bends or ripples. In the field of optics splines are employed for the spectacle lens, Gradal HS. The form of the spline surface is defined by optical requirements. Splines have contributed to the successful reduction to a lilinimum of the unavoidable aberrations present in progressive addition lenses and to the binocular equilibrium of the remaining aberrations. Conventional surface structures will be used as a comparison.
The methods of defining contact lenses used to date have presupposed a certain surface form: - that of a sphere, - that of a spheroid, - an appearance resembling these surfaces. The transition areas in the peripheral zones and the edge profiles were separately calculated and pieced together. This mathematical process resulted in a reduction of the free parameters. These difficulties are avoided by the spline algorithm presented herein. It is characterized by the following properties: 1. Every twice continuously differentiable surface can be represented. 2. Local influenceability without global effect. 3. EDP-friendly algorithm. ∫[f"(h)]2dh = min The parameterized form of the algorithm is of advantage for the definition of contact lens surfaces. Spherical surface elements can be very easily defined by modification of the process. This permits a reduction in the volume of data to be processed. An exact calculation of the system "eye - tear lens - contact lens" is performed as a practical example of the above. The deviation of the dioptric values of the decentred contact lens from the values in the centred position are graphically illustrated.
An analytical method for designing aspherical surfaces is described in this paper. Utilizing Buchdahl scheme for calculating abberratians, this method requires simply paraxial ray tracing data of two rays. The method aims to obtain aspherics with minimum asphericity , considering third and fifth order aberrations.
The significant aberrations in ophthalmic lenses are (oblique) astigmatism (spectacle lenses) and spherical aberration (contact and intra-ocular lenses). In many cases, they cannot be eliminated or sufficiently controlled by bending. Fortunately, aspherizing one or both surfaces allows much greater control over either of these aberrations and in the case of contact and intra-ocular lenses, allows the balancing of the spherical aberration in the eye itself.
In phase one of the program, three surface abrading techniques were evaluated for variability within sample sets and between test laboratory sites using lenses prepared from allyl resin homopolymer. Phase two of the program, assessed the ability of each machine-type to rank the performance of three commercially available lens coatings versus uncoated lenses.
During its use, a spectacle lens is subjected to numerous different abrasive conditions (e.g. cleaning ; wipping) which make two types of surface scratches : Viscoelastic and brittle scratches. We present here a scratch testing machine (rayoscope) and a scratch diamond test which allows to reproduce these two kinds of scratches and to caracterize the material by : - The "scratch rupture resistance" which is equal to the load, called critical load Cc, applied on a known penetrator, at which begins the chips formation. - The "scratchability curve" which expresses the whole behaviour of the material until the critical load value Cc and which is obtained by recording the scratch width as a function of the load. The scratch resistance R then equals Cc/∝B where ∝= slope of the asymptote of the curve B = ordinate at the origin of the asymptote Cc= critical load The scratch resistance values are in good correlation with the wearing test results.
This paper describes a variety of currently available coatings for plastic ophthalmic lenses in terms of their effectiveness, abrasion resistance, and durability. Specific test methods are described which have been found to be useful in comparing the quality of the coatings. The test methods are correlated with real life use.
The tumble abrasion test [J.D. Masso, SPIE-Proc., Vol.601] has been successfully applied to a variety of uncoated, organosilica coated, and vacuum deposition coated plastic ophthalmic lenses. The vacuum deposited coatings were predominantly antireflection coatings of various spectral types, as well as a few hard coatings. Light scattering measurements were used to quantify the test results. Accelerated lifetime characteristics were obtained by tumbling the lenses for 60 minutes. The results indicate that the thicker a vacuum deposited coating is, the more susceptible it is to abrasion. This observation has been supported by ultramicrohardness measurements which indicate that the brittleness of vacuum deposited coatings increases with thickness.
Coating of ophthalmic lenses is an application of high-vacuum coating technology which must satisfy not only physical and technical requirements but also customer demands with respect to aesthetics, color fidelity, and exchangeability of coated ophthalmic lenses. Because this application caters specifically to the consumer market, ophthalmic lenses are subject to certain fashion trends which frequently require quick adaptation of the coating technique. The state-of-the-art of ophthalmic lens coating is reviewed in this paper, with particular emphasis on the durability requirements in daily use by untrained consumers as well as on the applicable testing methods.
Based on computer calculations of the irradiance on the eye when laser range finders at wavelengths of 694 nm or 1064 nm are used, safety distances are derived for the worst case of directly looking into the laser source with the unaided eye, as well as with magnifying viewing instruments (binoculars, periscopes). From this, attenuation factors (optical densities) of protective filters for both applications are determined. Various means of attaining the necessary protection are discussed, and it is concluded that dielectric interference filters, sometimes in combination with spectrally absorbing glass substrates, are still the most reliable and economic solution, at least for eye protection through optical instruments. The mass production and widespread use of versatile and efficient goggles for individual personnel protection remains still a problem, particularly when corrective spectacles are included.
This paper presents a scleral lens specifically designed for the implementation of new electroretinography (E.R.G.) procedures including ganzeld ERG, pattern ERG and optic fiber ERG. Ganzfeld ERG requires a direct, uniform illumination of the retina and is usually obtained within a ball stimulation which provides precisely controlled stimulation conditions. Pattern ERG is related to the electrical activity of ganglion cells and is produced by a structured visual stimulus, for instance a cherkerboard reversal. Fiber optics stimulation is a promising new technique involving an optic fiber which connects the scleral lens to a remote light stimulator. The new scleral lens is made out of silicon. Silicon is an ideal material for ERG applications. It is highly permeable to oxygen. It eliminates the risk of corneal abrasion and it provides good comfort to the patient. Two layers of conductive silicon are used for recording the bioelectric potential difference of the cornea with respect to the eye lids. An optically transparent window allows for the projection of structured images on the retina. A wide angle conic aperture is provided for ganzfeld stimulation. The same cone is used as a blepharostat and as a plug for connecting the optic fiber, once the transparent window is centered with respect to the pupil entrance.
An optical device capable of detecting ocular disease states by remote sensing is described. Photometric analysis of images is explained. The clinical implications are noted for management of ocular disease in man.
The evaluation of visual functions requires technical means for stimuli generation, which play an important part in the final result quality. The cathode ray tube (CRT) technology presents many advantages for these applications. Most of the physiological parameters can be put under micro-processor control including pattern shape, luminance and contrast. CRTs present a familiar look for every body and are well accepted by patients.
The rationale for designing screening type eye examinations to document visual capabilities for specific jobs or changes in visual function following exposure to specific ocular hazards is discussed. Possible applications to clinical situations are also discussed. Specific tests meeting requirements of definite end point quantification, ease of administration, and reproducibility are given for contrast (glare) sensitivity, distortions in macular imaging (Amsler grid), and color vision. The selection is aetailed for tne individual test combinations of various populations such as automobile uriver license applicants, visual display operators, and persons exposed to lasers, including military as well as non-military installers and repairers of optical fibers for communications.
By the use of a slit pupil of variable orientation, placed in the focal plane of a Badal lens (sometimes coupled to a laser refractor), faint meridional differences in optical power are found in eyes clinically regarded as emmetropic. This subdlinical astigmatism is known to depend on a number of stimulus related factors. Amongst others, fresh material is now displayed indicating its dependence on the spectral composition of illumination, in particular, when passing from red to blue on CTV monitors, by using as test object the grain (or texture) of the screen. It cannot be excluded that these effects have a behavior related teleological meaning.
Tinted ophthalmic lenses are used primarily for eye comfort in a brightly lit environment. An ancillary benefit is the attenuation of ultraviolet radiation. Some national product standards specify quantitative limits for ultraviolet transmittances. Such limits ought to be founded on quantitative estimates of solar irradiances of ocular tissues, with actinic effectiveness taken into account. We use the equations of Green and coworkers for direct and diffuse solar irradiance at the earth's surface to calculate average sky and ground spectral radiances. We use the geometric factors derived by us for the coupling of radiation from these sources to the human cornea. Actinically weighted corneal spectral irradiances integrated over wavelength and time yield peak irradiances and accumulated exposure doses that are compared with recommended exposure limits. This provides the maximal effective ultraviolet transmittances of tinted ophthalmic lenses such that these exposure limits will not be exceeded in the selected exposure environment. The influences on corneal irradiation of such exposure parameters as solar zenith angle, altitude of the exposure site, characteristics of atmospheric aerosols, and ground reflectances are illustrated. The relationships between the effective transmittance (which is a function of the environmental radiation and any actinicweighting function) and readily determined characteristics of the lens itself, viz., its mean transmittance, and a selected spectral transmittance, are derived for three lens transmittance curves. Limits of lens transmittance for the UV-B and UV-A wavelength regions are presented for several representative exposure sites in Europe and the U.S.A.
The interdisciplinary research on a new hydrophilic polymer and on the lenses manufactured with the above material has evidenced a high biocompatibility considering the chemical and physical characteristics, and the chemical controls.
A bifocal electronic molecular lens is described which can be switched between two foci without any grossly visible change in displacement, geometry, temperature or chemical composition. The lens requires very little power to change focus (on the order of tens to hundreds of nanoamps) so that long-term remote operation is anticipated. The principle of operation is based upon electronic control of optical polarization through a birefringent lens. Switching of focus is accomplished by controlling the molecular alignment of a film of liquid crystal molecules and thereby selecting between two orthogonal optical polarizations (P1 and P2). Polarization P1 is associated with focal point S1 and P2 is associated with focal point S2 through the birefringent lens. A number of useful new products are made possible by this lens. Specifically discussed are an electronic intraocular lens and electronic eyeglasses, both of which can be powered independently.