1. Field of the Invention
The invention is generally directed to the field of ophthalmic vision correction, and more specifically to a method, a readable medium, and a system related to determining the optical zone associated with higher-order aberration correction provided by a custom contact lens, a custom IOL, a corneal inlay, or refractive laser surgery.
2. Description of Related Art
Until about the beginning of the last decade, vision correction consisted of approximately determining the lower order aberrations of a person""s eyes, namely defocus, cylinder (astigmatism) and the cylinder axis, and prescribing spectacle or contact lenses to approximately correct these aberrations.
More recently, however, the application of wavefront sensor technology (aberrometers) to the field of ophthalmic vision correction has allowed practitioners to accurately measure the higher-order aberrations of optical systems such as the eye. These higher-order aberrations include secondary astigmatism, spherical aberration, coma, trefoil; and others known to those skilled in the art. Moreover, advances in lens design and manufacturing, and laser vision correction methods and apparatus, have made it possible to correct certain of the higher-order aberrations with customized contact lenses, custom IOLs, and photoablative refractive surgery. In many patients, the theoretical limit of visual acuity, 20/8, has been achieved.
When measuring the ocular aberrations of a patient""s eye with an aberrometer or like instrument, the diameter of the measured zone is limited by the smaller of either the instrument""s aperture or the patient s pupil. In either case, the diameter of the measured zone is very likely to be smaller than the diameter of the optic zone of a contact lens, typically 7-8 mm, or the diameter of a photorefractive ablation zone, typically 5-6 mm, excluding the blend zone. Since many of the higher-order aberrations depend upon the size of the aperture that the light passes through (i.e., a person""s pupil), referred to hereinafter as the xe2x80x9coptical zone,xe2x80x9d it is important to know the identity and magnitude of the aberrations over the full optical zone in order to properly correct them with lenses, surgery, or otherwise. The current method for increasing pupil size is to use mydriatic drugs, like 2.5% phenylephrine hydrochloride, which dilate the pupil beyond its natural maximum size. This is undesirable for measuring ocular aberrations because pupil dilation is linked to accommodative mechanisms. Thus, when the pupil is artificially dilated, the crystalline lens may have an unnatural shape and may be decentered and tilted in ways different than during natural viewing. In addition, the pupil itself may be decentered when artificially dilated. This may alter the aberration contribution of the crystalline lens, and the total wavefront aberration may have errors. Cycloplegic drugs, like 1% tropicamide, which paralyze the ocular cilliary muscles may also be used. These drugs are even less desirable because they directly affect the shape of the crystalline lens.
The wavefront aberration over the measured zone is commonly described by Zernike polynomials with a normalization radius set to the half-diameter of the measured zone. Extrapolation of Zernike polynomials beyond their normalization radius is undesirable because Zernike polynomials behave wildly outside their normalization radius. Another possibility is to refit the data with Zemike polynomials having a normalization radius of 4 mm (i.e. 8 mm diameter). Although the Zernike polynomials will be better behaved, there is no reason to believe that the data created between the edges of the measured and optical zone is appropriate. Similar problems are encountered when extrapolation of other types of polynomial expressions are used to describing higher-order aberrations outside of the measurement zone.
In view of these shortcomings, the inventor has recognized a need for the ability to accurately and simply describes the optical aberrations beyond the typically measured zone, and extending over the optical zone of a contact lens or the patient""s eye.
The invention broadly relates to methods and apparatus allowing a simple and accurate determination of higher-order aberrations of an optical clement over an optical zone that is larger than the limited measured zone over which the aberrations typically are measured.
An embodiment of the invention is directed to a machine readable medium that includes an executable instruction directing a suitable machine or device to produce an aberration correcting surface on an optical element. For a custom surface (i.e., non-rotationally symmetric), the instruction directs the making of a meridional shape profile on the optical surface which fits a monotonic function to the aberration data that either increases or decreases smoothly and consistently with increasing radius to a peripheral limit of an optical zone that is larger than a peripheral limit of a measured zone of an optical aberration of the optical element, and extrapolating the aberration data over the optical zone of the element. Most preferably, the function is a conic. In one preferred aspect, the device is a laser suitable for ablating the optical surface of what will typically be a customized contact lens or other ophthalmic lens surface like an IOL or corneal inlay, for example, or a corneal surface. In another preferred aspect, the machine is a numerically controlled lathe such as, e.g, an Optoform 50/Variform(copyright) lathe (Precitech, Keene, N.H., USA). In a preferred aspect, the instruction directs the production of a corrective surface by fitting between 24 and 384 separate and independent, uniformly spaced conic functions to determine the full optical surface.
In a related embodiment, a system for making a corrective surface on an optical element includes a device cooperatively engageable with an optical element having a surface intended to be altered to provide an optical aberration correction, preferably a higher-order correction. The device is suitable for altering the surface of the optical element upon an executable instruction and, as such, can receive a readable medium including an executable instruction. The system alternatively includes a control system operatively associated with the device and adapted to receive a readable medium including an executable instruction, and to provide the instruction to the device for execution, and the medium itself The instruction directs the device to produce a meridional shape profile on the optical surface that fits a monotonic function to the aberration data that either increases or decreases smoothly and consistently with increasing radius to a peripheral limit of an optical zone that is larger than a peripheral limit of a measured zone of an optical aberration of the optical element, and extrapolating the aberration data over the optical zone of the element.
In another embodiment, a method for determining (which term includes designing, designating, specifying, or otherwise describing) a corrective optical surface for an optical element over an optical zone having a radius that is greater than that of a measured zone over which an optical aberration of the element has been measured, includes making a higher-order aberration measurement, fitting the aberration data to a function that either increases or decreases smoothly and consistently, and extrapolating the aberration data over the optical zone of the optical surface. In a preferred aspect, the function is a conic. More preferably, between 24 and 384 separate and independent conic functions spaced every 15 degrees to four degrees, respectively, are fit to the measured aberration data to accurately describe the total corrected optical surface.
These and other advantages and objects of the present invention will become more readily apparent from the detailed description of certain embodiments to follow. However, it should be understood that the detailed description and specific examples, while indicating the preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art based upon the description and drawings herein and the appended claims.