This invention relates to a method of prescribing bifocal spectacles for the low vision patient and to the resultant spectacles which are then fabricated according to the method.
It is well known in the field of optometry that when working with patients with reduced visual acuity, sometimes referred to as low vision patients, that these patients should be able to read. In regard to accommodating such patients, one employs relative distance magnification to enable such patients to read. Essentially the print or reading material is brought closer to their eyes to create an enlarged retinal image. The practitioner then prescribes high plus lenses to enable such a patient to maintain a clear focus at the near working distance. In any event, there are many considerations which are involved in prescribing such lens systems for patients with reduced visual acuity or low vision patients.
Thus, for example, in addition to the increased accommodative demand, near reading distances require considerably more convergence than are ordinarily employed. For this reason it is generally agreed that binocularity cannot be achieved when the add power of a high plus lens exceeds 12 diopters or more. Thus, in order to reduce the convergence demand practitioners include prism in their prescriptions or simply decenter the lenses to create a prismatic effect. These techniques are accommodated by rules of thumb or employ standard magnification glasses with built in prisms that are commercially available.
In any event, regardless of how the prescription is devised it is absolutely unclear how much convergence is required and it is unclear how to implement or check the final spectacle product to see that it conforms to the prescription generated by the practitioner. As will be explained, an optometrist or practitioner generates a preseciption for a patient with reduced visual acuity which prescription is solely concerned with the measurement of the far interpupillary distance (p.d.). The term for interpupillary distance si sued in this specification to denote the distance between the eyes of the patient indicative of the distance between their center of rotation. This measurement is made by use of a trial frame and when the patient is viewing a distant object The term p.d. is well understood by those skilled in the art and is as defined above. The term "far" p.d. is employed in this specification to denote the conventional p.d. as distinguished from a near p.d. which is part of the teaching of the method of this invention.
Thus, according to prior art techniques and as will be explained, the patient is fitted with a conventional trial frame. Such trial frames are well known and essentially consists of a frame which can accommodate a plurality of trial lenses. The frame can be adjusted to accurately align the frame with the patient's eyes so that the geometrical center distance between the eyes is accurately indicated by adjustment of the frame by means of a calibrated scale located on the frame. This distance is known as the interpupillary distance (p.d.) which is made and measured when the patient is viewing an eye chart where this p.d. is determined at the condition of far viewing to enable the practitioner to record this distance during the examination. The trial frame accommodates various lenses which are inserted into the frame in front of each of the patient's eyes and such corrective lenses are inserted in various combinations until the patient achieves optimum visual acuity indicative of a far or distant prescription.
As one can also ascertain, after a far or a distant prescription is achieved, the practitioner then inserts a high add lens for providing the patient with bifocal viewing. This high add lens is necessary to enable the patient to read at a closer working distance than the distance used for far viewing. As is well known, when high add lenses are used, such as lenses ofj 4 to 12 diopters one cannot obtain binocular viewing with such lenses due to the extremely short focal lengths. Thus the practitioner or optometrist mathematically computes the amount of base prism required. The base in prism is required to bend the light rays to enable the patient to achieve binocularity at these short focal lengths. Based on the extreme difficulties of such procedures, very few practitioners prescribe for binocularity in the high add range because it is too complicated and cumbersome.
For an example of some of the considerations, reference is made to an article entitled "Determining The Convergence Demand For A Patient Who Reads At A Close Working Distance" by Paul B. Bither published in the American Journal of Optometry and Physiological Optics, Vol. 64, No. 5, pp. 355-360 (May, 1987). This article describes the considerations that a practitioner must involve himself in when attempting to provide comfortable reading spectacles for a patient with reduced visual acuity. The article further contains a bibliography of other pertinent articles which address this problem. As indicated from the article, the author clearly shows and attempts to show the mathematics involved and presents certain charts whereby a practitioner, when obtaining the far interpupillary distance can now determine the amount of base in prism required in regard to various work distances for high add lenses. As the article indicates, while it is clear that the benefits of decentration can be considered, he clearly acknowledges the fact that these techniques are limited.
Thus, one will ascertain by perusing the prior art that a practitioner attempts to mathematically compute the amount of prism required to accommodate a high plus lens needed by a patient to enable the patient to read by means of a bifocal combination. This mathematical computation determines the amount of prism required to enable the patient to achieve binocularity at short focal lengths due to the high add lenses. The prior art attempted are relatively unsuccessful and hence many low vision patients cannot read with binocularity.
It is therefore an object of the present invention to provide a method in which a low vision patient is tested for near distance reading ability utilizing a conventional trial frame and based on the utilization of the trial frame bifocal spectacles, which accommodate both the far distance prescription as well as a near reading prescription, can be fabricated.
It is a further object of this invention to provide a pair of high add bifocal spectacles having unique optical properties to enable a person with reduced visual acuity to be able to both read and to perceive objects at a distance.
It is a further object of the present invention to provide an improved method which a practitioner can employ to determine both the near and distant prescriptions to be employed in providing spectacles for a patient with reduced visual acuity.