Refractive instruments are the most commonly used instruments in eye care. At present many of them have the capability for digital output, but their data is most often addressed by manual input into a paper or electronic record.
Refractive instruments address the power of a lens or of a patient's eye to bend light. In order for a patient to see well light must be focused on the retina in the back of the eye. If the natural optics of a patient's eye do not accomplish this, corrective lenses can bend incident light so that it will be focused on the retina after passing through the optics of the eye. The power of an optical system such as a spectacle lens or the eye is measured by its ability to bend light, and is measured in diopters (D). In practical clinical applications, this is measured to 3 decimal points, in increments of 0.125 D. The power of a lens is measured in at least two major meridians. A spherical lens power occurs when the power is the same in all meridians (0-180 degrees). A cylindrical lens power occurs when there is a difference in lens power across the various meridians. The shape of the anterior surface of the eye largely determines what type of correcting lens is needed. An eye that requires only spherical lens power is usually shaped spherically, more like a ball, while an eye that requires cylindrical lens power is ellipsoid and shaped more like a football.
Lenses can also bend light without changing its focal distance. This type of refraction simply displaces the position of the image laterally. The power of a prism to bend light is measured in prism diopters. In practical clinical applications this is measured to 1 decimal point, in increments of 0.5 prism diopters. Prism power is required in a pair of spectacles most commonly when both eyes are not properly aligned with the object of regard. Clinical prisms are considered to bend all light coming in from the lens either up, down, in toward the nose, or out away from the nose, in order to compensate for ocular misalignment.
Visual acuity is measured in various scales , all of which indicate a patient's vision as a fraction of what a reference standard patient would see at any given distance. For example, if a patient has 20/30 vision it means that he sees from a distance of 20 feet what a reference standard patient would see from a distance of 30 feet. These measurements are determined by presentation of standardized objects or symbols (optotypes) of varying sizes calibrated to reference standard vision (20/20). The smallest discernable optotype defines the patient’s visual acuity expressed in a variety of formats (letters, numbers, pictures, tumbling E, Landolt C, etc).
Visual acuity is measured in two categories of viewing distances : distance, and near. Distance visual acuity is measured at 20’ or six meters. This distance is roughly equivalent to optical infinity for clinical purposes. The near viewing distance can vary from 30cm to 75 cm depending on a variety of other conditions, but most commonly is measured at 40 cm.
Visual acuity is measured under several common viewing conditions : 1) Uncorrected vision is measured using the autoprojector to project the above mentioned optotypes for viewing, with no lenses in front of the patient's eyes. The line of smallest optotypes of which the patient can see more than half is determined, and that information is uploaded to a computer system. 2) The patient's vision using habitual correction is measured in a similar fashion using whichever vision correction the patient customarily wears. 3) Pinhole vision is measured in a similar fashion, with the patient viewing the optotypes through a pinhole occluder held in front of the eye. Pinhole visual acuity testing reduces retinal blur, providing an approximation of what the patient's vision should be with the best possible refractive correction (spectacles) in place. 4) Best corrected visual acuity is the visual acuity with the best refractive correction in place. 5) Crowding visual acuity measures the presence and amount of disparity in acuity between single optotype and multiple optotype presentations.
A patient’s spectacle prescription may or may not represent the same lenses that provided best corrected visual acuity in his refraction. Subjective comfort plays a role in determining the final spectacle prescription.
1.) Autolensometer: an autolensometer is used to measure the refractive power of a patient's spectacles. This is done by the automatic analysis of the effect of the measured lens upon a beam of light passing through it. Output from an autolensometer can be uploaded to a phoropter to provide a baseline for subjective refraction (discussed below), and it can be uploaded to a computerized medical record. Lenses may also be measured to confirm manufacturing accuracy.
2.) Autorefractor: an autorefractor is used to automatically determine, without patient input, what refractive correction should provide best corrected visual acuity. Output from an autorefractor can be uploaded to a phoropter to provide a baseline for subjective refraction (discussed below), and it can be uploaded to a computerized medical record.
3.) Phoropter (or phoroptor): an instrument containing multiple lenses, that is used in the course of an eye exam to determine the individual’s subjective response to various lenses (subjective refraction) and the need for glasses or contact lenses.. The patient looks through the phoropter lenses at an eye chart that may be at 20 ft or 6m or at a reading chart that may be at 40 cm. Information from the subjective refraction can be uploaded from an autophoropter to a computer. The best corrected vision that was obtained is displayed in an autoprojector, and that information can also be uploaded to a computer.
4.) Autokeratometer: an autokeratometer is used to measure the curvature, and thus the refractive power, of a patient's cornea. Two measurements are generally taken, one at the steepest and one at the flattest meridian of the cornea. The meridian measured is expressed in degrees, whole integers, in increments of 1 degree. If the measurement is expressed as power, the unit of measurement is diopters, to 3 decimal points, in increments of 0.125D. If the measurement is expressed as radius of curvature, the unit of measurement is millimeters, to 2 decimal points, in increments of 0.01 mm.