Which one is clearer, one or two; three or four? Anyone who wears corrective lenses knows exactly what that question pertains to; the ophthalmologist or optometrist is testing your eyesight and determining your prescription. But have you ever wondered how they get your glasses just right?
To understand how corrective lenses are made according to prescription, we must first understand how the eye itself works, why you need corrective lenses and how they work, as well as how to decipher the prescription.
How your eyes work
The eye is a sphere of several components that rely on each other, in order for us to see something. We begin to see when light rays reflect off of an object and then enter our eyes through the cornea.
The cornea, a transparent covering over the outer portion of the eye, bends the rays and passes them through the pupil, which is in the center of the eye. The colored portion of your eye is the iris, which opens and closes while regulating the amount of light coming through. The light rays then pass through the lens, which can change shape so as to bend the rays and focus them on the retina.
The retina is a complex layer of tissue in the back of your eye that contains light-sensing rods and cones. Located in the center area of the retina called the macula, cones detect colors and finer details in bright light. Outside the macula, rods extend to the edge of the retina and provide peripheral vision, motion detection, and help us to see better at night.
Both rods and cones convert light into electrical impulses for the optic nerve to send to the brain, where an image is created.
Why you need corrective lenses and how they work
There are a few reasons why you made need corrective lenses.
If the surface of your cornea is not smooth, it is an astigmatism which distorts your vision.
Nearsightedness or myopia is when a distant object appears blurry because the image was focused before it reached the retina. Myopia is corrected by using a minus (-) lens that moves the focus farther back.
Farsightedness or hyperopia occurs when an object that is close blurs because the image did not focus before it reached the retina. Hyperopia is common with age and can be corrected with a plus (+) lens.
Bifocals are a small segment of a plus lens (+) contained within a larger lens.
How to read your prescription
Oculus dexter (OD) stands for the right eye, oculus sinister (OS) is for the left eye, and oculus uterque (OU), means both eyes.
The shape of the lenses are typically spherical or cylindrical.
Sphere (SPH) means the correction for myopia or hyperopia is equal in all meridians of the eye, and indicates the power of the lens in diopters (D), which express lens strength by indicating how much light is refracted. The plus or minus in front of the diopter number are the type of lens.
Cylinder (CYL) indicates the lens for an astigmatism. This type of lens power means that its shape has no additional curvature, and so the perpendicular meridian contains the maximum power and curvature to help correct the astigmatism.
If a prescription includes cylinder power, it also contains an axis value. These values range from 1 to 180; with the numbers 90 corresponding to the vertical meridian of the eye, and 180 corresponding to the horizontal meridian.
The add is for the bifocal portion in the bottom of the lens, and is always a plus power.
Some prescriptions may contain a prism power that is measured in prism diopters (PD) and helps to compensate for eye alignment issues.
How the corrective lens is made
The prescription is input into a computer which then generates specific information on how to produce the lens required. Based on this, the technician then selects the appropriate lens blanks. The blanks have various curves that are pre-ground into the front to correspond to the prescription. The rest of the power must be ground on the back of the lens.
Placing the lenses in a lensometer, the technician uses an instrument to locate the optical center or point in the lens that will center over the customer’s pupil. Then, adhesive tape is placed on the front of each blank to prevent scratching during the blocking process.
Lenses are placed one at a time into a blocker machine that fuses the block to the front of the blank. The block hold each lens in place during the grinding and polishing processes.
Then, the blanks are placed into a grinding machine or generator that is set for the prescription. The generator grinds the curves into the back of the lens.
Next, the metal lens molds are selected according to prescription; and both lenses are placed in the fining machine with the back of the lens in the correct lap. The front of the lenses are then polished with soft sandpaper. After that, a smooth plastic is placed over the sandpaper and the lenses are polished again, while the fining machine rotates the pads and water flows over the lenses. When this process is complete, the pads are peeled off and discarded.
Then, each lap is removed from the lens and soaked in hot water. The laps are reattached to the lenses after a few moments of soaking, and then placed back in the fining machine for the final fining process. During the final fining process, the pads rotate in a circular motion while a polishing compound consisting of aluminum oxide, water, and polymers flows over the lenses.
Is it all becoming clearer? You now understand how your eyes work, why you may need a prescription and how to decipher it; as well as how the lenses work, and how they are made.
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