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Eyewear with Prescription Lenses for Sun ProtectionEyeglasses are a familiar sight everywhere around the world. Given how much we rely on the lenses within those frames to enhance our vision, it’s natural to wonder about the process behind crafting them.
In this article, we will explore how the eye focuses, the mechanics of lenses, how to understand a prescription, and the process of creating lenses—including the intricate steps involved in shaping plastic lens blanks to match an individual's prescription and frame.
Let’s begin by covering some fundamental concepts of vision.
Engage with this interactive activity from Discovery Channel—test your eyesight while learning how vision functions. Additional interactive features allow you to explore your body's systems and understand how they help you navigate daily life with ease.
How Your Eye Focuses
At the rear of your eye, there exists a sophisticated layer of cells called the retina. The retina responds to light and sends this data to the brain. The brain then processes the signals and converts them into a visual image. Since the eye is spherical, the retina’s surface is naturally curved.
When you focus on an object, three processes must take place:
- The image must be reduced in size to fit onto the retina.
- The scattered light must come together -- that is, it must focus -- at the surface of the retina.
- The image must be curved to match the curve of the retina.
To do all that, the eye has a lens between the retina and the pupil (the "peep hole" in the center of your eye that allows light into the back of the eye) and a transparent covering, or cornea (the front window). The lens, which would be classified a "plus" lens because it is thickest toward the center, and the cornea work together to focus the image onto the retina. (For more information on how the eye functions, see How Vision Works.)
- Aberrations: ghost images, halos, waves or rainbows caused by imperfections in the curve or lens surface
- Index of refraction: a ratio used to compare refractive power
- Plus lens (+): a lens that is thickest at the center; moves the focal point forward
- Minus lens (-): a lens that is thinnest at the center; moves the focal point backward
- Focal point: a spot in space where refracted light meets; may be actual (plus lens) or assumed (minus lens)
- Pupillary center: the point on a lens directly in front of the pupil
- Astigmatism: a condition caused by a distortion in the cornea that creates an additional lens power
Out of Focus
Sometimes, for different reasons, the eye doesn't focus quite right:
- If the surfaces of the lens or cornea are irregular, it may cause a distortion known as astigmatism, which manifests as streaks or blurs in vision.
- Accommodation problems can arise if the lens cannot adjust its curve to focus properly, leading to blurry vision.
- When the cornea is misshapen, it can cause unclear or blurred vision.
Many vision issues are a result of the eye's inability to properly focus an image onto the retina. Below are some common vision problems:
- Myopia, or nearsightedness, occurs when distant objects appear blurry because the focus point is reached before the light hits the retina. This condition can be corrected with a concave lens to push the focus back.
- Hyperopia, or farsightedness, causes nearby objects to look blurry because the image focus point is behind the retina. This condition, often worsened by aging, can be corrected with a convex lens. Bifocal lenses, which have both a plus and minus segment, can help people with hyperopia see better for both close-up and distant tasks.
- Astigmatism arises from an uneven curvature of the cornea, creating a second focus point. It can be fixed with a cylindrical lens to correct the distortion.
Lenses can also help correct double vision, particularly when the eyes don't align properly, such as with 'crossed eyes'. These lenses work by shifting the image so that both eyes can focus on it together.
Corrective lenses are designed to fix issues like aberrations, focus problems, or other eye abnormalities. For more information on vision problems, check out How Refractive Vision Problems Work.
How a Lens Works
A great way to grasp how light behaves when passing through a curved lens is to compare it to a prism. A prism is thicker at one end, causing light passing through to bend (refract) towards the thicker portion. Refer to the diagram below for clarity.
You can think of a lens as two prisms fused together. As light travels through it, it bends toward the thickest section of the prisms. In a minus lens (shown on the left), the thickest edges, called the base, are on the outer parts, while the thinnest part, the apex, is in the center. This setup spreads the light outward, pushing the focal point forward. The stronger the lens, the further the focal point moves away from it.
For a plus lens (shown on the right), the thickest part is in the center, and the outer edges are thinner. This causes the light to bend inward, shifting the focal point back. The stronger the lens, the closer the focal point is to the lens.
By placing the correct type and strength of lens in front of the eye, the focal point is adjusted to make up for the eye's inability to focus the image onto the retina.
Understanding Lens Power
The power of a lens depends on both the material it's made from and the curvature angle that’s shaped into the lens. This strength is quantified in diopters (D), which reflects the degree to which the light is refracted. A higher diopter means a stronger lens. Additionally, a plus (+) or minus (-) symbol in front of the diopter signifies the lens type.
You can combine plus and minus lenses, and the resulting lens power is the algebraic sum of the two. For instance, combining a +2.00D lens with a -5.00D lens results in:
Lens Shapes
In optometry, the two primary lens shapes that are commonly used are spherical and cylindrical.
- A spherical lens resembles a basketball that's been sliced in half, with a consistent curve across its entire surface.
- A cylindrical lens, on the other hand, resembles a pipe that's been cut lengthwise. The direction of its curvature, or axis, determines the orientation, bending light only along that axis. Cylindrical lenses are frequently used to correct astigmatism, as their axis can be aligned with the axis of the corneal aberration.
- Compound lens: a lens that combines both spherical and cylindrical components
- Cylindrical curve: a curve that extends along a straight line, similar to a pipe split lengthwise
- Diopter (D): the measure of a lens' refractive power; higher values indicate stronger lenses
- Refraction: the bending of light as it passes through a lens
- Spherical curve: a curve that is uniform in every direction, much like a basketball cut in half
Lens and Prescription
To create a lens, the first step is obtaining a lens blank. These blanks are produced in factories and then sent to labs to be crafted into eyeglasses. The raw material is poured into molds that shape the blanks into discs about 4 inches in diameter and between 1 and 1 1/2 inches thick. The mold's bottom creates a spherical curve on the front face. A small segment with a stronger curve can be added to the mold to create the section for bifocals or progressive lenses.
How to Read the Prescription
Most prescriptions have four parts:
- The base (spherical) strength and type (plus or minus)
- The cylinder strength and type
- The cylinder axis orientation (in degrees with 90 degree vertical; an "x" means "at")
- The strength of bifocal segment ("plus" indicating "in addition") and type
A short form prescription from the optometrist or ophthalmologist might read:
2.25 -1.50 x 127 plus +2.00
This means:
- A +2.25D spherical base curve (plus lens)
- A -1.50D cylinder at 127 degrees (a minus cylinder lens is added to the base curve)
- An additional bifocal segment of +2.00D
Total power of the lens with the cylinder is +2.25 + (-1.50) = +0.75D. At the segment, the power is (+0.75) + (+2.00) = +2.75D. And in case you've ever wondered, OD means right eye and OS, left eye.
- Base curve: a simple spherical curve; the primary lens curve
- Lens blank: basic spherical lens; the lab grinds the back of the blank to match the prescription
- Optical center: a spot on a spherical lens where light enters at a 90-degree angle to the lens plane
- Segment: the portion of a lens added for reading (bifocal or trifocal); it may be added separately to the lens blank or formed as a blended curve onto the base
Overview: How the Lens is Made
In the lab the patient's full prescription gives these exact details:
- The total lens power (measured in diopters) that the finished lens needs to have.
- The strength and dimensions of the segment (if required).
- The power and orientation of any cylindrical curves.
- Additional details like the location of the optical center and any necessary induced prism.
The technician selects a lens blank that includes the correct segment (called an add) and a base curve close to the prescribed power. To match the prescription precisely, another curve is then ground into the back of the lens blank.
- In most labs, the equipment is set up to grind minus curves, which is why a strong plus lens blank is often chosen.
- If the base curve is too strong, a minus curve is ground into the back to reduce the overall lens power.
For instance, a typical lens blank might be +6.00 diopters. If the prescription requires +2.00 diopters, a -4.00 diopter curve is ground on the back, resulting in: (+6.00D) + (-4.00D) = +2.00D. (See the illustration below.) If necessary, the cylinder curve is also ground simultaneously.
Even when the prescription calls for a minus lens, the +6.00 diopter blank can still be used. To create a -2.00 diopter lens, a -8.00 diopter curve is ground on the back: (+6.00D) + (-8.00D) = -2.00D.
Creating a Lens: Step 1
A lens blank is shaped to fit the patient's specific prescription.Phases 1 to 3
Corrective lenses can be produced from either glass or plastic, with plastic being the preferred material today. Although multiple plastic types are used for lenses, the manufacturing process remains consistent across them. Many of these steps also apply to glass, with a few notable exceptions that are mentioned later.
Whether automated or manual, a lab follows a 12-step process to create prescription lenses:
Step 1: The technician selects a lens blank made from the appropriate material with the correct base curve and, if necessary, the required add power.
Step 2: If the prescription includes a cylinder, a reference line is drawn on the front of the lens to represent 180 degrees, and another line is drawn to match the axis of the second curve. For lenses with a segment, the segment edge serves as the 180-degree line, with the optical center typically placed slightly above it. If there's no segment or induced prism, the lens may remain unmarked, and the cylinder axis will be determined after the lens has been ground.
The lens blank is marked to indicate where the cylinder axis will be positioned.Step 3: To preserve the integrity of the front surface of the lens, it is covered with a special protective tape.
The technician applies a protective layer to the front of the lens blank to prevent any potential damage.
- Generator: A specialized compound surface grinder used for shaping curves into the lens surface
- Induced prism: A method used to shift the optical center away from the pupil center
Making a Lens: Part 2
A compound grinder, known as a generator, carves the necessary curves into the rear of the lens blank. The large dials on the machine's console are used to set the spherical and cylindrical curves that need to be ground.Steps 4 through 6
Step 4: Depending on the equipment used, the lens must be readied for the generator, which is typically a compound surface grinder designed to grind two curves simultaneously.
A chuck receiver, also known as a block, is attached to the front of the lens over the protective tape. If a cylinder curve is present, the lens is positioned so the cylinder axis aligns with the cylinder sweep axis of the generator.
The chuck receiver, or lens block, needs to be affixed to the front of the lens so it can be properly mounted in the generator.The block's center will serve as the optical center of the lens. Depending on the type of equipment, the lens may be secured in place using special adhesive pads with an alloy that bonds the lens to the block or with plastic.
Step 5: The lens is placed into the generator.
The lens blank, now attached to the lens block, is positioned into the generator. The generator features pins that help align the lens correctly.The lens may require additional processing beyond the compound curves formed by the generator, which could involve tilting the lens in the chuck. This tilt can shift the optical center (referred to as induced prism), commonly used to create thinner lenses or address specific prescription needs.
The lens is ground inside a rubber-lined grinding chamber, with the cone-shaped quill (grinding wheel) located at the center. The quill, featuring a diamond cutting surface on its outer edge, is angled so that only the outer edge makes contact with the lens.Step 6: The machine is set to the desired curves and the lens is ground (generated). This can be either an automated process or done manually, where the operator moves the quill (grinding wheel) across the lens, gradually advancing it until the correct thickness is achieved. Lens thickness is influenced by factors like curve type (positive or negative), material (some plastics are harder and can be ground thinner), and specific needs (e.g., safety glasses are made thicker than regular lenses). If the lens becomes too hot during grinding, it may warp or tear, so water is used to cool it and wash away the excess material (known as scarf).
Making a Lens: Part 3
A cylinder machine can simultaneously sand two lenses. Air pressure is used to hold the lens and the sanding block together, while a timer ensures the machine stops after a predetermined time.Steps 7 through 9
Step 7: After being removed from the generator, the lens is placed in a special sanding machine, called a cylinder machine, to eliminate any marks left by the generator. Sandpaper is affixed to a block with matching reversed curves (e.g., a +2.00 base/+2.50 cylinder for -2.00/-2.50 generated curves). The lens and block are rubbed together while being kept cool and cleaned with water.
Once sanding is complete, the lenses undergo polishing on a similar machine. However, instead of sandpaper and water, felt polishing pads soaked in polishing compound are used. By the end of this process, the lens is optically clear with no visible scratches.
After sanding, the lenses are polished to achieve perfect clarity without any scratches. Liquid polish is applied to the lenses and flows into a reservoir, where it is recirculated for further use.Step 8: The block is detached from the lens, after which the lens is washed and thoroughly inspected. If necessary, special coatings may be applied. At this stage, the lens blank has had additional curves ground on the back and has been polished. However, the large diameter blank still needs to be resized and shaped to fit the frame chosen by the patient. Several methods exist to achieve this, depending on the equipment used, but they all follow a similar process.
Step 9: The lens blank is shaped using a linear lathe, also known as an edger, which employs either a ceramic or diamond grinding wheel, or stainless steel blades. Before shaping the lens, it is prepared to receive a chuck. As only the edge of the lens is being cut, a gentler system is used. A small chuck receiver is placed at the center of the lens, where the finished lens's geometric center will be. The lens is then aligned on the 180-degree axis. Typically, an adhesive pad is sufficient to hold the receiver in place. The lens is then chucked into the edger and secured by a pressure pad that holds it steady, similar to how a large coin is pinched between the thumb and forefinger at its center.
The lens is mounted onto the edger. The chuck on the edger rotates slowly as the lens is precisely cut to its desired shape.Making a Lens: Part 4
Frame patternsSteps 10 through 12
Step 10: A frame-shaped pattern is inserted into the edger to guide the cutting process. These patterns are typically made of plastic and can either be provided by the frame manufacturer or created within the lab.
A red pattern is used within the edger to define the final shape of the lens.Modern edgers no longer rely on physical patterns. Instead, a probe measures the frame's shape and stores the data in a computer. This computer then controls the edging process. As the lens slowly rotates, it is brought toward the fast-moving cutting surface, which can be either a grinding wheel or steel blades. The pattern, which rotates to match the lens, guides the cut. If the frame has a full rim, a bevel is cut along the edge to fit into the frame's groove, while the lens edge remains flat for frames without a full rim.
Step 11: The lenses, now trimmed to match the frame's shape, are prepared for insertion into the frame.
- If the lenses are to be tinted, the dyeing process takes place at this stage. Special dyes are heated and the lenses are immersed in them. The tint's intensity depends on how long the lenses are left in the dye. Lenses may be tinted partially (fade effect), have different colors at the top and bottom, or receive a custom tint through a mix of different dyes. Additionally, UV-blocking dyes can be applied in the same way.
Lenses that require tinting are immersed in hot dye.- If the frame is rimless, a groove is cut along the lens's edge to hold the string that attaches the lens to the frame. Any sharp edges are smoothed, and the edge can be polished on a buffing wheel if desired.
Step 12: The lens is placed into the frame. The fit and orientation are carefully checked, any worn screws or hinges are replaced, and the frame is aligned to ensure it's square. The completed eyeglasses are then thoroughly cleaned and packaged for delivery to the customer.
A technician inspects the finished lenses for any scratches or imperfections.Glass lenses are ground and polished in much the same way as plastic lenses, though diamond cutting surfaces are used, and certain details may vary. The blanks are made from relatively soft glass and must be tempered through chemicals or heat to strengthen them before they are inserted into the frame.
Automation is rapidly transforming the lens-making process. For instance, most labs now rely on computers to calculate curve parameters and select the appropriate lenses, and there is equipment available that can combine multiple steps or even handle the entire process automatically.
