Lesson 6 - Ophthalmic Lens Designs

Question 1

Single-vision lenses

Answer 1

Lenses with the same power throughout the lens

Question 2

Spherocylinder lenses

Answer 2

Lenses with a sphere (to correct nearsightedness or farsightedness) and a cylinder (to correct astigmatism)

Question 3

Multifocal lenses

Answer 3

Lenses in which the lower portion of the lens has an add that makes it more magnified (or plus) in power than the rest of the lens. A bifocal has two powers, and a trifocal has three.

Question 4

Chromatic aberration

Answer 4

When the lens does not focus all the wavelengths of color to one point

Question 5

Distortion

Answer 5

When the different points in the periphery of a lens have different amounts of magnification

Question 6

Spherical aberration

Answer 6

Occurs when parallel light rays move along the optical axis—that line that passes straight through the lens from front to back—converge in different places. As a result, an image appears blurred and out of focus.

Question 7

Coma

Answer 7

The type of lens aberration called coma occurs when parallel light rays going through a lens focus in a ring-shaped pattern with a V-shaped or comet-like flare on one end of the image.

Question 8

Curvature of field

Answer 8

Occurs when the things a person sees through the edge of a lens don’t focus on the retina at the same time as the objects the person sees through the center of the lens. This makes a flat surface look curved.

Question 9

Aspheric lens

Answer 9

An aspheric lens has a flatter base curve—that’s the curve on the front of the lens—and it’s much thinner and lighter than a regular spherical lens. It minimizes spherical aberrations. Distortion-free image. One minor drawback of aspheric lenses is that they tend to sit close to the face, and the people wearing them may notice reflections on the lenses. We can solve this problem easily by putting an antireflective coating on the lenses.

Question 10

Atoric lens (also called free-form lenses)

Answer 10

Different from the toric lens. Has both the front and back surfaces aspheric. This design maximizes the clarity of vision in the periphery of the lens in both meridians.

Question 11

Lenticular lenses

Answer 11

Have lesser or no power in the carrier portion of the lens

Question 12

Focal length

Answer 12

The inverse of the power of a lens in meters

Question 13

Vertex

Answer 13

The distance from the lens to a person’s eye. It plays a big role in determining the effective power of the lens

Question 14

A plus lens is made of two prisms placed how? A minus lens is made of two prisms placed how?

Answer 14

A plus lens is made of two prisms placed base to base (think rhombus/diamond pointing horizontally). A minus lens is made of two prisms placed apex to apex (think hourglass laying horizontally).

Question 15

Where on a plus or minus lens are the thickest?

Answer 15

Plus lenses are always thicker in the center, and minus lenses are always thicker at the edges.

Question 16

How do you describe the shape of a plus or minus lens?

Answer 16

A plus lens (also called a convex lens) looks like an elongated football. A minus lens is also called a concave lens. It’s shaped like a pillar with the middle squeezed or with the curves caving inward toward each other:

Question 17

Vergence

Answer 17

Describes how light rays diverge (spread apart) or converge (come together)

Question 18

How do plus lenses stack up when it comes to vergence? What is the image created called?

Answer 18

After going through a convex lens, light converges before it reaches the retina of a normal-length eyeball. We call the image created by a plus lens a real image.

Question 19

How do minus lenses stack up when it comes to vergence? What is the image created called?

Answer 19

After going through a concave lens, light rays diverge and focus farther out than they normally would. We call the image created by a minus lens a virtual image.

Question 20

The vergence of light entering a lens added to the power of the lens itself will be equal to what?

Answer 20

The vergence of light entering a lens added to the power of the lens itself will be equal to the vergence of light leaving the lens.

Vergence of light entering lens + lens power = vergence of light leaving the lens

Question 21

The light going through a plus lens will correct the vision of a ___ person. A ____ eyeball is shorter than a normal eye, so the light rays will focus ____ when uncorrected. When corrected with a plus lens, the light converges onto the retina of a short ___ eyeball, improving vision.

Answer 21

The light going through a plus lens will correct the vision of a hyperopic (farsighted) person. A hyperopic eyeball is shorter than a normal eye, so the light rays will focus behind the retina when uncorrected. When corrected with a plus lens, the light converges onto the retina of a short hyperopic eyeball, improving vision.

Question 22

A minus lens bends the light less, so the light converges where in relation to the retina of a nearsighted person whose myopic eyeball is longer than normal?

Answer 22

A minus lens bends the light less, so the light converges before it reaches the retina of a nearsighted person whose myopic eyeball is longer than normal.

Question 23

Plano

Answer 23

The surface of one side of a lens is flat.

Question 24

Biconvex

Answer 24

The front and back surfaces of a lens are both convex.

Question 25

Biconcave

Answer 25

The front and back surfaces of a lens are both concave.

Question 26

Meniscus lens

Answer 26

Has one convex and one concave side. Meniscus lenses can minimize distortion or blur in images, especially if the lens power is very high.

Question 27

Minus meniscus lens

Answer 27

A meniscus lens that’s thinner in the center than in the periphery

Question 28

Positive meniscus lens

Answer 28

A meniscus lens that’s thicker in the center than in the periphery

Question 29

An Abbe value is the measure of what?

Answer 29

An Abbe value is the measure of chromatic aberration. The higher the Abbe value, the less chromatic aberration the material has.

Question 30

Light will always bend towards the what?

Answer 30

Light will always bend towards the base of a prism.

Question 31

Diopter, or diopter of sphere. Sometimes abbreviated as D, DS, sph, or sphere.

Answer 31

This unit of power describes how much divergence or convergence occurs when light goes through a lens. Prescriptions show numbers in units of diopters. We also use diopters to measure the power of the eye itself

Question 32

The total power of the eye’s refractive system is?

Answer 32

The total power of the eye’s refractive system is about 59 diopters

Question 33

The total power of the crystalline lens alone is?

Answer 33

The total power of the crystalline lens alone is about 18 diopters

Question 34

Light going through a powered lens will focus behind the lens at a certain distance from the lens. To know where it will focus, we need to calculate the what? We get this number how?

Answer 34

This is the focal length. We get this number by calculating the inverse of the power of the lens in meters. The inverse is 1 divided by the power of the lens.

Question 35

Lens clock (also known as Geneva lens clock, or a Geneva clock)

Answer 35

It looks like a clock with plus powers 0-20 on the right and minus powers 1-19 on the left. One method for finding the power of a client’s lenses. When you use a lens clock, you’ll test both the front surface and the back surface of a lens. Then you’ll add the numbers to determine the power of the lens.

Example: The front of a lens reads -2.00 and the back surface is +4.00, the total power of the lens is -2.00 + 4.00, or +2.00.

Question 36

Dispersion

Answer 36

The colors of the rays passing through the lenses separate, much like a light ray passing through a prism separates into the colors of the rainbow.

Question 37

Pincushion effect

Answer 37

When distortion happens in a plus lens, the magnification increases from the center outward.

Question 38

Barrel effect

Answer 38

When distortion occurs in a minus lens the magnification decreases as you move outward.

Question 39

Oblique (or marginal) astigmatism

Answer 39

Occurs when rays of light pass through a lens at a slant. This creates two focal lines that are perpendicular to each other. The result is blurry vision. The difference between the two focal lines is the amount of astigmatism caused by the lens. The bigger the distance, the worse the problem is.

Question 40

Base curve

Answer 40

The curve that forms the basis for all other curves on a lens.

Question 41

We determine the power of a lens how?

Answer 41

We determine the power of a lens by adding the total powers of the front and back curves. So, lenses with the same power can have a wide range of front and back curves, if the sum of their powers is always the same

Question 42

Vogel’s Formula or Vogel’s Rule

Answer 42

The optimal base curve for a lens. The base curve of a PLUS lens is the spherical equivalent of the lens + 6 diopters. The base curve of a MINUS lens is half the spherical equivalent of the lens + 6 diopters. Spherical equivalent = taking half the cylinder amount and adding it to the sphere.

Question 43

A lens without power (a plano lens) will most likely have…

Answer 43

A lens without power (a plano lens) will most likely have a back surface curve of -6.00 and a front curve of +6.00.

Question 44

As the power of a lens becomes more minus in power, the front surface gets flatter, so the back surface must…

Answer 44

As the power of a lens becomes more minus in power, the front surface gets flatter, so the back surface must get steeper to maintain the same power.

Question 45

As the power of a lens becomes more plus in power, the front surface of the lens gets more curved (steeper), so the back curve must…

Answer 45

As the power of a lens becomes more plus in power, the front surface of the lens gets more curved (steeper), so the back curve must get flatter to maintain the same power.

Question 46

If both lenses are plus lenses, you’ll want to choose the…

Answer 46

If both lenses are plus lenses, you’ll want to choose the steeper base curve of the two.

Question 47

If both lenses are minus lenses, you’ll usually want to choose the…

Answer 47

If both lenses are minus lenses, you’ll usually want to choose the flatter base curve of the two.

Question 48

If one lens is a plus lens and the other is a minus lens, you’ll want to choose the…

Answer 48

If one lens is a plus lens and the other is a minus lens, you’ll want to choose the base curve that’s the higher of the two.

Question 49

If the difference between lenses is greater than two diopters, it’s better to use…

Answer 49

If the difference between lenses is greater than two diopters, it’s better to use the individual base curve selections.

Question 50

In general, if a prescription has no cylinder component, the front surface of the lens should be ___ and the back surface should be ___.

Answer 50

In general, if a prescription has no cylinder component, the front surface of the lens should be aspheric and the back surface should be spherical.

Question 51

If a prescription has a cylinder component, the front surface should be ___ and the back should be ___.

Answer 51

If a prescription has a cylinder component, the front surface should be aspheric and the back should be toric.

Question 52

Toric Lenses

Answer 52

A toric surface of a lens has two different curves. Each curve is perpendicular from the other, and each curve will have a different power. The difference in power between the two curves gives us the amount of astigmatism correction in the lens. Toric lens designs are found in both glasses and contact lenses.

Question 53

When you’re dealing with aspheric lenses, you’ll want to select a base curve that…

Answer 53

When you’re dealing with aspheric lenses, you’ll want to select a base curve that causes the least amount of curvature throughout the lens.

Many lens manufacturers make their lenses semi-finished and already ground with certain base curves to minimize distortion problems.

Question 54

The best solution for presbyopia isn’t longer limbs but what?

Answer 54

The best solution for presbyopia isn’t longer limbs but multifocal lenses. The add in each lens gives people the close vision they need, while the rest of the lens allows them to see well at a distance.

Question 55

Progressive Lens (also called progressive additions lenses and PALs)

Answer 55

Many people prefer these types of lenses because there’s no visible line in the lens—gradually change in power from top to bottom, so they combine distance and near powers in a single lens without a visible line between lens powers. An hourglass-shaped corridor of optimal vision runs down each lens vertically. This is called the progressive corridor. Outside of the corridor, vision will be slightly blurred. They vary in price, lens material availability, corridor length and width, amount of peripheral distortion, optical quality, and design. You can order aspheric progressives to help reduce spherical aberrations if the prescription is very high.
You can order short-corridor progressives if the vertical depth of the frame is small. Are available in photochromic materials and polarized versions

Question 56

Bifocals

Answer 56

In these glasses, there’s a distinct line between the main part of the lens and the reading add.

Question 57

Seg drop

Answer 57

The distance from the top of the full lens to the top of the segment.

Question 58

Seg height

Answer 58

The distance from the top of the segment to the bottom of the main carrier lens.

Question 59

Segment (shortened to seg)

Answer 59

The bottom part of the bifocal lens that contains the reading add.

Question 60

Seg depth

Answer 60

The depth of a bifocal segment (that is the distance from the top to bottom of the segment).

Question 61

The segments in bifocals (and trifocals as well) are attached to the main lens how?

Answer 61

The segments in bifocals (and trifocals as well) are separate lens pieces bonded to the front of the main lens.

Question 62

You can determine the power of the bifocal or trifocal add by…

Answer 62

You can determine the power of the bifocal or trifocal add by using a lensometer and reading the lens power from the backside of the lens.

Question 63

The difference between the power of the main lens and the power of the lens read through the segment is the power of the ___.

Answer 63

The difference between the power of the main lens and the power of the lens read through the segment is the power of the add.

Question 64

Image jump

Answer 64

A drawback of bifocals when the eye looks down and crosses over the segment line, there’s a sudden “jump” or shift in the position of the image the person is viewing

Question 65

List and describe the types of bifocal lens

Answer 65

Flat-top bifocal: have a half-circle design and are categorized by their width. Common widths are 22mm 25 mm, 28 mm, 35 mm, and 45 mm across the top.

Ribbon seg bifocal: flat on both the top and bottom. It’s typically 13 or 14 mm deep.

Kryptok segment: It’s round.

Double D bifocal: there is a traditional D-shaped segment on the bottom and another one on top with the two flat portions facing each other in the middle. We can make the power of the upper segment the same as the lower segment or only 50 percent or 33 percent as strong

Executive bifocal: like flat-top bifocals, but the bifocal line goes all the way across the lens. Also, the lens is one piece as opposed to a fused bifocal that has a reading segment fused to the front of the lens. These lenses provide a very wide reading area. However, they’re very heavy, have a thick ledge at the top of the reading segment, and are cosmetically unappealing because of the wide line across the lens.

Blended or curved-top bifocals: the edges of the bifocal segment are slightly buffed so there’s no hard line even though the segment is still there. What’s the drawback? There’s a zone about one to two millimeters wide around the buffed-out line that isn’t clear to look through. However, this style often works well for people who can’t adapt to a progressive lens.

Question 66

Trifocal Lens

Answer 66

The top portion of a typical trifocal lens contains the distance prescription. The top portion of the segment is for viewing objects at intermediate distances, such as looking at a computer or at shelves in a grocery store. And finally, the bottom portion is for reading. We can describe trifocals according to the width across the top of the segment.

Question 67

Cathode-ray tube (CRT) Trifocal

Answer 67

An old term for occupational trifocal design made for computer work, where the intermediate section is deeper but allows space to look though the top portion for distance viewing.

Question 68

Near variable focus lenses (NVF)

Answer 68

A type of progressive lens where the intermediate zone is much wider and the distance and near zones are much smaller.

Question 69

The seg height measurement of a progressive lens

Answer 69

The standard distance from a person’s pupil at a straight-ahead gaze to the bottom of the B measurement of a frame is typically 18 mm.

Question 70

Short corridor progressives

Answer 70

Have a shorter vertical depth. These frames may require a progressive lens with a shorter corridor. They only require about 13-15 mm seg heights.

Question 71

As long as frames are deep enough, progressive lenses work well with just about any frame shape. The only real challenges occur with what frame shape?

Answer 71

The aviator shape. The bottoms of the eyewires taper up sharply as they move inward toward the nose. If a person’s eyes are too close to the lenses and the person looks down to read, the eyes may not land in any part of the reading portion of the progressive lenses.

Question 72

Important suggestions for progressive lens wear

Answer 72

Try to make sure the frame sits fairly close to your client’s eyes (but not so close that it touches the eyelashes). The closer the lens, the wider the field of vision your client will have.

Also, recommend metal frames with nose pads. These are much easier to adjust for a progressive lens design than a zyl (a lightweight type of plastic) frame is. You can adjust the nosepads to change the height of the seg or the vertex of the frame a little, and you can adjust the temples to change the pantoscopic tilt (lens tilt of the horizontal axis) if you need to.

Finally, recommend a frame with a little bit of face form curve. This also maximizes the reading area when a person is looking down.

Question 73

When you’re considering progressive lenses, you’ll also want to know how much time your client spends at a computer. Why?

Answer 73

A conventional progressive lens has such a small, short intermediate area that it isn’t very practical for intermediate computer viewing distances. To solve this problem, some companies now make progressives designed specifically for computer use. In these lenses, the intermediate portion is a lot larger, and the distance portion is significantly reduced or non-usable. Be aware that people who use lenses for computer viewing will probably want a separate pair of glasses for other activities. Computer glasses aren’t optimal for outdoor activities.