11 Vision Correction

7/30/2019 11 Vision Correction

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Physics 1230: Light and Color

The Eye: Vision variants and Correction

http://www.colorado.edu/physics/phys1230


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What does 20/20 vision mean?

Snellen chart

http://www.bgsm.edu/eye/cornea/normal1.htm

Examples:

20/20 vision - Normal

20/40 vision - Worse than normal.- Can see at 20 feet what someonewith normal vision can see at 40 feet

- Can only see letters that are twice aslarge as someone with normal vision

20/15 vision - Better than normal

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Visual acuity is usually measured with a Snellen chart

Furthest distance that aperson with normal

vision can see object

Furthest distancethat the test subject

can see object


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How does the eye form an image? (revision)

T he eye uses 2 lenses to form a sharp image on the retina :

Cornea Strong lens of fixed focal length does most of the focusing Eyelens Weaker lens of variable focal length

- Enables us to focus on objects at different distances

eyelens

cornearetina


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Normal Myopic (Near-sighted) Hyperopic (Far-sighted)

Common Eye problems - Near and far-sightedness

Relaxed eyes viewing a distant object

retina

Primarily determined by:

Shape of cornea (crude focusing)

Length of eyeball


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Normal Eye

THE NORMAL EYE

For clear vision, objects focused exactly on retina Corneas curvature (or its focal length) exactly match to length of eyeball

http://www.lasersite.com/Eyeworks/index.htm


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Near-sighted eye (Myopic)

THE NEAR-SIGHTED EYE

- Cornea is too curved (focal length too short) or eyeball too long- Image of distant object forms in front of retina.- Image on retina is blurred


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What the world looks like with myopia

Foreground clear Distant objects blurred


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Who has myopia?

Approximately one in four Americans is near-sighted, totaling 70 millionpeople. The degree of myopia can vary from low (-1 to -3 diopters) to high(greater than 6 diopters). Myopia tends to start in the early teens (earlierfor high myopes), and increases as the eye grows in length during puberty. Ittends to stabilize after age 18.

Myopia is not caused by reading at an early age, prolonged reading, reading inthe dark, watching TV too closely, wearing glasses too strong, wearingglasses too weak, or other similar folklore. Some controversy exists overwhether myopia can be made worse by prolonged near work as a child, butthis has never been proven.

In studies, it has been correlated with higher I.Q.s., but this probably has todo with the fact that myopes tend to read more since that's where they cansee the best without glasses (see below). Myopia tends to run in families, so

it is probably inherited.

What causes myopia?

C t ti i


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Concept question on myopia

To correct for myopia or near-sightedness, your glassesneed to contain:A. Positive (convex) lensesB. Negative (concave) lenses

C. Bifocals

C t ti i


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Concept question on myopia

To correct for myopia or near-sightedness, your glassesneed to contain:A. Positive (convex) lensesB. Negative (concave) lenses

C. Bifocals


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Correction for myopia

Relaxed myopic eye

No glasses -This is the furthest objectthat can be seen clearly

With glasses withnegative lenses -A very distant object canbe seen clearly


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Far sighted eye (Hyperopia)

THE FAR-SIGHTED EYE

- Cornea is not curved enough (focal length too long) or eyeball too short- Image of distant object forms behind retina.- Image on retina is blurred


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What the world looks like with hyperopia

- Distant objects blurred- Foreground even more fuzzy


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Concept question on hyperopia

To correct for hyperopia, do you need glasses with:A: Convex lensB: Concave lens

C: Bifocals


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To correct for hyperopia, do you need glasses with:A: Convex lensB: Concave lens

C: Bifocals


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Correction for hyperopia

Relaxed hyperopic eye

a) Viewing nearby object -blurred

b) Viewing nearest clear object

c) Viewing nearby object withconvex glasses clear image


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Other facts about hyperopia

Hyperopia (i.e far-sightedness) is much less common than myopia oremmetropia. It is typically in the +1.00 to +4.00 diopter range, rarely it canbe as high as +8.00 diopters.

In contrast to myopia, hyperopia occurs when the eye is too short for thepower of its optical components (i.e. lens is too weak). In hyperopia, thecornea is not steep enough and light rays hit the retina before they comeinto focus. Distant objects appear blurred, and nearby objects are even morefuzzy. Most farsighted individuals need corrective eyewear to see clearly atall distances.

Correction of hyperopia requires a lens which is convex (i.e. thicker in the

middle than the edges). This acts as a magnifier, and causes objects toappear bigger by 2% per diopter. For this reason hyperopes while wearingtheir spectacle correction, appear to have "big" eyes. Optical aberrations anddecreased peripheral vision occur are likely to occur with large amounts of

hyperopia.


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Treatments for myopia

Glasses

Contact lenses

Eye surgery flatten the cornea

Radial keratotomy Photorefractive keratotomy (PRK)

LASIK


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Radial Keratotomy

Cut several spoke-like slits in the cornea with a diamond knife Causes central area to flatten and focal length increase. 85% of people achieve 20/40 vision or better


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Photorefractive keratotomy (PRK)

Uses a computer controlled eximer laser Laser emits pulses lasting only billionths of a second Laser vaporises microscopic slivers from the cornea theoretically making veryaccurate and specific modifications to its shape.

Procedure takes 1-2 minutes Procedure is pain free but 10-20% of people feel pain for weeks afterwards 95% achieve 20/40 vision or better

LASIK

Like PRK but performed deeper inside the cornea A surface flap is removed and then replaced to give access inside cornea. Generally results in quicker and less painful recovery than PRK Early studies suggest better final vision than RK or PRK
http://www.youtube.com/watch?v=aVTjyB0S7Rw&feature=related


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Side effects of corrective eye surgery

* May still need glasses With RK, approximately 10 percent to 30 percent ofpeople who have the procedure require some use of corrective lenses; with PRK,between 5 percent and 15 percent. Also, everyone's eyes degenerate with age, so byabout age 40, you may find yourself needing bifocals for sharp near vision.

* Night vision may be poor: Some may see "halos" around lights, which can make

night driving difficult. For some, this side effect passes. For others, it is permanent.* May see variation in visual acuity For some, eyesight may be better in themorning and less sharp at day's end. Some may experience light sensitivity and glare.

In very rare cases, there may be corneal scarring or rupturing Because thecornea is weakened in RK, there is a threat of corneal rupture at the incision site ifthe eye is hit.

Unknown or unexpected complications e.g. blindness at altitude in 1996 Everest

disaster


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DIOPTERS AND

YOUR PRESCRIPTION

Diopters are used to measure the refractive error of the eye and the

power of the lens needed to correct this error

-Negative number (myopia, nearsightedness)-Positive number (hyperopia, farsightedness).

-A -1.00 diopter myope is able to see objects at 1m clearly-A -2.00 diopter myope is able to see objects at 1/2m clearly

Your prescription contains 4 numbers:

1. Amount (in diopters) of myopia (- numbers) or hyperopia (+ numbers).2. Amount of astigmatism.3. Axis of the astigmatism.4. Amount of magnification needed for clear vision at near (bifiocals)


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Concept question - Adding lenses

Lens #1 Lens #2

F1 F2

What is the combined focal length of two convex lenses

A: Shorter than either F1 or F2?

B: Between F1 and F2?

C: Longer than either F1 or F2?

dd l


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Concept question - Adding lenses

Ftotal

F1

Lens #1 Lens #2

F2

What is the combined focal length of two convex lenses

A: Shorter than either F1 or F2?

B: Between F1 and F2?

C: Longer than either F1 or F2?

Formula for adding lenses


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m f g

F

To add lenses, add their POWERS (not their focal lengths)

Power (in diopters) = 1/F, and F is the focal length (in m)

So Ptotal = P1 + P2

or 1/Ftotal = 1/F1 + 1/F2

Example of adding lenses


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p g

F

Question: If F1 is 25 cm, and F2 is 1 m, what is Ftotal?

Answer: P1 = 4 D and P2 = 1 D

=> Ptotal = P1 + P2 = 5 D

=> Ftotal = 1/5 m = 20 cm

Another example of adding lenses


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p g

Question: If F1 is 0.5 m, and F2 is -1 m, what is Ftotal?

Answer: P1 = 2 D and P2 = -1 D

=> Ptotal = P1 + P2 = 1 D

=> Ftotal = 1 m

Concept Question on Adding Lenses


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p g

F

Question: If F1 is 50cm, and F2 is 1m, what is Ftotal?

A. -1mB. 33cm

C. 1.5m

Concept Question on Adding Lenses


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p

F

Question: If F1 is 50cm, and F2 is 1m, what is Ftotal?

A. -1mB. 33cm

C. 1.5m


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Calculating your prescription

To calculate your prescription, calculate thedifference in diopters between what you want tosee and what you can see

Calculating your prescription -Example


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Question: A near-sighted person wants to see to infinity

but does not see clearly beyond 2 meters. What kind andstrength of eyeglasses do they need?

Answer: -0.5 D

How did I get this answer?

Convert the infinity to zero diopters, and the 2 m to0.5 D. The difference is -0.5 D.

Why is the number negative?You need a negative lens to correct for near-sightedness

Calculating your prescription application of thin lens equation


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Calculating your prescription application of thin lens equation

Before correction (viewing far point)

1/xi + 1/ xo = 1/Feye

xo xi

xo xi

After correction (viewing new far point)

1/xi + 1/ xo = 1/Ftotal= 1/Feye + 1/Fglasses

Subtract before from after:

1/xo 1/xo = 1/FglassesChange in 1/(object distance) = Power of lens required (or your prescription)

Concept Question


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Question: A near-sighted person wants to see to infinitybut does not see clearly beyond 1 meter. What kindand strength of eyeglasses do they need?

Answer:A. -1 DB. +1 DC. -2 D

.

Concept Question


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Question: A near-sighted person wants to see to infinitybut does not see clearly beyond 1 meter. What kindand strength of eyeglasses do they need?

Answer:A. -1 DB. +1 DC. -2 D

Concept Question


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Question: A near-sighted person wants to see to infinitybut does not see clearly beyond 3 meters. What kindand strength of eyeglasses do they need?

Answer:A. -3.0 DB. -0.3 DC. +0.3 D

Concept Question


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Question: A near-sighted person wants to see to infinitybut does not see clearly beyond 3 meters. What kindand strength of eyeglasses do they need?

Answer:A. -3.0 DB. -0.3 DC. +0.3 D

.

Example on Hyperopia


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Question: A far-sighted person sees no closer than 0.5meters. They want to read at the normal 25 cm distance.What kind and strength of eyeglasses do they need?

Answer: +2D

How did I get this answer?Convert the 0.5 m to 2 D, and the 25 cm to 4 D. Thedifference is +2 D.

Why is the sign +?To correct for far-sightedness, you need a positive lens

Example on Hyperopia


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Question:A far-sighted person sees no closer than 0.5 meters. They want to

read at the normal 25 cm distance. What kind and strength of eyeglasses dothey need?

Answer: +2 D

How did I get this answer?Convert the 0.5 m to 2 D, and the 25 cm to 4 D. The difference is +2 D.

OR

Before correction -

1/xi + 1/ xo = 1/Feye

So 2 + 1/L = 1/Feye

After correction -

So 4 + 1/L = 1/Feye + 1/Fglasses = 2 + 1/L + 1/Fglasses

=> 2 = 1/Fglasses



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Question: A far-sighted person sees no closer than 1meter. They want to read at the normal 25 cmdistance. What kind and strength of eyeglasses do

they need?

Answer:A. -3 DB. +1.25 DC. +3 D



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Question: A far-sighted person sees no closer than 1meter. They want to read at the normal 25 cmdistance. What kind and strength of eyeglasses do

they need?

Answer:A. -3DB. +1.25 DC. +3 D



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Question: A far-sighted person sees no closer than 0.75meter. They want to read at the normal 25 cmdistance. What kind and strength of eyeglasses do

they need?

Answer:A. -2.7 DB. +1 DC. +2.7 D



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Question: A far-sighted person sees no closer than 0.75meter. They want to read at the normal 25 cmdistance. What kind and strength of eyeglasses do

they need?

Answer:A. -2.7 DB. +1 DC. +2.7 D

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11 Vision Correction

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