The light - EinDollarBrille...The light rays coming from remote objects are almost parallel. The lens of the eye is flat. The ciliary muscle is decontracted. Close-up vision In order

Martin Aufmuth - Germany 2013 - www.onedollarglasses.org

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The light

Light (normally spreads out straight ...

Refraction of light

But when a light ray passes from air into glas or water (or another transparent medium), it gets refracted (it looks like a broken stick):

light ray

normal

air

glas

The same happens to a single light ray passing through a lens:

And accordingly many light rays passing as a parallel light beam through the lens:

When the light ray comes out of the glass it gets refracted again.

... and into all directions.

θ1θ1

θ2θ2

θ3

light ray

normal

air

glas


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Lenses

Optical lens: If you cut the lens in half, from the side it looks like this:

Convex lenses are thick in the center and thin at the ends.

Optical power: +0.5; +1.0; +1.5; ... ; +6 diopters

Concave lenses are thin in the center and thick outside. Optical power: -0.5; -1.0; -1.5; ... ; -6 diopters

Positive lenses make things look larger.

Negative lenses make things look smaller.

With these lenses nothing changes.Optical power: 0.0 diopters

+2 + (-2) = 0 -2 + (+1) = -1+2 + (+2) = +2+2 = +4

Convex (positive) lenses

Concave (negative) lenses

lenses without optical power

Addition: calculating with the optical power of lensesYou can combine lenses to get different diopters:

+ = + = + = + =


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Light and Lenses

When parallel sun rays pass through a positive lens, they meet all in one spot, called focal point F. The light is converged.

In this point a paper would start to burn. The distance between lens and focal point is called focal lenght f.

A beam of light passing through a negative lens is diverged (spread).

The beam after passing through the lens, seems to come from a (not really existing) focal point in front of the lens.

focal point Ffocal point F

axis

Positive lenses bring the light rays closer together.

Negative lenses spread the light rays away from the axis.

f f


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The eyeThe ciliary muscle can change the thickness of the lens and thereby the refractive power.

This is called accommodation.

The ciliary muscle is contracted, the zonula fibers are loose (thick lens, for near vision).

The ciliary muscle is decontracted, the zonula fibers are tight (thin lens the eye is relaxed, for far vision).

How we can see a candle

We consider only the top point of the flame: From here light rays go straight in all directions.

Some of the light rays shine into the eye. They are refracted by the lens and then meet again in one image point on the retina.

The same thing happens to all other points of the candle. The image of the candle occurs on the retina.

This image is upside down and reversed.

Distance vision

The light rays coming from remote objects are almost parallel. The lens of the eye is flat. The ciliary muscle is decontracted.

Close-up vision

In order to focus the light rays of a near object, the lens of the eye must accommodate strong (get round) to refract strong.flat lens round lens

ciliary muscle

Vitreous bodyRetina

Choroid

Sclera

Fovea

Cornea

Pupil

Lens

Iris

24 mm

Ciliary Muscle

Optic nerve

Zonular fibers

Anterior Chamber

zonular fibers


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Defective Vision - Myopia / Hyperopia

Normally, the eyeball has a length of about 24 mm.

When seeing far, almost parallel light rays fall on the lens, are refracted and focused on the retina.

Light rays almost parallel:

The eye is too long

The eye is too short

Myopia - shortsightedness

The eyeball is too long. So the lens is not able to focus far objects on the retina but in front of it.

Those people only see close things in focus.

Shortsighted people need a negative lens for correction.

Hyperopia - farsightedness

The eyeball is too short. So the lens can‘t focus on near objects (the lens can‘t get round enough to focus the object on the retina.)

In extreme cases those people even can‘t focus on far objects too!

Farsighted people need a positive lens for correction.

Image is focused in front of the retina.

Image focused on retina.


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Defective Vision - Presbyopia

Presbyopia

With increasing age, the lens and the ciliary muscles loose their ability to focus on nearby objects.

Presbyopia usually starts between the age of 40 and 50 years.

Little babys can focus at 5 cm in front of their eyes (this equals 20 diopters). A 25 year old adult can still focus at 10 cm (or 10 diopters). A person at age 60 or older can only see clearly at a minimum distance of about 1 meter (this equals 1 diopter).

(The word presbyopia comes from the Greek word „presbys“, meaning „old man“)

The diopter - refractive power of the lens

When the eye focusses from far to a near object, it must change its refractive power (= optical power).

Looking at a near object, the ciliary muscle is contracted. The refractive power P of the eye is equal to the reciprocal of the focal length f (distance to the eye):

P = 1/f (in 1/m or diopter)

Looking at high distance, the ciliary muscle is decontracted. Parallel light rays are focussed on the retina. The refractive power of the eye is 0 diopters.

Glasses of +3.0 for an old-aged person with accommodation of only +1.0 adds up to +4.0. This means without glasses he/she can focus only on 1 meter, with glasses he/she can focus at 25 cm (1/4 = 0.25).

With a lens of +2.0 you can look at an object at 0.5 m distance with your eye relaxed as if the object would be very far away.

Refractive power of the lens:P = 1 / 0.25m = +4.0 diopters

f = 0.25m

f = 100m

P = 1/100m = 0.01 diopter = approx. 0 diopter

The problem is the lens.


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Defective Vision - Astigmatism

Besides myopia, hyperopia and presbyopia, there can be another common refractive error too.

Astigmatism

People suffering from myopia, hyperopia and presbyopia can be helped by „normal“ spheric glasses. (Spheric means, the lenses are shaped like a part of a sphere).

People suffering from astigmatism have a cornea which is deformed cylindrically.

Normally, light coming from a round circle falling through a positive spheric lens, is focussed in a single point.

The same light falling through a cylindric lens is focussed on a line (which can be vertical, horizontal or at any other angle). 2 cylindric diopters are as disturbing as 1 „normal“ spheric diopter.

At night, people with astigmatism don‘t see the stars like points but like ellipses.

There are +cylinders and -cylinders. Such a cylindric deformation must be compensated with the opposite cylinder in the glasses.

The positive cylinder of the eye can be compensated by a negative cylinder in the glasses (and vice versa).

People with a strong astigmatism must be sent to the professional optician.

sphere cylinder

Stars: seen with normal eye

-cylinder + cylinder

point

line

Stars: seen with astigmatism


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lens

notches shrink tubing

bridge

rim

temple

loop

45mmMeasures of the lens

28m

m

bead

u-wire spiral

hingefor folding temple

The glasses - technical terms

the size of the bridge

bridge b

size of bridge

Small Medium large

Important : Measure the size of the bridge inside between the wire!

13mm 19mm 24mm

54<DP<60.5 60.5<DP<66 66<DP<71 PD: Pupil Distance

OK! NOT OK!

7.5°

90°45° 45°

7.5° 22.5°

0° 0°

45° : 2 = 22.5° 22.5° : 3 = 7.5°

The right inclination of the temples


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Pupil distance

Typical pupil distances:

Small: 57.5 mm (yellow point frame)Medium: 63.5 mm (red point frame)Adults large: 68.5 mm (blue point frame)

If the glasses are too small or too big, the eyes are forced to look angular. The patient gets a headache.

Glasses are too small! Glasses are too small!

You (the optician) look face to face to the patient. You stand near in front of him. You close your left eye. The patient looks with both eyes to your right eye. With your right eye you look to the left of the patient.

Here, the left eye is in the small area (S).

Don‘t move your heads now!

Now the patient looks to your left eye. In this example the right eye is in the small area (S) too.

This patient needs a small (yellow point) frame.

Here, the pupil is in the medium (M) area.This patient needs a medium (red point) frame.

Here, the pupil is in the medium (L) area.This patient needs a large (blue point) frame.

Important: Both eyes must look through the center of the lenses!

How to measure the PD

R patient (1) L

G optician R

R patient (1) L

patient (2)

patient (3)

mesures des lentilles

PD (pupil distance)

L optician R

R L


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Pupil distance - test glasses

Take a lens of 0 diopters, a needle and a ruler.Scratch a horizontal line through the middle of the lens.

Now scratch a mark 21 mm from the left side into the lens.

Now scratch a second mark 24 mm from the left side into the lens.

Now scratch two vertical lines into the lens.

Write S, M and L on the lens and do all the same with the left lens.

horzontal line

mark 21

mark 24

60.5mm

change the bridge

Now take a medium (red) frame and adapt the bridge, so that the distance between S left and S right amounts to exactly 60.5 mm.


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Testing the eye (1)

Personal data After welcoming the patient, ask him for his surname, first name, age, address. Write this data into your book.

Ask for the problemAsk the patient if he has a problem with reading, seeing in the distance, aching eyes etc.. Ask for how long he already has the problem. Note it in your book.

Check eye diseases

1. Is the eye around the iris red (and not white)?

2. Is the center of the eye grey or white (and not black)?

3. If you illuminate the eye with your torch, the iris should become smaller. If it does not, the eye could be ill.

4. Has the patient any visible injuries or does something seem wrong with his eyes? Does he complain of severe pain in his eyes?

In these cases, you can test his eyes but you absolutely need to send him to an eye doctor!

Eye testPlace the patient 4 meters in front of the vision-chart on a chair. (If the patient needs reading glasses, first test his eyes with the vision chart too!)

Please considerTake care the sun does not shine into his face!Take care the light does not reflect from the chart!Don‘t cover the optotypes with your finger!Is the vision chart at the height of the patients eyes?Don‘t test in the dark evening!Clean the lenses before testing the patient.

Now check both eyes (without lens) first. Which row on the vision chart can the patient read?

white is red center is grey

Optotype (Test symbol)

!

!


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Testing the eye (2)

Start with one eyeNow cover the left eye with the opaque lens. If he has a problem to read the row 4, 5, 6 or 7 then start testing with -0.5 (if he has a problem with the first rows, start with -1.0 or even -2.0).

1. Case: He can see better

Continue with -1.0, -1.5, -2.0 etc. If two negative diopters are the same, take always the lower one! (Example: -2.5 and -3.0 are the same => take -2.5)

2. Case: He can see worse

Continue with +0.5, +1.0, +1.5 etc. When the patient says „it is very good“, then continue measuring!(Example: +2.0 is very good, but you continue with +2.5, +3.0 etc.)Continue until the patient says „now it is worse“. Then the lens before was the best. (Example +2.0 is very good, +2.5 is very good, +3.0 is very good, but +3.5 is worse. So you take +3.0)

3. Case: -0.5, -1.0, -1.5 etc. is not better and +0.5, +1.0, +1.5 etc. is not better

Now try the pinhole occluder.

=> If the patient can see better, than try again yourlenses, especially the strong ones (-6.0/+6.0)If you can‘t help, send him to an optician with professional equipment. (He needs either very strong glasses or cylindric glasses because of astigmatism.)

=> If the patient can‘t see better with the pinhole occluder, than most probably no optician can help him. Send him to an ophthalmologist (eye doctor). (The patient probably has a cataract, diabetic retinopathy, glaucoma etc.)

Retinoscopy bar

not good not good good good

good good good not good

pinhole occluder


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Testing the eye (3)

Reading Glasses

First you have tested the eyes of the patient at the long distance with the optical chart.

If he needs reading glasses, you look at this table:

Now add the value to the result of the far distance test.

Example I: A 48(+1.5)-years-old woman needs for far distances L -2.5 and R -3.0You give her Left (+1.5 - 2.5) = -1.0 Right (+1.5 - 3.0) = -1.5

Example II: A 70(+3.0)-years-old man needs for far distances L +1.5 and R 0.0You give him Left (+3.0 + 1.5) = +4.5 Right (+3.0 + 0.0) = +3.0

Now put both lenses into a frame and let the patient try to read.

Important: The patient should read in a relaxed position, the book on his knees or a little bit above his knees (this is normally a distance of 40-50 cm).

=> If he can read it‘s OK.

=> If he can‘t see clearly, increase the power on both eyes by +0.5 (or even +1.0), then check again.

Measure the PD (Pupil Distance)

When you have found the right lenses, then measure the PD.

Then adapt the length of the temples to the face/ears of the patient.

At the end write the data in your bookPrescription L... / R... , PD (Pupil Distance)Which frames you have sold (simple ones / folding temples), selling prize, signature of patient

age 45 +1.0age 50 +2.0age 60 (or older) +3.0

Table: Age and diopters


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Testing the eye (4) – Common Problems

Big difference between left and right eye

If the difference between the left lens and the right lens is 2 diopters or more (e.g. left eye -0.5, right eye -3.0), the patient should test the glasses for some hours.

If he gets problems like a headache, then change the stronger lens in order to decrease the difference (e.g. take -2.0 instead of -3.0).

Some weeks later he can try the stronger lens again.

Patient has problems with high diopters

If a patient has glasses with high diopters for the first time in his life (e.g. L-4.5 and R-4.0) it might happen that he has problems with it.

In this case you could try to correct him first with a reduced power (e.g. L-2.5 and R-2.0). Then you wait some weeks and then try the regular power again.

These lenses causeproblems.

Reduced lens power.

Some weecks later youtry the regular power again.

Reduced lens power.

High difference between left and right eye.

G -0.5 -3.0 D

G -0.5 -2.0 D

G -4.5 -4.0 D

G -2.5 -2.0 D

G -4.5 -4.0 D


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Test (1)

1. What happens with the light ray? Complete the drawing.

2. What happens with the light beam? Complete the drawing.Draw in the focal point F and the focal lenght f.

3. What is the difference between a convex and a concave lens?How do they change objects, when looking through?

4. You have a convex lens but you don‘t know the power of the lens. What can you do? (You can use negative lenses from -0.5 to -6.0 diopters)

5. Draw the picture of an eye and explain its parts.

6. How does the accommodation of the eye work? Draw an eye a) reading and b) looking into the distance (with ziliary muscle, lens and zonular fibers)

7. How does the image of the candle occur on the retina? Complete the drawing.

8. What is Myopia and how can you correct it? 9. What is Hyperopia and how can you correct it? 10. What is Presbyopia and how can you correct it?

11. You look at an object which is 50 cm in front of you eye. What is the refractive power of your eye now?

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12. How do people with astigmatism see the stars?

13. Describe the different parts of the glasses

14. What are the measures of the lens?

15. What are the different sizes of the bridges?

16. How many degrees is the inclination of the temples?

17. How do you measure the pupil distance of a patient?

18. Describe the different steps of testing the eyes.

19. A 65-year-old woman wants to have reading glasses. For looking into the distance she needs L+1.0 and R+1.5. What kind of reading glasses would you give her?

20. A 50 years old man gets glasses for the first time in his life. He needs L +4.0 and R+4.5. He complains about headache after he has tested the glasses for half an hour. What can you do?

21. A young man needs glasses for looking into the distance. For his left eye -3.0 is as good as -3.5. What lens will you give him?

22. A young girl needs glasses for looking into the distance. For her right eye +3.0 is as good as +3.5 but +4.0 is worse. What lens will you give her?

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Test (2)

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The light - EinDollarBrille...The light rays coming from remote objects are almost parallel. The lens of the eye is flat. The ciliary muscle is decontracted. Close-up vision In order