P I - 3 : V I S I O N OBJECTIVES

General Objectives 1. To understand the basics of refraction and its abnormalities and the correction by

observing a human eye model (PASCO). 2. To understand the basics of refraction and its abnormalities and the correction on

humans. 3. To understand the basic principles of visual field measurement by using a

perimeter. 4. To understand the mechanism of diplopia. 5. To understand the mechanism of direct and indirect (consensual) pupil reflects. 6. To understand the events happening in the eye during seeing far and near

objects. 7. To understand the location of the blind spot relatively to the fovea centralis in the

retina. Specific Learning Objectives 1.1. To explain the corresponding parts of a PASCO eye model to the human eye

and their functions. 1.2. To demonstrate various conditions of refraction and the correction by using a

human eye model (PASCO): a. image formation on the retina b. emmetropic eye with or without accommodation c. spherical aberration event and the correction d. myopic eye and the correction e. hypermetropic eye and the correction f. astigmatic eye and the correction g. aphakia and the correction

2.1. To explain the correlation between two point discrimination with minimum angle

of vision. 2.2. To explain the basic principles of constructing a Snellen chart. 2.3. To explain the definition of visual acuity and refraction on humans. 2.4. To explain the basic principles of determining a persons visual acuity by using a

Snellen chart. 2.5. To demonstrate various refraction abnormalities and the correction principle on

humans: a. myopic eye and the correction b. hypermetropic eye and the correction

2.6. To demonstrate the presence of astigmatism by using a picture of Lancaster-Regan dial

3.1. To explain the basic principles of visual field measurement by using a perimeter. 3.2. To explain the criteria of normal visual field for white and various colors of the

right and left eye. 3.3. To demonstrate visual field measurement for white by using a perimeter.

4.1. To explain the mechanism of diplopia. 4.2. To demonstrate diplopia.

5.1. To explain the mechanism of direct and indirect (consensual) pupil reflects. 5.2. To demonstrate direct and indirect (consensual) pupil reflects. 6.1. To explain the three events happening in the eye during seeing far and near

objects. 6.2. To demonstrate the three events happening in the eye during seeing far and

near objects. 7.1. To explain how to make external projection of blind spot. 7.2. To demonstrate external projection of blind spot relatively to the fovea centralis. 8.1. To determine the presence of organic color blindness and the type of color

blindness by using the pseudoisochromatic book 8.2. To demonstrate how to induce functional color blindness and explain the

mechanism INSTRUMENTS 1. PASCO human eye model and its accessories 2. Snellen chart 3. A set of lenses 4. Measuring rope 5. A picture of Lancaster-Regan dial 6. Perimeter + examination form 7. Eye patch 8. Flashlight 9. White paper 10. Ishihara pseudoisochromatic book 11. Red and green glasses PROCEDURES

I. EYE AS AN OPTICAL ARRANGEMENT Study the PASCO human eye model with its accessories (See Figure PI-1.): 1. Container, may or may not be filled with water until 1-2 cm from the top. 2. Corneal Lens. 3. Retina Screen, can be placed on 3 different places (NEAR, NORMAL, FAR).

Q-PI.1. What is the purpose of having 3 different locations for the retina? 4. An object that produces light (light source). 5. Lens holder containing:

a. Pupil aperture b. Four spherical lenses each with focal length: +120 mm, +62 mm, +400 mm, -

1000 mm c. Two cylindrical lenses each with focal length: +307 mm, 128 mm

Q-PI.2. How do you differentiate

a. a negative spherical lens from a positive spherical lens? b. a spherical lens from a cylindrical lens?

Q-PI.3. How do you calculate the optical power of a lens? Calculate each lens optical power and write the results in the review sheet.

Q-PI.4. What is the better way to determine the type and optical power of a

lens?

Figure PI-1. PASCO human eye model and its accessories

A . I M A G E F O R M A T I O N O N T H E R E T I N A

1. DO NOT fill the eye model with water yet. Put the retina screen in the middle slot, marked NORMAL. Put the +400 mm lens in the slot labeled SEPTUM.

2. Turn on the light source and place it in front of the human eye model to get a focused image.

3. Observe the image formed on the retina.

B . A C C O M O D A T I O N

1. DO NOT fill the eye model with water yet. Replace the lens in the SEPTUM slot with the adjustable focus lens. Position the eye model about 35 cm from the light source.

2. Move the syringe plunger to adjust the lens and form the clearest image possible.

3. Move the eye model closer from the light source. Adjust the lens again to form the clearest image.

4. Repeat step #3 by moving the eye model farther from the light source. Adjust the lens again to form the clearest image.

5. Replace the adjustable focus lens with the +400 mm lens in the SEPTUM slot. Adjust the distance of the light source to form a clear image. Mark the position of the eye model so you can return it to the same place after you fill it with water.

6. Fill the eye model with water to within 1 or 2 cm of the top. Return it to the same position as in step #5.

7. Observe the image formed. Q-PI.5. a. What is the function of water in the container?

b. What is the corresponding part of water in the container to the human eye?

c. What effect do the water have on the focal length of the eyes lens system?

8. Place the eye model about 35 cm from the light source. Replace the +400

mm lens in the SEPTUM slot with the +62 mm lens. Move the eye model as close as possible to the light source while keeping the image in focus. Record this distance, which is the near point of the eye model when equipped with the +62 mm lens.

9. Add the +400 mm lens to slot B. Move the eye model as close as possible to the light source while keeping the image in focus. Record this distance.

10. Keep the +400 mm lens in slot B and replace the lens in the SEPTUM slot with the +120 mm lens. Move the eye model as close as possible to the light source while keeping the image in focus. Record this distance.

C . P U P I L D I A M E T E R A N D S P H E R I C A L A B E R R A T I O N

1. Remove both lenses and place the +62 mm lens in the SEPTUM slot. Adjust the eye-source distance to the near point distance for this lens (section B step #8) so that the image is in focus.

Q-PI.6. Name the corresponding condition happening in the eye model to

the condition happening in the human eye.

2. While looking at the image, place the round pupil in slot A. Keep the pupil in slot A until the end of the experiment. Q-PI.8. Why did the shadow become clearer after the round pupil is

placed?

3. Position the eye model so that it is looking towards a distant object. Replace the lens in the SEPTUM slot with one that makes a clear image of the distant object; this is the far-vision lens. Record the focal length marked on the handle of the lens.

D . H Y P E R M E T R O P I A

1. Set the eye model to normal near vision (put the 62 mm lens in the SEPTUM slot, remove other lenses, and make sure the retina is in the NORMAL position). Position the eye to look at the nearby light source. Adjust the eye-source distance to the near-point distance so that the image is in focus.

2. Move the retina screen to the forward slot, labeled FAR. Describe what happens to the image.

3. Turn the eye model to look at the distant object, and describe the image. 4. Return the eye model to looking at the nearby light source. You will now

correct the hypermetropia by putting eyeglasses on the model. Find a lens that brings the image into focus when you place it in front of the eye in slot 1. Record the focal length of this lens.

E . M Y O P I A

1. Set the eye model to normal near vision (put the 62 mm lens in the SEPTUM slot, remove other lenses, and make sure the retina is in the NORMAL position). Position the eye to look at the nearby light source. Adjust the eye-source distance to the near-point distance so that the image is in focus.

2. Move the retina screen to the back slot, labeled NEAR. Describe what happens to the image.

3. You will now correct the myopia by putting eyeglasses on the model. Find a lens that brings the image into focus when you place it in front of the eye in slot 1. Record the focal length of this lens.

4. Remove the eyeglasses. 5. Turn the eye model to look at the distant object. Replace the lens in the

SEPTUM slot with the normal far-vision lens (section C step #3). Describe the image formed. Note: The lens in the SEPTUM slot represents the crystalline lens in its most relaxed state, with its longest-possible focal length.

F . A S T I G M A T I S M

1. Set the eye model to normal near vision (put the 62 mm lens in the SEPTUM slot, remove other lenses, and make sure the retina is in the NORMAL position). Position the eye to look at the nearby light source. Adjust the eye-source distance to the near-point distance so that the image is in focus.

2. Place the -128 mm cylindrical lens in slot A. The side of the lens handle marked with the focal length should be towards the light source. Describe the image formed.

3. Rotate the cylindrical lens. What happens to the image? 4. You will now correct the astigmatism with eyeglasses. Place the +307 mm

cylindrical lens in slot 1. The side of the lens handle marked with the focal length should be towards the light source. Rotate the corrective lens and describe what happens to the image. Find the orientation of the eyeglasses lens at which the image is sharpest. Q-PI.9. How do you know the axis of the lens? Q-PI.10. How do you state the axis of a cylindrical lens?

G . A P H A K I A

1. Remove the crystalline lens to produce aphakia, which is absence of the crystalline lens of the eye. Turn the eye model to look at the distant object. Describe the image formed.

2. Place the +400 mm lens in slot 1 to act as an eyeglasses lens. 3. Turn the eye model to look at the nearby light source. 4. Replace the eyeglasses lens in slot 1 with the +120 mm lens.

II. VISUAL ACUITY 1. Perform this experiment on at least one subject. Instruct the subject to sit facing

the Snellen chart at a 6 m distance. (= d)

Q-PI.11. Why is the reading distance 6 m?

2. Have the subject wear special glasses and close his/her left eye with a special black eye patch provided in the lens box.

3. Check the subjects visual acuity by instructing him/her to read the letters you are pointing. Start with the row with the largest letter (all the letters) until the row with the smallest letters (all the letters) that can be read without any mistake.

Q-PI.12. If on the examination the subject can only read without any mistake

the line of letters marked with 30 Ft (9.14 m), what is the subjects right eye visual acuity?

Q-PI.14. What is the basic principle of constructing a Snellen chart?

4. Write down the subjects right eye visual acuity. 5. Repeat the test on:

a. the left eye b. both eyes simultaneously

6. Write down the results.

III. REFRACTION AND CORRECTION From the visual acuity test above (point II), the subjects visual acuity is examined without using any lens. On the next test you will examine the refractive power of the optical system (eye refraction).

A. REFRACTION

If the subjects visual acuity without lens = 6/6, than the eye refraction IS NOT myopic (M). The eye refraction might be emmetropic (E) or hypermetropic (H).

Q-PI.15. a. Can a persons visual acuity be better than 6/6? b. Why does a hypermetropic eye have a visual acuity of 6/6?

To distinguish whether a subjects eye refraction with a visual acuity of 6/6 is emmetropic or hypermetropic, the following test is performed: 1. Have the subject wear special glasses and close his/her left eye with a

special black eye patch. 2. Place a +0,25D spherical lens in front of the subjects right eye and check

his/her visual acuity one more time.

Q-PI.16. If the subjects visual acuity is now worse, what is your conclusion? Q-PI.17. If the subjects visual acuity is still 6/6, and the subject even feels

that he/she can see clearer, what is your conclusion?

3. If the subjects right eye visual acuity is E, the examination is stopped. 4. If the subjects right eye visual acuity is H, continue placing the lens by each

time placing a stronger +0,25D lens.

5. The strongest positive lens that gives the subject a maximum visual acuity is the degree of his/her hypermetropia stated in dioptry (D).

6. Write down the subjects degree of H in dioptry.

B. CORRECTION

If the subjects right eye visual acuity without lens is less than 6/6, the subjects eye refraction is usually M. To determine the subjects degree of myopia, the following test is performed: 1. Have the subject wear special glasses and close his/her left eye with a

special black eye patch. 2. Place a negative spherical lens in front of the subjects right eye, starting

from 0,25D and each time adding a stronger 0,25D lens. 3. Check the subjects visual acuity every time after each lens placement. 4. The weakest negative lens that gives the subject a maximum visual acuity is

the degree of his/her myopia stated in dioptry (D). 5. Write down the subjects degree of M in dioptry.

Q-PI.18. If the subjects right eye visual acuity without lens is less than 6/6, what other refraction abnormality can be found besides M?

Q-PI.19. If an elderlys visual acuity without lens is less than 6/6, what

refraction abnormality can be found? Q-PI.21. Is it possible to get a visual acuity of 6/6 on an elderly? Explain

your answer.

If the subjects visual acuity still does not reach 6/6 with spherical lens, then a refraction abnormality called astigmatism might be present. The correction for astigmatism is by placing cylindrical lens with the following method: 1. Have the subject wear special glasses and close his/her left eye with a

special black eye patch. 2. Place a negative spherical lens in front of the subjects right eye until a

maximum visual acuity is reached. 3. Instruct the subject to look at the picture of Lancaster-Regan dial. If all the

black lines on the meridian looks equally sharp, the subjects refraction is not astigmatic. Stop the examination. If a blurry line is seen, determine the meridian of the sharpest black line seen by the subject.

4. Now add a positive or negative cylindrical lens in front of the negative spherical lens with the axis of the cylindrical lens placed perpendicularly to the meridian of the sharpest black line seen by the subject, so all the black line will look equally sharp.

5. Instruct the subject to see the Snellen chart one more time. Determine and write down the type, optical power and axis (for the cylindrical lens) of the lenses that give a maximum visual acuity for the subject.

Q-PI.22. Name another instrument used to diagnose the presence of

astigmatism.

IV. VISUAL FIELD MEASUREMENT (PERIMETRY) 1. Perform this experiment on at least one subject. Instruct the subject to sit facing

the perimeter with his/her back facing the light. 2. Close the subjects left eye with an eye patch.

3. Place the subjects chin on the left chin support and adjust the height of the support so the lower edge of his/her right eye is as high as the top of the chin supports vertical bar.

4. Place the examination form for the right eye on the back of the perimeter as follows: 4.1. Rotate the arc of the perimeter so it is horizontal and the examination form

peg is at the top of the disc. 4.2. Clip the examination form on the disc so the 180 0 line of the form is

superimposing with the 180 0 line of the perimeter and the concentric circle of the form is located according to the scale on the perimeter.

5. Instruct the subject to concentrate his/her sight on the fixation point at the center of the perimeter. During the examination, the subjects sight must always be fixated on that point.

6. Used the removable object on the perimeter arc to examine visual field. Choose a white circle with a medium diameter (( 5 mm/circle no.3) to be used on the object.

Q-PI.23. How do you choose the color and adjust the diameter of the circle?

7. Move the white circle slowly on the arc, from the subjects left edge to the middle. Stop moving the object exactly when the subject started seeing the white circle. Repeat this step 3 times.

8. Every time the object stops, note where it stops and write down the largest degree on the examination form.

Q-PI.24. How do you write down the degree on the examination form?

9. Repeat steps no. 7-8 on the opposite side of the arc without changing the position of the arc.

10. Repeat step no. 7-9 after each time rotating the arc is 30 clockwise to the examiner, until the position of the arc is vertical.

11. Return the arc to its horizontal position. No measurement is performed on this position.

12. Repeat steps no. 7-9 after each time rotating the arc is 30 counter clockwise to the examiner, until the position of the arc is 60 from the horizontal plane.

13. Do a visual field measurement for the left eye with the white circle.

Q-PI.25. What is the minimum value for normal visual field for white and various colors?

14. Compare your result to the minimum value for normal visual field.

Minimum value of normal visual field Temporal 850 Nasal 600 Lower Temporal 850 Upper Nasal 550 Bottom 650 Top 450 Lower nasal 500 Upper Temporal 550 Full visual field 5000

Figure PI-2. Visual field standard of the left and right eye

Figure PII-2. Visual field standard for various colors

Figure PI-3. Perimeter

V. DIPLOPIA 1. Look at an object with both eyes.

B = blue G = yellow R = red Gr = green W = white

2. Press your left eye ball from the lateral side to create a shift on the eye axis to medial.

3. Notice the occurrence of double vision.

Q-PI.26. What is the mechanism of double vision in diplopia experiment?

VI. PUPIL REFLEX 1. Shine the subjects right eye with a flash light and observe the change in pupil

diameter on that eye.

Q-PI.28. What event did you see and what is the mechanism of that event?

2. Shine the subjects right eye with a flash light and observe the change in pupil diameter on his/her left eye.

Q-PI.29. What event did you see and what is the mechanism of that event?

VII. NEAR RESPONSE

1. Instruct the subject to look at the examiners finger placed at a m distance in front of his/her eyes.

2. While observing the subjects pupil, move your finger closer to the subject so you can see convergence of the subjects eyes.

Q-PI.30. What change do you see in the pupil?

VIII. BLIND SPOT EXAMINATION 1. Draw a small cross in the middle of a sheet of white paper. Put the paper on the

table. 2. Instruct the subject to close his/her left eye, placing his/her right eye directly

above the cross at a 20 cm distance, and directing his/her sight to the cross. 3. Move the tip of the pencil slowly from the cross to the lateral side of the

examined eye, until the tip of the pencil becomes invisible and then visible again. Mark the spot on the paper when the tip of the pencil becomes invisible and when it becomes visible again. Determine the center of the two dots (T). With T as the center point, draw 8 lines according to the 8 directions of the compass. Move the tip of the pencil across T according to the 8 lines with the subjects sight still focused on the cross. Mark the spot on the paper every time the tip of the pencil becomes invisible and when it becomes visible again (you will have 8 dots excluding T).

4. Connect all these dots. This is the external projection of the blind spot of the subjects right eye.

Q-PI.32. Where is the location of the blind spot projection relatively to the small

cross and why?

IX. ORGANIC AND FUNCTIONAL COLOR BLINDNESS

I. ORGANIC

1. Instruct the subject to identify the numbers or pictures in the Ishihara pseudoisochromatic book.

2. Write down your result on the provided form.

II. FUNCTIONAL

1. Instruct the subject to see through the red or green glass for at least 10 minutes to a bright field (white clouds).

2. Immediately afterwards, examine the color blindness that occur by using the Ishihara pseudoisochromatic book.

3. Write down your result on the provided form. P-PI.34. What is the mechanism of functional color blindness? Explain

your answer!

A N S W E R S

Q-PI.1. To get different lengths of the eye axis, so that we can replicate myopic, emmetropic, and hypermetropic eye.

Q-PI.2. a. By moving the lens above a row of letters. Movement of a positive lens will make the letters move in the opposite direction to the lens, and a negative lens will give the opposite result.

b. By rotating the lens on a parallel plane above a row of letters. Rotation of a positive lens will cause deformation of the letters, while rotation of spherical lens will not.

Q-PI.3. The optical power (P) of a lens is defined as the inverse of its focal length, thus = !!, and the measurement unit is called diopter (d).

Q-PI.4. A better way is by using a lensometer.

Q-PI.5. a. As an optical media b. aqueous humor and vitreous humor c. Due to waters higher refractive index, lens immersed in water will have higher focal length

than lens surrounded by air

Q-PI.6. An emmetropic eye without accommodation with wide pupil.

Q-PI.7. Mention the effect of wide pupil to the clarity of the shadow on the retina.

Q-PI.7. Wide pupil causes spherical aberration, so the shadow on the retina is blurry.

Q-PI.8. Because smaller pupil eliminates light entering the edges of the eye and reduces the effect of spherical aberration.

Q-PI.9. The axis of the lens is marked by the notches in the edge.

Q-PI.10. The axis of the lens is read by the examiner from a horizontal line, clockwise and stated in degrees.

Q-PI.11. 6 m is the closest distance for an object to be seen clearly by a normal eye without accommodation.

Q-PI.12. How do you examine a persons visual acuity? (Q-PI.13).

Q-PI.13. Visual acuity is stated with a formula: V = d/D (Snellens formula)

V = visual acuity

d = distance between the examined eye and the Snellen chart (= 6.1 m)

D = reading distance of an emmetropic eye at the row with the smallest letters readable by the subject.

6,1

So the subjects right eye visual acuity = -------

9,14

The ratio of d/D cannot be simplified, so we can always know the method of the examination by knowing the result.

For example, if the subjects right eye visual acuity = 6/9, it cannot be simplified to 2/3.

Q-PI.14. The distance between 2 separable points is the function of minimum visual angle (1 minute) and the reading distance.

See Ganongs review of medical physiology, 23rd ed; 2010, about Visual Acuity, page 184.

Q-PI.15. a. Yes, it can.

This condition shows that the persons visual acuity exceeds normal, in other words, his/her

minimum visual angle is less than 1 minute.

b. Because H eye can compensate by accommodation.

Q-PI.16. The subjects right eye is emmetropic.

Q-PI.17. The subjects right eye is hypermetropic.

Q-PI.18. Severe hypermetropia.

Q-PI.19. H with less accommodation or M.

Q-PI.20. In such condition, how should the eye examination be performed?

Q-PI.20. For an elderly with a visual acuity less than 6/6 without lens, the examination should start with positive spherical lens. If the visual acuity is better, the elderlys eye refraction is H. If the visual acuity is worse, the elderlys eye refraction is M.

Q-PI.21. Yes, its possible, if the refractive power of the optical system (cornea, aqueous humor, lens, and vitreous humor) is normal.

Q-PI.22. Placido Keratoscope.

Q-PI.23. On the edge of the removable object there are 2 discs that can be rotated. The top disc is to adjust the diameter and the bottom disc is to adjust the color of the circle.

Q-PI.24. If the circle is stopped at number 50 on the nasal side of the arc, mark the nasal side of the concentric circle number 50 on the examination form. Do the same for the temporal side.

Q-PI.25. The minimum value of normal visual field for white is shown in Figure PII-1.

The comparison of minimum value of normal visual field for various colors is shown in Figure PII-2. (W = white, B = blue, G = yellow, R = red, Gr = green).

Q-PI.26. Pressure on one of the eye ball will cause the shadow of the object to not fall on the corresponding points.

Q-PI.27. What are corresponding points?

Q-PI.27. Corresponding points are the points on which the image of an object must fall if it is to be seen binocularly as a single object.

Q-PI.28. Direct pupil reflex in the form of right pupil constriction.

See Ganongs review of medical physiology, 23rd ed; 2010, about Other Pupillary Reflexes, page 189.

Q-PI.29. Indirect (consensual) pupil reflex in the form of left pupil constriction.

See Ganongs review of medical physiology, 23rd ed; 2010, about Other Pupillary Reflexes, page 189.

Q-PI.30. Miosis/pupil constriction.

See Ganongs review of medical physiology, 23rd ed; 2010, about Accomodation, page 189.

Q-PI.31. What are the events happening in near response? Explain the mechanism.

Q-PI.31. See Ganongs review of medical physiology, 23rd ed; 2010, about Accomodation, page 189.

Q-PI.32. On the temporal side, below the horizontal line.

Q-PI.33. Where is the location of the blind spot relatively to the fovea centralis in the retina?

Q-PI.33. On the nasal side of fovea centralis.

See Ganongs review of medical physiology, 23rd ed; 2010, about Retina, page 182-184.

Q-PI.34. Discuss within your group!