Lec.4 (Eyes and Vision

Sub.Medical physics Physics of Eyes and Vision مدرس المادة
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Sense of vision
- Eyes (Fig.1).
- Millions of optic nerves.
- Visual cortex.
Figure 1: Cross-section of the left eye as seen from above.
Analogy with CCTV (Fig.2).
Figure 2: The sense of sight is in many ways similar to a closed circuit color TV system.
Special features.
- Large viewing angle (Fig.3).
Sub.Medical physics Physics of Eyes and Vision مدرس المادة
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- Blinking built-in lens cleaner and lubricator for the front lens
(cornea).
- A rapid automatic focusing system permits viewing objects as close
as 20 cm one second and distant objects the next. Under relaxed
conditions the focus for normal eyes is set for "infinity" (distant
viewing).
- Range of light intensity: brilliant daylight to very dark night (1010 to
1).
- Iris: automatic aperture adjustment
- Built-in scratch remover in cornea
- Self-regulating pressure system for internal pressure of about 20
mmHg keeps the eye in shape.
- Bone provides protection and fat provides cushion.
- Automatic image correction from the upside down image in retina.
- Three-dimensional viewing using two eyes.
- Flexible eye movement by muscles.
1. Focusing Elements of the Eye
Sub.Medical physics Physics of Eyes and Vision مدرس المادة
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2. Mostly water and contains many components of blood.
3. Supply nutrients to nonvascularized cornea and lens.
4. Surplus escapes through drainage tube (the canal of Schlemm).
5. Blockage of the drainage tube increased eye pressure glaucoma
Vitreous humor (vitreous body)
1. Clear jelly-like substance filling the large space between the lens and the
retina. 2. Keep fixed shape of the eye. 3. Permanent.
Sclera
1. Tough, white, light-tight covering over all of the eye except the cornea.
2. Protected by a transparent coating called the conjunctiva.
3. The Retina – the Light Detector of the Eye
Convert light image into action potentials
Light p h o t o n ( 3 e V ) photochemical r e a c t i o n i n p h o t o r e c e p t o r
action potentials.
IR has insufficient energy to produce action potential not seen.
UV are absorbed before the retina not seen.
Most vision is restricted to a small area called the macula lutea,
or yellow spot. A l l d e t a i l e d vision takes place in a very small area
in the yellow spot (~ 0.3mm in diameter) called the fovea centralis
(Fig. 1). In Fig. 4, 0 is the object size, I the image size, P the object
distance, and Q the image distance, usually about 2 cm. Thus
we can write 0/P = 1/Q or 0/1 = P/Q. I = (QIP) O (Example.1).
Figure4: T h e r e is a simple relationship between the object and image
sizes and the object and image distances. The image on the retina is
small because of the short image distance of about 2cm.
Sub.Medical physics Physics of Eyes and Vision مدرس المادة
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Example 1:
How big is the image on the retina of a fly on a wall 3.0 m away? Assume the
fly is 3 mm(0.003 m) in diameter and Q=0.02 m.
I = 0.02/3 ? 0.003= 2 ?10-5 m = 20 ?m
Photoreceptors: Fig.5, 6, 7, and 8.
There are two general types of photoreceptors in the retina: the cones and the
rods.
1- Rods
- Vision in dim light, photopigment is rhodopsin night or scotopic vision.
- Peripheral vision.
- ~ 120 million in each eye in most of the retina.
- Hundreds of rods are connected to one optic nerve better sensitivity.
- Maximal sensitivity at 510 nm (blue-green light).
2- Cones
- Color vision in bright light, 1 to 3 photopigments daylight or
photopic vision
- ~ 6 5 million in each eye in the fovea centralis
Sub.Medical physics Physics of Eyes and Vision مدرس المادة
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Figure 5: The light must pass through various cell layers to reach the rods and cones. In the
fovea centralis much of this tissue is pushed to the side, permitting better detail vision in this
area. Blood vessels also the light to some rods and cones.
Figure 6: The distribution of the rods and cones in the retina of the left eye; notice the blind
spot with no sensors. A parametric angle of 300 to the left or right when the eye is looking
straight ahead
Sub.Medical physics Physics of Eyes and Vision مدرس المادة
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Figure 7: The rods are much more sensitive than the cones. The vertical axis is a log scale;
each division represents a factor of 10 in sensitivity. The best sensitivity of cons is at about
550 nm, while the best sensitivity of rods is at about 510 nm.
Sub.Medical physics Physics of Eyes and Vision مدرس المادة
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5- Diffraction Effects on the Eye
Light waves passing through a small opening diffraction (point light
source rings on the retina)
Diffraction pattern on the retina due to the iris: Fig.9.
All lenses have defects (aberrations). The effect of such aberrations is
reduced if the lens opening is made smaller. In the eye, a small pupil
improves visual acuity. However, if the pupil is made very small the acuity
becomes worse due to diffraction effects. There is an optimum size for the
pupil; best acuity for an emmetropic eye is obtained with a pupil size of 3 to
4 mm its normal size under good illumination.
Figure 9: Diffraction in the eye. (a) Monochromatic light from a distant point source is
brought to a focus at the fovea centralis in the retina. (b) The diffraction pattern on the retina
produced by a pupil 3.0 mm in diameter consists of a central bright spot 8 ?m in diameter
surrounded by a ring of light of reduced intensity.
Sub.Medical physics Physics of Eyes and Vision مدرس المادة
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6- How Sharp Are Your Eyes?
The familiar eye charts used to determine whether we need
corrective lenses test the property of our eyes called visual acuity.
A physicist calls visual acuity the resolution of the eyes.
The optometrist u s u a l l y uses a Snellen chart (Fig.10) to test visual
acuity.
The visual acuity or resolution of the eye is primarily determined by
the characteristics of the cones in the fovea. A common way of
testing resolution is to use a pattern of alternating black and white
lines that become increasingly narrower. A combination of one white
line and one black line is called a line pair (lp). Under optimum
conditions, the eye can just barely resolve as separate lines a pattern of
about 3o lp/mm; when the eye is twice as far away, it can only resolve
15 lp/mm. The resolution is often given in terms of the angle
subtended from the eye. The minimum angle between two black
lines that can be seen as separate is about 0.3 milliradians.
The resolution rapidly gets worse as the image moves away from the
fovea centralis. At IOo from the fovea, the acuity is worse by a factor
of 10. If the lighting is not optimum the resolution also deteriorates
Sub.Medical physics Physics of Eyes and Vision مدرس المادة
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Figure10: A Snellen chart to test vision is usually viewed from 20 ft.
Figure11: Visual acuity improves with better lighting .The top curve shows the acuity for
the Landolt C ring, where the gap direction must be recognized .Vernier acuity (the ability
to align two lines) is much better than acuity for the Landolt C.
Sub.Medical physics Physics of Eyes and Vision مدرس المادة
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7- Defective Vision and Its Correction
In order to discuss the strength of a corrective lens for a defective
eye we need to review the basic equations of simple lenses. There is a
simple relationship between the focal length F, the object distance P,
and the image distance Q of a thin lens (Fig.1 2 ).
1/F=1/P+1/Q
IF F is measured in meters, then 1/F is the lens strength in diopters
(D). That is, a positive (converging) lens with a focal length of 0.1 m
has a strength of 10 D. The focal length F of a negative (diverging) lens
is considered negative. A negative lens with a focal length of -0.5 m
has a strength of -2 D.
Sub.Medical physics Physics of Eyes and Vision مدرس المادة
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Example:
Assume lens A with focal length FA= 0.33 m is combined with
lens B with focal length FB = 0.25 m. What is the focal length
of the combination? What is the dioptric strength of the
combination?
1/F= 1/FA+1/ FB=1/o.33+1/0.25=1/0.143
or F = 0.143 m. Note that lens A is 3D and lens B is 4 D. The
combination is simply the sum, or 7 D.
Let us consider the image distance Q of the cornea and lens of the
eye to be 2 cm, or 0.02 m (17 mm is a more correct value but the
arithmetic is harder). When the normal eye is focused at a great
distance (infinity), the focal length F of the eye is the same as Q,
or 1/F = 1/Q = 1/0.02 m, or the eye has a strength of 50 D. If the
eye focuses on an object at P = 0.25 m, then 1/F = (1/P) + (1/Q)
gives us 1/F = (1/0.25) + (1/0 .02) = 4 +50=54 D.
Focusing ( refractive) problem: Fig.13 and Table 1.
Ametropia affects over half of the population of the United
States. It is often possible to correct it completely with glasses. –
- There are four general types of ametropia: myopia near -
sightedness), hyperopia or hypermetropia (far-sightedness)
astigmatism (asymmetrical focusing), and presbyopia ( o l d sight),
or lack of accommodation.
1- The myopic individual usually has too long an eyeball or too
much curvature of the cornea; distant objects come to a focus
in front of the, retina, and the rays diverge to cause a blurred
image at the retina (Fig.14b). This condition is easily corrected
with a negative lens.
T a b l e 1 : A s u m m a r y o f v a r i o u s f o c u s i n g p r o b l e m s a n d
T h e i r c h a r a c t e r i s t i c s .
Sub.Medical physics Physics of Eyes and Vision مدرس المادة
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Sub.Medical physics Physics of Eyes and Vision مدرس المادة
Lec. ناهدة حمود الجراح 4
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A hyperopic eye has a near point further away than normal and uses some of its
accommodation to see distant objects clearly. The usual cause of hyperopia is
too short an eyeball (Fig .14 c). A positive lens is used to correct this
condition.
Example
Let us consider a far-sighted eye with a near point of 2 .0 m .What
power lens will let this person read comfortably at 0.25 m?
The strength of a good eye focused at 0.25 m is given by (1/0.25) +
(1/0.02) = 4 +50=54 D. An eye focused at 2m has a strength of (l /2.0)
+ (1/0.02)=0.5+50=50.5 D. A corrective lens of 54-50.5= +3.5 D would be
prescribed for this eye.
Figure 14 : Focusing properties of the eye. (a) The normal, or emmetropic, eye
focuses the image on the retina. (b) The near-sighted, or myopic,
eye focuses the image in front of the retina. This problem is corrected
with a negative lens. (c) The far-sighted, or hyperopic, eye focuses the
image behind the retina. This problem is corrected with a positive
lens.
Sub.Medical physics Physics of Eyes and Vision مدرس المادة
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8- Testing for myopia or hyperopia:
You can check to see if you are myopic or hyperopic. Look through
a pinhole in a well-illuminated distinct object, for example,
a streetlight (Fig.15).Move the pinhole up and down in front
of your eye. If you are an emmetrope, you will not see any
motion; if you are myopic, the image on the retina will move
in the direction opposite that of the card and will be
interpreted by the brain as moving in the same direction;
and if you are hyperopic, the motion of the image on the
retina will be in the same direction as the card and will appear to
be moving in the oppositive direction.
You can also easily check whether glasses have positive
or negative lenses by looking at an object through one
lens held some distance away. When you move the lens,
the object also lens, it is a negative lens; if it moves in
the opposite direction, it is a positive lens. Another test
is to hold the lens over some printing. If it enlarges the
printing, the lens is positive; if it makes the printing
smaller, the lens is negative.
Sub.Medical physics Physics of Eyes and Vision مدرس المادة
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Figure 15: Moving a pinhole in front of the ametropic eye while it is looking at a distant object
causes apparent motion of the object. Emmetropes see no motion.
Figure16: A simple test for astigmatism. An eye with astigmatism sees lines
going in one direction more clearly than lines going in other
directions.
Figure 17: Astigmatism is corrected by adding a cylindrical lens to a spherical lens. (A)
Converging or (B) diverging.
Sub.Medical physics Physics of Eyes and Vision مدرس المادة
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Figure 18: Loss of accommodation with age. The decrease in accommodation usually
becomes noticeable after age 40.
9- Use contact lens :
1- Contact lenses are used primarily for cosmetic reason by young people.
The direct contact between a contact lens and the cornea, however,
Sub.Medical physics Physics of Eyes and Vision مدرس المادة
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cornea over a period of hours. The lens is soaked in the medication before
it is put in the eye.
10- Instruments Used in Ophthalmology
1- Ophthalmoscope: to examine the interior of the eye
2- Retinoscopy: to determine the prescription of a corrective lens
3- Keratometer: to measure the curvature of the cornea
4- Lensometer: to measure the focal length of a lens
5- Tonometer: to measure eye pressure, applanation tonometer