Optics and the Eye

1
Optics and the Eye
2
The Visible Spectrum
3
(No Transcript)
4
Some similarities between the eye and a camera
5
Refraction - the basis of optics
Light bends when it goes from one medium to
another The amount or bending, or refraction,
depends on the angle incidence, and the nature of
the two media
6
Objects-Lens Distance
7
Optical Power greater the power the closer the
image is to the lens
8
Convex lenses have positive optical power
Concave lenses have negative optical power
9
The precise distance from the lens to the focused
image depends on lens power and the distance to
the object according to the following equation
P 1/F 1/do 1/di (lensmaker equation)
P lens power F the focal distance do the
distance to the object di the distance to the
image (all distances are expressed in meters)
10
(No Transcript)
11
(No Transcript)
12
Focussing --Accomodation
In humans, fine focussing is controlled
through changing the shape of the lens.
13
Accomodation

• Changing the lens shape is controlled by the
  ciliary muscles

14
(No Transcript)
15
Average human eyes power is 60 diopters (cornea
and lens together) Focal distance of such optics
is 1/60 0.0167 m 16.7 mm Posterior nodal
distance of average eye is 16.7 mm - good fit!
16
(No Transcript)
17
(No Transcript)
18
(No Transcript)
19
Near and Far Points

• Near point - closest distance that an object can
  still be kept in focus.
• Will change with age
• Far point
• Normally at optical infinity

20
(No Transcript)
21
Astigmatism
Astigmatism Target
Sometimes a lens can be emmetropic for light
waves in one orientation (say, for horizontal
lines), but be badly hyperopic or myopic for
other orientations (say, vertical lines). Thus,
people with astigmatic vision see sharp lines
and contours in some orientations, and blurry
contours in others.
22
Someone with an astigmatism in the vertical
orientation might see the target such that the
horizontal lines are in focus and of high
contrast, but as the lines become more vertical
they go out of focus, becoming blurry and lower
in contrast.
23
The Human Eyeball
24
(No Transcript)
25
The Retina
The cells of the RETINA act as transducers. A
transducer changes one form of energy into
another.
26
(No Transcript)
27
(No Transcript)
28
Rod Photoreceptors

• about 120 million per eye
• only one kind
• most sensitive to light of about 505 nm
• approx. 10 times more sensitive than cones
• used in night/scotopic vision
• psychophysical physiological data indicate that
  rods can respond to a single photon!

29
Cone Photoreceptors

• about 8 million per eye
• 3 kinds, each most sensitive to 440, 530 or 560
  nm
• basis of colour vision
• approx. 10 times less sensitive than rods
• used in day/photopic vision

30
Distribution of Rods Cones

• rods are most dense in periphery
• no rods in the center of the macula
• cones are most dense in fovea
• no receptors in blind spot

31
(No Transcript)
32
Eye Movements
33
Why move our eyes?
One reason we move our eyes is that we have a
relatively narrow field of vision and must move
our eyes around to sample the visual world
extensively. A second reason we move our eyes is
that our retinas are not uniformly sensitive.
Our retinas have the highest acuity in the region
called the fovea. To see something clearly we
orient our eyes so that the image will be
projected onto the high resolution fovea.
34
Two Main Classes of Eye Movements
1. Conjugate (Version) - Both eyes move to the
same degree and in the same direction
2. Vergence - eyes rotate in opposite directions
- e.g. inward to look at a close object
35
Conjugate Eye Movements
36
Vergence Eye Movements
37
(No Transcript)
38
(No Transcript)