This ppt may usefull to all optometry students.....###

1. NAZIRSALAH.K
Bsc . OPTOMETRY
AL-RAYHAN COLLAGE OF OPTOMETRY
KONDOTTY

2. a, anterior surface of cornea
b, posterior surface of cornea
c, anterior cortex
d, anterior core
e, posterior cortex
f, posterior core
V, anterior pole of the eye
g, posterior poles of the eye
line jh, visual axis

3.  Perfectly aligned optical system
 Paraxial rays
◦ Rays close to axis
◦ Small angle of incidence
 No spherical aberration
◦ Pupil size 2mm

7.  cornea + tear layer - separates air from aqueous
humor
 Lens - separates aqueous from vitreous humor
 Rays refracted first (most) at the first surface of
the cornea - large difference in index of
refraction at the air-to-cornea interface
 Second surface of the cornea has negative power
 Cornea - over 70% of 64 diopters (D) of refractive
power of the unaccommodated eye
 lens supplies the remaining refractive power
 Accommodation - additional power is supplied
by the lens, which assumes a rounder form

8.  Europe until the Renaissance
 psychic spirit
 moved through a hollow optic nerve to the
retina and crystalline lens into the anterior
chamber
 projected out of the eyes as an emanation of
rays that made objects in space visible
 lens was the main receptor that created the
visual sensation that traveled back as a visual
spirit through the optic nerve to the brain

9.  Alhazen (965–1039) – book of optics
 light emanated from luminous sources such
as the sun and was reflected from the object
to the eye
 an image was formed in the eye
 unsure of its precise nature because of
inadequate appreciation for the refractive
properties of the ocular media

10.  Eye worked like pin hole camera

11.  Kepler (1571-1630) - role of the crystalline
lens in the image-forming process
 Points in space were imaged on the retina to
form an inverted, real image caused by
refraction by the cornea and lens
 Proof by Scheiner (1573-1650) - removed
part of the sclera and choroid from
enucleated sheep eyes to reveal the back of
the retina
 Pointing the eye toward a bright object, he
observed a small inverted image on the retina

12.  Scheiner - cornea of the eye is convex mirror whose
reflex image could provide a measure of the
curvature of the cornea
 a series of small glass marbles of various sizes from
about 10 to 20 millimeters in diameter
 Patient was seated opposite a bright window where
the image of the crossbars could be observed
 one marble and then another was inserted in the
corner of the eye until at length one was found which
gave a reflex image as nearly the same size as
possible as that seen in the cornea
 Inferred that the radius of curvature of the anterior
surface of the cornea was at least nearly the same as
that of the marble

13.  Invented ophthalmoscope
 refined ophthalmometer
 No accurate data on the crystalline lens
 Ophthalmophacometer by Tscherning -
separate Purkinje images of all refracting
surfaces could be formed
 depth of the anterior chamber and the
curvatures of the anterior and posterior
crystalline lens surfaces and the lens
thickness calculated trigonometrically

14.  refined the Helmholtz schematic eye
 invented the photokeratoscope to
photograph the corneally reflected image of a
target consisting of concentric circles
 Measurements of the spacing of circles in the
image reveal whether the cornea is spherical,
aspheric, or astigmatic
 If the images are elliptic, the cornea is
astigmatic, that is, toroidal
 peripheral portions of the cornea could be
investigated and its entire contour mapped

15.  six spherical refracting surfaces, two for the
cornea and four for the crystalline lens
 lens is seen as a central double convex core
surrounded by a cortex that has a lower
index of refraction

16. a, anterior surface of cornea
b, posterior surface of cornea
c, anterior cortex
d, anterior core
e, posterior cortex
f, posterior core
V, anterior pole of the eye
g, posterior poles of the eye
line jh, visual axis

17.  Light is assumed to travel from left to right
 Positive distances are measured from left to
right
 negative distances are measured from right
to left
 Object distances are measured from the
optical element to the object point
 Image distances are measured from the
optical element to the image point

18.  the object distance from the lens to the
object point is negative, that is, it is
measured from right to left, and the image
distance is positive.

19.  Light diverging from the
object point - negative
vergence
 spherical wavefronts grow
larger as their radial
distances from the source
increase
 curvature is the reciprocal
of the radius of curvature
 farther the wavefront is
from the object, the
smaller its curvature will be
 Wavefront vergence in
diopters equals the
reciprocal of the radial
distance in meters

20.  Vergence = 1/Distance in meters
 Light that is converging toward an image has
positive vergence
 Wavefronts become increasingly curved as
they approach the image point, and the
vergence increases correspondingly
 At distance of 4 meters, the vergence is
 ¼ = + 0.25 D
 2 meters, the vergence is
 ½ = + 0.5 D

21.  paraxial characteristics of a complex optical
system can be determined readily by reducing
the system to six cardinal points
◦ 2 focal points
◦ 2 principal points
◦ 2 nodal points

22.  When light from an infinitely distant source
found to the left of an optical element strikes
the element, the collimated paraxial rays will
be converged to F‘
◦ real image point for positive elements
◦ virtual image point for negative elements
 Light originating from the first focal point F
will be collimated by the optical element,
forming an image at infinity

23.  Positions of the first and second focal points
formed by a positive thin lens in air and
positive single refracting surface.

24.  Positions of the first
and second focal
points formed by a
negative thin lens in
air and a negative
single refracting
surface.

25.  plane defining the position of a thin lens that
theoretically could replace the lens system

26.  pair of axial points
 in calculating image sizes
 An incident ray directed toward the first nodal
point will appear to emerge from the second
nodal point with unchanged direction
 points of unit angular magnification
 slope of the ray directed toward the first
nodal point is the same as the slope of the
ray that appears to emerge from the second
nodal point

27.  principal and nodal points all coincide at the
vertex of the lens

28.  first and second nodal points coincide with
the first and second principal points

30.  complex series of refracting surfaces that
forms an image in vitreous of an object in air
 All six cardinal points
 Schematic eye

33. P = equivalent refracting power
P1= refracting power of the first element
P2= refracting power of the second element
D= reduced distance

35.  Catoptric images
 Dioptric images

36.  1 = corneal reflex
◦ Brightest
◦ virtual
 2 = weak reflex
◦ Virtual
 3 = virtual
◦ Depends on accommodation
 4 = real
◦ Depends on accommodation

37.  Refractive indices
 Radii of curvature
 Position of refractive elements

38.  n0 (air) = 1.00
 n1 (cornea) = 1.376
 n2 (aqueous) = 1.336
 n3 (lens cortex) = 1.386
 n4 (lens nucleus) = 1.406
 n5 (vitreous) = 1.336

39.  r1 (cornea anterior surface) = +7.70 mm
 r2 (cornea posterior surface) = +6.80 mm
 r3 (lens anterior surface) = +10.00 mm
 r4 (lens nucleus anterior surface) = +7.91
mm
 r5 (lens nucleus posterior surface) = -5.76
mm
 r6 (lens posterior surface) = -6.00 mm

40.  d1 (cornea anterior surface) = 0 mm
 d2 (cornea posterior surface) = +0.50 mm
 d3 (lens anterior surface) = +3.60 mm
 d4 (lens nucleus anterior surface) = +4.15
mm
 d5 (lens nucleus posterior surface) = +6.57
mm
 d6 (lens posterior surface) = +7.20 mm

69. =
+58.64D

71. +1.35 mm

72. +1.60 mm

73. -17.05
mm

74. +22.78
mm

75. +7.08 mm
+7.46 mm

76. +22.78 mm

77.  Theoretical optical specification of an idealized
eye, retaining average dimensions, omitting
complications
 Assumption - Refracting surfaces co-axial
 Real – lens is decentered and tilted
 P = +58.64D
 F1 = -17. 05 mm
 f2 = +22.78 mm
 H = +1.35 mm
 H’ = +1.60 mm
 N = +7.08 mm
 N’ = +7.46 mm

78.  HJ – Pupil
 E0 – pupil centre

79.  Object – pupil
 Image formed by cornea
 Centre E

80.  Object – Pupil
 Image formed by lens
 Centre – E’

81.  An incident pencil of rays directed towards and
filling the entrance pupil would pass through the
entire area of the real pupil, after refraction by
the cornea, and on finally emerging into the
vitreous body, limited by the exit pupil

82.  A ray directed towards E passes through E’
after refraction
 E and E’ are conjugate
 If a ray directed towards E makes an angle u
with the optic axis, the conjugate refracted
ray will make an angle u’ where u’/u=0.82

83.  Entrance pupil
◦ 3 mm behind ant surface of cornea
◦ 13% larger than real pupil
 Exit pupil
◦ Close behind real pupil
◦ 4% larger

84.  The differences between the prediction of the
paraxial ray method and the actual image are
called aberrations.
◦ Spherical aberration
◦ Coma
◦ Astigmatism
◦ Chromatic aberration

85.  optically homogeneous lens with spherical
refracting surfaces would produce spherical
aberration
 Marginal rays have different focus

86.  Positive spherical aberration - rays near the
edge of the lens have an effective focal point
that is closer to the lens than rays that strike
the lens near the axis
 Negative spherical aberration - rays near the
edge of the lens have an effective focal point
that is at a greater distance from the lens
than rays that strike the lens near the axis

87.  increases with the diameter of the lens
 minimized by limiting the opening of the lens

88.  cornea is not spherical – steep at center, flat
at periphery
◦ Reduce spherical aberration
 Lower index in the outer zones of the lens
◦ Marginal rays refracted less
 Constriction of the pupil
◦ Reduce spherical aberration

89.  During accommodation
◦ curvatures of the lens become steeper
◦ axial thickness increases
◦ pupil constricts
 Enable the eye to focus sharply near objects
on the retina
 Allows front surface of the lens to bulge in
the center while keeping the periphery less
curved
 Control spherical aberration

90.  The differences between the prediction of the
paraxial ray method and the actual image are
called aberrations.
◦ Spherical aberration
◦ Coma
◦ Astigmatism
◦ Chromatic aberration

91.  This aberration affects rays that come from
an object that is not at the center of the lens.
 magnification of a lens is different for
marginal and paraxial rays
 coma positive - the image of an object
produced by off-axis rays is slightly larger
than the image produced by paraxial rays
 coma negative - the image produced by the
off-axis rays is slightly smaller

92.  several different images of different sizes all
of which are in focus on the same screen
 bright image formed by the paraxial rays and
a series of smaller (or larger) images formed
by the rays that hit the lens far away from the
axis
 like a comet – a clear image with a fuzzy tail
oriented along the screen and composed of
weaker images formed by the off-axis rays

95.  The differences between the prediction of the
paraxial ray method and the actual image are
called aberrations.
◦ Spherical aberration
◦ Coma
◦ Astigmatism
◦ Chromatic aberration

96.  off-axis effect
 When an object point is quite far off of the
central axis of the lens, the effective radius of
curvature (and hence the effective focal
length of the lens) in one direction is not the
same as in a perpendicular direction.
 produce a slight difference in the focal length
for rays that are in the longitudinal plane of
the lens and for rays that leave this plane

97.  A point on an off-axis object has two image
points – one for the rays that strike the lens in
the plane of the object and the central axis of the
lens and another for rays that strike the lens
perpendicular to this plane
 At either of these two image points, the rays
forming the other image are somewhat out of
focus, and often form a small line segment
 At intermediate points between the two image
points the rays combine to form an image that
sometimes looks like a small + sign.

100.  The differences between the prediction of the
paraxial ray method and the actual image are
called aberrations.
◦ Spherical aberration
◦ Coma
◦ Astigmatism
◦ Chromatic aberration

101.  Combination of positive and negative lenses
will have a net refractive power but their
opposing dispersions will cancel
 short wavelength light is refracted more
strongly than long wavelength light
 Resulting in chromatic aberration

102.  Eg:
 If light of wavelength 550 nm is in focus on
the retina, the image in ultraviolet light of
wavelength 350 nm will be out of focus

103.  lens acts as a filter transmit the visible spectrum
but absorbs the near ultraviolet light of
wavelengths shorter than 400 nm
◦ Near ultraviolet increases chromatic aberration
 sensitivity of the eye shifts toward the red end of
the spectrum as the illumination is increased
◦ rods - peak sensitivity at 500 nm (bluegreen)
◦ cones - peak sensitivity 562 nm (yellowgreen)
◦ Cones respond to long wavelength – less aberration
 pigmentation of the macula lutea
◦ Absorption of violet and blue regions