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Optics of normal eye
Dr Cynthia Arunachalam
Professor and Head
Department of Ophthalmology
Yenepoya Medical College
Yenepoya University
Properties of light
• Part of electromagnetic
radiation
• Non ionising radiation
• Range of visible light 400700 nm
• Visible light – VIBGYOR
(passing through prisms)
• Violet- longer wavelength,
red – short wavelength
Wavelength
In phase
Out of phase
Amplitude
frequency
Properties of light
Interference
• superposition of individual
waves when they cross
paths.
• Constructive interference is
when the amplitude of the
resultant wave is greater
than the individual waves
• destructive interference is
when the amplitude of the
resultant wave is less than
the individual waves
Coherent
monochromatic
Properties of light
Diffraction
• phenomena exhibited
by light when it
interacts with barriers
and obstacles.
• Secondary waveforms
produced which are
out of phase with the
primary wave form –
diffraction patterns
Properties of light
Scattering
• Deflection of a ray of light
from a straight path by
any irregularities in the
propagating medium
• Wavelength dependant
• Frequency dependant
Properties of light
Polarization
• Only those wave forms
are transmitted through
a polarizing medium
which are perpendicular
to the direction of the
polarizing substance in
the medium
(angle of incidence = to the
polarizing angle of the
medium)
Optical radiation
Absorption spectra of the eye
<270nm
<315nm&>1400nm
<350nm
ALL
Gullstrand’s schematic eye
•
•
•
•
2 principal points
2 principal foci
2 nodal points
Eye is a refracting ideal
spherical surface,
separating two media of
refracting indices of 1.00
and 1.33.
• Ant corneal surface –
spherical with ROC 8mm
• Optical axis connects the
COC of cornea and lens
to the fovea
Listing’s reduced eye
• Thick lens system
• One lens system with an
optical centre ( nodal
point)
• Single ideal refracting
surface with ROC5.73mm
• Power – 58.6 D
• AFD- 15.7mm
• PFD- 24mm
• Principal axis – COC of
the ant & post surface of
the lens to fovea
Refractive media of the eye
Reduced eye
• Assumes power of
60D at the corneal
surface
• Anterior focal point
at approx. 17mm
• Length of the eye
22.6mm
• Nodal point 5.6mm
behind the cornea
Reduced eye
• Retinal image size
may be determined
easily using the
reduced eye, because
the nodal point is at
the centre of
curvature of the
single anterior
refracting surface
OPTICAL ABERRATIONS
• Imperfections or lapses in the optical system
• Though they normally exist to a small degree,functionally they
are immaterial.
• Affect mainly the peripheral rays
Diffraction of light
• The actual pattern of a
diffracted image point
produced by the pupil is
a series of concentric
dark and bright rings.
• At its centre is a bright
spot known as the Airy
Disc.
• Diffraction blur increases
with the small size of the
pupil
Spherical aberrations
• Spherical lens
refracts peripheral
rays more strongly
than paraxial rays
• As a result the
incoming rays do not
come to a point focus
Spherical aberrations
Factors contributing to the
diminution of spherical
aberrations in human
eye are1. Lens is flatter at the
periphery than the
centre.
2. Central portion of the
lens has greater density
and curvature than the
periphery.
3. Iris blocks the peripheral
rays and allows only
paraxial rays
Chromatic aberrations
Occurs due to the fact that
the ref of light through
any transparent medium
varies with the
wavelength of the
incident light.
emmetropic eyehypermetropic for red
and myopic for blue &
green
This forms the basis of the
duochrome test in
subjective refraction.
Chromatic aberrations
Chromatic aberrations in the eye is minimized by
• Yellow rays form most sharply defined images on
the retina.
• Fovea lacks blue cones.
• Narrow spectral sensitivity band of long &
medium wavelength cones.
coma
• This is an off-axis
aberration. The light rays
entering the optical system
away from the middle at an
angle are focused at
different points than those
entering the optical system
on or near the optical axis.
This results in a comet-like
image being formed away
from the middle of the
image.
Sturms conoid
• A lens with cylinder
power produces an
astigmatic focus.
• A vertical focal line,
corresponding to the
focus of the horizontal
principal meridian,
• A horizontal focal line,
corresponding to the
focus of the vertical
principal meridian.
Sturms conoid
Sturms conoid
• The region between
these two lines is known
as the conoid of Sturm or
Sturm's interval.
• At the dioptric mid-point
between these two focal
lines, the astigmatic
focus forms a circular
patch known as the circle
of least confusion
Sturms conoid
• The location of the circle
of least confusion is
equal to the spherical
equivalent of the
prescription:
Sturms conoid
• An image that falls on the retina can be thought
of as being made up of many dots, just like a
photo in the newspaper.
• If an ametropic eye , is not optically corrected,
then the image will consist of many blur circles
instead of sharp dots. The more out of focus the
image is, the larger the circles are. If an
astigmatic eye is not optically corrected, the blur
circles will be distorted into ellipses
Sturms conoid
• example, if an
emmetropic person
is looking at a cross,
it may be
represented as an
image with sharp
dots, like this—
Sturms conoid
• A minus 2D myope,
without correction,
may see the image
like this—
Sturms conoid
• Sometimes we do not supply the patient with his full
astigmatic correction: such as patients with 1.00D or less
cylinder correction when performing a visual field exam,
or perhaps the soft contact lens patient with 1.00D or
less astigmatism in one eye. In these situations we use
the spherical equivalent.
• These patients will not see the sharply focused dot
image. The spherical equivalent, representing the
"Circle" of Least Confusion, provides a blur circle instead
of a blur ellipse. The basic idea is that if the image is
going to be a little blurry, it is better that it not be
distorted also.