The eye and vision

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The eye and vision

• Medical optics

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1. Structure and function of the eye
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Structure and function of the eye

• The human body is sensitive to most
  electromagnetic radiations.
• Microwave and infrared produce the sensation of
  warmth. Ultraviolet and ionising radiations can
  produce chemical changes and cause biological
  damage such as skin cancer.
• The eye is the one organ of the body design to
  respond specifically, to receive (or perceive) a
  part of the electromagnetic spectrum- visible
  light- having wavelengths between 380 and 760 nm.

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Spectrum of Electromagnetic radiation
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Structure- function

• 1. Conjunctiva -Is a thin protective covering of
  epithelial cells. It protects the cornea against
  damage by friction (tears from the tear glands
  help this process by lubricating the surface of
  the conjunctiva)
• 2. Cornea -Is the transparent, curved front of
  the eye which helps to converge the light rays
  which enter the eye
• 3. Sclera -Is an opaque, fibrous, protective
  outer structure. It is soft connective tissue,
  and the spherical shape of the eye is maintained
  by the pressure of the liquid inside. It provides
  attachment surfaces for eye muscles

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Structure- function

• 2. Choroid - Has a network of blood vessels to
  supply nutrients to the cells and remove waste
  products. It is pigmented that makes the retina
  appear black, thus preventing reflection of light
  within the eyeball.
• 3. Ciliary body- Has suspensory ligaments that
  hold the lens in place. It secretes the aqueous
  humour, and contains ciliary muscles that enable
  the lens to change shape, during accommodation
  (focusing on near and distant objects)
• 4. Iris -Is a pigmented muscular structure
  consisting of an inner ring of circular muscle
  and an outer layer of radial muscle. Its function
  is to help control the amount of light entering
  the eye so that
• - too much light does not enter the eye which
  would damage the retina
• - enough light enters to allow a person to see
• 5. Pupil -Is a hole in the middle of the iris
  where light is allowed to continue

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Structure- function

• 6. Lens -Is a transparent, flexible, curved
  structure. Its function is to focus incoming
  light rays onto the retina using its refractive
  properties
• 7. Retina -Is a layer of sensory neurones, the
  key structures being photoreceptors (rod and cone
  cells) which respond to light. Contains relay
  neurones and sensory neurones that pass impulses
  along the optic nerve to the part of the brain
  that controls vision
• 8. Fovea (yellow spot) - A part of the retina
  that is directly opposite the pupil and contains
  only cone cells. It is responsible for good
  visual acuity (good resolution

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Structure-function

• 9. Blind spot -Is where the bundle of sensory
  fibres form the optic nerve it contains no
  light-sensitive receptors
• 10. Vitreous humour -Is a transparent, jelly-like
  mass located behind the lens. It acts as a
  suspension for the lens so that the delicate
  lens is not damaged. It helps to maintain the
  shape of the posterior chamber of the eyeball
• 11. Aqueous humour -Helps to maintain the shape
  of the anterior chamber of the eyeball

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• Optic Nerve - The biological pathway to the brain
  stem, which forwards electrical energy to the
  occipital lobe.
• Occipital Lobe - The part of the brain that
  converts electrical energy into an image.

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How the Eye Functions?

• As light enters the eye, it first passes through
  the cornea, the clear outer portion of the eye.
  Because the cornea is curved, the light rays
  bend, allowing light to pass through the pupil to
  the lens. The iris, or colored part of the eye,
  regulates the amount of light that enters the eye
  with the ciliary muscles. These muscles cause the
  pupil to contract when exposed to excess light or
  to dilate when there is too little light.
• When light hits the curved surface of the lens,
  it is refracted and brought into focus on the
  retina. The retina then turns the light into
  electrical energy. This energy passes through the
  optic nerve to the brain stem and finally into
  the occipital lobe, where it is converted into an
  image.

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Refraction and the Eye

• Refraction is the phenomenon which makes image
  formation possible by the eye as well as by
  cameras and other systems of lenses.
• Most of that refraction in the eye takes place at
  the first surface, since the transition from the
  air into the cornea is the largest change in
  index of refraction which the light experiences.
• About 80 of the refraction occurs in the cornea
  and about 20 in the inner crystalline lens.
  While the inner lens is the smaller portion of
  the refraction, it is the total source of the
  ability to accommodate the focus of the eye for
  the viewing of close objects. For the normal eye,
  the inner lens can change the total focal length
  of the eye by 7-8.
• Common eye defects are often called "refractive
  errors" and they can usually be corrected by
  relatively simple compensating lenses

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Refractive index

• The greatest change of direction, or bending of
  the rays, occurs where the difference of
  refractive index is greatest, and this is when
  light passes from air into the cornea, the
  refractive index of the corneal substance being
  1.3376
• the refractive indices of the cornea and aqueous
  humour are not greatly different, that of the
  aqueous humour being 1.336 (as is that of the
  vitreous) thus, the bending, as the rays meet
  the concave posterior surface of the cornea and
  emerge into a medium of slightly less refractive
  index, is small.
• The lens has a greater refractive index than that
  of its surrounding aqueous humour and vitreous
  body, 1.386 to 1.406, so that its two surfaces
  contribute to convergence, the posterior surface
  normally more than the anterior surface because
  of its greater curvature (smaller radius).

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Physical structure of human retina

• In adult humans the entire retina is
  approximately 72 of a sphere about 22 mm in
  diameter.
• The entire retina contains about 7 million cones
  and 75 to 150 million rods. An area of the retina
  is the optic disc, sometimes known as "the blind
  spot" because it lacks photoreceptors. It appears
  as an oval white area of 3 mm².
• Temporal (in the direction of the temples) to
  this disc is the macula. At its center is the
  fovea, a pit that is most sensitive to light and
  is responsible for our sharp central vision.
• Human and non-human primates possess one fovea
  as opposed to certain bird species such as hawks
  who actually are bifoviate and dogs and cats who
  possess no fovea but a central band known as the
  visual streak.
• Around the fovea extends the central retina for
  about 6 mm and then the peripheral retina. The
  edge of the retina is defined by the ora serrata.
  The length from one ora to the other (or macula),
  the most sensitive area along the horizontal is
  about 3.2 mm.

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Retinal structure
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The retina is a seven-layered structure
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Retinal structure

• The retina is a seven-layered structure involved
  in signal transduction. In general, dark
  "nuclear" or "cell" layers contain cell bodies,
  while pale "plexiform" layers contain axons and
  dendrites.
• Trace the signal through the retina- Light
  enters from the GCL side first, and must
  penetrate all cell types before reaching the rods
  and cones.- The outer segments of the rods and
  cones transduce the light and send the signal
  through the cell bodies of the ONL and out to
  their axons.- In the OPL photoreceptor axons
  contact the dendrites of bipolar cells and
  horizontal cells. Horizontal cells are
  interneurons which aid in signal processing.-
  The bipolar cells in the INL process input from
  photoreceptors and horizontal cells, and transmit
  the signal to their axons.- In the IPL, bipolar
  axons contact ganglion cell dendrites and
  amacrine cells, another class of interneurons.-
  The ganglion cells of the GCL send their axons
  through the OFL to the optic disk to make up the
  optic nerve. They travel all the way to the
  lateral geniculate nucleus.

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Rods and cones

• The retina contains two types of photoreceptors,
  rods and cones. The rods are more numerous, some
  120 million, and are more sensitive than the
  cones. However, they are not sensitive to color.
• The 6 to 7 million cones provide the eye's color
  sensitivity and they are much more concentrated
  in the central yellow spot known as the macula.
  In the center of that region is the " fovea
  centralis ", a 0.3 mm diameter rod-free area with
  very thin, densely packed cones.
• The experimental evidence suggests that among the
  cones there are three different types of color
  reception. Response curves for the three types of
  cones have been determined. Since the perception
  of color depends on the firing of these three
  types of nerve cells, it follows that visible
  color can be mapped in terms of three numbers
  called tristimulus values.
• Color perception has been successfully modeled in
  terms of tristimulus values and mapped on the
  chromaticity diagram.

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Electron microscopy image
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rhodopsine (11 cis-retinal-opsine)
The rods employ a sensitive photopigment called
rhodopsin
The chromophor 11-cis retinal
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• Photochemical reaction and conversion to
  all-trans retinal

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• The cones are less sensitive to light than the
  rods, as shown a typical day-night comparison.
• The daylight vision (cone vision) adapts much
  more rapidly to changing light levels, adjusting
  to a change like coming indoors out of sunlight
  in a few seconds.
• Like all neurons, the cones fire to produce an
  electrical impulse on the nerve fiber and then
  must reset to fire again. The light adaptation is
  thought to occur by adjusting this reset time.
• The cones are responsible for all high resolution
  vision. The eye moves continually to keep the
  light from the object of interest falling on the
  fovea centralis where the bulk of the cones
  reside.

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Luminous Efficacy The curves represent the
spectral luminous efficacy for human vision. The
lumen is defined such that the peak of the
photopic vision curve has a luminous efficacy of
683 lumens/watt. This value for the scotopic
peak makes the efficacy the same as the photopic
value at 555 nm. The scotopic vision is
primarily rod vision, and the photopic vision
includes the cones. The response curve of the
eye along with the spectral power distribution of
a luminous object determine the perceived color
of the object.
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Spectral responseThe Color-Sensitive Cones
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• In 1965 came experimental confirmation of a long
  expected result - there are three types of
  color-sensitive cones in the retina of the human
  eye, corresponding roughly to red, green, and
  blue sensitive detectors.
• Painstaking experiments have yielded response
  curves for three different kind of cones in the
  retina of the human eye.
• The "green" and "red" cones are mostly packed
  into the fovea centralis. By population, about
  64 of the cones are red-sensitive, about 32
  green sensitive, and about 2 are blue sensitive.
  
• The "blue" cones have the highest sensitivity and
  are mostly found outside the fovea. The shapes of
  the curves are obtained by measurement of the
  absorption by the cones, but the relative heights
  for the three types are set equal for lack of
  detailed data. There are fewer blue cones, but
  the blue sensitivity is comparable to the others,
  so there must be some boosting mechanism. In the
  final visual perception, the three types seem to
  be comparable, but the detailed process of
  achieving this is not known.

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Spectral response

• The minimum or threshold intensity needed to see
  a flash of light is very wavelength dependent.
• The cornea is opague to wavelength shorter then
  300 nm, and the lens to wavelength below 380 nm.
• Rods and cones do not detect wavelength above 700
  nm.
• Although the rods do not give any color
  information, they are most sensitive to light in
  the green part of the spectrum , with wavelength
  of about 510 nm.

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Sensibilitatea spectrala a conurilor si
bastonaselor. Conurile si bastonasele sunt
sensibile la lungimi de unda diferite ale
spectrului vizibil.
Deci retina contine patru foto-receptori
bastonasele si trei tipuri de conuri, fiecare
dintre ele specializate pentru absorbtia unei
alte portiuni a spectrului vizibil - Conuri
care absorb lungimi de unda mari (rosu) -
Conuri care absorb lungimi de unda medii (verde)
- Conuri care absorb lungimi de unda mici
(albastru) .
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Accommodation
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Accommodation

• When the eye is relaxed and the interior lens is
  the least rounded, the lens has its maximum focal
  length for distant viewing . As the muscle
  tension around the ring of muscle is increased
  and the supporting fibers are thereby loosened,
  the interior lens rounds out to its minimum focal
  length..

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2. OPTICAL DEFECTS AND THEIR CORRECTION
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Eye Diseases and Refractive Errors

• Refractive Errors
• Nearsightedness (myopia) - Nearsighted vision is
  caused by an irregularly shaped cornea that
  results in light focusing in front of the retina,
  rather than directly on the retina. People who
  are nearsighted have difficulty seeing objects at
  a distance.
• Farsightedness (hypermetropia) - Farsighted
  vision is caused by an irregularly shaped cornea
  that results in light focusing behind the retina
  instead of directly on the retina. People who are
  farsighted have difficulty seeing nearby objects.
  
• Astigmatism - The most common of all eye
  disorders, astigmatism is a condition in which
  the eyeball is shaped more like a football than
  its naturally spherical shape. This odd shape
  causes light to focus on two points of the
  retina, rather than one.
• Presbyopia (old sight) - Presbyopia occurs when
  the lens of the eye becomes less flexible,
  necessitating the use of reading glasses for near
  vision. Specifically, the lens becomes stiffer,
  and the muscles that control the lens become
  weaker, hindering its ability to bend and flatten
  in order to focus light on the retina.

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Myopia

• Is a refractive defect of the eye in which
  collimated light produces image focus in front of
  the retina when accommodation is relaxed.
• Those with myopia see nearby objects clearly but
  distant objects appear blurred. With myopia, the
  eyeball is too long, or the cornea is too steep,
  so images are focused in the vitreous inside the
  eye rather than on the retina at the back of the
  eye. The opposite defect of myopia is hyperopia
  or "farsightedness" or "long-sightedness"this is
  where the cornea is too flat or the eye is too
  short.

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Myopia and correction with diverging lens
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Thin lens formula

• fc focus distance of the eye lens (biconvex
  crystal)
• X2 the distance between the crystal and retina
• XR distance between the object and retina
• The optical system consist of the eye crystal
  with fc and the glasses with f0the corrected
  image is on the retina.
• (2) - (1) ?, fo lt 0 ?
• Diverging lens for correction

(1)
(2)
(1) (2)
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Example

• Consider a man whose far point is 0,5 m. The
  distance between the eye crystal and the retina
  is 20 mm 0.02 m.
• The power of his eye when fully relaxed is
• P 1/ f .
• To have his far point at infinity , the man needs
  the power to be
• When wearing the glasses, the effective power is
  the algebraic sum of the powers of his eyes and
  his lenses, assuming the lenses are close to the
  eye
• P Pi Pf 50-52 -2 dioptres . So he need
  diverging lenses to see distant object clearly.

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Hypermetropya

• Hyperopia, also known as farsightedness,
  longsightedness or hypermetropia, is a defect of
  vision caused by an imperfection in the eye
  (often when the eyeball is too short or when the
  lens cannot become round enough), causing
  difficulty focusing on near objects, and in
  extreme cases causing a sufferer to be unable to
  focus on objects at any distance.
• As an object moves toward the eye, the eye must
  increase its power to keep the image in focus on
  the retina. If the power of the cornea and lens
  is insufficient, as in hyperopia, the image will
  appear blurred.
• Hyperopia, and restoring of vision with convex
  lens.
• People with hyperopia can experience blurred
  vision, asthenopia, accommodative dysfunction,
  binocular dysfunction, amblyopia, and strabismus.

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Hypermetropia, correction with converging lenses.
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Thin lens formula

• fc focus distance of the eye lens (biconvex
  crystal)
• X2 the distance between the crystal and retina
• Xp distance between the object and retina
• The optical system is made of the crystal (fc)
  and the glasses (fo) and will bring the near
  point of an object placed at ? -25 cm to the
  retina.
• (1)-(2)

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Example

• Consider a man whose far point is 2 m and the
  distance from the eye crystal to the retina is
  20 mm 0.02 m.
• The power of his eye when fully relaxed is
• To focus at 0.25 m the man will need a power
• So the correction must provide a power of 4
  dioptres to give him a near point of 0.25 m.
• P 54 - 50 4 dioptres

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Astigmatism

• This is usually caused by the cornea being not
  spherically curved, so that it has different
  curvatures in different directions. Images are
  seen in shaper focus in one direction, e.g.
  vertical, than in others. It is corrected by
  lenses that have a cylindrical curvature in the
  correct orientation to compensate for the cornea.
  

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Eye Diseases

• Cataracts - A cataract is a condition
  characterized by a clouding of the eyes natural
  lens. This clouding occurs when protein begins to
  clump together in the lens.
• Glaucoma - Glaucoma is an eye disease that occurs
  when elevated intraocular pressure (IOP) causes
  damage to the optic nerve.
• Macular degeneration - Macular degeneration is a
  degenerative eye disease that is characterized by
  a loss of central vision. It occurs when the
  macula (a tiny area on the retina) becomes
  damaged.
• Diabetic retinopathy - Diabetic retinopathy is a
  degenerative eye disease that occurs in patients
  with diabetes and is characterized by abnormal
  blood vessel growth. This can eventually lead to
  a detached retina and blindness.

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A hawk's eye
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Animation movie
http//www.physpharm.fmd.uwo.ca/undergrad/medsweb/
L1Eye/Eye.swf