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Assessment of Vision 1 > Week 11: Visual Optics & Aberrations > Flashcards

Week 11: Visual Optics & Aberrations Flashcards

1
Q

Describe light

A
  • Light is only a very small part of the electromagnetic radiation spectrum
  • Electromagnetic radiation is a transverse wave motion composed of an electric (E) and a magnetic (H) field
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2
Q

What are the Laws of Geometrical Optics?

A
  • Light travels in straight lines
  • Adjacent rays are independent of one another
  • Light obeys the laws of refraction
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3
Q

Describe point source of light

A

Emits waves

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4
Q

Describe extended object

A

An infinite number of point sources

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5
Q

What is pencil of light?

A
  • A bundle of light rays
  • The pencils may be formed of light rays that are diverging, converging or parallel
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6
Q

What does vergence describe?

A

The curvature of optical wavefronts

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7
Q

Describe Divergent Rays

A
  • Vergence of diverging light is always negative
  • The amount of divergence = reciprocal of distance to a point source (m) = 1/ l (m)
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8
Q

Describe Convergent Rays

A
  • Formed by a magnifying lens
  • Always positive
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9
Q

Describe vergence

A
  • A concept in which distances are converted to a dioptric power
  • Lenses change the vergence of a beam of light
  • Dependent on the distance between the beam, lens AND refractive index of the lens
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10
Q

Describe Object vergence L

A
  • Object distance (l) has a negative sign (if left of lens)
  • Travels through a refractive index n1
  • Vergence L = n1/-l
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11
Q

Describe Image vergence L’

A
  • Has a positive sign (image to the right of the lens)
  • Travels through refractive index n2
  • Vergence L’ = n2/l’
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12
Q

What is Refraction?

A
  • Bending of light when passing from one medium to another
  • Refractive index (n): describes how fast light travels through a material
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13
Q

Explain Snell’s Law

A

The ratio of the sines of the angles of incidence and refraction of a wave are constant when it passes between two given media

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14
Q

Describe positive & negative lens forms

A
  • A spectacle lens is a transparent medium bound by two polished surfaces one of which must be curved
  • Positive & negative lenses can have either a bi-convex or meniscus form
  • A meniscus lens is one with both a convex and concave surface
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15
Q

List the Forms of Spectacle Lenses and briefly describe

A
  1. Spherical = same curvature throughout the lens segment of a sphere
  2. Cylindrical = curved one side and flat the other
  3. Toroidal = bent cylinder
    Aspheric = both sides curved but not spherical – flatter form
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16
Q

Describe Convex lenses

A
  • Plus power
  • Rays are focused at a point behind the lens
  • Have a positive focal length
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17
Q

Describe Concave lens

A
  • Minus power
  • Rays appear to come from a point in front of the lens
  • Have a negative focal length
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18
Q

Focal Length and Lens Power

A
  • The power of a lens (F), in Dioptres (D), is equal to the reciprocal of its focal length in metres (f’)
  • Longer focal length = weaker lens
  • Shorter focal length = stronger lens
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19
Q

Curvature of the lens

A

Power of lens depends on two things;
- Curvature of the surface
- Refractive index of the material

20
Q

Describe Spherical Lenses

A
  • Is one for which brings parallel light to a single point focus
  • The surface is perfectly symmetrical, and can be thought of as a slice off the side of a sphere (+ve lens) or the mould from the side of a sphere (-ve lens)
21
Q

Describe Cylindrical Lenses

A
  • If any refracting surface in the eye is not perfectly spherical, then light will not be able to form a single point focus on the retina
  • This most commonly occurs when the cornea is not perfectly spherical, having 2 primary curvatures at 90o to each other
  • A cylindrical lens will refract light in one meridian whilst leaving light unrefracted in the perpendicular meridian
22
Q

Describe Axis Meridian

A
  • Meridian along which parallel rays will not be refracted
  • There is no power along this meridian
23
Q

Describe Power Meridian

A
  • Meridian along which there is maximum refraction of light.
  • This lies at 90o to the axis meridian
24
Q

Sphero-Cylindrical Lenses

A
  • In most cases of refractive error, we are required to correct both myopia or hyperopia with a spherical lens, along with astigmatism with a cylindrical lens
  • The majority of lenses prescribed are sphero-cyl lenses
  • Hence, rays are refracted in both meridians, just in differing amounts
25
Q

Describe Aberrations

A
  • All optical systems have aberrations that degrade the quality of the image
  • Aberrations are important in the design of spectacle and correction of refractive errors
  • Monochromatic aberration = aberration produced by a single wavelength of light
  • Chromatic aberration = aberrations produced by multiple wavelengths light
26
Q

Describe Paraxial Assumptions

A
  • Paraxial equation is useful as it allows to locate the images produced by spherical optical systems
  • Light rays with very small angle of incidence with the refracting surface are called paraxial rays
  • Paraxial equation becomes less accurate if the angle of incidence increases
27
Q

List the Seidal or Classic aberrations

A
  1. Spherical aberrations
  2. Coma
  3. Oblique astigmatism
  4. Curvature of field
  5. Distortion
28
Q

Describe Longitudinal Spherical Aberration (LSA)

A
  • Spherical lenses suffer from positive (undercorrected) longitudinal spherical aberration
  • Non paraxial rays focus closer to the lens than paraxial rays
  • Positive longitudinal spherical aberration is present in both plus & minus lenses
  • The amount of longitudinal spherical aberration is dependent on: radii of curvature of front and back surface of a lens
29
Q

Describe LSA & Spectacles Lens Design relationship

A
  • As peripheral rays of that emerge from a spectacle lens are blocked by the iris, preventing them from reaching to retina and participating in image formation
  • LSA in spectacles lenses generally does not reduce retinal image quality
30
Q

What is the role of aspherical lenses then?

A
  • The surface becomes flatter
  • Aspheric surface improves the cosmetic appearance of a plus lens
  • Flatter front surface results in less spectacle magnification
31
Q

Describe LSA & Human Eyes

A
  • Unaccommodated eye typically manifest positive SA, which increases with age
  • SA would be greater if not the aspheric nature of the cornea
  • As the eye accommodates, the amount positive of SA decreases
32
Q

LSA & Human Eyes relationship with night time vision

A
  • Under dim light, pupil dilates, exposing the retina to non-paraxial rays, which focus in front of the retina, making the eye myopic
  • This is contributing factor to night myopia – that is present only under low illumination
  • Clinically, prescribing slightly more minus power (or less plus) for those patients who do considerable night time driving
33
Q

Describe coma

A
  • Results when light rays are oblique with respect to the optical axis
  • Because of coma, an off-axis point source results in image with a comet-like shape
  • When the tip of the comet is pointed towards the optical axis, the coma is said to be positive and when it is pointed away, the coma is negative
34
Q

Describe Oblique Astigmatism (OA)

A
  • If the point source is on the optical axis of a spherical lens, the angle of incidence for the horizontally diverging rays is equal to the angle of incidence for the vertically diverging rays
  • This occurs even the rays emerging from the off-axis object pass through the centre of the lens
  • OA is important in the design of the spectacle lens
35
Q

Oblique Astigmatism (OA) & Lens Design

A
  • Even OA is minimised by proper selection of the front surface power, it can be a problem when the lens is tilted with respect to the eye
    -E.g. wrap-around sunglasses may be tilted with respect to horizontal plane of face
  • Face foam refers the power in the vertical meridian of the lens
36
Q

Describe Curvature of Field

A
  • The image plane for the flat object is curved
  • Not all points on the extended object are the same distance from the spherical converging lens
  • It can be minimised by proper selection of the lens front surface power
37
Q

Describe Distortion

A
  • The central and peripheral regions of a spherical lens do not produce the same amount of lateral magnification
  • Barrel distortion = minus lenses as minification is greater than in its periphery
  • Pincushion distortion = greater magnification in the periphery of a plus lens compared to its centre
38
Q

Describe chromatic aberration

A
  • Occurs only with polychromatic light, a mixture of different wavelengths
  • The transmission speed within a refractive medium depends upon the wavelength with shorter wavelengths travelling more slowly than longer wavelengths
  • Consequently, the refractive index, which is the ratio of the speed of light in a vacuum to that in a given medium, is different for each wavelength
39
Q

Chromatic Aberration: Dispersive Power

A
  • A single refractive index is specified for a refractive medium
  • To quantify the amount of dispersion produced by a prism or lens, we use 3 wavelengths: 486, 589 and 656 nm
40
Q

Chromatic Aberration: Constringence

A
  • The reciprocal of dispersive power is defined as the constringence (also called as Abbe number or ν) of the refractive element
  • As the Abbe number increases, dispersion decreases
41
Q

Lateral (Transverse) Chromatic Aberration

A
  • Chromatic dispersion for two wavelengths of lights, which appear red (656 nm) and blue (486 nm)
  • Chromatic aberration produced by a prism is referred as lateral chromatic aberration, transverse chromatic aberration or chromatic power
42
Q

Do lenses manifest lateral chromatic aberration?

A
  • Lenses have dioptric & prismatic power, which increases with increasing distance from the optical centre of the lens
  • As the lens’s prismatic power increases toward the periphery of the lens, the lateral chromatic aberration also increases
  • To avoid the lateral CA, it is important to align patient’s pupil with the optical centre of the lens
43
Q

Describe Longitudinal Chromatic Aberration (CA)

A
  • Because lens’ index of refraction is different for each of wavelengths constituting white light, its focal length is different for each
  • The difference in dioptric power for wavelengths of 486 and 656 nm is defined as longitudinal CA
44
Q

Chromatic Aberration in the Human Eye

A
  • For electromagnetic radiation from 380 to 760 nm, the human eye exhibits 2.50 D of longitudinal CA, corresponding to a linear distance of 0.93 mm
  • Although we are not aware of longitudinal chromatic aberration, it is thought to be stimulus to accommodation
45
Q

Chromatic Aberration & Red-Green Refraction Technique

A
  • Red-Green refraction technique also called as duochrome or bichrome refraction technique
  • This refraction technique takes advantage of the eye’s longitudinal CA
  • When a patient views a chart that is green on one side and red onother side, the green image is focused anterior to the red image
  • If the patient is myopic, both green and red images are focussed anterior to the retina and both will appear blurred
46
Q

Red-Green Refraction Technique

A
  • When duochrome test is performed in the clinic, plus lenses may be placed in front of the patient’s eye to ensure that both the green & red images fall anterior to the retina
  • Under these conditions, the optotypes on the red appear clearer than on the green

Assessment of Vision 1 (10 decks)

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