Week 11: Visual Optics & Aberrations Flashcards
(46 cards)
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
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
3
Q
Describe point source of light
A
Emits waves
4
Q
Describe extended object
A
An infinite number of point sources
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
6
Q
What does vergence describe?
A
The curvature of optical wavefronts
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)
8
Q
Describe Convergent Rays
A
- Formed by a magnifying lens
- Always positive
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
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
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’
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
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
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
15
Q
List the Forms of Spectacle Lenses and briefly describe
A
- Spherical = same curvature throughout the lens segment of a sphere
- Cylindrical = curved one side and flat the other
- Toroidal = bent cylinder
Aspheric = both sides curved but not spherical – flatter form
16
Q
Describe Convex lenses
A
- Plus power
- Rays are focused at a point behind the lens
- Have a positive focal length
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
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
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
Describe Aberrations
- 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
Describe Paraxial Assumptions
- 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
List the Seidal or Classic aberrations
1. Spherical aberrations
2. Coma
3. Oblique astigmatism
4. Curvature of field
5. Distortion
28
Describe Longitudinal Spherical Aberration (LSA)
- 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
Describe LSA & Spectacles Lens Design relationship
- 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
What is the role of aspherical lenses then?
- The surface becomes flatter
- Aspheric surface improves the cosmetic appearance of a plus lens
- Flatter front surface results in less spectacle magnification
31
Describe LSA & Human Eyes
- 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
LSA & Human Eyes relationship with night time vision
- 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
Describe coma
- 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
Describe Oblique Astigmatism (OA)
- 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
Oblique Astigmatism (OA) & Lens Design
- 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
Describe Curvature of Field
- 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
Describe Distortion
- 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
Describe chromatic aberration
- 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
Chromatic Aberration: Dispersive Power
- 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
Chromatic Aberration: Constringence
- 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
Lateral (Transverse) Chromatic Aberration
- 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
Do lenses manifest lateral chromatic aberration?
- 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
Describe Longitudinal Chromatic Aberration (CA)
- 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
Chromatic Aberration in the Human Eye
- 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
Chromatic Aberration & Red-Green Refraction Technique
- 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
Red-Green Refraction Technique
- 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
