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
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
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
- Spherical aberrations
- Coma
- Oblique astigmatism
- Curvature of field
- 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
