Optics Flashcards
What is High Order Aberrations?
A distortion acquired by a wavefront of light when it passes through an eye with irregularities of its refractive components, such as: tear film, cornea, aqueous humour, crystalline lens and vitreous humour.
Electromagnetic Spectrum
The range of frequencies of electromagnetic radiation and their respective wavelengths and photon energies.
Visible Light
Electromagnetic radiation at wavelengths which the human eye can see.
We perceive this radiation as colours ranging from:
- Red = Longer wavelengths - 700nm
- Violet = Shorter wavelengths - 400nm
What does the Electromagnetic Spectrum consist of?
1) Radio Waves
2) MicroWaves
3) Infrared
4) Visible Light
5) Ultraviolet
6) X-ray
7) Gamma Ray
Electromagnetic Radiation
Refers to the waves of the electromagnetic field, propagating through space, carrying electromagnetic radiant energy, in which electric and magnetic fields vary simultaneously.
Frequency
cycles per second - Hertz.
Wavelength
metres
Energy
electron Volts
Radio Waves ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ Gamma Rays
Long Wavelength
↓
Short Wavelength
Lower Frequency
↓
Higher Frequency
Lower Energy
↓
Higher Energy
Lightray
A ray is an idealised model of light, obtained by choosing a line that is perpendicular to the wavefronts of the actual light, and that points in the direction of energy flow.
Incident ray
An incident ray is a ray of light that strikes a surface. The angle between this ray and the perpendicular/normal of the surface is the angle of incidence.
What order does light travel through the Eye?
Conjunctiva ↓ Cornea ↓ Aqueous humour ↓ Lens ↓ Vitreous humour ↓ Retina/ macula – fovea
What are the stages in Visual Process?
1) Normal stimulus to vision i.e. light
2) The formation of an image when the optical image falls on the retina
3) Photochemical reaction between light falling on the retina and chemical light-absorbing substances within it.
4) Rod reaction is mediated by the substance known as visual purple
What are the Retinal Photoreceptors?
Rods
Cones
Rods
Rods become fully active at Low scotopic levels of illuminance (dim light)
Cones
Cones become fully active at High photopic levels of illuminance (bright light)
What is the ratio of Rods:Cones in the Human Eye?
7million CONES and 120million RODS
Distribution of Cones
Fovea Centralis is densely packed with cones only, there are virtually no rods present.
The distribution of cones decreases from the Central pit to the Periphery at low density.
Distribution of Rods
Rods are concentrated at the outer edges of the Retina and are used in Peripheral Vision.
Sensitivity of Photoreceptors
Rod cells are more sensitive than Cone cell and are almost entirely responsible for Night Vision.
How is the Macula responsible for Critical Vision?
The High Spatial Density of Cones along with the absence of blood vessels at the Fovea accounts for the High Visual Acuity capability at the fovea.
Basic Sign Convention
Concave
Convex
Practical Implication for Concave lenses?
If you have Myopia or Nearsightedness, you would use Diverging, Concave lenses to shift the focus of your eye lens backwards so that it can focus on the Retina.
Practical Implication for Convex lenses?
If you have Hypermetropia or Farsightedness, you would use Converging, Convex lenses to shift the focus of your eye lens closer so that it can focus on the Retina.
Direction of the Incident Light is….
always positive
Ray diagrams are drawn from….
left to right so Cartesian sign convention applies.
Laws of Optical Image Formation:
1) Diagrams are drawn from left to right because incident light initially propagates in this direction.
2) Object and image distances, focal lengths and the radii of curvature are measured from the lens, mirror or surface concerned.
3) Distances measured in the same direction as that in which the incident light is travelling is positive in sign.
4) The vertical distance from the optical axis to a point above it is taken as positive, and to a point below it as negative.
5) Acute angles are positive when produced by anticlockwise rotation from the optic axis, and negative when produced by clockwise rotation.
6) The angle between a ray and the optical axis is measured from the ray to the axis
Define Optical Axis?
Line passing through the centre of cornea, lens and meets retina on nasal side of fovea.
Define Visual Axis?
Line joining fixation point, nodal point and fovea
Define Fixation Axis?
Line joining fixation point and centre of rotation.
Coaxial
Have a common axis
Relationship between Optical AND Visual Axis
The angle between the Optical and Visual axis is the angle Alpha and is considered positive when the Visual axis in object space lies on the nasal side of the Optical axis.
- Positive value of 5’ is common
Donders’ observation that the angle tends to be smaller in myopia and greater in hyperopia.
As for the vertical plane, the visual axis in object space is generally inclined in an upward direction from the optical axis, 2’ is common.
Define Object Space?
The space in relation to an Optical System in which are located the objects to be imaged by the system.
Dioptre
Unit of measure for the refractive power of a lens i.e. the ability of a lens to converge or diverge light.
- 0.25D increments
What is the equation for Dioptre?
D = 1/f
The power of a lens, D = 1/focal length (m)
Reciprocal of focal length in m
What Corrective lens do Myopes require?
Myopia can be corrected by placing a negative, concave lens in front of the eye, causing the incoming light rays to diverge.
What Corrective lens do Hypermetropes require?
Hyperopia can be corrected by placing a positive, convex lens in front of the eye to increase the convergence of the incoming light rays.
Refractive Index
Ability of a medium to bend light
What is the symbol for RI?
n
The presence of a dash or (prime) shows…
that the symbol refers to a quantity after refraction/reflection
What is the symbol for:
Objective Distance Image Distance First Focal length Second Focal length Radius of curvature Objective Height Image Height
l
l’
f
f’ r h h’
Law of Reflection
States that the Incident ray, the Reflected ray, and the Normal to the surface of the mirror all lie in the same plane.
Furthermore, the Angle of Reflection is equal to the Angle of Incidence.
Both angles are measured with respect to the Normal to the mirror.
Why are Symbols in Italics?
Denoting a quantity in italics
e.g.
ƒ is the power of the lens or surface
Why are Symbols in Roman Capitals?
Denoting geometrical points
e.g.
F is the first principle focus
RI Equation
n = c/v
Velocity of light c of a given wavelength in empty space divided by its velocity v in a substance.
Real Object
One from which incident rays diverge
Virtual object
One towards which incident rays are converging as the result of a previous refraction or reflection.
Real image
One towards which refracted or reflected rays converge and is therefore capable of being received on a screen.
Virtual image
One from which refracted or reflected rays appear to originate.
Reduced distances
Distance or thickness of material transversed by a pencil of rays, divided by the refractive index of the given medium.
Gullstrand’s Simplified Schematic Eye
A mathematical or physical model that represents the basic optical features of the real eye.
- The average eye
3 surface model:
- Single surface Cornea
- Crystalline lens considered as a single homogenous entity with 2 refracting surfaces.
Theoretical Optical specification of an idealised eye with average dimensions includes the following 3…
- Curvature of the refracting components of the eye
- Refractive indices of the transparent media in the eye
- Axial dimensions of the components of the eye
To calculate Cardinal points…
The radii of curvatures and distances separating the refractive surfaces must be known.
It is presumed that the cornea and crystalline lens…
Lie on a common Optical Axis so all the refracting surfaces are coaxial.
What is the difference between Schematic Eye and Reduced Eye Models?
Schematic = Multiple refracting surfaces Reduced = Only one refracting surface
Nodal point
Either of two points which are located on the axis of a lens or Optical System so that any incident ray directed through one will produce a parallel emergent ray directed through the other, leaving the system with the same direction.
Listing and Donder’s Reduced Eye Model
The reduced eye model replaces the several refracting bodies of the eye by an ideal air/water interface surface that is located 20 mm from a model retina.
- Cornea is represented as the only source of the refractive power of the eye.
- Lens is completely neglected
Total power of the plane
1 Nodal point
Homogenous RI of Water = 1.33
But the Cardinal point held a relationship to the normal Phakic eye.
What is the power of the eye?
+60D
Define Refraction?
Change in direction of any wave as a result of its travelling through one medium and another or through a medium of varying density.
Light will be refracted in two places when it enters the eye:
- Cornea
2. Lens
Cornea account for…
2/3 of total refraction of the eye
Lens account for…
1/3 of the total refraction of the eye
What is the Dioptric power of the Cornea?
+40D
What is the Dioptric power of the Lens?
+20D
Refractive Error
The difference between the power of the eye’s optical system and the length of the eye.
Emmetropia
An eye that is able to refract incident parallel light rays exactly on to the retina without the use of the accommodating power of the lens.
Ametropia
An eye that is unable to refract incident parallel rays on to the retina and hence there is refractive error.
Myopia
An eye that refracts light in front of the retina.
The focal point is in front of the macula.
Hypermetropia
It is the refractive state of the eye where parallel rays of light coming from Infinity are focused behind the Retina with accommodation being at rest.
The posterior focal point is behind the retina which receives a blurred image.
Accommodation
Involuntary change in the shape of the lens to increase the power.
Whats the function of Accommodation?
- Theroetically will allow the eye to focus at all distances that are less than infinity.
- Activated if there is disparity between the power of the eyes Optical system and the Axial length.
Why does the Image form in front of the Retina in Myopes?
- The cornea being too curved
- The lens is too powerful
- The axial length is too long
Why does the Image form behind the Retina in Hypermetropes?
- The cornea being too flat
- The lens is too weak
- The axial length from the cornea to the retina is too short
Astigmatism
This is a compound refractive error where light will come to a focus in different points/multiple focal points due to irregular curvature of the cornea or lens.
Astigmatism Correction
Correct one meridian with a sphere.
Correct the second meridian with a cylinder.
Articulate Myopia to a Professional
In Myopia (Near-sightedness), because the cornea and lens are too strong or because the eyeball is too long, parallel light rays are brought into focus in front of the retina.
Myopia can be corrected by placing a concave lens in front of the eye, causing the incoming light rays to diverge.
Articulate Hypermetropia to a Professional
In Hypermetropia (Far-sightedness), the distance from the cornea and retina is too short for the refractive power of the cornea and lens, thereby causing images that would come into focus behind the retina.
For clear vision, the eye must accommodate to increase the lenticular power to bring distant objects in focus on the retina. This requires contraction of the ciliary muscle.
Hyperopia can be corrected by placing a convex lens in front of the eye to increase the convergence of the incoming light rays.
Pencil of light
A geometric construct used to describe a beam or portion of a beam of electromagnetic radiation or charged particles, typically in the form of a narrow cone or cylinder.
Axial Myopia
Axial length greater than schematic - 23.30mm
If the Total Refractive Power of an eye remains constant, but Axial length increases, causing Myopic shift.
Constant Variables:
- Fe = +60D
- Ne = 4/3
Refractive Myopia
Refractive power of the eye is too large because the cornea is too steep or lens is too powerful.
Fe = greater than +60D
Constant Variable:
- AL = 23.30mm
- Ne = 4/3
Index Myopia
Refractive Index of Cornea/lens is greater than 4/3 or 1.333333.
Constant Variables:
- AL = 23.30mm
- Fe = +60D
Ocular Refraction
The refraction of light produced by the media of the normal eye and resulting in the focusing of images upon the Retina.
Spectacle Refraction
The correcting lens placed in front of the eye in the spectacle plane to make the Ocular Refraction project onto the Retina.
Calculate Ocular Refraction -
K = K’ - Fe
Overall power
= Ocular Refraction
Far point in Myopia lies…
a finite distance from the front of the eye.
Far point in Hypermetropia is….
beyond infinity thus VIRTUAL behind the retina.
Far point in Emmetropia is…
Infinity
Define Far Point
In Visual Perception, the far point is the farthest point at which an object can be placed for its image to be focused on the retina within the eye’s accommodation.
Infinity??
Define Near Point
In Visual Perception, the near point is the closest point at which an object can be placed and still form a sharp, focused image on the retina, within the eye’s accommodation range.
25 cm
Axial Hypermetropia
Total refractive power of the eye is normal but there is axial shortening of the eyeball.
AL less than 23.30mm
Constant Variables:
- Fe = +60D
- Ne = 4/3
1mm Shortening = 3D Hypermetropia.
Refractive Hypermetropia
Total refractive power of the eye is less than +60D.
Constant Variables:
- AL = 23.30mm
- Ne = 4/3
Index Hypermetropia
Refractive Index is less than 4/3
Constant Variables:
- AL = 23.30mm
- Fe = +60D
Physiologically changes with age
Pathologically changes with Diabetes/treatment
What are the 2 types of Corneal Curvature Irregularities?
- With-the-Rule
2. Against-the-Rule
With-the-Rule Astigmatism
Meridian of maximum curvature i.e. steepeness is vertical, 90’ - More plus.
Against-the-Rule Astigmatism
Meridian of maximum curvature i.e. steepness is horizontal, 180’
What patient is WTR Astigmatism prevalent in?
Younger, Myopic eyes
What patient is ATR Astigmatism prevalent in?
Older, Cataract eyes
Regular Astigmatism
Where the principle merdians are 90’ perpendicular to each other.
Functional Hypermetropia
Due to paralysis of accommodation.
Seen in patients with 3rd Nerve Palsy and Internal Ophthalmoplegia
Total Hypermetropia =
Latent + Manifest
Total Hypermetropia
It is the total amount of refractive error, estimated after complete Cycloplegia.
Latent Hypermetropia
The amount of farsightedness that is ‘masked’ when the accommodative muscles are used to increase the eye’s focusing power and is corrected by Inherent tone of the ciliary muscle. This is revealed after abolishing ciliary tonus by Cycloplegics.
Usually about 1D
Manifest Hypermetropia
The component of Total Hyperopia that cannot be corrected by a physiologic response of the ciliary muscle; after abolishing voluntary Accommodation.
Consists of:
- Absolute
- Facultative
Absolute Hypermetropia
Residual, uncorrected Hypermetropia
Facultative Hypermetropia
The degree of Hypermetropia corrected by accommodative effort.
Positional Hypermetropia
Results from posteriorly placed crystalline lens of the eye.
Circle of Least Confusion
An optical spot caused by a cone of light rays from a lens not coming to a perfect focus when imaging a point source.
Interval of Sturm
Found between the first and second focal point
Astigmatic blurring
Generally an elongation or an ellipse
The requirements for a sharp retinal image are that, after refraction by the eye, the image forming pencil of rays are…
homocentric – free from astigmatism
Presbyopia
Long-sightedness caused by loss of elasticity of the lens of the eye, occuring typically in middle and old age.
‘Old sight’, age-related changes
- Not strictly a refractive error
What occurs in Presbyopia?
Occurs as the eye ages – the lens loses its ability to change focus due to:
- The change in the curvature of the lens decreases
- The decline in focusing ability starts in youth but is generally only really noticeable around age 40-45.
- The lens will stiffen until age 60+
The small amount of accommodation left after age 60 is actually depth of field so pupil getting smaller.
MS = S + (C/2)
Cylindrical blur is assumed to have ½ the effect of spherical blur, because the circle of least confusion lies on the retina and the cylinder axes are orthogonal.
Frequency distribution
Presentation of the frequency of numerical data arranged according to magnitude.
Normal distribution
Specific shape of the frequency distribution that results from random variation i.e. bell-shaped curve.
KURTOSIS
The sharpness of the peak of a frequency-distribution curve.
Refractive error in infants:
Average new-born infant is hyperopic
Mean refractive error of +2D
- Rapid decline in hyperopia between 6 months and 2 years of age
- Gradual decrease up to 6yrs
Emmetropisation
Rapid reduction in refractive error over first few years of life towards emmetropia.
Refractive error changes with age:
- Less hyperopic with age
- More hyperopic when become presbyopic (lens induced changes)
- Less hyperopic
- Astigmatism
- Shape of cornea changes with age
- Young people ‘with the rule’
- Older people ‘against the rule’
Spatial resolution is much lower due to..
Spatial summation of rods i.e. a number of rods merge into a bipoar cell, in turn connecting to a ganglion cell and the resulting unit for resolution is larger but acuity is small.
Spatial resolution is much lower due to..
Spatial summation of rods i.e. a number of rods merge into a bipoar cell, in turn connecting to a ganglion cell and the resulting unit for resolution is larger but acuity is small.
Pupillary distance, PD:
The distance between one centre of a person’s pupil to the other in mm -
Sometimes called interpupillary distance = IPD
Normal Adult PD:
54-74 mm
Normal Child PD:
43-58 mm
Distance PD is different to Near PD because
Pupils converge so it is important to ensure that a patient’s pupil is lined up with the optical centre of glasses.
Aberrations
A property of Optical Systems such as lenses that causes light to be spread out over region of space rather than focused to a point.
What do Aberrations cause?
The image formed by the lens to be blurred or distorted.
The ability of a spectacle lens to produce Aberrations is dependent upon the material it is made from.
The dispersive ability is measured by the ________
Abbe Number
Name 6 Types of Aberrations:
1) Chromatic Aberration
2) Spherical Aberration
3) Coma
4) Oblique Astigmatism
5) Curvature of Field
6) Distortion
Name 5 Types of Monochromatic Aberrations:
1) Spherical Aberration
2) Coma
3) Oblique Astigmatism
4) Curvature of Field
5) Distortion
What is Chromatic Aberration?
The effect produced by the refraction of different wavelengths of light through slightly different angles resulting in a failure to focus light at the same focal plane leading to dispersion of colours.
It also produces a blurred image with coloured fringes.
What is Spherical Aberration?
A type of Aberration found in Optical Systems where light rays that strike a spherical surface off-centre are refracted or reflected; this deviation reduces the quality of images produced by optical systems.
What is Coma Aberration?
Comatic Aberration, in an Optical System refers to Aberration inherent to certain optical designs or due to imperfection in the lens or other components that results in off-axis point sources such as stars appearing distorted, appearing to have a tail (coma) like a comet.
What is Oblique Astigmatism?
An Aberration of off-axis rays that causes radial and tangential lines in the object plane to focus sharply at different distances in the image space.
Chromatic Aberrations cause 2 things?
- Colour fringing
2. Dispersion
Name 2 types of Chromatic Aberrations?
- Axial = Longitudinal
2. Transverse = Lateral
The Refractive Index is greater for Shorter Wavelengths
Blue light is refracted to the greater extent because it has a shorter wavelength.
Red light is refracted least because it has the longest wavelength.
Why are spectacle lenses tinted yellow?
Yellow lens tints are helpful in low-light conditions as they enhance contrast and depth perception and make the environment appear brighter.
Also these lenses block out blue light
Blue light with its shorter wavelength scatters easier than other colours and makes focusing on objects difficult.
Removing blue light improves sharpness and depth perception and reduced Asthenopia and fatigue.
Yellow is where _______
The Circle of Least Confusion lies.
Longitudinal Chromatic Aberration:
Axial aberration occurs when different wavelengths of light are focused at different distances from the lens (focus shift).
Transverse Chromatic Aberration:
Transverse aberration occurs when different wavelengths are focused at different positions in the focal plane, because the magnification and/or distortion of the lens also varies with wavelength.
Chromatic Aberration can affect many aspects of Vision, such as:
1) Retinal Image Contrast
2) Coloured Fringing
3) Contrast Sensitivity
4) Visual Acuity
5) Colour perception
6) Depth perception
7) Diagnostic imaging of the Fundus
What are the Uses of Chromatic Aberration?
Duochrome
When light is no longer monochromatic, there are defects that occur in the retinal image formed by an optical system i.e.
Chromatic Aberration
Anisometropia
Where the two eyes have unequal refractive power, generally a difference in power of 2D and more.
Abbe Number/V Number
An approximate measure of the material’s dispersion (change of refractive index versus wavelength), with high values of V indicating low dispersion.
High Abbe Number
Better optical performance due to:
Better peripheral optics
Minimised Chromatic Aberrations
Human cannot detect the Chromatic Aberration if the Abbe Number is above _____
40
Decentration
Decentration is any displacement of the Centration point from the standard Optical Centre position - boxed centre and is necessary when the boxed centre distance of the frame and the subject’s Inter-pupillary distance are not the same.
What are the Optical Effects of Decentration?
Decentration causes:
- Prismatic Effect
- Formation of Ghost Images
- Reduces the lens power effectivity = the point where the Paraxial Rx is most effective is no longer in front of the pupil/
How would you reduce the Aniseikonia produced by an Anisometropic Rx?
- Choose a frame with a Small Vertex Distance
- Keep the eye size small
- Px is an Anti-metrope
- Thin the lenses
- Decrease BVD
