OPTOM 263B Flashcards
What aberrations are rotationally and not rotationally symmetric
Spherical aberrations are symmetric
Monochromatic aberrations are not symmetric
Types of spherical aberrations ( SA )
Positive and Negative SA
How to correct Positive and Negative Spherical aberrations
Positive SA use Negative lens
Negative SA use Positive lens
How does Spherical aberration arise
From spherical surfaces and Larger pupils
What do eyedrops do to vision
Vision becomes blurry and should not drive
What type of spherical aberration is induced with accommodation and no accommodation
Negative spherical aberration on accommodation and Positive spherical aberration without accommodation
What is Coma, its effects and where on the retina it occurs
Off axis aberration that produces flare and non-uniform lighting on the retina, it also occurs on the fovea due to assymetry
What is astigmatism
The lack of rotational symmetry in a reference axis in at least 1 surface like the anterior cornea
Occurs when objects are more than 20 degrees off axis and is the main aberrator
How is central astigmatism corrected for, and what if the astigmatism isnt due to the cornea
Central astigmatism is corrected with lenses or LASIK. If astigmatism not from cornea then only corrected using lenses
Name the two loci in Astigmatism
T and S loci ( Tangential and Saggital )
Where is the T loci relative to the S loci
T is always left of S
How to quantify amount of astigmatism
The difference between the T and S loci power
Characteristics of larger glasses
Gathers more light but more astigmatism
Characteristics of smaller glasses
Gathers less light but reduces astigmatism
What size glasses are better optically
Smaller glasses
What is Field of Curvature and effects on vision
Off axis astigmatism where the image is formed behind the retina on an imaginary plane called the petzval surface.
It affects peripheral vision more than central vision
What is Distortion and effects on vision
Objects in the periphery and off axis known as primary monochromatic aberrations. The image lands on the Gaussian image plane so it is less likely to affect vision
What factor deteriorates image quality more
Larger pupil size deteriorates vision more than diffraction induced vision loss
What size pupil gives the best VA and why
2-3mm diameter as it is the least amount of diffraction
What do ocular aberrations affect
Image quality and pupil size
Which aberration affects the pupil size most significantly
Distortion
Why is Eye-Instrument alignment important
So accommodation and convergence does not occur when viewing binocularly and may also give more aberrations
What is the most common ophthalmic device and what is its purpose
Glasses, to have good foveal viewing at different gazes
What is the stop aperture in Glasses
The Pupil
What is the function of a stop aperture
Ensure light goes through a single point and in this case, through the pupil
Why is peripheral aberrations ignored
Due to poor peripheral resolution
What does Contact lenses do and compare with glasses and its optical characteristics
They correct vision, it differs since it rotates with the eye so off axis aberrations do not happen. Contact lens also fit well so spherical aberrations are the main aberrator
What replaces the crystalline lens
Artificial intraocular lens
Purpose and characteristics of an artifical intraocular lens
Replaces crystalline lens during cataracts, it moves with the eye so spherical aberrations is main aberrator
What happens if the artifical intraocular lens is decentered/tilted
Sagittal and Tangential power errors may occur
Types of chromatic aberration and how they occur
Longitudinal and Transverse, occurs from effects of dispersion
How does dispersion occur and what does it mean
From the varying refractive indices of different wavelengths, means that different wavelengths bend differently in the same refractive index
What is the relationship between refractive index and wavelength and what does it mean for the eye
Increasing wavelength decreases refractive index, so the eye has less power with a longer wavelength so red is focussed behind the retina in LCA
What is longitudinal chromatic aberration
Different wavelengths focus at different points along the optical axis
Characteristics of LCA
Chromatic difference of power and chromatic different in refraction
What is chromatic difference of power
The change in power with wavelength
What is chromatic difference in refraction
The difference in vergence between short and long wavelengths, this difference is greater for shorter wavelengths than longer ones
What is transverse chromatic aberration
Associated with foveal vision where different wavelengths are focussed on different parts on the retina
Characteristics of TCA
Chromatic difference in position
What is chromatic difference in position
The way to measure TCA known as the Angular measure of transverse chromatic aberration
How to measure Transverse chromatic aberration
Using chromatic magnification by comparing the image size between different wavelengths
Methods to measure longitudinal chromatic aberration
Best focus method, Vernier method, Double pass technique
What is the best focus method
A back illuminated target lit with different colours is moved back and fourth until the px says it is in focus
Can also use different power lenses instead of moving the target
What is the Vernier Method
Subjective measurement where two narrow targets of different wavelengths is imaged onto the fovea through a small aperture. There is a position where both targets seem to be aligned and a a position where the targets are aligned where the distance between the targets that seems aligned is the amount of LCA
What is the double pass technique
A narrow beam of light is formed on the fundus and it reflects some light to form an image outside the eye, a trial lens is used to minimise the width of the image formed for all colours
Very time consuming
What is more in focus for low accommodation
Longer wavelengths
What is more in focus with accommodation
Shorter wavelengths
Rate of LCA increase per 1D accommodation or 1D refractive error
Increase by 2.5% per 1D
How does age affect LCA
With age there is less accommodation so therefore less LCA
What CA does not affect foveal vision and what increases TCA
TCA
Decentered pupil and dislocated pupil causes more TCA which decreases VA
Why is LCA important in vision
Humans cant accommodate well in monochromatic light, LCA helps accommodation system, so increasing LCA has minimal effect on accommodation accuracy but lowering LCA may reduce accommodation accuracy
Why is chromatic aberration effects lost in vision
Effects of CA are lost due to the spectral sensitivies of the eye
What are achromatising lenses and their drawbacks
Removes chromatic aberrations where these lenses do not have power but has equal and opposite LCA to the eye
Does not improve VA and CS in white light so are useless for correction purposes
What is chromostereopsis
Allow objects of different colour to be seen as 3D where red appears closer than blue. This is binocularly driven where TCA is combined with BV
What induces more chromostereopsis
Larger pupils give more chromostereopsis due to more TCA
What colour is macular pigment
Yellow
What does macular pigment contribute towards and how it affects vision
The luminosity efficiency function as there is no macular pigment in the periphery so more aberrations occur there meaning less chromatic aberrations in the fovea
What is light
A transverse EM wave that transmits energy through space
Property of Transverse waves
Oscillations of magnetic and electric fields are perindicular to each other and to the direction of travel, can also be polarised to its components
What are harmonic waves
Repeating waves where its intensity is proportional to the square of the wave amplitude
How does a vacuum or non-vacuum environment relate to wavelength
Wavelength stays the same in a vacuum while in the eye the wavelength changes as it travels through it due to differing refractive indices.
What is superposition and what does it produce
The constructive and deconstructive interference of waves that produces interference patterns
What is the ripple tank analogy
2 vibrators producing circular waves in a tank, the final ripple is the sum of ripples from the vibrators
What is a standing wave and how is it created
When the ripple/wave does not seem moving at all. Produced by putting 2 vibrators half a wavelength apart and also works with light
How to produce coherent pattern
If phase difference between two monochromatic sources is constant
How to produce incoherent patterns
If phase difference between two monochromatic sources are not constant
Property of incoherent patterns
Distributes energy randomly in all directions
Example of coherent patterns
Laser pointer where the light travels in one direction and has a homogenous everage irradiance
What is the Youngs double slit experiment
light passes through a pinhole then through a double pinhole to produce coherent light. The outcome is an interference pattern of dark and bright fringes giving a standing wave
Youngs double slit experiment in health
Check visual health where two spots of coherent light passed through the pupil and fringes form on the retina where the orientation, spacing and intensity of the firnges is changed by tweaking the laser
Used on cataract px
What do thin films achieve
Produce colourful interference fringes
How do thin films produce colourful interference fringes
Due to multiple inner-thin-layer reflections
What does more reflections mean
Fringes spaces apart more where it is very dense as you move down
How is light lost and how do we prevent this
Light lost from reflections and anti-reflection coatings are used to counter this to increase transmission but a coloured pattern is produced
What happens to the image which more reflections
The image brightness is dimmer
How to let more wavelengths through a lens
Multiple anti-reflection coatings
Drawback of multiple anti-reflection coatings
More transmission of light but the lens becomes grey and becomes unfashionable and may let in UV light that damages the eye
What is diffraction
Ability of light to bend around corners where each wavefront is its own source
How to observe diffraction
Diffraction is on a small scale so to see it you need a dark envrionment since any background light will mask the diffraction
What is single slit diffraction
When monochromatic light goes through a slit of certain width and produces ripples on a screen where it is brightest in the centre
Slit width in relation to the diffraction pattern
Decreasing slit width reduced diffractioin
Single slit with polychromatic light
Centre of pattern is white and the periphery is coloured with red being the outermost
Why is red the outermost in the pattern
Longer wavelengths are diffracted the most
Two types of diffraction
Far and Near field
What is far field diffraction
When slit-screen distance is more than 20 metres
What is near field diffraction
When Slit-screen distance is closer than 20 metres
What does the diffraction pattern depend on in near field
The slit-screen distance
What does near field diffraction mean for vision
Floaters that form from debris and comes in all shapes and sizes
Are floaters normal
Yes, but too many may be an abnormality
What type of system is the eye
Diffraction limited system
Why is the eye a diffraction limited system
When all factors like refractive error, aberrations and scatter are corrected for, diffraction still exists
Where does the diffraction come from in the ey
Small pupil size
Pros and cons of large pupils
Better light and resolution but more aberrations that increases light spread so the ability to distinguish between two points diminishes
Pros and Cons of smaller pupils
Aberrations are reduced by introduced diffraction what increases PSF so blur is introduced
What does resolution is a diffraction system depend on, and what does mean for the resolution of the eye
The wavelength of the light where longer wavelengths diffraction more than shorter wavelengths.
Shorter wavelenths give better resolution due to less diffraction
How can pinholes be used to assess vision
If VA reduction is due to optical reasons like diffraction then pinholes would increase VA
If VA reduction is pathological then pinholes would not improve the VA
How are Halos produced
When white light goes through an aperture resulting in an airy type diffraction pattern
Halos outer and inner most colours
Outermost is red and inner most is blue due to diffraction
How are halos relevant to the eye
Corneal swelling from fluid deposits between stormal fibres produce halos when you look at light
What do halos in vision may indicate
High uncontrolled Intra-ocular pressure from uncontrolled glaucoma
How to treat swelling cornea
Using hypertonic saline droplets
What does scattering do to vision
Reduces image quality due to inhomogeneities in a medium
What does scattering depend on
Particle size, Particle distance and Strength of interaction between light and the particles
Name two types of scattering
Coherent and Incoherent
How does incoherent scattering arise
Due to small/medium/large particles in a medium
Large particles scatter light foward and Small particles scatter light in all dirextions
What happens in the wavelength is similar in size to the particle
Scattering in one and all directions simultaneously occur
Relation between particle size and wavelength
Smaller particles scatter smaller wavelengths more
Incoherent scattering examples
Blue sky and white smoke
How does multiple incoherent scattering occur
When particles are far from each other
What is single incoherent scattering
When there are more large particles in the liquid than small particles
What is Coherent scattering
When light has the same frequency and wavelenfth and occurs when particles are much closer than the wavelength of light
What happens in coherent scattering
Superposition occurs between the scattered waves from the particles since the medium acts as a diffraction grating
What does each successive scatter mean for the light
It adds more component to the phase shift which explains why light travels at different speeds in mediums
How is coherent scattering relevant
This is how the cornea works to maintain transparnency where the collagen fibrils are spaced apart and also wide of half the wavelength of light
How much of the light is scattered in the cornea
10%
What happens if the cornea swells
The stromal fibres space apart more than half the wavelength of light so coherent scattering is less effective so the cornea becomes less transparent
How is scattering relevant to the sclera
The collagen size and spacing is larger so incoherent scattering occures which renders the sclera to be white.
Scleral thinning makes the sclera bluer and less white due to small particle scattering
Scattering in the lens
Lens has a yellow pigment so it scatters more light. In catarcts the proteins aggregate which also scatters more light
Scattering in the vitreous humor
Has very small collagen fibers so it scatters 0.1% light. Age causes the collagen to aggregate forming fluid pockets causing more scattering
Scattering in the retina
Healthy retina scatters same amount as cornea, if the retina is damaged then fluid collects in the nerve fibre layer that increases scattering. Mily gray spots on the retina would be seen
How does the iris get its colour
Due to the amount of pigmemtation where less pigment gives blue eyes, more pigment means brown eyes.
What are spatial frequencies
The details in an image
What is the relation between spatial frequencies and the optical system
Low spatial frequencies are transmitted even when high spatial frequencies are gone where low frequencies are seen as blur.
What is contrast and how is it affected
The luminance difference between an object and a background where blurry images decreases contrast
What is MTF
How well an optical system can reproduce fine details and contrast with a ratioj
What is the ratio of a perfect MTF system
1
Factors that change MTF
Spatial frequency, photoreceptor density, refractive error, accommodation error etc
What is the final luminance and contrast made of in a linear system
The sum of all components in a system
The lower frequency gives gross shape of the final wave and the higher frequencies give the smaller details on the final wave
Why is optical system design important
An optical system with a high cut off frequency may not be the best to use as our visual system is not able to perceive it anyways
What is the ideal optical system design
The highest frequency cut off point is similar to the human cut off point
What type of aperture will increase MTF and why
Square apertures since it reduces diffraction and airy disc formation
What effect do square apertures do to an image
It enhances contrast and preserves more high spatial frequencies
Factors that affect MTF
Mis-focus produces blur that decreases MTF
Diffraction decreases MTF and occurs with 2mm or less pupil size
Larger pupil size induces aberrations which also decreases MTF
Longer wavelengths decline in MTF slower with larger pupils compared to shorter wavelengths
Halving pupil diamater increases VA by two times
Smaller pupil means slower MTF decline
A higher resolution of the eye also lowers the MTF and relates to the CSF
What is an obstructed telescope
A telescope with a secondary mirror
What type of telescope would you use to view high spatial frequencies
Obstructed telescope
What type of telescope to use to view low spatial frequencies
A non-obstructed telescope
How to calculate the MTF for a system
The product of the MTFs of each component in a system is the MTF for the system
What type of filter is the eye
A low pass filter
What does a low pass filter mean
It detects low spatial frequencies better without information loss
Why type of neural pathway filters do we have more of
Low pass filters
Function of low and high pass filters
Low pass filters give the shape information and High pass filters give the small details of the system
How are refracted waves produced
By coherent scattering and it travels through the medium at a particular speed
What does a larger refractive index mean
Light travels slower
Which part of a refractive index graph shows opaqueness
The peak of the refractive index function in a medium shows which wavelength it is opaque to
What is strabismus and what does it cause
Both eyes not fixing on same object and it causes diplopia
What can treat strabismus and what are its effect on vision
Prisms, and they tend to have dispersive effects so you might see different colours of the object
What is the relationship between dispersion and prism thickness
More dispersive with more thickness
Property of EM waves
Transverse so can be polarised
What is unpolarised light
Light vibrates in all planes
Why might unpolarised light be useful
Makes surroundings appear to be illuminated by the same source
What is needed to have superposition in waves
The wave needs to oscillate in the same plane
How does polarisation work
Vertical components are absorbed so vertical light is passed through since the horizontal electrons are restricted from conduction
What stops light from being polarised
Is the vertical wires are smaller than the wavelength of light
What is an ideal polaroid
Lets 50% of light through, either vertical or horizontal light through, HN-50
In reality, what occurs with the ideal polaroid
4% of light is reflected due to surface reflections at each surface so in total there are 8% reflections so the ideal polaroid is now known as HN-42, so only 42% of light is let through
How to prevent all light from being transmitted
If the two polaroids are oriented perpendicularly
3D glasses polarisation features
Has clockwise and anticlockwise circular polarisation so each eyes sees differently for a 3D view
How is polarisation relevant to vision
Gets rid of reflections and improves vision quality and used in glasses
What is birefringence and how does it happen
Effect of double refraction which splits beam into two paths where each beam travels at a different speed and has its own refractive index. This is due to anisotropia
What is an anisotropic material
Where the medium has a different refractive index in different directions
What is a factor that refractive index depends on
Absorption strength for V and H polarised light
What is retinal birefringence scanning technique
Detects central fixation of the eye that is not used and can detect strabismus. This is non-invasive but needs patient cooperation.
How is light lost in the eye
Reflection, Scatter and Absorption. So not all light entering the eye reaches the retina
What is the problem with too much light
Eye damage including UV
Factors changing eye light sensitivity
Time of day changes sensitivity like photopic and scotopic vision. Scotopic is sensitive to short wavelengths and photopic is sensitive to longer wavelengths
What drives photopic vision
Cones
What contributes to the luminousity efficiency function
All three cones
Who has CVD
People with one or two missing types of cones
Factors that change sensitivity to light and how
Sensitivity to blue light decreases with age due to absorption of blue from cataracts, less blue light reaches retina
Which photoreceptor should have less variability in performance for scotopic vision
Rods, since there is only one type of it
What is luminous flux
Measured in lumens and is the density of light
What is Luminous Efficacy
Ratio of lumens to watts
What is luminous intensity
Measured in Candelam it is the luminous flux in a direction
What is luminance
Objective brightness measure of a light source in cd/m^2
What is illuminance
Luminous flux density on a surface in Lumens/m^2
What is the threshold to detect light on retina
Luminous flux
How is increased sensitivity to light achieved
Light falls on many photoreceptors and spatial summation occurs which enhances sensitivity to dim light
What does the threshold depend on
The amount of light collected by the retina
What does stimulus size mean for sensitivity
Larger stimulus means spatial summation is less critical as more photoreceptors are excited. So the amount of retinal illuminance is more important than luminous flux
Luminance vs Illuminance
Brightness observed is luminance, The brightness of a particular area is illuminance
How is light lost in the eye
specular reflection, absorption and scatter
Where does specular reflection mainly occur
In the cornea
Why is specular reflection useful and its drawbacks
Can find radius of curvature of the cornea but makes it hard to assess due to glare
Effects of light transmission on different parts of the eye
Transmission decreases in the cornes with age
Aqueous humor transmittance is maintained throughout life
Lens decreases transmittance of shorter wavelengths with age from cataracts
Vitreous transmittance not affected by age
How is eye tissue damaged from light
Water in the eye has strong absorption to longer wavelengths causing heating of water resulting in thermal damage.
UV strongly absorbed by the lens causing damage
Lasers are absorbed by different parts of the eye
Effects of scatter in the lens
Lens is made of more cells than cornea so it has more scatter with age. Cataracts cause scatter due to less transparency from protein aggregation increasing inhomogenity and anisotropy in the lens
What is fluorescence
Absorption of wavelength and re-emitting at a longer wavelength
What type of emission is fluorescence and does it affect vision
Isotropic where energy is emitted in all directions causing veiling glare onto the retina for people with cataracts decreases VA at low contrasts
What fluoroscent compounds are in the lens
Tryptophan and two fluorogens
What is intrinsic Birefringence
Difference refractive indices in different directions
Refers to properties of a medium
What is form birefringence
Biological structures have less order but have more order at a structual level
Refers to structural order of the medium
Why does the eye exhibit birefringence
The collagen arrangement which affects light transmission and polarisation
Birefringence in the cornea
Fibrils are cylindrical and regularly spaces, but each layer is at a large and different angles so birefringence occurs
If light is incident perpendicularly then no birefringence occurs due to alot of layers and orientations so it cancels the birefringence
Birefringence in the Lens
Fibres are radial so the birefringence is different for the lens and the cornea
What are the 4 pigments in the eye
Macular pigment, Visual pigment, Melanin in Pigmented epithelium and Haemoglobin in the choroid
All absorbs light
Light pathway in the retina ( Basic ) and retinal characteristics
Hits retina and reflected in the Inner limiting membrane. 6 layers between ILM and photoreceptors are transparent
What is the macular pigment
Xanthophyll
Recall the efficiency of light in the visual pathway
92% light enters the eye and macular pigment absorbs some meaning 53% of the light reaches the cones outer segment, only 67% of this light absorbed by the visual pigments go into a photochemical reaction
What is the retinal efficiency
12%
How much of incoming light goes to the brain as a signal
7%
Light absorption in the layers of the eye
Absorption occurs in the pigmented epithelium, some light passes through and enters choroid which has Hb so absorbs short wavelength light ans scatters long wavelengths.
Rest of the light is absorbed by the sclera
Why is retina reflex red
Most absorption is in the shorter wavelengths
What colour visualises axons better
Blue
Does retinal birefringence affect vision
No
Where is retinal birefringence used
In scanning laser polarimetry
What is scanning laser polarimetry
Estimates nerve fibre loss due to glaucoma
How does scanning laser polarimetry work
Red lasers polarised and passed into retina and scatters deep in retinal layers, the choroid reflects and passes back out of the eye
Polarisation plane is changed twice through the nerve fiber layer, amount of polarisation gives nerve fibre layer thickness
Thicker retina means more polarisation
What does a thinner retina indicate
Glaucoma
Effect of age on corneal stroma
Decreased interfibrillar space and increased crossed sectional area of the collagen
Other age effects on scatter
Increased endothelial degeneration and AH entering cornea increasing scatter
What light transmission decreases with age an due to what reason
UV and Visible light transmission decreases due to cataracts with age
How does the lens change with age and its relation to anterior chamber depth
Lens increases in volume and mass with age and due to increased axial thickness in the cortex. The anterior chamber depth decreases at the same rate as the lens axial length change
What is the lens paradox
Becomes more myopic with age from lens thickening, after 30 goes more hyperopic as lens becomes less curved
What happens to the eye after age 70
Shifts back to the myopic direction and introduces astigmatism from corneal shape change
What is presbyopia
Decreased ability to accommodate with age, occurs around age 40
General trend with presbyopia
People lose accommodation in their 50s before any physiological functions are affectd
Two theories of presbyopia
Lenticular and Extra-Lenticular
What is lenticular theory
Change in mechanical or geometrical change of lens and its capsule, assues lens hardens with age and maintained ciliary muscle strength
What is Hess-Gullstrands theory
Zonule slackness means lens is more gravity influenced for older people
What is Fincham theory
Lens capsule gets more rigid with age so more pressure is applied on the lens, ciliary muscle works harder to over come capsule resistance
What is Geometric theory
Increasing lens curvature with age causes change in lens orientation so zonules apply tension less radially so zonule relaxation does not change lens shape
What is the Extra-Lenticular theory
Change in ciliary muscle or elastic compoments of the zonules with age
What is senile miosis
Pupil diamater decrease with age
What also decreased with age regarding the pupil
Pupillary reaction speed
What age is max pupil size
Teenage years
What does senile miosis mean for MTF
MTF decreases with age due to senile miosis due to reduce ocular transmittance from smaller pupils
What is Abbe value
The dioptric difference between blue and red convergence points in LCA, shows amount of chromatic aberration for a material in the lens
What does a higher Abbe value mean
Means less chromatic aberration
What is transverse chromatic aberration
Difference between image magnification between blue and red images
What does the prismatic effect of lenses depend on
Distance form optical axis of the lens
Where is there more prism on a lens
The periphery
What does this mean in real life
We see through the centre of glx in real life so prismatic effects are less noticeable, but if lens is decentered then this effect is amplified
Is prismatic effect more or less in the periphery of a high power lens
More
What is an achromatic lens and how is it made
Does not have chromatic aberration where all wavelengths are focussed onto a point
Combining a plus lens of one material with a minus lens of a different material where the sum of the powers is the correction needed to focus the blue and red light
What factors are taken into place for lens designs
Vertex distance, lens thickness, refractive index and back lens surface power
Aims to correct spherical aberrations, Coma, Astigmatism and Distortion
What is spherical aberration
the power difference between periphery and centre of lens and it On Axis
Function of the pupil
eliminate peripheral rays to reduce spherical aberration
What is Coma
When object is off axis and a magnification difference between rays going through difference zones of a lens
What images do Coma produce
Comet shaped images where the point of the comet points towards the optical axis. This aberration is not an issue for the human eye
What is oblique astigmatism
Light focus at T and S points, difference between T and S is amount of astigmatism
How can oblique astigmatism be induced with lenses
When lens is tilted so objects that were on axis are now off axis
What are wrap around lenses and its function
A Prescribed protection glasses that protect from wind and sunlight, and common for sports use
Problem with wrap around lenses
Tilting glasses changes its power, and changes its astigmatism correction
Considerations when designing wrap around lenses
Assume lens is used tilted, so we tilt lens in direction light is coming from. Base in prisms used to retain BSV
What are sports vision trainers
Glasses made of curved plastic LCD lenses where it switches between clear and blocked to train athletes to anticipate whats coming in the periphery and helps with hypersensitive motion cues
Field of curvature aberration in lens design
Occurs even when lens made astigmatism free. The dioptric difference between where the image is focussed and where is should focus is the Image Shell Error
Why should we use. the manufacturers base curves
To reduce oblique astigmatism and power errors
What is Distortion
Difference in magnification in different areas of the lens periphery compared to the centre of the lens. Magnification increases in periphery for positive lenses which increases image size
What are multifocals
Allows clear vision at many distances
Who to prescribe multifocals and how to design the lens
Presbyopic patients, top portion of lens powerless and bottom portion with a plus lens for near vision
What is Near Addition ( Near Add )
The additional power needed on top of distance correction for near distance viewing in the lower portion of the lens
How does near addition work with astigmatism
Sum the sphero correction with the near addition, everything else stays the same
What is a trifocal lens
These have an intermediate portion for normal viewing distances between the distance and near correction
What is generally the intermediate prescription
Half the near add
When to use tri-focals
When you cant accommodate well or if the add is more than +1.50D
When would you not use a tri-focal lens
If you can accommodate then you can see intermediate objects if you see through the distance correction and then accommodating
3 Ways Bi/Tri-focals are made
Fused, One-Piece and Cemented
What is a fused Multi-focal lens
Only available in glass, the segment is made from a higher refractive index glass compared to the distance lens
What is a one-piece multi-focal lens
Made from glass or plastic, the change in segment kens power due to the change in lens curvature. The One-piece multifocals are found by feeling the segmented border
What is a cemented lens
Custom made lens with a glued small segment on the distance lens
Types of Bi-focal designs
Round segments, Flat-top segments, curve top, panoptik, Franklin and Executive style
( Also available tri-focals )
What are occupational lenses
Lens chosen carefully and positioned for special viewing conditions
Common occupational lens designs
Double D segment, Quadrafocal and Rede-rite
Who are Double D segment lenses for
Those who need intermediate and near viewing like plumbers, the lens recreates workers situations to get the working distance to get the upward power
What is a Quadrafocal lens and who is it for
A double D segment with a flat top trifocal on the bottom and an upside down flat-top segment on the top, for those who need both trifocal and double segment lenses and has 4 viewing areas
Only available in glass material
What are Rede-rite lenses and who is it for
An Upcurve bi-focal and has a large round segment at the top. For those who want a segmented lens and need a full, near working area. They also want to see clear in the distance wihtout taking glasses off
Where is more prism effect seen in a lens
Towards the periphery away from the optical centre
What is image jump
Sudden change in displacement of the image due to the sudden change in prism between the distance and the near correction
What are progressive lenses
Multifocals that look the same as single vision lenses but has clear viewing at all distances without any lens lines in bi/tri-focals
Power of lens changes gradually and eliminates image jump but may be hard for some people to use
Who needs bifocals sooner, Hyperopes or Myopes
Contact lens hyperopes do not need bifocals sooner than contact lens myopes as the spectacles is the reason why there is an accommodation difference
Hyperopes can accommodate to near, Myopes cannot accommodate to near
What consideration is needed when prescribing occupation lenses
The working distance of the person needs to accommodate the same as they did their normal glasses
Conditions when designing contact lenses
Need to sit well on the cornea with minimal lateral movement so reduce asymmetric aberrations
Diamater of optic zone needs to be more than the pupil size in all lighting conditions
What will be visible to a patient after LASIK
Halos
Pros and Cons of soft contact lenses
Shapes themsleves so back surface resembles shape of anterior corneal surface for a good fit but is less well at correcting astigmatism
Why my soft contact lenses have on-eye power changes
Flexibility and hydration variations
The flexibilty means that the contact lens changes curvature and thickness so on-eye power changes
Changes to hydration means refractive index also changes causing on-eye power change
Eye has higher temperature so it may change the hydration and therefore refractive index
What is checked for soft contact lenses before insertion
The back vertex power checked at room temperature in a fully hydrated state
How much can hydration change in a contact lens
Can drop by 5-10% after 1-2 minutes of wear
Thinner vs Thicker contact lens hydration equilibrium
Thinner lenses reach equilibrium in 5 minutes, thicker and high power lenses can keep dehydrating after 30 mins of insertion which the time depends on the material
Why causes contact lenses to dehydrate more
High water content, it reaches equilibrium faster than low water content soft contact lenses of the same thickness
Can also be due to lots of near work as dehydration occurs due to less blinking
Are power changes more significant for negative or positive lenses and why
Positive lenses and they are thicker
Contact lens design and its effects on the eye
Well centred contacted lenses with steep surfaces introduce spherical aberrations. The eye and the contact lens moves together so spherical aberrations are induced.
How are transverse chromatic aberrations induced in glasses, how is this different in contact lenses
The prism effects during oblique viewing due to the airgap between the glasses and the eyes, contact lenses do not have this gap so it reduced TCA
Considerations for contact lenses to be mass produced
The spherical aberration for everyone should be similar, the spherical aberration is the major aberrator, spherical aberration should be the same off eye and any decentration of the contact lenses should not introduce any significant aberrations
Spherical aberration in relation to age
Spherical aberration becomes more negative with age , this affects myopes more
Spherical aberration increases with age
What is A^opt
The optical lens surface power adjustment needed to minimise spherical aberration
Problems with correcting spherical aberration
Chromatic aberration and astigmatism still exist
When is astigmatism generally corrected for
When is exceed 0.75D
What still exists regardless of large pupils in mesopic and scotopic conditions
Monochromatic aberrations
Characteristics of Rigid contact lenses
Back surface of the Contact lens maintains shape so on eye power is the same
The overall optical system giving the final image is based on the rigid contact lens, Tear lens and the eye itself
What does the power of the tear lens depend on
The geometry of the optic zone at the back surface of the rigid contact lens and the anterior surface of the cornea
How does the power of the tear lens change with the steepness of the back optic zone radius ( BOZR ) and how to counter this
Power increases by +0.25D for each 0.05mm the BOZR steepens, so the back vertex power of the rigid contact lens needs to be changed by -0.25D per 0.05mm the BOZR steepens to compensate
How to find the BOZR
Trial lenses used to give BOZR which gives fit for a particular lens design. The power is the sum of the back vertex power of the trial lens and the over-refraction.
What is the over-refraction for
Determines additional power needed with the trial lens to give clear vision
Where do contact lens correct astigmatism for
For the cornea and not the lens
How does VA change with contact lens power
VA gets worse with contact lens powers of more than +3.00D where the aberration from the cornea is much less than the aberrations introduced by the contact lens
Contact lens power change considerations with high and low order aberrations
Px w high-order aberrations wearing CL will reduce aberrations
Px w low-order aberrations wearing CL might increase the aberrations
