OV1014 Visual Optics and Measurement techniques Flashcards
What is Ametropia?
Abnormal refractive eye condition (myopia, hyperopia or astigmatism) which images fail to focus on the retina
What is vision?
- Perception of difference in the external world resulting from stimulation of the retina by light
- Smallest line of letter read with the unaided eye
Why assess vision?
Quantify what people can and cannot see
What were Snellen’s innovations?
- Special characters (optotypes/test types)
- Letter chart to test distance vision
- Defined ‘the standard observer’ for comparison
In a snellen chart what do the number by each line represent?
The letter size in meters
The distance at which a letter height subtends 5’ arc
or
The gap/limb width subtends 1’ arc
Why is ‘serif’ font used in snellen charts?
Helps readability in large sections of texts
How can you increase reliability of tests for Snellen charts?
- Include one or more difficult letters on each line
- Is ‘common confusion pairs’ (UV, FP. HN)
What is the progression of letter in Snellen chart?
No common ration so no geometric progression
What is the letter/line spacinging Snellen charts?
- Letter width - 4 units
- 20 mm between successive lines
What is the standard observer?
Visual acuity that just allows him/her to identify a letter with a stroke width subtending 1’ arc
What is form vision?
The ability to recognise an object from its form
What is resolution?
The ability to detect fine detail
What requires both resolution and form vision?
Recognition of letters as they get progressively smaller
What is visual acuity?
The smallest line of letters that can be read with spectacles or contact lenses worn ( provided all defects of focus are corrected)
What is the minimum angle of resolution?
Angular separation of two points/lines at the resolution acuity limits
What is the MAR for the standard observer?
1’
What to do if VA is below 6/60?
- Reduce distance to chart until Px can read top letter - 3/60
- Counting finger - Record CF at 25 cm if Px can count
- Hand movements - Record if Px can detect your HM
- Perception of light - Record if Px can detect PL ad which quadrant light is in
- No perception of light - Record NPL if patient cannot detect light
What are the advantages of Snellen Chart?
- Quick and Reliable
- Sensitive to ametropia
- Narrow
- Less affected by guessing compared to grating acuity etc
- Enshrined in legislation
What are the limitations of Snellen Chart?
- Unequal number of letters per line, so tasks in not equivalent at differing levels of VA
- Non-uniform change in letter size for successive lines
- Line by line scoring
- Fixed row spacing
- Unequal letter spacing, creates crowding
What are the features of LogMAR chart?
- Same number of letters per line (5)
- Letter of similar legibility chosen
- Approx equivalent task at each level
- Uses a fixed raitio between each line (0.1 Log unit)
- Each letter represents 0.02 LogMAR unit
- Letter spacing = letter width
What are Geometrical optics?
- That rays can be represented by straight lines along which light propagates
- Assumes light travels as rays in homogeneous medium
What are wavefronts?
- Surfaces of constant phase or more easily constant optical distance from the surface
- Perpendicular to the rays
What is the quantity of reduced vergence inked to ?
Curvature of wavefronts
What does vergence tell us?
How rapidly the wavefronts are converging or diverging
What do axial objects/images show?
the location of the object/image
What do off axial objects/images show?
The height of the object/image
What is the thin lens approximation?
- Light does not converge/diverge significantly between the surface
- Refraction effects can be added
- Represent lens as one surface
What happens to light for a distant object?
Becomes less divergent as it strikes the lens
What is light from an infinitely distant object represented by?
Parallel rays
L= 0
Where does an image form from an infinitely distant object?
F (Back focal point)
What is F’?
A point which is conjugate with an object at infinity
Where does an image form when an object is placed at F ?
Infinity
What does surface power depend on?
Radius of curvature and difference in refractive index
What is the fovea centralis?
Most sensitive point on the retina
Where does visual axis pass?
Passes through pupil and hits fovea centralis
What is angle alpha?
Angle between the visual and optical axis
Why does light fall on fovea centralis?
To use best visual acuity
What do we approximate for a reduced emmetropic eye?
Cornea as one surface
Lens as a homogeneous n
Fe = +60D
n = 4/3
What is optic neuritis?
Px is at risk to multiple sclerosis
Why do people carry out index test?
Quicker / easier than diagnosis
Cheaper
Invasive
What is the bench mark test called ?
Gold Standard/ Reference standard
What does the light leaving the negative lens depend on?
The vergence of the incident light
What are the two types of ametropia?
Spherical ametropia
Astigmatism
Define Emmetropia?
Second focal point Fe’ coincides with the retina for an unaccommodated eye
Where is the far point for an emmetropic eye?
Infinity
Define Far point Mr?
Object point conjugate with the retina in the unaccommodated eye
Define Myopia?
Second focal point Fe’ falls in front of the retina for an unaccommodated eye
Where is the Mr in Myopic eye?
In front of the retina - real point
What is the effect of myopia?
Blurred vision
Objects closer to the accommodated eye can be seen clearly
Anything beyond Mr would not be sharply in focused
What is ocular refraction
The vergence of light striking a reduced eye from the far point.
When a contact lens is needed to correct ametropia what is the ocular refraction
The thin lens power
When are L and L’ equal to K and K’?
When the image is formed at the retina and the object is at the far point
Define hypermetropia?
Second focal point (image of a distant object) falls behind the retina in the unaccommodated eye
Where is the MR located in a hypermetropic eye
far point (MR) lies at behind the eye
Define Accommodation
Increase in power of the eye with an effort to focus at near
What happens in accommodation
Lens changes to increase its dioptric power
When does eye have its weakest power
Unaccommodated eye
What occurs in presbyopia
Accommodation declines with age
Ability to focus on near objects affected unless myopic
Define Presbyopia
Condition where accommodation is insufficient for near work
What occurs in accommodation in hypermetropia
Increasing the dioptric power of the eye can place the optical image on the retina
What causes spherical ametropia
Failure in co-ordination during growth of the optical component eye
Ametropia larger that +_4 - eye is too long or short
Pathology
When is a negative lens required
When the second focal point of the spectacle lens Fsp coincides with MR
What happens when ray incident on the eye appear to come from the MR
Forms a sharp image on the retina
When is a positive lens required
Second focal point of spectacle lens Fsp’ coincide with MR
When does the vergence remain the same
Parallel lines travelling from infinity
what is spectacle effectivity
Effective power of a spectacle lens at another plane (usually the eye). It is caused by the change in reduced vergence as the light propagates from the lens
Whats the effectivity for myopes
Magnitude of Fsp is higher than K
Whats the effectivity for hypermetropes
Magnitude of K is higher than Fsp
Define transverse magnification
Ratio of image height to object height
What is the magnification in the eye
Eye is a high positive powered optical system
Magnification is negative so image is inverted
Magnification is smaller than 1 so retinal image is much smaller than the object
Define Nodal points N and N’
Conjugate points along optical axis of unit angular magnification
Coincident for a single surface
Define principal points
Conjugate points along the optical axis of unit transverse magnification
Where are the principal points for a single surface/think lens
Always at the surface/lens
Define blur ratio
Ratio of the blur circle diameter to the basic blurred retinal image height
Does it matter whether blur circle is +ve or -ve
No
What is blur analysis based on
Geometrical optics where in an ideal system points are imaged as points
What does the Pinhole do
Artificially reduces the pupil size significantly decreasing the blur circle
What is the effect of Pinhole
Reduces light entering the eye
Improves vision if blurring is significant
Reduces light level adversely affecting vision
Increases diffraction if vision is already good (possibly decreasing vision)
If the vision improves with pin hole what does this mean
Indication that the deficit is due to retinal image blur
Define basic blurred image height
The height of the principal ray at the retina
What does letter recognition depend on
The size of the blur circle and blur ratio
Define projected blur
The retinal blur circle projected back on to the object.
What is important about the object when referring to the projected blur super imposed
It will only be a scaled version of the image
Can be used for comparison with an experiment of perceived blur
Why may the actual blur perceived be different fro our values?
Edges of real blurred objects are unclear
Objects often appear to have structure possibly from diffraction in the eye
What does blur ratio and projected blur provide?
First approximation to understanding letter recognition with blur present
What does it mean when the angle subtended by h1’ at eh eye (u) is less than w
The retinal image is smaller than in the uncorrected eye
Define Spectacle Magnification
Ratio of the retinal image height in the corrected eye to the basic blurred retinal image height in the uncorrected eye
What happens to the retinal image for spectacle corrected myopes
Gets smaller
What happens to the retinal image for spectacle corrected hypermetropes
Gets larger
