What are 3 examples of illumination systems and their automatic equivalents?
- direct and indirect ophthalmoscopes
- retinoscopes = autorefractors
- slit lamp biomicroscopes
What defines an observation system?
has focusing lens that focuses an image on px’s retina
What are the main 2 challenges of using these equipments?
- minimising reflections from optical and ocular surfaces
2. maximise alignment of illumination and viewing axis, via use of fenestrated and semi-silvered mirrors
What are the 3 types of projector charts and their advantages and disadvantages
Back illuminated vision charts:
- Advantages: Minimal technology, simple to operate and low maintenance
- Disadvantages: Memorisation of the chart, limited chart types
Projector Charts:
- Advantages: Compact, variety of charts, movable between locations
- Disadvantages: Maintenance, memorisation of letters, expensive
Screen-based LCD/LED charts:
- Advantages: compact, multiple charts, portable, letters can be randomised, constant illumination, size of room less of a problem, wear and tear decreased
- Disadvantages: requires PC for operation.
What formula do we use when finding the image and object distance for projector charts
gauss’s law
F = L’-L
How is the projection lens of the projector charts placed?
the projection lenses are 2 positive lenses positioned apex to apex and movable to change letter size
When we look for a reflex, divergent light from source will form a fundus blur patch so we use positive lenses to converge light and observe corneal reflex at working distance.
Describe the reason for the different variation in reflexes between emmetropes, myopes and hyperopes
- Emmetropic eye = bright and full reflex viewed through site hole at C as parallel rays exit and converge after hitting plus lens and focus at sight hole –> full reflex
- Myopic eye = dark annulus surrounds less bright reflex as converging rays from cornea will converge again at plus lens before sight hole –> partial reflex
- Hyperopic eye = diverging rays will exit and focus behind sight hole after converging at plus lens –> partial reflex
Define neutralisation point and how can it be achieved?
changing reflex from partial to full
dioptric power of eye (K) = Fs (neutralisation point)
- optom moving closer to focal point to achieve full reflex
- placing lenses either positive or negative to bring focal point back to sight hole
What is the relationship between reflex and ametropia?
Reflex size, brightness and speed of reflex relative to optom is inversely proportional degree of Ametropia
relative speed = angular momentum of reflex/mirror rotation = W-X/W-K
- formula requires a known working distance ie distant of px to sight hole
What is the perceived movement of reflex relative to light source of a myope, hyperope and astigmatic and explain how
myopia = against direction because if optom moves the equipment top to bottom, px will perceive it as bottom to top as the image gets inverted inside the eye, due to strong converging power
hyperopia = with direction because if optom moves the equipment top to bottom, px will perceive it as top to bottom as the focal point is behind the eye so the image viewed is erect
astigmatic = oval-shaped reflex will be seen from a spot light source. The axis of stretch = rough axis of correcting cylinder to correct it, a streak light source is effective to firstly, correct one axis then correct the other to achieve a full reflex
- when the reflex is not clear, thin the light source by rotating the vergence adjusting lens to determine axis
What is the function, 3 main components and extra features of a retinoscope?
function = observe size, brightness, shape and reflex movement to indicate ametropia type and degree
- fenestrated mirror
- vergence adjusting lens = controls vergence that enters pxs eye by moving up and down
- rotating line filament bulb = filter which produces slit of varying heights and widths
o Brow rest for spectacle wearers
o Battery handle that is interchangeable with an ophthalmoscope head
o Sealed to keep out dust
o Polarising filter to reduce unwanted reflections off lenses and the cornea
o Separate bulbs to switch from spot to streak
What is the function of having 2 different sight hole diameters?
large and small have their own applications
- smaller sight hole diameter –> narrower range of far point positions for full reflex appearance –> greater accuracy of K-value
Define optometers and their disadvantages
Optometers = subjective device for measuring the refractive state of the eye, with a single optometer lens and movable target
Disadvantages =
o Proximal accommodation
o Movement closer to eye further stimulates accommodation
o Inaccuracies due to depth of focus = even if accommodation is controlled, there is a range of focus that the px can still see clearly
o Dioptric scale not linear
o Target size on retina varies with target position
o No astigmatism
What were some other earlier models of subjective instruments and how were they used?
Early model eg Simple, Badal, Young’s Optometer = instrument projects target on patient’s retina and optom observes target reflected from patient’s retina
Made sharp by =
- Adding lenses to path of reflex ie retinoscopy
- Adjusting vergence of projected light (amount of adjustment, measured by dioptric scale present, determines amount of refractive error)
Therefore, accuracy of results dependent on control of patient accommodation
Describe the 7 components of the Aston refractometer
- light source
- movable target up and down
- dioptric scale measures amount of
- optometer lens = positive lens
- fenestrated mirror
- reaches px eyes
- compensating lens which moves simultaneously with movable target to maintain clear image for optom
What are the 5 components of the objective modern autorefractors?
- target (infra-red source) = visible, steady and slightly blurred to relax accommodation with infra-red component
- photocell (infra-red detector) = interprets non-visible light reflected off retina
- cylinder adjustment = contains stokes lens ie one plus and one minus placed on top of each other and powers are changed by rotating it horizontally
- collimating lens = provides parallel rays to eye enabling measurement of distance
- condensing lens = decrease divergence
What is the difference between direct and indirect ophthalmoscopes?
direct = erect image and 15x magnification
indirect = inverted image and 2-5x magnification
Describe the 5 components of a DO
- light source
- condenser lens = decrease divergence of light from source
- aperture stop = controls amount of light
- projection lens = control amount of vergence that enters the eye
- beam splitter ie semi-silvered mirror = so optom can see image of retina
If there is a big corneal reflex then it is harder to see because the reflections will interrupt images seen so what is recommended while using DO?
smaller aperture = smaller corneal reflex so start big and transition to small as you go closer towards the eye for a larger FOV
What are the 3 advantages of using the indirect ophthalmoscope
- stereoscopic vision from high powered plus lens = permits binocular vision with depth perception
- wider FOV
- not affected by pxs refractive state
Describe the set up of the indirect ophthalmoscope (6 stages)
- light source = tungsten filament
- condenser lens = converges light from source
- mirror = diverges light towards projection lens
- projection lens = high plus lens converges light towards pupil
- Light reflected from the retina passes back through the lens creating a real, horizontally and laterally inverted image of the fundus situated between the lens and the examiner
- focusing lens = focuses image on optom’s retina
Identify the function of the keratometer
utilises the properties of the cornea to measure its radius of curvature
Rigid contact lenses are fitted in what steps?
0.05mm steps
What formula is used to measure radius in keratometry?
r=2dh'/h where r = radius of curvature d = working distance or distance between object and virtual image h = object height h' = image height
