Flashcards in OPTICS Deck (40):
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?
1. 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
- 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
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)
1. optom moving closer to focal point to achieve full reflex
2. 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
1. fenestrated mirror
2. vergence adjusting lens = controls vergence that enters pxs eye by moving up and down
3. 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
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
1. light source
2. movable target up and down
3. dioptric scale measures amount of
4. optometer lens = positive lens
5. fenestrated mirror
6. reaches px eyes
7. compensating lens which moves simultaneously with movable target to maintain clear image for optom
What are the 5 components of the objective modern autorefractors?
1. target (infra-red source) = visible, steady and slightly blurred to relax accommodation with infra-red component
2. photocell (infra-red detector) = interprets non-visible light reflected off retina
3. 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
4. collimating lens = provides parallel rays to eye enabling measurement of distance
5. 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
1. light source
2. condenser lens = decrease divergence of light from source
3. aperture stop = controls amount of light
4. projection lens = control amount of vergence that enters the eye
5. 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)
1. light source = tungsten filament
2. condenser lens = converges light from source
3. mirror = diverges light towards projection lens
4. projection lens = high plus lens converges light towards pupil
5. 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
6. 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?
What formula is used to measure radius in keratometry?
r = radius of curvature
d = working distance or distance between object and virtual image
h = object height
h' = image height
What is the magnitude of error from incorrectly focussed eyepiece?
1D = 0.01mm
What are the 2 main types of keratometer?
1. B&L design = variable doubling system, fixed mire position, one position instrument = both principal meridians are perpendicular to each other
2. Javal-Schiotz design = fixed doubling system, variable mire position, two position instrument = rotate the instrument for the secondary meridian which allows measurement of two non-perpendicular principle meridians
What is the main structure of the keratometer?
1. px's cornea is illuminated with the image of the mire
3. doubling prism
Define the role of the doubling prism and how it can be used to measure the height of an image
specific prism power allows a deviated image to be formed on the eyepiece. Based on the principle that the Plano prism will deviate exactly to the size of the image
Since two images will be produced, the height h2’ can be found by moving the deviated image to lightly touch the second real image that is on the axis. A linear scale on the drum knob will define the height
What principle does the radiuscope rely on and thus equivalent equation used to measure radius?
Drysdale's principle for curved images = radius of curvature = difference between self-reflecting point positions
r = S - C
S = centre of mirror surface
C = centre of mirror curvature
From Drysdale's principle, how would you operate a radiuscope?
Start high up and scroll down to capture the first image (which means rays are converging at C) and then scroll knob down to next clear image (which is at point S)
Calculate difference to determine distance
What are 5 advantages of the slit lamp biomicroscopes?
• Adjustable height, width, orientation and tilt
• Adjustable brightness
• Uniformly illuminated beam with sharp edges
• No chromatic aberrations = important because we sometimes want to check redness of eye
• Offer optic section through cornea and lens
• Provides a range of magnification of the eye
• Parfocal magnification = having corresponding focal points all in the same plane due to presence of isocenter ie having sets of objectives or eyepieces mounted so that they may be interchanged without varying the focus of the instrument
What is the main structure of the slit lamp?
1. light source = tungsten filament or halogen-filled for higher light intensity
2. condenser lens = decreases chromatic aberration
3. adjustable slit width and height
4. projection lens = controls amount of vergence that enters eye
5. image of slit projected on eye
What is the use of a doubling prism in a compound microscope?
initially, eyepiece focuses on inverted image to give a magnified image but 2 prisms can be used to re-invert image and shorten length of microscope
What 4 techniques can be used to vary magnification during slit lamp examinations?
1. Different objectives = 2 magnifications but varies between different slit lamps
2. Different eyepieces = changing objectives and eyepieces won’t give you a range of magnification so it is inconvenient
3. Zoom system = change distance between objective and eyepiece, smooth, range of magnification
4. Littmann-Galilean telescope principle = - eyepiece towards examiner, + objective lens, 2 objective lens ie slit lamp and littman Galilean, object is reflected from person’s eye, objective lens should always be positioned towards patient for a larger final image
a. Galilean telescope = plus on one side means negative on the other of the rotating drum, rotate to increase and decrease magnification, designed so that a negative lens never goes in front of the px
What is the set up of a slit lamp with the galilean telescope?
2. relay lens
3. galilean telescope
A slit lamp requires a fundus lens to be placed before the objective lens to see the retina. What are the 2 types of fundus lenses?
• High power negative (Hruby) lens = -55D, erect image of fundus, small FOV approx. 4mm in diameter excess convergence decreases so move as close as possible, then focus, then place Hruby lens
• High-power positive lens = +90D or +78D, inverted aerial image between lens and slit lamp, large FOV approx. 12mm for 9mm aperture focus in air which means diverging rays will form then put positive lens to converge the rays again, therefore place further away ie need to pull back till you see a clear image
What principle are the stereoscopes based on?
Perceive depth when something is not real = left and right eye image are different, so it is not seen at the same angle which fuses to create depth.
What happens when you increase distance between two lens?
creates more retinal disparity so more depth perception
What are the clinical applications of sterescopes?
- accommodative training
- convergence and divergence training