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Clinical Skills H > optics of indirect ophthalmoscope > Flashcards

optics of indirect ophthalmoscope Flashcards

1
Q

what are the two components of an indirect ophthalmoscope

A
  1. illumination system

2. observation/imaging system

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2
Q

what is the purpose of the illumination system in an indirect ophthalmoscope

A

you need light in order to see the imaging system i.e. its how you illuminate the retina (imaging system)
but at the same time you need to control the light that is illuminating our object

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3
Q

what is the purpose of the illumination system in an indirect ophthalmoscope

A

(you need light) in order to see the imaging system i.e. its how you illuminate the retina (imaging system)
but at the same time you need to control the light that is illuminating our object

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4
Q

what is the purpose of an observation system in indirect ophthalmoscopy

A

its how we are able to see the retina

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5
Q

what is the imaging system in indirect ophthalmoscopy

A

the retina

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6
Q

how are you able to see the retina with direct ophthalmoscopy

A

two eyes are put in front of each other = should be able to see the retina, given there is sufficient light

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7
Q

with direct ophthalmoscopy, what 2 components determines the amount of light that goes from the patient to the examiner

A

a sight hole and an aperture

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8
Q

what is a consequence of being further away with direct ophthalmoscopy

A

the further away we are from the patient, the less light goes into the patients eye and the less we can see

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9
Q

what is the main problem with direct ophthalmoscopy, and explain how it occurs

A

observer can only see what (i.e. part of fundus) goes through the sight hole

light emerging from inside of the patients pupil, depending on where it is coming from in the retina is coming to our FOV & the moment it exits the patients eye it goes off axis, so most of it will be going away from the optical axis.
so different beams of light from different parts of the retina will be going away from the optical system

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10
Q

what is the main problem with direct ophthalmoscopy, and explain why it occurs

A

observer can only see what (i.e. part of fundus) goes through the sight hole

because, light emerging from inside of the patients pupil, depending on where it is coming from in the retina is coming to our FOV & the moment the light exits the patients eye it goes off axis, so most of it will be going away from the optical axis.
so different beams of light from different parts of the retina will be going away from the optical system

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11
Q

what is a possible solution to the observer only being able to see what goes through the sight hole with direct ophthalmoscopy and how can this solution still not be 100% possible

A

to move the ophthalmoscope as close as possible to the patient’s eye as this improves FOV

but this is still not enough and the only way to fix this is bringing the pupils close together and on top of each other e.g. brining the sight hole of the ophthalmoscope directly onto the pupil in order to collect all of the light beams emerging from the patient’s eye, but this is only theory as the pupil of the eye is behind the cornea

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12
Q

what is the imaging system in indirect ophthalmoscopy

A

(the object) the retina/fundus

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13
Q

what is the observation system of an indirect ophthalmoscope

A

a high powered +ve lens e.g. volk lens, held at a certain distance from the patients eye

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14
Q

where does light emerge from with indirect ophthalmoscopy

A

from the patient’s retina which is the object (fundus)

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15
Q

how is the image of the object (fundus) viewed with indirect ophthalmoscopy

A
  • the object (O - fundus) goes into the +ve lens
  • an image is formed (O’) in mid air of focal length/point of the volk lens ( formed inbetween the volk lens and the observer’s eye)
  • = an intermediate image and with 2 optical systems.
  • the image of one optical system becomes an object for the other
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16
Q

how is the image of the object (fundus) viewed with indirect ophthalmoscopy

A
  • the object (O - fundus) goes into the +ve lens
  • an image is formed (O’) in mid air of focal length/point of the volk lens ( formed inbetween the volk lens and the observer’s eye)
  • = an intermediate image and with 2 optical systems.
  • the image of one optical system becomes an object for the other hence indirect
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17
Q

how is an indirect image formed by indirect ophthalmoscopy

A

the observer is looking at the patient’s retina but looking at an image formed by the ophthalmoscope lens

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18
Q

at which distance is the intermediate image formed with an indirect ophthalmoscope

A

at the focal point of the ophthalmoscope lens i.e. for a +50D lens, the intermediate image will be 2cm away from the lens

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19
Q

how is the intermediate image formed/viewed by the observer

A

intermediate image is inverted

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20
Q

what is the role of the observation system in indirect ophthalmoscopy

A

to relay the image of the retina of the patient to the examiner
the further away the observation system/lens = the larger the diameter needs to be

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21
Q

name 2 ways of increasing FOV with indirect ophthalmoscopy

A
  • larger diameter lens

- larger power lens

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22
Q

what is the relationship between the examiner’s and patients’ pupil during indirect ophthalmoscopy

A

patient and examiner’s pupils are conjugate to each other

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23
Q

what is the only way to collect all the light in indirect ophthalmoscopy

A

patient’s and examiner’s pupils must be conjugate with each other

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24
Q

as well as pupils of the examiner and patient having to be conjugate with each other, what else has to be conjugate

A

the retina of the patient but be conjugate with the examiner’s

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25
Q

where on the examiner is the image of the patient’s pupil formed

A

on the examiner’s pupil

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26
Q

as well as pupils of the examiner and patient having to be conjugate with each other, what else has to be conjugate

A

the retina of the patient but be conjugate with the examiner’s

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27
Q

where on the examiner is the image of the patient’s pupil formed and how so

A

on the examiner’s pupil - two if the rays at the edge of the patient’s pupil diverge and reach the examiner’s pupil

28
Q

what size is the examiner’s pupil in relation to the patient’s pupil in indirect ophthalmoscopy

A
  • examiner’s pupil is optically smaller than the patient’s

- patient’s pupil is magnified so is larger

29
Q

what is the result of the examiner’s pupil being optically smaller and the patient’s pupil being magnified so larger in indirect ophthalmoscopy

A

only light rays emerging from the part of the patient’s pupil that is the diameter of the examiner’s pupil that is smaller can be seen, so can only see a small part of the patient’s pupil

30
Q

what does the diameter if the ophthalmoscopy lens determine

A

the field of view

31
Q

although a larger lens = larger FOV, what is a drawback to producing a larger lens

A

this produces more spherical aberrations and in the periphery, they will have non paraxial rays

32
Q

what does moving the ophthalmoscopy lens closer to the eye do

A

gives a larger FOV

33
Q

list all of the things that can be done to the ophthalmoscopy lens to give a larger FOV in indirect ophthalmoscopy

A
  • larger diameter ophthalmoscopy lens
  • moving the lens closer
  • larger power of ophthalmoscopy lens
34
Q

although a larger power ophthalmoscopy lens results in larger FOV, whittles does it result in

A

smaller magnification hence smaller image size h’

so can have both high mag/power and large FOV/image i.e. its a trade off

35
Q

what is the equation for the size of an image, if the object is infinitely far away

A

h’ = -ntani/F

36
Q

as well as pupils of the examiner and patient having to be conjugate with each other, what else has to be conjugate with indirect ophthalmoscopy

A

the retina of the patient but be conjugate with the examiner’s

37
Q

where on the examiner is the image of the patient’s pupil formed and how so in indirect ophthalmoscopy

A

on the examiner’s pupil - two if the rays at the edge of the patient’s pupil diverge and reach the examiner’s pupil

38
Q

what is the result of the examiner’s pupil being optically smaller and the patient’s pupil being magnified so larger in indirect ophthalmoscopy, in indirect ophthalmoscopy

A

only light rays emerging from the part of the patient’s pupil that is the diameter of the examiner’s pupil that is smaller can be seen, so can only see a small part of the patient’s pupil

39
Q

what does the diameter if the ophthalmoscopy lens determine in indirect ophthalmoscopy

A

the field of view

40
Q

although a larger lens = larger FOV, what is a drawback to producing a larger lens, in indirect ophthalmoscopy

A

this produces more spherical aberrations and in the periphery, they will have more non paraxial rays

41
Q

what does moving the ophthalmoscopy lens closer to the eye do in indirect ophthalmoscopy

A

gives a larger FOV

42
Q

although a larger power ophthalmoscopy lens results in larger FOV, what else does it result in, in indirect ophthalmoscopy

A

smaller magnification hence smaller image size h’

so can have both high mag/power and large FOV/image i.e. its a trade off

43
Q

what will satisfy L’ = L + F in indirect ophthalmoscopy

A

the patient’s pupil must be conjugate with the examiner’s pupil

44
Q

what do you have an image of, if your pupil is off centre from the patient’s pupil (not conjugate) and how is this, in indirect ophthalmoscopy

A

have an image of the light source and image of the patient’s retina

the light will first diverge, then converge to a point of the patient’s pupil

45
Q

with binocular indirect ophthalmoscopy, how is it set up

A
  • one of patient’s eye and two of the examiner’s
  • both pupils are conjugate with each other
  • both eyes of the examiner at looking at the object (fundus) which forms an intermediate image O’
46
Q

with binocular indirect ophthalmoscopy, how is it set up

A
  • one of patient’s eye and two of the examiner’s
  • both pupils are conjugate with each other
  • both eyes of the examiner at looking at the object (fundus) which forms an intermediate image O’
47
Q

what can you not control with binocular indirect ophthalmoscopy and what problems can this cause

A

the pd’s of the examiner’s eyes

image of the right and left pupil ends up in the periphery which causes a problem because the examiner can only see whatever light goes into the patches formed by their two pupils (which are also optically smaller than the patient’s) which rest on the pupil border of the patient , as a result the examiner can see most of the patient’s iris which is no good

48
Q

what is a solution to the problem where the examiner is only being able to view most of the patient’s iris in binocular indirect ophthalmoscopy as the pd’s cannot be altered

A

if viewing binocularly, need to add mirrors in order to see into the patient’s pupil, as mirrors has the effect of bringing the examiner’s pupils closer together hence allowing to see into the pupil, as a result both of the examiner’s pupils are collecting light coming from the patient’s pupil

49
Q

what affect does mirrors have on the angle of the light going into the examiner’s pupils in binocular indirect ophthalmoscopy

A

mirrors makes the angle smaller, so both of the examiner’s pupils are collecting the light coming from the patient’s pupil

50
Q

what is the binocular indirect ophthalmoscope composed of

A
  • head piece
  • light coming out from the middle
  • mirrors
51
Q

what does a slit lamp use instead of mirrors in order to bring the pupil images of the examiner together in binocular indirect ophthalmoscopy

A

prisms which has the same affect as mirrors

the slit lamp looks at the intermediate image O’ in this case and magnifies it

52
Q

what creates a corneal reflection with binocular indirect ophthalmoscopy

A

all the light that it focussed onto one point = corneal reflection

53
Q

what must the corneal reflection not obscure in binocular indirect ophthalmoscopy

A

the viewing channels

54
Q

what is a result of the corneal reflection overlapping the viewing channels in binocular indirect ophthalmoscopy

A

a corneal reflex, therefore corneal reflection (i.e. illumination) and both viewing channels must be separate

55
Q

how do you avoid getting a corneal reflex in binocular indirect ophthalmoscopy

A

make sure the light source doesn’t over lap the pupils

56
Q

what happens when the light source is not focussed correctly during binocular indirect ophthalmoscopy

A

corneal reflex will enter the viewing channels

57
Q

what 2 things is the correct distance and position of the indirect ophthalmoscope important for

A
  • max FOV

- no corneal reflections

58
Q

what can be done to a patient to more likely avoid corneal reflection overlapping the viewing channels

A

pupil dilation is often necessary

59
Q

what can scatter from other ocular media do

A

it can still enter the viewing path

60
Q

what powers are volk/ophthalmoscopy lenses available in

A

between +15D (31 deg) and +40D (69 deg) = much larger FOV than direct ophthalmoscopy

61
Q

how much FOV in degrees can a +15D ophthalmoscopy lens produce

A

31 degrees

62
Q

how much FOV in degrees can a +40D ophthalmoscopy lens produce

A

69 degrees

63
Q

what type of lens does a ophthalmoscopy/volk lens have and what are the advantages to this lens type

A
  • high index, glass and aspheric surfaces
  • to maximise lens diameter and power while minimising aberrations
  • low dispersion glass: minimises chromatic aberrations (if not there will be different wavelengths refracted by different amounts)
  • AR coating and colour filters: reduces reflections, scatter, chromatic aberrations and harmful radiation
64
Q

the principle of what other imaging device is similar to indirect ophthalmoscopy but with more complicating optics

A

optics of the fundus camera is very similar in principle, but with more complex optics and recording of image

65
Q

define indirect ophthalmoscopy

A

a positive lens which is used to form a real inverted image of the patient’s fundus. this intermediate aerial image is viewed by the observer

Clinical Skills H (18 decks)

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