optics of indirect ophthalmoscope Flashcards
what are the two components of an indirect ophthalmoscope
- illumination system
2. observation/imaging system
what is the purpose of the illumination system in an indirect ophthalmoscope
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
what is the purpose of the illumination system in an indirect ophthalmoscope
(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
what is the purpose of an observation system in indirect ophthalmoscopy
its how we are able to see the retina
what is the imaging system in indirect ophthalmoscopy
the retina
how are you able to see the retina with direct ophthalmoscopy
two eyes are put in front of each other = should be able to see the retina, given there is sufficient light
with direct ophthalmoscopy, what 2 components determines the amount of light that goes from the patient to the examiner
a sight hole and an aperture
what is a consequence of being further away with direct ophthalmoscopy
the further away we are from the patient, the less light goes into the patients eye and the less we can see
what is the main problem with direct ophthalmoscopy, and explain how it occurs
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
what is the main problem with direct ophthalmoscopy, and explain why it occurs
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
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
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
what is the imaging system in indirect ophthalmoscopy
(the object) the retina/fundus
what is the observation system of an indirect ophthalmoscope
a high powered +ve lens e.g. volk lens, held at a certain distance from the patients eye
where does light emerge from with indirect ophthalmoscopy
from the patient’s retina which is the object (fundus)
how is the image of the object (fundus) viewed with indirect ophthalmoscopy
- 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
how is the image of the object (fundus) viewed with indirect ophthalmoscopy
- 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
how is an indirect image formed by indirect ophthalmoscopy
the observer is looking at the patient’s retina but looking at an image formed by the ophthalmoscope lens
at which distance is the intermediate image formed with an indirect ophthalmoscope
at the focal point of the ophthalmoscope lens i.e. for a +50D lens, the intermediate image will be 2cm away from the lens
how is the intermediate image formed/viewed by the observer
intermediate image is inverted
what is the role of the observation system in indirect ophthalmoscopy
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
name 2 ways of increasing FOV with indirect ophthalmoscopy
- larger diameter lens
- larger power lens
what is the relationship between the examiner’s and patients’ pupil during indirect ophthalmoscopy
patient and examiner’s pupils are conjugate to each other
what is the only way to collect all the light in indirect ophthalmoscopy
patient’s and examiner’s pupils must be conjugate with each other
as well as pupils of the examiner and patient having to be conjugate with each other, what else has to be conjugate
the retina of the patient but be conjugate with the examiner’s
where on the examiner is the image of the patient’s pupil formed
on the examiner’s pupil
as well as pupils of the examiner and patient having to be conjugate with each other, what else has to be conjugate
the retina of the patient but be conjugate with the examiner’s
where on the examiner is the image of the patient’s pupil formed and how so
on the examiner’s pupil - two if the rays at the edge of the patient’s pupil diverge and reach the examiner’s pupil
what size is the examiner’s pupil in relation to the patient’s pupil in indirect ophthalmoscopy
- examiner’s pupil is optically smaller than the patient’s
- patient’s pupil is magnified so is larger
what is the result of the examiner’s pupil being optically smaller and the patient’s pupil being magnified so larger in indirect ophthalmoscopy
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
what does the diameter if the ophthalmoscopy lens determine
the field of view
although a larger lens = larger FOV, what is a drawback to producing a larger lens
this produces more spherical aberrations and in the periphery, they will have non paraxial rays
what does moving the ophthalmoscopy lens closer to the eye do
gives a larger FOV
list all of the things that can be done to the ophthalmoscopy lens to give a larger FOV in indirect ophthalmoscopy
- larger diameter ophthalmoscopy lens
- moving the lens closer
- larger power of ophthalmoscopy lens
although a larger power ophthalmoscopy lens results in larger FOV, whittles does it result in
smaller magnification hence smaller image size h’
so can have both high mag/power and large FOV/image i.e. its a trade off
what is the equation for the size of an image, if the object is infinitely far away
h’ = -ntani/F
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
the retina of the patient but be conjugate with the examiner’s
where on the examiner is the image of the patient’s pupil formed and how so in indirect ophthalmoscopy
on the examiner’s pupil - two if the rays at the edge of the patient’s pupil diverge and reach the examiner’s pupil
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
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
what does the diameter if the ophthalmoscopy lens determine in indirect ophthalmoscopy
the field of view
although a larger lens = larger FOV, what is a drawback to producing a larger lens, in indirect ophthalmoscopy
this produces more spherical aberrations and in the periphery, they will have more non paraxial rays
what does moving the ophthalmoscopy lens closer to the eye do in indirect ophthalmoscopy
gives a larger FOV
although a larger power ophthalmoscopy lens results in larger FOV, what else does it result in, in indirect ophthalmoscopy
smaller magnification hence smaller image size h’
so can have both high mag/power and large FOV/image i.e. its a trade off
what will satisfy L’ = L + F in indirect ophthalmoscopy
the patient’s pupil must be conjugate with the examiner’s pupil
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
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
with binocular indirect ophthalmoscopy, how is it set up
- 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’
with binocular indirect ophthalmoscopy, how is it set up
- 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’
what can you not control with binocular indirect ophthalmoscopy and what problems can this cause
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
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
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
what affect does mirrors have on the angle of the light going into the examiner’s pupils in binocular indirect ophthalmoscopy
mirrors makes the angle smaller, so both of the examiner’s pupils are collecting the light coming from the patient’s pupil
what is the binocular indirect ophthalmoscope composed of
- head piece
- light coming out from the middle
- mirrors
what does a slit lamp use instead of mirrors in order to bring the pupil images of the examiner together in binocular indirect ophthalmoscopy
prisms which has the same affect as mirrors
the slit lamp looks at the intermediate image O’ in this case and magnifies it
what creates a corneal reflection with binocular indirect ophthalmoscopy
all the light that it focussed onto one point = corneal reflection
what must the corneal reflection not obscure in binocular indirect ophthalmoscopy
the viewing channels
what is a result of the corneal reflection overlapping the viewing channels in binocular indirect ophthalmoscopy
a corneal reflex, therefore corneal reflection (i.e. illumination) and both viewing channels must be separate
how do you avoid getting a corneal reflex in binocular indirect ophthalmoscopy
make sure the light source doesn’t over lap the pupils
what happens when the light source is not focussed correctly during binocular indirect ophthalmoscopy
corneal reflex will enter the viewing channels
what 2 things is the correct distance and position of the indirect ophthalmoscope important for
- max FOV
- no corneal reflections
what can be done to a patient to more likely avoid corneal reflection overlapping the viewing channels
pupil dilation is often necessary
what can scatter from other ocular media do
it can still enter the viewing path
what powers are volk/ophthalmoscopy lenses available in
between +15D (31 deg) and +40D (69 deg) = much larger FOV than direct ophthalmoscopy
how much FOV in degrees can a +15D ophthalmoscopy lens produce
31 degrees
how much FOV in degrees can a +40D ophthalmoscopy lens produce
69 degrees
what type of lens does a ophthalmoscopy/volk lens have and what are the advantages to this lens type
- 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
the principle of what other imaging device is similar to indirect ophthalmoscopy but with more complicating optics
optics of the fundus camera is very similar in principle, but with more complex optics and recording of image
define indirect ophthalmoscopy
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
