Chapter 14: Instruments Flashcards
Describe how a direct ophthalmoscope works
The instrument consists of a system of lenses which focus light from an electric bulb on to a mirror where a real image of the bulb filament is formed. The mirror reflects the emitted light in a diverging beam which is used to illuminate the patient’s eye. The mirror contains a hole through which the observer views the illuminated eye. The image of the bulb is formed just below the hole so that its corneal reflection does not lie in the visual axis of the observer.
Factors that increase the field of view in direct ophthalmoscopy
Hypermetropic eye
Dilated pupil
Observer is as close as possible to patients eye
Factors that reduce the field of view in direct ophthalmoscopy
Myopic eye
Small pupil pupil
Observer eye is far from patient eye/ sight hole (either the hole in mirror of ophthalmoscope or the pupil, which ever is smaller
In direct ophthalmoscopy, what is the sight hole
Either hole in the mirror of scope or the pupil, whichever is smaller
In direct ophthalmoscopy, is the field of view evenly illuminated? Why?
No. Real image formed of the light source is below the sight hole (pupil or whole in mirror, which ever is smallest). As a result, lower bit of retina may not be as brightly illuminated. Largely eliminated with newer instruments.
Secondly, total internal reflection by the lens can cause dark shadows to be seen in the periphery of the retina.
Using a direct ophthalmoscope, what happens to the image size when a patient is myopic.
(observer is emmetropic)
Larger than emmetropia
Using a direct ophthalmoscope, what happens to the image size when a patient is hypermetropic. (observer is emmetropic)
Smaller than emmetropia
In an ammetropic patient, what must we add to an ophthalmoscope to ensure that the image is focused on the observers retina. (observer is emmetropic)
A lens of equal and opposite power to the ammetropic state of the patient
Eg, if patient is myopic, need to add concave lens to the ophthalmoscope so that rays enter observers emmetropic eye parallel and can focus on their retina. If not, it forms a blur circle on the observers retina.
I’m emmetropic eye, images are seen at infinity
For a myopic patient, describe the field of view and image size when looking through a direct ophthalmoscope. (observer is emmetropic)
Increased image size but reduced field of view
For a hypermetropic patient, describe the field of view and image size when looking through a direct ophthalmoscope (observer is emmetropic)
Increased field of view but reduced image size.
Why is it difficult to examine a patient with axial myopia with a direct ophthalmoscope?
Field of view is small
True or false, you can clearly assess the fundus of a patient with high degrees of astigmatism
False, the correcting lens is spherical and can only correct one meridian at a time. As a result, image will be distorted to some degree
If a patient has ammetropia, how can they use a direct ophthalmoscope
Can either keep glasses on and add a correcting lens equal and opposite to that of the patients ammetripia. This isn’t ideal as field of view will be reduced as distance between the observer and sight hole is increased.
Alternatively, can take glasses off and add a lens of equal and opposite power to the sum of patient and observers refractive error.
Approximately how much magnification does a direct ophthalmoscope provide?
15x
M= F/4
If eye is 60D, M =15
How do we use direct ophthalmoscoped to visualise micro aneurysms
They incorperate red free filters. Green light causes the aneurysm to show up as black relative to green background
What does the image formed from an indirect ophthalmoscope look like compared to direct?
Indirect= vertically and horizontally inverted, real
Direct= vertical
True or false, indirect ophthalmoscope has a larger field of view compared to direct ophthalmoscope
True
Indirect = 25 degrees
Direct= 6 degrees
True or false, indirect ophthalmoscope has a larger magnification compared to direct ophthalmoscope
False.
Direct= 15x
Indirect= 3-5x
Remember, as magnification increased, FOV decreases.
How does an indirect ophthalmoscope form an image and what kind of image is it.
Light from a lamp mounted on a head piece is refracted through a condensing lens (+20D or +30D) into the patients retina. The light is reflected off the patients retina and is then refracted through the same condensing lens. This forms a real inverted image between the observer and the condensing lens.
Using an indirect ophthalmoscope, where is light focused on the patients retina?
Vitreous (L)
In indirect ophthalmoscopy, how does the refractive state of the patients eye effect the field of view. Picture below is of direct ophthalmoscope for comparison
Opposite effect as direct ophthalmoscopy (because rays are divergent)
Myopic eye = large FOV
Hypermetropic eye = small FOV
What is the sight hole of an indirect ophthalmoscope?
-The observers pupil, not the subjects pupil!!!
-Only light passing through the observers sight hole (image) influences the size of the field of view.
-In direct, it’s the slit in the mirror or the subjects pupil, whichever is smaller.
Basically, direct and indirect ophthalmoscope have opposite properties
What factors influence the field of view when using an indirect ophthalmoscope
The observes pupil size and the aperture of the condensing lens.
Fill in the gap.
With indirect ophthalmoscopy, the field of illumination is determined by …1….
Meanwhile, the field of view is determined by …2….. and …3…..
1) subjects pupil size
2)observer pupil size
3) condensing lens aperture