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
Why is the condensing lens used in indirect ophthalmoscopy aspheric
To reduce spherical aberration.
When using an indirect ophthalmoscope, why does the observer need to accommodate?
The the image formed is inbetween the condensing lens and the observers eye.
Let’s say the condensing lens is 13D, the image will be focused at F2 of the lens which is 8cm away from the lens (1/13). The observer holds the condensing approve 40-50cm away from them. As a result, the rays that were focused on F2 are now divergent.
For the light to be focused on the observers retina, they need to accommodate of have correcting lenses added.
How do you calculate the linear magnification using a indirect ophthalmoscope
Focal length of the condensing lens (CF)/ distance between the nodal point and the subjects retina (BN).
What happens to the magnification caused by a condensing lens used in indirect ophthalmoscopy when the lens power is increased
As lens power increases, magnification decreases
13D= 5x mag
20D= 3x mag
When using an indirect ophthalmoscope, where does the image lie, relative to the condensing lens for a myopic and hypermetropic patient.
Myopic, between lens and F2
Hyperopic, between F2 and observer
When using an indirect ophthalmoscope, where does the image lie, relative to the condensing lens for a myopic and hypermetropic patient.
Myopic, between lens and F2
Hyperopic, between F2 and observer
In a myopic patient, what happens when the condensing lens is moved away from the patient
Image gets bigger
In a hyperopic patient, what happens when the condensing lens is moved away from the patient
Image gets smaller
Which ophthalmoscope is preferred when assessing and operating on patients with retinal detachment?
Indirect as it gives a better FOV.
Laser energy can also be directed through if for photocoagulation .
What do we use retinoscopy for?
Objective measure of the refractive state of the eye
What are the 3 stages of retinoscopy
- Illumination
- Reflex
- Projection
During the illumination stage of retinoscopy, when a plane mirror is moved, what happens to the illumination on the retina
Light on the retina moves in the same direction as the direction of mirror
During the illumination stage of retinoscopy, when a concave mirror is moved, what happens to the illumination on the retina
Light on the retina moves in the opposite direction to the movement of the concave mirror
In retinoscopy, what is the neutral point?
Also known as the point point of reversal, it is when the patients far point coincides with with the observers nodal point. In this case, observer sees diffuse bright red reflex that doesn’t move.
In the projection stage of retinoscopy, what happens to the light reflex in a myopic patient
Goes in the opposite direction to the illumination, against the motion.
Describe the steps of retinoscopy
Look at the diagram. With the motion = hypermetropic
Against the motion= myopic.
Too much power of a convex lens makes someone myopic = start getting against the motion for power above the neutralisation point.
Don’t forget to take away the working distance having written the entire formula
What is placidos disk what do we use it for
Concentric black and white circle, with a central aperture containing a convex lens that the observers looks through. Rings are reflected from the circles onto the eye and then into the aperture for the observer to see. Allows you to see regularity of anterior cornea. Astigmatism. Eg, smaller the image with smaller gaps between circles= smaller radius of ant cornea.
What is the formula to calculate the radius of the cornea from a keratometer
R= 2u * I/O
U= constant (distance between telescope and the cornea)
I= image size
O = object size
How does the von Helmholtz keratometer differ from the javal schiotz keratometer
VH: object is fixed and the image is adjusted to measure radius
JS: object size is varied.
Using a javal Schoitz keratometer, how are images doubled?
Wollaston prism is used. It is made of 2 quartz prisms cemented on each other.
Using a javal schoitz keratometer, the corneal curvature is measured accurately when…
The doubled images touch each other(steps touch rectangle). Can do this by adjusting the mires
NB: JS = size adjusted
How can a javal schoitz keratometer be used to assess astigmatism
Mires can be adjusted relative to each other. Each step in the Mire corresponds to 1D. If the images match up perfectly in one meridian and overlap by 1.5 steps in the other meridian, there is 1.5D of astigmatism.
NB: When an astigmatic cornea is examined, the two images are displaced vertically in all but the two principal meridians of the cornea.
Describe computer videokeratograpgy (CVK)
An automated method of assessing corneal topography.
Placido’s disk is projected onto cornea and the reflection is converted into a digital image.
Where the cornea is steeper (ie lower radius), the rings appear more bunched together.
From this, a colour coded analysis is produced where the Same diopteric power is shown as the same colour
Describe how a compound microscope
Composed of 2 convex lenses, separated by the sum of their focal lenghts.
1. Object is placed outside of the anterior focal point (F0) of the objective lens. This forms a real inverted image.
2. The real inverted image lies on/ close to the focal point of the eyepiece lens (Fe). This makes the eyepiece lens act as a loupe.
3. An image is formed that is vertically and horizontally inverted behind the object. THis can be corrected using a Porro prism.
nb gallilean telescope produces an erect magnified image
What do we mean by smooth change when zooming in using a compound microscope. How can we achieve this?
The change in zoom doesn’t change the position of the object or the image.
This can be achieved by placing a moveable concave lens between the eyepiece and objective lens. The more mobile lenses placed in the system, the less the image shifts (from Elkington p196)
True or false, in a slit lamp, the focal point of the lens meets with the focus of the light.
True
How are the microscope tubes in a slit lamp shortened and how what is used to magnify images on a slit lamp?
Prism are used to shortened the tubes. They also invert the image so that it is upright and and not flipped sideways (remember, slit lamp produces images that are vertically and horizontally inverted: shown in image).
Images are magnified by incorporating several Galilean telescopes into the system.
Using a slit lamp, what is direct focal illumination?
Slit been focused on part of the eye directly under inspection
Using a slit lamp, what is diffuse illumination?
Light is all over the area of focus. Can be used to look at anterior capsule
Using a slit lamp, what is lateral illumination?
Aka, indirect illumination. Eg, beam of light on the margin of the pupil makes the sphincter muscles more apparent
Using a slit lamp, what is retro illumination?
Using a structure as a reflective surface so that the structure anterior to it can be illuminated from the back
Using a slit lamp, what is speculated reflection?
Way of assessing corneal endothelium. Patient is asked to look in between the angle of illumination and the microscope.
Using a slit lamp, what is sclerotic scatter?
When light is directed on the limbus, the whole limbal area glows. This is because light is reflected between the two internal limiting surfaces causing light to be scattered circumferentially around limbus. It is brightest if light is shone at 3 o’clock on the limbus.
How can we assess a patients fundus using a slit lamp
Need a lens to allow light to focus onto the slit lamp.
This is because the refractive power of the lens and cornea renders light emerging from them parallel and so no image falls in the focal range of the slit lamp. For this to happen, light needs to be divergent.
A lens need to refract light to do this like hruby lens
What is hruby lens used for, where is it placed, what is it dioptric power and what kind of image does it form?
Used to look at fundus using slit lamp.
Placed directly in front of the eye.
Power of -58.6D
Image is virtual, erect, in front of the fundus and diminished
What is rodenstock panfunduscope and how does it work.
Contact lens that gives a wide field of view but poor magnification. Used in conjunction to a slit lamp.
Composed of a high powered convex lens which forms a real inverted images inside a spherical glass element. The spherical element then flattens and directs the image towards the observer.
NB: The panfunduscope contact lens forms a real, inverted image of the fundus.
Using the Goldman and Schmidt applanation tonometer, how do we assess IOP
Assessor looks through applanation head, and adjusts the pressure so that inner aspect of each circle overlap, that signifies a surface contact of 3.06mm
Using the Goldman and Schmidt applanation tonometer, what happens if a patient has astigmatism and how can we accommodate of this?
Operator wont see semicircles but rather eliptical shapes. This will caused 1mmHg pressure error for every 4D of astigmatism!
This can be corrected if contact is made 43 degrees to the meridian of the minus cylinder
What is a pachymeter and how does it work?
Intrustment used to measure corneal thickness.
Uses purkinje sanson images.
Image 1 and 2 used to to measure corneal thickness.
Images 2 and 3 used to measure anterior chamber depth.
Describe how jaeger pachymeter works
Similar concept to applanation tonometry
Eye is illuminated by perpendicular light and is seen at 40 degrees.
Consists of 2 plates. Upper one can be tilted.
To measure corneal thickness, adjust upper plate tilt so that anterior cornea of one image lines up with poster cornea of second image.
To measure anterior chamber depth. Adjust tilt of upper plate so that posterior cornea of one image lines up with anterior lens of the other image.
NB all images are doubled.
Describe the Scheiner principle
Very early method of finding a patients far point.
2 pinholes are placed infront of a pupil with light coming from infinity.
If the patient is emmetropic, they see 1 dot.
If they see 2, they are ammattropic.
Can adjust the distance of the light source until they see 1 dot.
How does an optemeter work and what does it measure?
Instrument used to measure the refractive power of the eye.
Convex lens is placed infront of the eye. Infront of the lens is a moving target.
If the moving target is at f1 of the lens, light hits the cornea parallel and focuses on the retina of an emmetropic patient. If image is not clear, they have some form of ammetropia
The machine is calibrated such that the position of the target corresponds to the power of the eye.
