Chapter 9: Refraction By The Eye Flashcards
In the thick lens formula, what are the principle points/ planes?
They are hypothetical lines at which any light incident on p1, leaves p2 at exactly the same vertical distance from the principle axis.
NB: the light ray can then be deviated at p2 unliked the nodal point
What is the nodal point in the thick lens formula
Intersect between principle plane and axial plane.
Light incident on N1 leaves N2 undeviated.
NB: Nodal and principle points only coincide if the medium on either side of the lens are the same.
Define anterior or posterior vertex focal length, aka AVFL or PVFL
Using a focimeter, you are not given the exact focal length of lens, instead you are given avfl and pvfl.
NB AVFL and PVFL are not equal.
In the thick lens theory, where does P1 and P2 lie in a concave and convex meniscus lens respectively
Concave = infront of the lens
Convex = behind the lens
(towards F1)
As a results, focal lengths are different from that produced from focimeter which use front and back vertex distances.
Define back vertex power
Measured in dioptres, it is the reciprocal of the posterior vertex focal length
Define equivalent power of the lens
The true power of a lens. Using a focimeter, you are calculating back vertex power based on the pvfl which is not the same as the the true focal length. This can result in significant visual disturbances in high powered lenses.
Optoms have a tables to make corrections
For a lens to focus an image on the retina, the second principle focus (f2), must coincide with what?
The far point of the eye is equal to f2 of the lens
This allows parallel rays to focus on the retina.
What is the refractive index of the cornea
1.376
What is the refractive index of the lens
1.386–1.406
WHat is the total refractive power of the eye
+58.64 D
In gulstrands schematic eye, where are the nodal points
They straddle the posterior pole of the crystalline lens.
NB, principle points and nodal points do not coincide because the refractive material on either side of the refractive system has a different refractive index (air vs vitreous).
Nodal points are closer to the side with the higher refractive index
Why does a posterior pole cataract effect vision so significantly?
Nodal points of the eye straddle it
Using gulstrands reduced eye, what are the cardinal point measurements
F1
P
N
F2
NB: all measurements are from the anterior cornea
F1= -15.7mm
P= 1.35mm
N= 7.08mm
F2= 24.13mm
What is the power of the lens in the eye
15D
Outside of the eye it is 19D
What is the overall power of an aphakic eye
43D
Total eye = 58D
Lens = 15 D
How to calculate the size of a retinal image.
H= tan(alpha) *F1
H= image height
Alpha= angle substending image at nodal point
F1 = first focal point
As an object approaches the eye, what happens to the image.
Gets bigger as angle substended by object and nodal point get larger.
And tan(alpha) gets bigger). Therefore h*tan(alpha) is bigger
During accommodation, what happens to the anterior surface of the lens
Bulged forward towards the cornea.
Define the far point of an eye
In a relaxed eye without accommodation, what is the largest distance at which an rays of light fall on the retina. In a normal eye, this is infinity.
NB when ciliary muscle are relaxed, suspensory ligaments are coiled up and so lens is flat
Define the near point of the eye
When the eye has accommodated to its maximum (ciliary muscles contracted and lens is at its thickest and most anterior), what is the closest an object can be to the eye for it to focus on the retina
Define range of accommodation
Near point - far point
Define static refraction
Power of the eye in dioptres at rest, ie not accomodated
What is dynamic refraction
Power of the eye in dioptres when eye is accomodated
What is the amplitude of accommodation
1/near point - 1/far point
How do you calculate the power required to focus on an intermediate point between the far and near point?
Power needed= 1/intermediate point distance - 1/farpoint
When an emmitropic eye has to focus on an object 1m away, what is the accommodative power
1/1- 1/0= 1dioptre
NB 1/0 as the far point is infinity Ina n emmitropic eye
What is the normal range for accommodative convergence/accommodation ratio (AC/A)
3:1-5:1
How to calculate AC/A ratio (Accommodative Convergence to Accommodation Ratio) using the heterophoric method
Calculated heterophoria method is based on the difference in deviation between distance phoria & near phoria and this method takes account of patient Interpupillary distance (IPD), which is an important factor if accommodative changes are induced by placing the target at near.
Measures ocular deviation
AC/A = interpupilary distance + ((Dd - Dn)/ D)
IPD is the interpupillary distance in cm
Dd is the ocular deviation for distance
Dn is the ocular deviation for near
D is the near fixation distance in dioptres.
(By convention a positive (+) value denotes an esodeviation and a negative (–) value an exodeviation.)
https://o.quizlet.com/zpleDQ8r8OC1nb523QrSHw.png
How to calculate AC/A ratio (Accommodative Convergence to Accommodation Ratio) using the gradient method
Gradient method which uses a minus lens rather than a near object to stimulate accommodation.
AC/A = (Dd - Dn)/D
Dd is the ocular deviation for distance
Dn is the ocular deviation for near
D power of minus lens using to induce accommodation
What is convergence esotropia and how is it managed.
eyes are straight for distance but break down to a convergent squint for near.
Convergence excess esotropia may be controlled with bifocal spectacles, the distance portion incorporating the full hypermetropic distance correction and the near portion having further plus power so that little or no accommodation is required for near, and binocular near vision is facilitated.
If surgery is appropriate, recession of both medial recti is effective in convergence excess esotropia
How is esotropia without convergence managed
If surgery is appropriate, recession of both medial recti is effective in convergence excess esotropia whereas recession of one medial rectus with resection of the ipsilateral lateral rectus is more effective in esotropia without convergence excess.
Vital to differential between eso with and without convergence as management differs
What are catoptric images
catoptric images or Purkinje–Sanson images are reflections of light formed on the surface of
1 anterior cornea
2 posterior cornea
3 anterior lense
4 posterior lens
In catoptric images or Purkinje–Sanson images, describe what images 1 2 and 3 look line
Are composed of
1 anterior cornea
2 posterior cornea
3 anterior lens
All act as convex mirrors
Images are therefore virtual and erect
In catoptric images or Purkinje–Sanson images, describe what image 4 look line
Image 4 is from the posterior aspect of the lens which acts like a concave mirror
The image is therefore image is inverted and real
How does refraction effect purkinje images when making measurements?
When using images II, III and IV to make measurements, account must be taken of refraction of the reflected light as it re-emerges from the eye.
In the diagram below, images appear as though they are coming from behind the cornea and within the lens for refracted imaged 1-4 (black)
In reality, they are deeper (red)
1 and 2 are just behind anterior lense
3 is in the vitreous
4 is inverted and in the anterior lens substance (still inverted)
What is the first catoptric image used for and how is it measured?
Image 1 = anterior corneal surface
Used to measure surface curvature using placidos disks and radius using a keratometer
What are catoptric images 3 and 4 used for
They are reflections from the anterior and posterior surface of the lens respectively. These images can be used to assess the structure of the lens during accomodation
