prescription and lenses ( lecture 2) Flashcards
What does a lens do?
can describe the action of a lens by looking at its focal length and principle focus
What is a principle focus?
point at which it focuses light coming from infinity
What is the 1st principle focus?
when light is travelling from right to left.
What is the 2nd principle focus (positive lens)?
(WE ASSUME LIGHT TRAVELS FROM LEFT TO RIGHT) the light converges to a real image at its 2nd principal focus (F’)
(light from infinity represents straight parallel lines (left) and passes through lens and light converges at a point (F’) )
What is F’?
2nd principle focus
What is f’?
2nd principle focal length- describes distance from lens to its principle focus (f’)
WHAT IS a focal length?
focal length of a lens is from the vertex of the lens to th 2nd principle focal point
What is a negative lens?
diverges light
What is the 2nd principle focus (negative lens)?
its always negative
-to find focal point need to find virtual point from where the light diverges from.
What is an optical axis?
where light passes through lens and deviated.
What does it mean when you have a lens with short focal length?
it is more powerful, stronger, steeper curved lens
What does it mean when you have a lens with a longer focal length?
it is a less powerful, weaker , a flatter curved lens
What is the power of lens measured in?
D (Dioptres)
What is the power of a lens in dioptres?
the reciprocal of its focal length in metres (m) D=1/f’
How do we measure Dioptres?
0.25D steps
What do dioptres represent?
the power of spheres or cylinders
How must D be written?
with a + or - sign and MUST be to 2 DECIMAL PLACES
What is emmetropia?
light from point objects in all distances are focused on retina as a point image - which means image is clear
-No optical correction required to patient
What is the axial length of eye?
24mm
Overall power of optical system?
58D
What is hypermetropia (long sightedness)?
-condition when the incident light is focused behind the retina - blurred image
What are the 2 reasons of hypermetropia?
- optical power of system is too weak to focus light
- overall dimensions of light are too short
How do we correct hypermetropia?
correct with a positive spherical lens
What is spherical lens?
has the same power in every direction
What is the principle of this correction?
to make sure the 2nd focal point of lens co insides with the far point of the eye
What does the positive spherical lens do for
increases convergence and brings the light forward onto the retina (image on retina) as focal point is brought forward.
What is accomodation?
- some patients have ability to increase optical power of the lens by making it thicker and convex
- ability to do this reduces from age
What is the effect of accomodation?
can cause eye strain (astenopia)
What is myopia?
incident light focused infront of retina leaving a blurred image on retina
How do we correct myopia ?
with a negative lens
What is negative spherical lens?
diverges incident light before it reaches cornea and patient sees a virtual image
- When diverging a negative lens placed infront of the cornea and light is focused on the retina and can see clear image
What does astigmastism mean?
non-point forming
a general patients eye being able to see clearly?
when cornea is spherical, which means all rays entering the eye focus on a single point on the retina
What is astigmatism?
- when the patients cornea is aspherical therefore power of lens is different in different directions.
- it causes light to be bent more in one direction more than other (so a higher power than the other)
- this creates 2 principal powers , a maximum power in one meridian and minmum power in the 2nd meridian jwhich falls perpendicular (90) to the first.
- This means a patient looking at a point object doesnt become a point object on the retina- which means the image is distorted and blurred on the retina.
How is astigmatism formed on a diagram ?
2 line foci produced instead of a one dot image
What does astigmatism result in?
results in a image blurry more in one direction than the other
How to correct astigmatism?
use cylindercal lens
- Lens must have 2 principal powers at perpendicular merdians to one another
- Simplest form of achieveing 2 perpendicular powered meridians is on the basis of a cylinder
- these lenses have 1 surface with no power which is 90degrees away from a surface with maximum power.
What is an axis meridian?
surface of cylinder with no power (flat side)
What is the power meridian?
surface with maximum power ( curved)
What is the power meridian?
surface with maximum power ( curved)
What is the cylinder power?
90 degrees from axis
What are toric lenses?
sum of spherical and cylindercal lenses for patients with hypermetropia, myopia and astigmatism.
how to write these prescriptions?
1) how myopic or hypermetropic patient is (+1.00/or/-1.00)
2) 2nd component represents power of cylinder (as astigmatic so been corrected)
3) axis and power of cylinder lies in 3rd component
+1.00/-0.75 x180
How to write prescription in positive cyl?
+1.00/-0.75 x180
```
Transpose prescription
+0.25/+0.75 x90
add spherical and cylindercal
(keep cylinder positive)
(-90degrees)
~~~
