Lensometry, prism, and hand neutralization Flashcards
Where is the far point for a myopic eye?
before the cornea
Where is the far point for a hyperopic eye?
after the retina
Primary focal point
object-space focus. Distance from the optical surface to the primary focal point
Primary focal point for a plus lens
point from which light must originate to emerge parallel from the lens. Image is at infinity
Primary focal point for a minus lens
Point towards which the incident light must be directed in order for the image rays to emerge parallel
collimated
rays are parallel
secondary focal point
image-space focus
secondary focal point for a plus lens
parallel rays from a distant point object are rejoined to form an image at that point.
secondary focal point for minus lens
diverging rays seem to come from, after a parallel bundle of rays are refracted by a negative lens
Plus lenses
magnify
minus lenses
minify
lensometer
lensmeter/focimeter/vertometer
What does a lensometer measure?
sphere, cylinder, axis, add power, prism
Down stream vergence
P/(1-[t/n2)P
lens power
(n’-n)/r
Approximate power
- P1+P2.
- Matches closely with back vertex power of minus lens.
- matches closely with thin lens
- cannot be used for manufacturing because error in app can be significant
- convenient/simple
Front vertex power
also called the neutralizing power.
=P2/(1-[t/n2]P2) + P1
Used in hand neutralization
Back vertex Power
Badal
Standard spectacle lens correction
Used for specifying power of spectacle lenses
=P1/(1-[t/n2]P1) +P2
equivalent power
P1 + P2 + (t/n2)P1P2
Different lenses with the same EP can provide differring corrective powers
Effective power
P/(1-[t/n2]P)
- used for things at a distance, needs to be addressed when RXing CL
- the further away, the more power is needed
Why is effectivity important?
accomodative demand
CL calculations
Refractive surgery calcs
Practice problem in this slide show
do it
astigmatism
common vision condition that causes blurred vision. It occurs when the cornea is irregularly shaped or sometimes because the curvature of the lens in the eye
spherecylindrical lenses
axes cannot exceed 180
How does the angle increase for cyl
from patients left to right
lensometry
- measures back vertex power in diopters
- uses a standard lens of known power
What is the effect of prism?
- deviation of light rays towards the base of prism
- the image will appear to move towards the apex of the prism
where does the image appear to move towards in a prism?
the apex
magnification
m=-i/o
Abbe number
- constringence or V-number
- transparent material’s chromatic dispersion
- shows the variation of refractive index versus the wavelength
- High Abbe, Low dispersion
what can affect refractive index?
wavelength and temperature
what color has the lowest index of refraction?
red
what color has the highest index of refraction
violet
How do you measure prism?
1 prism diopter creates a deviation of 1cm at 1 m
5 prism diopters creates a deviation of 5cm at 1m
describe orientation of image in BI
image goes out
describe orientation of image in BO
image goes in
describe orientation of image in BU
image goes down
describe orientation of BD
image goes up
amount of prism
prism=P x d (in cm)
prentice’s rule
prims=P x d (in cm)
hand neutralization for minus lens
with motion
upright
minified
hand neutralization for plus lens
against motion
inverted
magnified
How do you find front vertex?
hand neutralization
how do you find back vertex?
lensometer
What is the back surface power of first lens equal and opposite to in thick lens?
the neutralizing power in the second lens
On a thick lens, if the unknown lens is placed in back…
you are determining the neutralization power of the unknown lens using the back vertex power of the known lens
in thick lens, if the known lens is placed in back, you are determining…
the back vertex power of the unknown lens based on the neutralization power of the front lens (which might be different than the known back vertex power
spherocylindrical
turning this type of lens gives a scissors motion
