Lensometry 4% Flashcards
A way to represent the Rx in a visual form
Optical cross
Optical cross is the same theory as ……
lensometry and retinoscopy
Optical cross:
If you work in plus cyl your sphere is ____
Your most minus meridian
Optical cross:
If working in minus cyl sphere …..
the most plus meridain
Optical cross:
how do you get the cyl
difference between the meridians (like a number line….not algebraically)
Optical cross:
Axis is….
Sphere meridian
What are the advantages to automated lensometry?
objective measurements
easy to learn and to use
no need to focus eye piece, align target or compute cyl power
Disadvantages to automated lensometry
Not as accurate as manual especially with prism
How many types of targets does the manual lensometer have?
2
What are the 2 types of manual lensmeter targets?
American or Cross Line
Corona or European Dot
Manual lensmeter consists of:
Eye piece
Lens platform
Power Wheel
Illumination source
Lens marking device
Range is to +/- 30D
Bring dots or lines into sharp focus, then read power
Prism Compensation Device (PCD)
What is the range of manual lensmeter
+/- 30D
Any lens >3D in power you must measure
from the back of the lens
What is the first thing to do when performing lensometry
focus eye piece and insure that the lens is flat on the stage and centered.
This device measures the approximate prescription of a pair of glasses. It measures the radial curvature of the lens in D and each clock is calculated to a specific (index of refraction)
Geneva Lens Clock
How do you use a Geneva Lens Clock?
Place on lens, rotate 90 degrees
If no change in the reading =no cyl in lens
If there is a change the Algebraic difference in reading = cyl power
Axis is an approximate measurement
Sphere is an algebraic sum of the measurements from the front of the lens and the back of the lens.
If there is no change when you rotate the Geneva lens clock then…
There is no cyl in lens
If there is a change when you rotate the geneva lens clock 90 degrees then …..
there is cyl in the lens
How do you calculate the cyl of a lens using the geneva lens clock
It is the algebraic difference between the readings you get at the 90 degree rotation of the clock
Is the axis accurate when using the geneva lens clock
no it is approximate
How do you get the sphere when using a geneva lens clock
algebraic sum of measurements from front and back of lens
Temporary press on prism
Fresnel prism
How high does the Fresnel prism go
40D
Fresnel prism_____images
magnifies
Prism that press on and is nearly weightless, is cut into shape of the spectable lens, is temporary/removable, magnifies lenses, and goes up to 40D
Fresnel prism
Prism used for vertical prism >1D, large horizontal deviations, and convergence insufficiencies.
Ground in prism
How do you know that there is prism present in a pair of glasses?
Mires will not center in the manual lensometer
There are concentric rings that will show up in the lensmeter target
How do you measure the amount of prism in the lens?
By using prism rings, each ring corrresponds to 1 PD, except for the tiny circle in the middle of the target(this is 1/2 PD)
Prism can be ____(BI, BO, BU, BD) or _______(BI&BO, BD&BO)
simple or compound
Prisms bend light rays toward their____
Base
What is the flat wide part of the prism
base
What is the sharp pointed part of the prism
apex
Lens used to correct hyperopia
convex (two triangles base to base)
This lens converges light rays
convex (each prism bends light rays towards their base)
Lens that treats myopia
Concave(two triangles apex to apex)
Lens that diverges light rays
Convex (light rays go towards the base of the prism)
Formula that gives approximate amt of prism present at any given point on a lens based on the dioptric power of the lens and the distance that point is from the optical center of the lens.
(Gives the approx amt of induced prism when the lens is decentered)
Prentice’s Rule
What is the formula for Prentice’s Rule?
P=dxD/10
P=amt of prism present at a particular point
d=distance of that point from the optical center, in mm
D=Dioptric power of lens
Pt should be looking through the _______ of the lens
optical center
Why would we intentionally decenter a lens?
If pt needs only a small amount of prism.
What are the 2 pieces of information needed to determine how much prism is induced when the lens is decentered?
power of the lens
how far the lens is decentered
If the lens is -8.00 and is decentered by 20mm calculate the amount of induced prism.
P=20x8/10
P=16
If distance is presented in cm you would have to convert to mm for formula to work. There are 10 mm in a centimeter, 100cm in a Meter.
If the lens is a plus lens the induced prism will be oriented in the ____direction of the decentration.
same
If it is a minus lens the induced prism will be oriented in the ________direction of the decentration.
opposite
A condition where the refractive error of the two eyes differ significantly….. >3D between eyes.
Anisometropia
a significant difference in perceived image size
aneseikonia
What causes aniseikonia?
Anisometropia
What does slabb off correct?
vertical imbalance that results from anisometropia
Slabb off is only required for the _____portion of the pt glasses
near
Excessive prismatic imbalance can result in _____
an inability for pt to fuse the two images
Bicentric lens (Base up prism) is placed in most ______ or _______lens
most minus or least plus lens
Prismatic effect causes ___ ocular muscle imbalance through Bifocal seg.
2 degree
The first number on the temple piece of glasses is____
lens width or eye size
The second number on a glasses temple is____
bridge
What is the third number on the glasses temple?
The temple length
How is the temple length measured?
From the screw to the tip of the temple (including curve of temple)
