Base Curves Flashcards
What does the base curve influence?
it is one of the spectacle lens parameters that most directly influences the patient’s vision as well as the appearance of the lens.
What does single vision Rx usually have?
A spherical front curve which is the base curve. Technically it is possible to have a plus cylinder form spherocylinder that hs two curves on the front where the weaker or flatter of the two curves is the base curve.
What happens if the wrong base curve is chosen?
the patient will lose clarity as they move their eyes through the lens periphery due to increased lens aberrations.
What are aberrations commonly controlled with?
base curve selection that are:
spherical aberration, marginal or oblique astigmatism and distortion.
How does spherical aberrations happen?
when parallel light rays enter a large area of spherical lens surface.
What happens to the peripheral rays due to the curvature of the lens?
It will focus sooner than the central rays and cause a slight blurring of the image. In general this aberration is not a significant problem because the small aperture of the pupil is effective in keeping out of these aberrant peripheral rays.
When does a marginal or oblique astigmatism occur?
when a small bundle of light strikes a spherical lens surface from an angle.
With the situation of the marginal or oblique astigmatism, what does the rays in the opposite meridians tend to do?
to focus at different points.
What is the distance between the two points in marginal or oblique astigmatism?
The distance between the two points of focus equals the degree of astigmatism created.
What led manufactures to develop with the situation of marginal or oblique astigmatism?
to develop of corrected curve lens series.
When does distortion occur?
as the light rays move from the center of the lens toward the periphery.
As the rays approach the edge of the lenses in distortion, what increases?
increases magnification causes the image to be distorted.
In the case of strong plus lenses, what is distortion seen as?
an inward curvature known as the pincushion effect.
In the case of a strong minus lens, what is the distortion seen as?
an outward bulging known as the barrel distortion.
What is the Corrected Curve Theory?
Formulas concentrate on correcting errors experience when looking more than 30 degrees away from the enter.
power| base curve:
-8.00D to -12.00D | plano
………
What are the base curves range in minus lenses?
from plano to 6.00D
What are the base curves range in plus minus lenses?
from 6.00D to 10.00D.
What does minus side curves range from?
-4.00D and -7.00D deliever the clearest vision.
As the minus powers increase, the selection of a base curve should?
flatten.
As the plus power increases the selection of a base curve should?
steepen.
When changing from a lower index of refraction it may be necessary to slightly reverse these rules.
What do you do when replacing one lens of an existing pair of eyeglasses?
when ordering an identically powered second pair of eyeglasses the base curve should be specified.
What does the ANSI standards recommend?
the lens supplied is within +/- 0.75D.
Ex:
if you are duplicating a +2.00 base curve lens and it measures +2.50. it is within the 0.75D ANSI tolerance allowed. But, if it measured +3.00D, it would be more than the ANSI standards allow and would not be within the tolerance.
A different base curve may be necessary if?
there is a change in the Rx.
of lens material.
an aspheric design.
In the simplest form, how do you determine the lens power?
F1+F2=Ftotal
How are base curves measured with?
a lens clock, aka a Geneva lens clock or lens measure.
What are the three pins that the lens clock have?
that when pressed against the lens and will read the dioptric curvature of a lens surface.
What are the two outside pins in the lens clock?
are stationary while the center pin moves up depending on the curvature of the lens surface.
What is the face of the lens clock have?
has plus numbers in black and minus numbers in red. Most lens clocks are calibrated to measure lenses with a 1.523 index of refraction.
