base curves Flashcards
Single vision for base curves is usually what?
Single-vision lenses usually have a spherical front curve
If the wrong base curve is chosen what happens to the patient?
The patient can lose clarity through the lens periphery.
What are the commonly controlled base curve selection for Aberrations?
- Spherical
- Marginal or Oblique
- Distortion
What is Spherical Aberrations?
- Parallel light rays enter a large spherical surface
- Peripheral rays focus sooner than central rays
- Not a significant problem
What is Marginal/oblique Aberrations?
- A small bundle of light strikes a spherical lens from an angle
- Distance between two points of focus is a degree of astigmatism created
- This led to the development of the corrected curve lends series.
What is Distortion Aberrations?
- As rays approach the lens edge, magnification increases and distorts image.
- Strong plus lenses “pincushion” distortion.
- Strong minus lenses “barrel” distortion
What is the Corrected Curve Theory?
Formulas concentrated on correcting errors experienced when booking more than 30 degrees away from the center.
What range do the minus lens base curves range from?
Plano to 6.00D
What range do the plus lens base curves range from?
6.00D to 12.00D
What range do minus side curves deliver the clearest vision?
-4.00D and -7.00D
As minus power increases, what should the base curve do?
Flatten
As plus power increases what should the base curve do?
Steepen
A different base curve may be necessary when there is a what type of change?
- Rx
- Of lens material
- To an Aspheric design
What is the formula for determining lens power?
F1+F2=Ftotal
Front curve plus back curve equals total power
Find the total power of this problem.
Front curve= +4.00D
Back curve = -3.00D
(+4.00D) + (-3.00D) = +1.00D
What tool is use to measure the base curve?
Lens clock
What is a Lens Clock?
- It is a tool that measure the base curve
- Two outside pins are stationary
- Center pin moves to measure curvature
- Plus numbers in black, minus numbers in red
- Most calibrated to measure lenses with 1.523 index of refraction
What if the lens clock doesn’t have a 1.523 index of refraction on it?
Plastic 1.49
Polycarbonate 1.59
Within ANSI Tolerance
What is the Sagittal Depth?
The perpendicular distance between the geometric center of the back of the lens and the diameter if a line is drawn straight across
How do you Calibrate the lens clock?
- Press all three pins perpendicularly against a flat, hard surface.
- The lens clock should read zero
How do you measure the lens base curve?
- Hold the lends clock perpendicular to the lends surface at the optical center.
- Do not tilt the lens clock or an incorrect reading will result.
How do you verify your results on the lens clock?
- Clock the surface at 180 and 90 degrees
- The two readings should be the same
- Significant differences in the two measurements indicate lens warpage
Is it a different process to measure a glass duces bifocal lens?
No, it is done in the same manner as a spherical lens.
How do you measure a piece of bifocal lens?
Placing all three contact points horizontally above the bifocal lines.
