Optician Formulas, Abberations, and more.... Flashcards
Focal length
Inverse of the power of a lens in meters.
example:
+2.00
becomes
1/2= .5 meter
Vertex
Distance from lens to the front of the eye.
Meridians
Imaginary lines that help describe the location of axis powers Of a lens with correction for astigmatism.
Index of refraction
Amount a material is able to bend light. The higher a materials index, the more it bends light.
Abbe value
Amount of chromatic abberration produced by a lens material
Snell’s Law
The angle of refraction depends on the angle of incidence of
- light
- the indices of refraction between the two mediums
Index of refraction
Describes the density of a material
Law of refraction
Light bends toward the normal when it enters a medium more dense than the one it came from.
Refraction
How light bends when passing through a medium.
Vogel’s Formula
Formula to choose a base curve
Plus lens- Spherical Equivalent, add 6 Diopters
Minus lens- 1/2 Spherical Equivalent, Add 6 Diopters
Spherical Equivalent
- Take 1/2 of Cylinder
- Add to Sphere
Always round up to nearest quarter Diopter
Rules for Vogel’s Formula
Both lenses Plus- Choose steeper base curve
Both lenses Minus- Choose flatter base curve
One Plus, one Minus- Choose highest base curve of two lenses.
Vogel’s formula- power becomes more minus. What happens?
Front surface gets flatter. Back surface has to become steeper to maintain same power.
Vogel’s formula- Power becomes more Plus. What happens?
Front surface becomes more curved( steeper). Back surface has to become flatter to maintain same power.
Transposition
Converting a prescription:
Minus to plus or Plus to minus
How to transpose a prescription?
Add cylinder and sphere
Change cylinder sign to opposite sign
Change Axis by 90
Axis cannot be more than 180
Chromatic Aberration
Caused by the refraction of different wavelengths and results in colored fringes around images.
Spherical Aberration
Occurs when parallel light rays move along the optical axis.
Chromatic Aberration
When lens does not focus all wavelengths of color to one point.
Coma
Lens aberration- Occurs when parallel light rays through a lens focus in a ring-shaped pattern with a V-shaped or comet-like flare at one end of an image.
Visable Spectrum
380 to 760 nm
measured in nanometers
Cylinder power 30 degrees away from axis?
25%
Cylinder power 60 degrees away from axis?
75%
cylinder power at 45 degrees
50% of cylinder added to the sphere
