Use LEFT and RIGHT arrow keys to navigate between flashcards;
Use UP and DOWN arrow keys to flip the card;
H to show hint;
A reads text to speech;
13 Cards in this Set
- Front
- Back
|
Vertex power allowence |
Difference between the compensated power and the nominal surface powers. |
|
|
What will the vertex power allowence always be? |
Negative |
|
|
What the formula for VPA on the front surface of a lens |
F1 - F1n |
|
|
What the formula for VPA on the back surface of the lens |
F2 - F2n |
|
|
What is effectivity |
The new power required when a spec lens is moved from its original position to a new position in front of the eye |
|
|
What is the formula for effectivity |
Feff= f/ 1-df |
|
|
What is vertex distance |
The distance between the back surface of the lens and the front surface of the cornea |
|
|
What is the definition of the back vertex power (FV') |
The reciprocal of the back vertex focal length in meters. |
|
|
What is the back focal length |
The distance from the back vertex of the lens to the paraxial focus for helium'd' light incident parallel to the optical axis from left to right |
|
|
What is nominal surface power |
The surface power which would be worked if the lens were considered to be thin |
|
|
What is compensated (reduced) surface power |
The actual surface power which needs to be worked taking the thickness of the lens into account.( will always be more negative than nominal) |
|
|
What's the formula for front to surface compensation |
F1 = F1n 1 + t/nF1n |
|
|
What is the back surface compensation formula |
F2 = Fv' - F1 1 - t/nF1 |
