va Flashcards
What is the primary objective of hand neutralization of spectacle lens power?
To determine the power of unknown lenses without a lensometer
Hand neutralization is particularly useful for minus lenses and low-powered plus lenses.
What is the difference between back-vertex power and front-vertex power?
Back-vertex power is specified in a spectacle lens, while hand neutralization gives front-vertex power.
What type of motion is observed in a plus power lens during hand neutralization?
Against-motion.
What type of motion is observed in a minus power lens during hand neutralization?
With-motion.
In a sphero-cylinder power lens, what can be observed besides image motion?
Image break.
What are the two types of breaks observed in sphero-cylinder lenses?
- With-break * Against-break
What causes image motion and image break to be observed in lenses?
Variable prismatic effects of a lens away from its optical center.
What determines the speed of motion observed when neutralizing a lens?
The lens power: slow motion indicates low power, fast motion indicates high power.
What does the presence of scissors break indicate when observing a sphero-cylinder lens?
It indicates the lens is a sphero-cylinder lens.
What is the first step in the hand neutralization procedure?
Find an appropriate visual target.
What should be done to the optical center of the lens before starting hand neutralization?
Mark the optical center of the lens.
During hand neutralization, what happens when the target line moves down in a minus sphere lens?
With-motion is observed.
During hand neutralization, what happens when the target line moves up in a plus sphere lens?
Against-motion is observed.
What is the procedure for neutralizing a sphero-cylinder lens?
- Check for scissors-break * Align principal meridians * Mark optical center and meridians * Neutralize each principal meridian with sphere lenses
What is the significance of the with-break meridian in a sphero-cylinder lens?
It designates the minus cylinder axis.
