Lensometry and Verification Flashcards
The term lensometer has become the generic term for the instrument that is used to neutralize lenses. What is the correct name of the instrument?
Focimeter
Rather than reading the power of lenses, what are we actually doing?
We are neutralizing the power of the lens by measuring the focal length of the lens. The instrument converts the focal length to dioptric power.
When viewing through the eyepiece, as you move a lens or glasses to the right, and the image viewed also goes to the right, what type of lens are you neutralizing?
A minus power lens
When you are going to neutralize a pair of eyewear in the lensometer, what is the first step of the process?
Focus the eyepiece
How do you focus the eyepiece in a lensometer?
With the instrument off, place a white piece of paper on the lens stop. Rotate the eyepiece far to the left, then slowly rotate it to the right. When the mires are clear, stop. Rotating it back and forth will stimulate accommodation, so do it slowly and simply stop when clear.
What kind of lens are you looking at when both the sphere and cylinder lines are clear?
A sphere lens
If the sphere lines are clear and the cylinder lines are blurred, what kind of lens are you looking at?
A sphero-cylinder lens
How can you use a lensometer to tell if a lens has prism?
If the eyewear is sitting at the lensometer so that the optical center of the lens is in the center of the field of view, and the mires are anywhere other than the center of the field of view, the lens has prism.
If you are neutralizing a lens and get the sphere lines clear at +3.00 at 180 degrees and the cylinder lines come in clear at +1.00, what is the power of the lens?
+3.00 -2.00 x180
If you are neutralizing a lens and get the sphere lines clear at +2.50 at 045 degrees and the cylinder lines come in clear at -1.00, what is the power of the lens?
+2.50 -3.50 x045
You are neutralizing a pair of SV glasses and you center the mires on the OD. Then, you move to the OS and the target is below the center. What should you do next?
Panic! Just kidding. You should identify which lens is stronger and center that one first, then switch to the opposite lens to determine how much prism is in the lens and how much.
Explain the process for neutralizing the power of a multifocal lens.
Neutralize the distance portion just as if it’s a SV lens in back vertex power. To determine add power, turn the lens around (front vertex power) and read the sphere rx and note it. Then move to the segment and read the sphere power. The difference between the two powers is the add power.
If you are neutralizing a multifocal lens and in the distant portion of the lens the sphere lines come into clear focus at +2.50 @ 145 and the cyl lines come into clear focus at +1.00, and then as you move down into the segment the sphere comes into clear focus at +5.00 and the cyl comes into clear focus at +3.50, what is the power of the lens? Is it necessary to see the focus for the cyl in the reading segment?
+2.50 -1.50 x 145 ADD 2.50
No, it is not necessary to focus the cyl
If you are neutralizing a multifocal lens and in the distant portion of the lens the sphere lines come into clear focus at -4.50 @ 145 and the cyl lines come into clear focus at -2.50, and then as you move down into the segment the sphere comes into clear focus at -2.50, what is the power of the lens?
-4.50 +2.00 x145 ADD 2.00
