Bifocals

Question 1

Another name for Inset

Answer 1

Seg Inset

Question 2

Inset

Answer 2

On a person: Difference between the far and near monocular PDDivide this number by two if there is a binocular PD On a pair of glasses: Horizontal Difference between the distance OC and the segment OC

Question 3

Binocular PD

Answer 3

Total distance between your pupils ie. 66 PD

Question 4

Monocular PD

Answer 4

Distance between the bridge of the nose to your pupil on each side ie. 32/34

Question 5

Near PD

Answer 5

The amount a pair of eyes converge to view an object in the near field of view Usually 16 inches or 40cm It is measured on a pair of lined bifocals by measuring from the outside edge of one segment to the inside edge of the other segment

Question 6

Segment Height

Answer 6

In spectacles: The measurement in mm from the uppermost portion of the segment to the lowest point of the bevel apexBevel apex is not usually directly below the segment In humans: Measured from the deepest bevel of the frame to the lower lash line (usually)

Question 7

Segment Drop

Answer 7

A calculation to determine the correct vertical placement of a segment on an ophthalmic frame Calculate by measuring the B dimension and dividing it by 2, then subtract the seg height Half the B minus the seg ht The number of mm’s that the tip of the segment has to be dropped or raised from the 180 degree line during blocking

Question 8

If the segment drop is positive…

Answer 8

Drop the seg ht A certain number of mm under half the B

Question 9

If the segment drop is negative…

Answer 9

Raise the segment drop above the 180 line during blocking

Question 10

Golden Rule

Answer 10

The segment width should represent in size, the percentage in time the patient spends viewing at near

Question 11

Another name for Near PD

Answer 11

Segment PD

Question 12

Measure Near PD

Answer 12

Measure the distance from the inside of one segment to the outside of the other

Question 13

Segment Height

Answer 13

Measure from the uppermost portion of the segment to the Deepest part of the frame and include the bevel Be sure to measure to the deepest part of the frame regardless of where the segment is located in relation to this point

Question 14

Neutralization Procedure for Bifocals

Answer 14

How do we read Bifocal prescriptions?

Question 15

Step 1 : Neutralization of Bifocals

Answer 15

Read the Distance RX as you would normally read a single vision lens

Question 16

Step 2: Neutralization of Bifocals

Answer 16

Next, turn the glasses around in the lensometer so that the concave side is now facing youReread the Distance Rx

Question 17

Step 3: Neutralization of Bifocals

Answer 17

Slide the glasses up until you are in the segment Has to be the same distance that you were ABOVE the seg when you reread the distance RX

Question 18

Step 4: Neutralization of Bifocals

Answer 18

Take a reading trough the segment The difference between the distance RX taken from the Concave side of the glasses and the reading through the segment is the Reading addition or the Add Power

Question 19

Step 5: Neutralization of Bifocals

Answer 19

If you are unable to find the target when taking a reading through the segment, use an auxiliary prism Put its base in the opposite direction of the prismatic displacement of the lens Usually the problem occurs with high plus lenses which cause base up prism, so place the auxiliary prism base down

Question 20

How does the ADD lens work?

Answer 20

Designed to take light diverging in all directions from a near object and change direction of that light so it apparently comes from a great distance Light becomes parallel after going thru the ADD lens