DISP - Spectacle Frame Specification and Lens Centration - Week 2

Question 1

What are the two forms of frame measurement systems and which is older? Which is most often used?

Answer 1

Datum
Boxing - used more often

Question 2

What is the datum line?

Answer 2

It is a line made halfway between two horizontal lines encasing the top and bottom of the lens (and not the frame).

Question 3

Describe how to find the datum centre.

Answer 3

Once the datum line is found, the datum centre is the midpoint between the edges of the lens using the datum line as a reference.

Question 4

Define eye size in the datum system.

Answer 4

It is the width of the lens at the datum line.

Question 5

Define bridge size according to the datum system.

Answer 5

It is the distance between the two lenses at the datum line.

Question 6

What was the boxing system introduced for?

Answer 6

Elongated and asymmetrical lenses

Question 7

Define the effective diameter and describe how it is related to the geometric centre of the lens. What system uses this?

Answer 7

Used by the boxing system, the effective diameter is the largest diameter possible for a given lens.
It is twice the longest distance from the geometric centre to the farthest rim/edge of the lens.
It is used by the boxing system.

Question 8

Is the geometric centre the same as the datum centre?

Answer 8

No

Question 9

Describe the lens/eye size in the boxing system and compare it to the datum system. What is done to eye size in the boxing system?

Answer 9

In the boxing system, the lens size is the width of the box around the lens and not the width of the datum line, as it is in the datum system.
In the boxing system, 1mm is added to allow for the groove of the frame.

Question 10

Define bridge size for the boxing system and compare it to the datum system. what is usually done to bridge size in the boxing system?

Answer 10

It is the distance between the boxes of each lens - ie the smallest distance between each lens.
In the datum system, it is the distance between each lens at the datum line.
In the boxing system, 1mm is subtracted to allow for the groove of the frame.

Question 11

How is the geometric centre found?

Answer 11

It is in the centre of the box in the boxing system.

Question 12

How is frame size/frame PD found?

Answer 12

Eyesize plus bridge size

Question 13

What three things are typically not the same between the boxing and datum systems?

Answer 13

Eye size
Bridge size
Box centre ≠ datum centre

Question 14

What is the reference point for all frame measurements, both datum and boxing systems?

Answer 14

The peak of the bevel

Question 15

Describe the format for frame specification and mention where in the spectacles it can be found.

Answer 15

Eye size/bridge size temple length make, model and colour
i.e. 52/18 140 MW Atlas Grey
Markings can be found on either side of the nose pad, or somewhere on the temple arm

Question 16

Where should the measurement for the temple arm begin?

Answer 16

At the screw

Question 17

Briefly describe how to calculate decentration.

Answer 17

Add frame eye size and distance between lenses.
Take the given PD and divide by 2, subtract the sum by the monocular PD.

Question 18

list three reasons why horizontal decentration is important.

Answer 18

Spectacle lens optical centres should correspond to the wearer’s visual axis.
Advanced lens designs like aspherics must be positioned accurately to avoid distortion.
Also to avoid unwanted prosmatic effects from mismatching centration distances and pupil centres, especially in high Rx powers.

Question 19

How can PD be approximated in uncooperative children?

Answer 19

By measuring inner canthus in one eye to outer canthus in other eye

Question 20

How can PD be measured in patients with strabismus?

Answer 20

Cover the eye not being observed so that the turned eye takes up fixation.

Question 21

In what four cases are monocular PDs important?

Answer 21

When the wearer’s face is asymmetrical
Strabismus
Is a must for PALs and aspheric single vision (distance and near)

Question 22

List some sources of error when measuring PD.

Answer 22

Parallax - practioner doesnt close unobserving eye, or isnt directly in line with the patient
Incorrect ruler position
Pupillometer not steady and positioned properly
If there is a significant difference between the PD of the patient and practitioner
Failure by the patient to fixate
Poor instructions

Question 23

Is the effective diameter the longest diagonal or the longest axis?

Answer 23

Neither

Question 24

Define the minimum size uncut and how to calculate it.

Answer 24

Size of the blank
It is the effective diameter + (2 time the mono lens decentration)

Question 25

Briefly describe how to estimate the minimum lens size.

Answer 25

Measure the longest axis
Measure the frame size
Measure PD
MLS = longest axis + frame size - PD