Week 2: Optics of Instruments, Focimetry, Lens Neutralisation

Question 1

What is a Focimeter/Vertometer/Lensmeter?

Answer 1

• Measure the prescription of a lens
• Find the optical centre of a lens
• Prepare a lens for edging

Question 2

Situations Focimeter Would be Used? Give 5 examples.

Answer 2

• New patient to clinic
• Find optical centre
• To check prescription
• To check px’s specs are correct
• prepare for lens edging

Question 3

How do you find Optical Centre of Lens

Answer 3

• Optical centre (OC) of a lens is the point at which light is not refracted
• Typically want the patient to be looking through the optical centre of the lens, as there is no distortion here

Question 4

What is a meridian?

Answer 4

• Any line through the centre of the lens is a meridian
• All meridians in a spherical lens will have the same power

Question 5

How do you measure dark Iris?

Answer 5

• In this case we measure from the edges of the visible iris (limbus to limbus)

Question 6

How do you measure strabismus ?

Answer 6

• Covering the eye that is not being measured to ensure the eye we are measuring is looking straight

Question 7

How do you measure dark asymmetrical pupils?

Answer 7

If one pupil is bigger than the other:
1. Take a measurement from nasal pupil in RE to nasal pupil in the LE
2. Temporal pupil in the RE to the temporal pupil in the LE
3. Average of these two measurements to get the PD

Question 8

How do you measure babies

Answer 8

• An average of measurements from nasal to temporal canth

Question 9

How to measure heights with single vision lenses

Answer 9

• Monocular PDs
• Centre of Rotation Rule must be used to determine optical centre heights

Other single vision lenses typically aspheric lenses are fit:
- On datum (line midway of the frame)
- Using binocular PDs

Question 10

What is the Principal Axis/Centre of Rotation Rule

Answer 10

• Patient tilts head back so that lens is perpendicular to floor
• Mark pupil centre
• OC will naturally drop 1 mm for every 2° of tilt

Question 11

How do you fit bifocal heights?

Answer 11

Distance from the top of patient’s lower eyelid to bevel of the lowest edge of the frame

Question 12

How do you fit trifocal heights?

Answer 12

Distance from the lower edge of the patient’s pupil to the bevel of the lowest edge of the frame

Question 12

How do you fit trifocal heights?

Answer 12

Distance from the lower edge of the patient’s pupil to the bevel of the lowest edge of the frame

Question 13

How do you fit PAL Fitting Height?

Answer 13

Distance from the centre of the patient’s pupil to the bevel of the lowest edge of the frame