4.1.2: Measures and verifies optical appliances taking into account relevant standards where applicable

Question 1

Cyl tolerance

Answer 1

0.125 – 0.25: 16
0.375 – 0.50: 9
0.625 – 0.75: 6
0.875 – 1.50: 4
1.625 – 2.50: 3
> 2.50: 2

Question 2

BVP (SVDs): Tolerances

Answer 2

• 0.00-12.00DS, tolerance <0.25DS
• > 12.125DS, 0.25DS tolerance
• Cyl power >6.00, 0.25DC tolerance

Question 3

BVP (progressives)

Answer 3

> 12.25DS, 0.25DS tolerance

Question 4

Positioning of segments/fitting heights

Answer 4

• Horizontal & vertical within 1mm
• Tilt – no more than 2 degrees from horizontal

Question 5

Positioning tolerances

Answer 5

1mm PDs

1mm heights

2 degrees tilt from horizontal

Question 6

Manual: How to

Answer 6

• Mark up varifocal – check measurements
• British standards – heights should be pupil to HCL
• Mono PDs not Binoc
• Any coatings etc
• Front or back surface (front = sides facing away)
• Focus least plus meridian first
• Focus 2nd meridian – difference = cyl power
• Reading add – move to reading section, difference between 2 most vertical lines = reading add

Question 7

Prisms

Answer 7

Triangles opposite way, add:
BI < > BI = add
BU V, ^ BD = add

Triangles same way, subtract
BI < < BO = subtract
BD V V BD = subtract

Question 8

Prism in varis

Answer 8

• There is always prism at the prism reference point due to prism thinning; mainly for cosmetic purposes to ensure same level of thickness in lens despite reading addition
• To check for prism in varifocals
• Measure both lenses at prism reference point
• If there is a difference, then this residual prism gives prism in specs
  Examples
• 3BD RE 3BD LE cancels out = no residual
• 3BD RE none LE = residual prism of 3BD
• Plus lenses – base up prism should be removed to reduce thickness, therefore if + lens, the prism should be BASE DOWN for prism thinning
• Minus lens – base up prism for thinning