Lesson · ABO
Frames and measurements
The boxing system, frame PD, decentration, minimum blank size and segment placement: the measurements behind every pair of glasses.
After this lesson you can
About 12 minutes- Name and measure A, B, DBL and ED on any frame
- Work out frame PD and decentration per lens
- Find the minimum blank size for a job
- Place a bifocal or trifocal segment from its height
The boxing system
Draw a box around each lens shape with its sides touching the lens edge. Every frame measurement is taken from those boxes.
| Measurement | What it is |
|---|---|
| A | Horizontal width of the box around one lens |
| B | Vertical height of the box |
| DBL | Distance between lenses: the gap between the two boxes at the bridge |
| ED | Effective diameter: twice the longest radius from the box centre to the lens edge |
| Datum line | The horizontal line halfway up the B measurement |
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Frame PD and decentration
The boxing centres of the two lenses are one frame PD apart. The patient’s pupils are usually closer together, so each lens OC is moved in (decentred) to sit in front of the pupil.
Frame PD = A + DBL
Decentration per lens = (frame PD − patient PD) ÷ 2
Decentration
A 52, DBL 18; patient PD 64 mm.
- Frame PD = 52 + 18 = 70 mm
- (70 − 64) ÷ 2 = 3 mm
Answer: Decentre each lens 3 mm in
Minimum blank size
The minimum blank size (MBS) is the smallest uncut lens that will still fill the frame after it has been decentred.
MBS = ED + total decentration
Total decentration is both lenses together: frame PD − patient PD.
Minimum blank size
Same frame: ED 56, frame PD 70, patient PD 64.
- 56 + (70 − 64) = 62 mm
Answer: 62 mm
Segment placement
Seg height is measured from the lowest point of the lens box up to the top of the segment. The lab needs to know where that is relative to the datum line.
Segment placement = seg height − (B ÷ 2)
A negative answer means below the datum line.
Where does the segment go?
B 40 mm, seg height 18 mm.
- 18 − (40 ÷ 2) = -2
Answer: 2 mm below the datum line
Lens surfaces and the lens clock
The front (base) curve and back curve add up to the lens power, ignoring thickness. A lens clock reads the curve of one surface at a time.
Nominal power = front curve + back curve
Nominal power
Front curve +6.00, back curve −8.00.
- +6.00 + (−8.00) = −2.00
Answer: −2.00 D
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