Feature, Optical Dispensing

Finding the minimum lens blank size – Part 2

Selecting the right lens blanks can avoid many headaches for optical dispensers. In the second part of his special series, LEIGH ROBINSON explains how to get measurements right every time.

In last month’s issue, we discussed some important understandings of lenses, and that it is the primary role of the dispenser to position a spectacle lens in direct relationship to the wearer’s pupil.

The Major Reference Point (MRP), the optical centre, frame measurements and the Effective Diameter (ED) are all important to getting it right, and avoiding a call from your lab about ‘cut out’.

Lenses and frames combine to make spectacles, so we need to be in control of the order from the beginning.

Firstly, when quoting patients on lens costs, we must know whether a stock lens will cut out of the available stock lens diameters, and secondly, if the order needs to be a grind lens. It is a lot easier to quote ‘grind prices’ from the beginning.

If the wearer’s binocular Pupillary Distance (PD) and the Frame Centre Distance (FCD) are equal, no decentration is required (Fig 1), although this rarely happens.

When the wearer’s PD is smaller than the FCD, which is common, the lens must be decentred in, towards the bridge (Fig 2).

In the uncommon situation where the PD is wider than the FCD, the lens is decentred out, towards the temple.

Decentration           = FCD – PD / 2
Frame: 52/18          = 70mm
PD                           – 62mm
Decentration           = 8mm IN or 4mm each eye
(The lens is decentred IN, because the PD is smaller than the frame)

Figure 1

It is recommended that Monocular PD measurents be used for all prescription lens orders, especially for aspheric, high-powered, and progressive addition lenses. To determine decentration here, we halve the FCD.

Decentration                      = FCD / 2 – Mono PD
Frame: 52/18 = 70/2          = 35
minus RE Mono PD          – 31
= 4 mm decentration IN RE
(The lens is decentred IN, because the PD is smaller than the frame)

Figure 2

Minimum size uncut

Determining the Minimum Size Uncut (MSU) requires an understanding of the relationship between the lenses, the frame and the wearer’s PD.

The effective diameter of the frame helps us to determine the MSU: the absolute minimum stock lens to be fitted to the desired frame within prism tolerance (Australian standards, AS/NZS ISO 21987: 2011).

But why not just use the biggest lens possible? The answer is thickness. A bigger plus lens will add more thickness to a lens, and lead to an unhappy customer.

MSU is directly affected by the amount of decentration. If the lens needs to be decentred away from the geometric centre by 1 mm, the lens needs to be 1 mm wider in both directions. For example, for a lens decentred by 4mm the blank will need to be 8mm larger.

The larger the frame or the smaller the PD, the more decentration is required and the bigger the lens will have to be. Understanding this can make a difference in every job.

MSU                        = FCD – PD + EFFECTIVE DIAMETER (ED)
Frame: 52/18           = 70
PD                            – 62
Decentration            = 8
(ED)                         + 55
= 63
+ 2 = 65mm Minimum Blank Size
(Always add 2mm to allow for blishes on the edge of the lens)

Figure 3

It is important to use the effective diameter rather than the ‘widest point’ or ‘longest axis’. These measurents are not always accurate and sometimes lead to disagreents between the lab and the practice.

There are lots of us that don’t understand formulas, so there are other methods we can use, although they are not quite as accurate.

The alternate method for calculating the minimum blank size is to measure the frame while on the wearer. The distance from the pupil centre to the widest point of the rim of the frame, then doubled, provides the minimum blank size. Don’t forget to add the extra 2 mm to allow for bevel and lens edge witnesses.

The same method can be used from a dotted pupil centre. This method uses the radius of the frame’s lens opening, plus the decentration, to provide the minimum lens size.

Be in control of your jobs before they return to bite you.

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