Importance of accurate measurements – Nicola Peaper

In an Insight article last year Grant Hannaford, director of the Academy of Advanced Ophthalmic Optics, compared the accuracy of various methods of measuring and concluded that a measuring tower was the most accurate with claimed precision of 0.01mm and systemic uncertainty of 0.03mm.[1] This compared to using a ruler with claimed precision of 1mm and systemic uncertainty of 3mm. But how accurate do our measurements need to be? I am constantly told that, as a laboratory cannot fit to 0.1mm accuracy why, do we need that accuracy in measuring?

We have seen swingeing changes in the optimisation of lens designs to reduce aberration since digital surfacing was developed. Put simply, when a lens design is optimised the angle of incidence of light to the lens needs to be known. The angle it leaves the lens and subsequent angle of incidence at the eye is then calculated. This happens at thousands of points across a lens for all vision points for the eye. The width of usable corridor is increased with careful calculation of aberration to match as the eyes move across the lens (Figure 1).

To perform these complex calculations, it is essential to know where the eye sits. If the eye starts in the wrong place the calculations will be off and aberration will be increased.

A laboratory may not be able to fit to 0.1mm, but if a practitioner rounds their measurements, then tolerances allow for larger errors to eventually creep in. For instance, if a PD is measured as 31.4mm but ordered as 31mm (rounded), then with a 1mm tolerance the lab could fit at 30mm. This would be 1.4mm out. On a progressive lens with a high add of over +2.00 and a corridor length of 14mm, the best the patient can hope for is a lens that is just usable, at worst the intermediate portion may be too narrow to use and reading severely compromised.

If we consider a single vision aspheric lens that is not fitted with height dropped for pantoscopic tilt, then the height may be incorrect by 4mm or more. Whilst the lens will still be wearable the patient will not be getting what they have paid for which should be very low aberration clear vision.

This then raises the next question. When should we include face form angle (FFA), pantoscopic tilt (PT) and corneal vertex distance (CVD) in our lens measurements and subsequent lens design? This is a question that I am frequently asked and the general opinion I hear is that this accuracy and technology should be used for high scripts only.

Certainly, if we tilt a +6.00D lens by a couple of degrees we will induce more aberration than if we tilt a +2.00D lens. However, the congruity of the asymmetrically modified zones will also be affected. In other words, the aberration pattern that is calculated so the right eye and left eye match as they scan the lens will no longer give the required effect and performance will be reduced. By performance we mean corridor width and swim.

When a patient complains of swim on a new progressive lens, often changing PT is the first thing trialled. However, it can be seen from Figure 2 below that FFA has a much larger effect on lens performance.[2] A change from standard FFA (5°) by only 3° will cause a drop in lens performance of 25% and a face form angle of 10°, which is common with today’s larger frame designs, causes a drop to less than 50% in the performance.