Dispensing, Feature

A guide to spectacle lens compensations — Nicola Peaper

Compensations of the lens power aim to give the patient the closest experience to the power measured during refraction. NICOLA PEAPER outlines key considerations.

Compensating the power of a spectacle lens simply means changing the power of the lens (read flat on a vertometer) so that the patient experiences the same power that they did under refraction conditions.

Nicola Peaper.

The most common compensation is that for position of wear. When we refract using a phoropter head or trial frame, the lens is presented to the patient with zero pantoscopic tilt (PT), zero face form angle (FFA) and a measurable corneal vertex distance (CVD). When the lens is dispensed into a spectacle frame it will invariably present with degrees of PT and FFA, and a different CVD. The power of the spectacle lens needs to be compensated for the variances in power the position of wear causes.

The compensation that we are perhaps the most familiar with is that for CVD as it is used during contact lens fitting where the wearing CVD is zero. With a refracting CVD of 13.5 mm and a refracted power over +/-4.00D, the compensation for contact lens fitting is around 0.20D. The internet has numerous calculating tools for these compensations.

Table 1: Compensated power for change in CVD of a 4.00DS lens.

It is important to note that both a refracting CVD and a wearing CVD are necessary for calculating. If the refracting CVD is not given, then an average is used, usually around 13 mm. In 2002, Weiss et al1 found that the average phoropter head CVD over a group of 189 patients was 20.4 mm. If we compare the results of compensating a 4.00D lens (Table 1) it can be seen from this that use of the correct CVD is essential. Compensations of PT and FFA are less well known but practitioners have long been able to calculate compensated powers for these. It is only recently, with the introduction of digital surfacing, that it has become commonplace for lens manufacturers to perform these calculations.

During a refraction light from a distant object is incident at right angles to the lens at the optical centre. When the lens is fitted to a spectacle frame the PT and FFA mean that light is incident at an oblique angle causing oblique astigmatism i.e. inducing a cyl. This means that if the script is -4.00DS, when the lens is worn in a spectacle frame some cyl will be present. The amount of cyl will be dependent both upon the power of the lens and the angle of tilt.

Figure 1: Tilt angle of a spectacle lens.

If we consider FFA alone and look at the effect different angles have (figure 1):

Ordered power -4.00DS, FFA 20⁰, n=1.5

FSPH = F (1 + sinÇ θ/2n )

FSPH = -4.16

FCYL = FSPH tanÇ θ

FCYL = -0.55

Power experienced at vision point

-4.16 / -0.55 x 90

Ordered power -4.00DS, FFA 5º, n=1.5

FSPH = F (1 + sinÇ θ/2n )

FSPH = -4.01

FCYL = FSPH tanÇ θ

FCYL = -0.03

Power experienced at vision point

-4.01 / -0.03 x 90

A second compensation needs to be calculated for induced prism due to base curve:

△ = 100tanθ t/n F1

△ = 0.3Δ Base Out

As this compensation includes base curve and centre thickness there is only a small amount of induced prism for a minus lens. If the power was +4.00D then the induced prism would be 1.2Δ Base Out.

A fully compensated lens is therefore calculated for the effect PT, FFA and CVD have on power including induced prism.

It is obvious that with the FFA of 20 it is essential to compensate the lens power otherwise the power the patient is experiencing is outside of tolerance and vision will not be optimal. However, questions are frequently asked about the validity of compensations on low powers with low amounts of FFA and PT, with respect to patient perception and manufacturing limitations.

It is important that a lens is produced with the greatest accuracy possible and, to this end, manufacturing tolerances are employed. The more accurate the order, the better the tolerances work. Take for example the fitting of a progressive lens. We can measure PD to 0.01 mm but we cannot possibly fit with that accuracy. Tolerance to PD is 1.00 mm (independent of power). If the PD is 34.45 mm and is rounded to 34 mm then it is possible that the manufacturer can supply 33 mm. This would be inaccurate by 1.45 mm and outside tolerance on the original measurement.

Each of these compensations of the lens power are designed to give the patient the closest experience to the power measured during refraction. In each instance, the desired outcome is to optimise the ‘as worn’ spectacle lens performance to the patient’s visual experience outside the consulting room.

References

1. Richard A. Weiss et al, Clinical Importance of Accurate Refractor Vertex Distance Measurements Prior to Refractive Surgery. Journal of Refractive Surgery Volume 18 July/August 2002. P444-448.

All formulae are taken from Brooks and Borish System for Ophthalmic Dispensing (3rd edition) p411 – 414

ABOUT THE AUTHOR: Nicola Peaper spent 20 years working as an optometrist in the UK. For the past 15 years she has worked within the lens manufacturing industry and is currently professional services manager for Rodenstock Australia.

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