DISPENSING
SPECTACLE LENS COMPENSATIONS COMPENSATIONS OF THE LENS POWER AIM TO GIVE THE PATIENT THE CLOSEST EXPERIENCE TO THE POWER MEASURED DURING REFRACTION. NICOLA PEAPER OUTLINES KEY CONSIDERATIONS.
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ompensating 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 POWER OF THE SPECTACLE LENS NEEDS TO BE COMPENSATED FOR THE VARIANCES IN POWER THE POSITION OF WEAR CAUSES”
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. It is important to note that both a refracting CVD and a wearing CVD are necessary for calculating. If the refracting CVD is not
Figure 1: Tilt angle of a spectacle lens.
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. 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 + sin² θ/2n ) FSPH = -4.16
Refraction with trial frames results in zero PT, FFA and CVD, but when the lens is dispensed into a spectacle frame it needs to be compensated due to the position of wear.
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FCYL = FSPH tan² θ FCYL = -0.55 Power experienced at vision point -4.16 / -0.55 x 90
INSIGHT June 2022
Rodenstock_AI_
