OA Corner Part 40

Sue Deal FBDO R

What lens type is best for sports?

In last month’s article, we looked at how single vision lenses correct different refractive errors. This month, we will look at some different spectacle lenses available and their associated features and benefits.

Spectacle lenses are available in different materials and fall into two broad categories – glass and plastic. Plastic lenses are more impact resistant and lighter than glass lenses but will scratch more easily. If impact resistance is required for safety purposes , such as for certain sports, then polycarbonate or Trivex lenses are the better option as they will withstand a greater impact than other lens materials.

Plastic and glass lenses are also available in different refractive indices, and the higher the refractive index, the thinner the lens will be. Generally, however, as the refractive index increases the abbe number decreases, which leads to increased aberrations when the patient looks away from the centre of the lens.

The abbe number determines the dispersion of light by the lens and is due to white light splitting into coloured components. These aberrations manifest as colour fringing under high contrast conditions and blurring under low contrast conditions in the lens periphery. Some patients are more sensitive to this than others, but it is worth asking the patient if they have had high index lenses in the past, and if they have experienced these problems before.

Aspheric lenses have flatter curves, which leads to thinner, lighter lenses, and a reduction in centre thickness for positive prescriptions, which results in reduced spectacle magnification. This means the wearer will have a smaller retinal image size with an aspheric lens, compared to a non-aspheric lens, and their eyes will look less magnified behind the lenses.

This will be a benefit to most patients, although care is needed if a patient needs as much magnification as possible, for example, in the case of a low vision patient (remember that dispensing to patients with a visual impairment can only be undertaken by, or under the supervision of, dispensing opticians or optometrists registered with the General Optical Council (GOC). Also, some patients do experience difficulty adapting to an aspheric lens – as the lens form may be different to the lens form they have had in the past. In the case of a negative prescription, there will be a small reduction in edge thickness due to the aspheric form.

A combination of an aspheric design and a high index lens will provide a thinner lens, and the weight can be reduced by choosing a lens with a low specific gravity. The specific gravity (SG) of CR39 is 1.32, compared to 1.49 for a Tokai 1.76 lens. On first glance, as the SG of the 1.76 lens is higher than CR39, and the indication is that the 1.76 lens will be heavier. However, the reduction in volume of the 1.76 lens is about 34 per cent less than CR39, and so although the lens may be denser than CR39, the reduction in volume can mean the lens is lighter.

When dispensing lenses, especially higher powers, it is important to consider the frame size and the pupillary distance (PD) of the patient, because if this is not chosen wisely, the lenses can be thicker than the patient expects. This can cause a problem if the patient has chosen higher index lenses based on the recommendation that the lenses will be thinner.

It is best to keep decentration to a minimum as this will result in a thinner lens compared to a lens with excessive decentration. Practically, this means you should try to keep the box centre distance (BCD) of the frame as close to the patient’s PD as possible. The BCD is the horizontal box lens size plus the distance between lenses. For example, if the frame is marked as 50 x 18, add 50 and 18, which is 68mm. If the PD of the patient is also 68mm, there will be no decentration required, and this will give the best finish to the final lenses.

In conclusion, these are just some aspects to consider when dispensing spectacle lenses. It is important to remember that there is more to reducing the lens thickness than just the refractive index of the lens.

Sue Deal FBDO R is a practising dispensing optician, ABDO College examiner, senior tutor and supervisor for dispensing opticians. She is also a practice visitor and external moderator for ABDO. She was recently awarded the ABDO Medal of Excellence for her outstanding services to the profession.