OA Corner Part 8: Single vision lens basics

AR-coated lenses are ideal for screen use

Single vision (SV) lenses can be dispensed for distance, intermediate or near vision – and they can be tinted, photochromic or anti-reflection (AR) coated.

SV lenses can be made from different materials including glass and plastic, and they can be high index. So, what are the relative advantages and disadvantages of each type?

Glass lenses are heavier than plastic and more scratch resistant, although plastic lenses can be hard coated. The main disadvantage of glass is the safety aspect as they can shatter on impact, potentially sending shards of glass towards the eye. Glass lenses can be toughened, but this adds to their weight.

Both glass and plastic lenses are available in high refractive index materials. The higher the refractive index, the thinner the lens. The highest index available in glass is 1.9. For plastic it is 1.76, which means that glass lenses are available in the thinnest materials.

Plastic lenses are more impact resistant than glass lenses and, if they do break on impact, the fragments are less sharp. Plastic lenses are available in Trivex and polycarbonate lens materials, and these are classed as safety lenses. They are used for industrial purposes in safety spectacles and are an ideal choice when increased impact resistance is needed, for example, in sports spectacles.

There are disadvantages of higher index materials as the Abbe number, or V-value, tends to be lower. This means there is increased dispersion within the lens. This can manifest itself as colour fringing under high contrast conditions, and peripheral blurring under low contrast conditions when the patient looks away from the optical centre of the lens.

As the refractive index of the lens increases, so does the reflectance, which is why it is advisable to AR coat high index lenses. In fact, very high index lenses cannot be supplied without this coating. One advantage of an AR coating is that it increases light transmission through the lens, and so reduces reflections, which can cause glare. This is valuable when driving at night when the light from car headlights reflects from the spectacle lenses, or in the case of reflected light from screens such as computers and mobile phones.Glass and plastic lenses can both be tinted. Plastic lenses are usually dyed in a tint bath and the tint will be an even colour over the lens regardless of the prescription. Glass lenses can either vacuum coated, which is a process similar to AR-coating, and this will give an even colour to the lens. They can also be tinted by mixing the tint into the molten glass, but this will give an uneven colour where the lens is thicker.

Photochromic lenses are available in glass and plastic. Photochromic lenses in their inactivated state have only a very pale tint and darken in response to ultraviolet light. Plastic lenses have a uniform colour, but glass lenses will be darker where the lens is thicker as glass lenses incorporate a solid tint.

This article has hopefully given an insight into some different single vision lens types. In next month’s article, we will look at occupational and progressive lenses.

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.

OA Corner Part 1: What makes a good OA
OA Corner Part 2: Communications
OA Corner Part 3: A question of strategies
OA Corner Part 4: Understanding bifocals
OA Corner Part 5: Our amazing eyes
OA Corner Part 6: Frame fitting basics
OA Corner Part 7: Frame styling factors