From time to time, optical assistants (OAs) may come across prescriptions that incorporate prism. In this month’s OA Corner, we will look at what a prism does and why some people need prism in their spectacles.
If we start by looking at myopia and hypermetropia; myopia means the light doesn’t focus on the retina but in front of it, and so a concave (diverging) lens is needed to push the light back onto the retina. Hypermetropia means the light will theoretically focus behind the retina, and so a convex lens is needed to focus the light closer and on to the retina. Both these conditions illustrate that the light does not focus on the retina in the uncorrected eye.
Prisms are different to this, and we need to consider binocular vision where prisms are concerned. We have two eyes, and so two images are formed: one on the right eye retina and one on the left. The eye’s visual system then works with the brain to form a single image. Sometimes the two images don’t perfectly align, and patients can then experience symptoms such as double vision, blurred vision, headaches and difficulty changing focus distance to near.
A prism will change the direction of the light, but will not bring it to a focus, unlike a concave or convex lens. When you look through a prismatic lens, you will see the object you are looking at displaced from its original position. Light is deviated towards the base of the prism and the image is displaced towards the apex.
If you think of a triangle, the prism base is the thickest part of the triangle, and the apex is the thinnest part. Prism base directions in prescriptions can be base up, base down, base in or base out. So if you looked at a glazed pair of spectacles and the nasal edge was much thicker than the temporal edge, and the spectacles incorporated prism, this would be base in prism – as the base would be the thickest part. A prism of power one dioptre will deviate a light ray by 1cm at a distance of one metre from the lens.
Prism can be incorporated into a lens either by surfacing or by decentration. Surfacing is done in the lab when the lens is being manufactured, and decentration means moving the optical centre of the lens away from its original position. According to Prentice’s rule, P = Cf where P = the prism, c is the decentration in cms and F is the power of the lens.
So if we have a lens of power +4.00D and we need to incorporate two prism dioptres, we can use P = Cf to find the amount of decentration required by P = cF – and so 2 = c x 4 and so 2/4 = c. This means c = 0.5cm, or 5mm. As this is a positive lens, we would need to decentre the lens 5mm in from its original position, but if this was a -4.00D lens, we would need to decentre it 5mm out from its original position.
There are also stick-on temporary prisms called Fresnel prisms, which can be fitted to the back surface of the spectacle lens. Fresnel prisms are available in very high powers, up to 40 prism dioptres. This is much greater than could be incorporated into a lens by either surfacing or decentration. The disadvantages of Fresnel prisms include reducing the visual acuity through the lens by a line or two on the Snellen test chart.
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.