Optical Terminology Explained

Field of View
How far the patient can see up, down, left and right. Essential for driving, helps balance and used in everyday tasks.

Glaucoma
An eye disease characterised by elevated unstable pressure of the fluid, causing characteristic loss of field of view, various causes (hence pre-screening processes - for early detection).

Retina
Positioned at the back of the eye where information is received similar to the film in a camera.

Cataract
Loss of transparency of the crystalline lens, various causes (cloudy appearance of the lens).

Dioptre
The unit of measurement to give the refractive power of a lens.

Focimeter
An instrument used to measure the power of a spectacle lens in dioptres, determines the prism, sphere, cylinder and axis. Can be used to determine what prescription the patients currently wearing.

Crystalline lens
A liquid filled lens in the eye able to change shape (accommodate) when looking at far away or closer objects.

Near	=	fatter lens more of the light is bent thus focussing at shorter point
Far Away	=	thinner lens less of the light is bent thus focussing at longer point

Visual Acuity (VA)
A measurement of how distinctly the details of an object appears.

The VA is measured by finding out which is the smallest size text the patient can see with each eye on its own, and then using both eyes to assess how they are working together.

The text is read off of the chart in the test room, this is done in controlled conditions (hence the darkened room). The patient has to read the text at 6 metres away (equivalent). The size of text is given a scaled number and this is recorded as:

• 6/6 (at 6 metres away the patient comfortably can read print size 6)
• 6/18(at 6 metres away the patient comfortably can read print size 18)

(N.B. 20/20 Vision refers to the patients being able to read line 20 at 20 feet which is approximately the same Visual Acuity as reading line 6 at 6 metres away, although this terminology is not used in the UK)