This section contains definitions of some of the functions the eye can perform, without attempting to provide too much detail. Superficially the eye resembles a camera in so far that it has a lens, which throws an image onto the light sensitive back surface, which is called the retina. Focussing is not achieved by altering the distance between lens and retina (as with a camera) but by changing the shape of the lens. In the retina there are two types of receptor, rods, which are highly light-sensitive and are principally responsible for detection of shape and movement, and cones, which are less sensitive to light, but can distinguish colours.
Accommodation
Accommodation is the ability of the eye to focus on objects at varying distances from the eye. This is achieved by changing the focal length of the lens of the eye using the ciliary muscles. This ability varies with age of the individual and state of tiredness and also with the luminance of the visual scene.
Adaptation
This is the mechanism by which the eye changes its sensitivity to lighting levels. Adaptation from normal lighting levels to dark conditions can take up to 10 minutes, adaptation from dark to light is more rapid.
Convergence
Almost invariably, we focus both our eyes on the same target. When that target is distant the lines of sight are in parallel. However when we look at a nearby object our lines of sight intersect at the target. Convergence allows the eyes to rotate inward so that both eyes focus on this object.
Visual Acuity
The ability to differentiate between closely spaced visual stimuli. This can vary from person to person and also is strongly linked with the background luminance and observation time.
Contrast Detection
Most of the visual information we receive is the result of luminous variations in the field of view. We call this contrast. Contrast can take two forms, which mostly occur together, contrast in colour and contrast in luminance.
Glare
There are two forms of glare, discomfort glare and disability glare. Discomfort glare is a sensation of annoyance or pain, probably as the result of frequent changes in pupil size caused by excessive brightness contrasts. Disability glare is the result of interference in the visual process, there are two sub-categories of disability glare, “veiling glare” and “adaptive glare”. This subject is in more detail in subsequent chapters.
