Stereoscopic and Binocular Vision

Stereoscopic vision/depth perception is how a person judges how far away an object is from him/her, combining such factors as the apparent size of the object, its apparent rate of motion, the object's height in the field of vision, the image's clarity and various shadows.

It is dependent on the person possessing good binocular co-ordination but also is a visual skill which even in the absence of any other eyesight defect or problem, is not equally well developed in everybody.

When a person has only one usable eye or a squint, none of the symbols will "stand out". If the vision of one eye is much worse than that of the other, the person also may experience difficulty with this test.

Usually there is no optical remedy for low stereopsis and therefore poor performance on this test alone should not lead to "referral".

This of importance in making critical judgement of distances and locations within the range of 20 metres. Beyond that distance, stereopsis is of lesser consequence.

These are of significant consequence at all viewing distances and the more so for persons using their eyes under critical and/or on-going conditions, which clearly includes drivers (especially at high speeds), users of fire arms, and Display Screen Equipment users.

Binocular vision defects such as heterophoria (Phoria test on the NRK Medical Screener) once detected should be referred for a full professional eye examination. These conditions are specifically mentioned in the section on Vision in the publication by the Medical Commission for Accident Prevention, entitled 'Medical Aspects of Fitness to Drive'.

(N.B. Uncorrected uncompensated 'Phorias' can cause excessive fatigue, visual misjudgement, lack of concentration and even transient double-vision.)

The NRK Medical Screeners assess all the aforementioned eye functions and for this reason are so widely used for vision testing of Police and Armed Security personnel.