Methods of Vision Screening of Employees

Two techniques for screening vision may be used:

1. Modified clinical technique (MCT);
2. Instrument screeners.

Modified clinical technique

This method of screening is carried out by qualified personnel, e.g. optometrists, who assess the visual functions considered to be particularly important to job performance. The screening examination may therefore consist of any of the following:

1. History and symptoms; This is particularly useful as the history of a squint or amblyopic eye may be noted and this may reduce unnecessary referral. The presence of eyestrain when performing certain tasks can also be assessed.
2. Visual acuity at any distance; The measurement can be taken at the exact distance required, with the appropriate size of letters or task.
3. External eye examination and ophthalmoscopy; This allows detection of pathology of the eye that may otherwise go unnoticed. Ocular diseases do not always cause a reduction in vision that results in the person seeking medical attention.
4. Retinoscopy; This permits the precise degree of hyperopia or myopia to be assessed objectively. It willl also indicate whether or not poor vision is due to the fact that spectacles are required.
5. Amplitude of accommodation; The result of this test will indicate whether the person can focus clearly and comfortably on a near task. Spectacles will generally be required by the older presbyopic person to see the task clearly.
6. Binocular vision assessment; This can include a cover test at the viewing distance required, motility, near point of convergence and stereopsis. To avoid eyestrain, binocular vision needs to be stable and these tests will indicate whether the binocular functions are under stress for a particular task. The presence of a squint or phoria will be detected by cover test.
7. Visual field; This can be assessed quite simple by a confrontation test, to determine whether there is any marked reduction such as a homonymous hemianopia or quadratic defects. The central visual field can be assessed using Amsler Chart. When held at 30cm, this chart covers central 10 degrees of the field either side of fixation.
8. Colour vision; This can be assessed rapidly using tests such as the Ishihara Pseudo-isochromatic plates or Saturated D15. Tha Ishihara plates can be used to detect congenital colour vision defects and the Saturated D15 will detect acquired defects. However, if the task requires very fine discrimination of colours then a more thorough test, such as the FM100 Hue, may be used.

The MCT for vision screening has both advantages and disadvantages:

Advantages

1. Flexibility: tests can be selected depending on visual functions considered to be important for the task and the tests can be performed at the appropriate distance.
2. Type and magnitude of refractive error can be assessed.
3. Ocular pathology may be detected.
4. Presence of a squint will be detected.
5. Very few false referrals.

Disadvantages

1. Expensive method of screening due to the use of professional personnel, such as optometrists.
2. Time-consuming.
3. Shortage of professional people willing to participate in screening programmes.

Instrument vision screeners

There are many different instrument vision screeners available but most are basically modified stereoscopes. The eye piece lenses are arranged so that their prismatic components simulate the viewing of a distant object and the vision is simulated by either:

1. Moving the targets closer
2. Decreasing the separation between the half stereograms
3. Changing the lens power, i.e. introduce negative lenses

The screener usually have internal lighting and the targets are mounted either on a rotary drum or separately on cards, which are changed manually or by remote control. The types of tests commonly included in the instrument screener are:

1. Distance and near visual acuity
2. Horizontal and vertical heterophoria at distance and near
3. Distance and near stereopsis
4. Fusion
5. Colour vision
6. Fogging test
7. Visual field
8. Astigmatism.

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