The causes of reduction of the visual field, whether anatomical or physiological, are very numerous: narrowing of the pupil; opacity of the lens; pathological conditions of the retina, visual pathways or visual centres; the brightness of the target to be perceived; the frames of spectacles for correction or protection; the movement and speed of the target to be perceived; and others. A good visual acuity is the ability to distinguish fine details.
Visual acuity defines the limit of spatial discrimination. The retinal size of an object depends not only on its physical size but also on its distance from the eye; it is therefore expressed in terms of the visual angle usually in minutes of arc. Visual acuity is the reciprocal of this angle. In clinical and occupational practice, the recognition task, in which the subject is required to name the test object and locate some details of it, is the most commonly applied.
In clinical practice Snellen charts are the most widely used tests for distant visual acuity; a series of test objects are used in which the size and broad shape of characters are designed to subtend an angle of 1 minute at a standard distance which varies from country to country in the United States, 20 feet between the chart and the tested individual; in most European countries, 6 metres.
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Larger test objects which form an angle of 1 minute of arc at greater distances are also provided. In optometric practice, the objects are often letters of the alphabet or familiar shapes, for illiterates or children. However, when the test is repeated, charts should present unlearnable characters for which the recognition of differences involve no educational and cultural features.
This is one reason why it is nowadays internationally recommended to use Landolt rings, at least in scientific studies. Landolt rings are circles with a gap, the directional position of which has to be identified by the subject.
Vision: The Visual System, the Eye, and Color Vision
Except in ageing people or in those individuals with accommodative defects presbyopia , the far and the near visual acuity parallel each other. Most jobs require both a good far without accommodation and a good near vision. Snellen charts of different kinds are also available for near vision figures 2 and 3. Figure 2. Example of a Snellen chart: Landolt rings acuity in decimal values reading distance not specified. Example of a Snellen chart: Sloan letters for measuring near vision 40 cm acuity in decimal values and in distance equivalents.
For occupational practice, several types of visual testers are available on the market which have similar features; they are named Orthorater, Visiotest, Ergovision, Titmus Optimal C Tester, C45 Glare Tester, Mesoptometer, Nyctometer and so on. They are small; they are independent of the lighting of the testing room, having their own internal lighting; they provide several tests, such as far and near binocular and monocular visual acuity most of the time with unlearnable characters , but also depth perception, rough colour discrimination, muscular balance and so on.
Near visual acuity can be measured, sometimes for short and intermediate distance of the test object. The most recent of these devices makes extensive use of electronics to provide automatically written scores for different tests. Moreover, these instruments can be handled by non-medical personnel after some training. Vision testers are designed for the purpose of pre-recruitment screening of workers, or sometimes later testing, taking into account the visual requirements of their workplace.
Table 1 indicates the level of visual acuity needed to fulfil unskilled to highly skilled activities, when using one particular testing device Fox, in Verriest and Hermans Table 1. Visual requirements for different activities when using Titmus Optimal C Tester, with correction. It is recommended by manufacturers that employees are measured when wearing their corrective glasses. Fox , however, stresses that such a procedure may lead to wrong results—for example, workers are tested with glasses which are too old in comparison with the time of the present measurement; or lenses may be worn out by exposure to dust or other noxious agents.
It is also very often the case that people come to the testing room with the wrong glasses. Other deficiencies of vision testers are referred to later in this article. VA meets its first limitation in the structure of the retina. In night vision, acuity is very poor or nil at the centre but may reach one tenth at the periphery, because of the distribution of cones and rods figure 4.
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Density of cones and rods in the retina as compared with the relative visual acuity in the corresponding visual field. The diameter of the pupil acts on visual performance in a complex manner. When dilated, the pupil allows more light to enter into the eye and stimulate the retina; the blur due to the diffraction of the light is minimized. A narrower pupil, however, reduces the negative effects of the aberrations of the lens mentioned above. Thanks to the process of adaptation it is possible for the human being to see as well by moonlight as by full sunshine, even though there is a difference in illumination of 1 to 10,, Visual sensitivity is so wide that luminous intensity is plotted on a logarithmic scale.
On entering a dark room we are at first completely blind; then the objects around us become perceptible. As the light level is increased, we pass from rod-dominated vision to cone-dominated vision. The accompanying change in sensitivity is known as the Purkinje shift. The dark-adapted retina is mainly sensitive to low luminosity, but is characterized by the absence of colour vision and poor spatial resolution low VA ; the light-adapted retina is not very sensitive to low luminosity objects have to be well illuminated in order to be perceived , but is characterized by a high degree of spatial and temporal resolution and by colour vision.
After the desensitization induced by intense light stimulation, the eye recovers its sensitivity according to a typical progression: at first a rapid change involving cones and daylight or photopic adaptation, followed by a slower phase involving rods and night or scotopic adaptation; the intermediate zone involves dim light or mesopic adaptation. In the work environment, night adaptation is hardly relevant except for activities in a dark room and for night driving although the reflection on the road from headlights always brings some light.
Simple daylight adaptation is the most common in industrial or office activities, provided either by natural or by artificial lighting. However, nowadays with emphasis on VDU work, many workers like to operate in dim light. In occupational practice, the behaviour of groups of people is particularly important in comparison with individual evaluation when selecting the most appropriate design of workplaces.
The results of a study of office workers in Geneva Meyer et al. It may be seen that, once adapted to daylight, most of the tested workers with eye correction reach a quite high visual acuity; as soon as the surrounding illumination level is reduced, the mean VA decreases, but also the results are more spread, with some people having very poor performance; this tendency is aggravated when dim light is accompanied by some disturbing glare source figure 5. In other words, it is very hard to predict the behaviour of a subject in dim light from his or her score in optimal daylight conditions.
Recovery time after disabling glare may last several seconds, depending on illumination level and contrast Meyer et al. Figure 6.
Response time before and after exposure to glare for perceiving the gap of a Landolt ring: Adaption to dim light. Local disadaptation is usually accompanied by the continued image of a bright spot, coloured or not, which produces a veil or masking effect this is the consecutive image. Afterimages have been studied very extensively to better understand certain visual phenomena Brown in Graham et al. After visual stimulation has ceased, the effect remains for some time; this persistence explains, for example, why perception of continuous light may be present when facing a flickering light see below.
If the frequency of flicker is high enough, or when looking at cars at night, we see a line of light. These afterimages are produced in the dark when viewing an enlighted spot; they are also produced by coloured areas, leaving coloured images. It is the reason why VDU operators may be exposed to sharp afterimages after looking for a prolonged time at the screen and then moving their eyes towards another area in the room. Afterimages are very complicated. Another experiment showed that an orange-red field appeared momentarily pink, then within 10 to 15 seconds passed through orange and yellow to a bright green appearance which remained throughout the whole observation.
When the point of fixation moves, usually the afterimage moves too Brown in Graham et al. Such effects could be very disturbing to someone working with a VDU. Ergophthalmologists also describe discomfort glare, which does not reduce visual acuity but causes uncomfortable or even painful sensation IESNA The level of illumination at the workplace must be adapted to the level required by the task.
If all that is required is to perceive shapes in an environment of stable luminosity, weak illumination may be adequate; but as soon as it is a question of seeing fine details that require increased acuity, or if the work involves colour discrimination, retinal illumination must be markedly increased. Table 2 gives recommended illuminance values for the lighting design of a few workstations in different industries IESNA Brightness contrast and spatial distribution of luminances at the workplace.
From the point of view of ergonomics, the ratio between luminances of the test object, its immediate background and the surrounding area has been widely studied, and recommendations on this subject are available for different requirements of the task see Verriest and Hermans ; Grandjean It thus varies from 0 to 1.
As shown by figure 7, visual acuity increases with the level of illumination as previously said and with the increase of object-background contrast Adrian This effect is particularly marked in young people. A large light background and a dark object thus provides the best efficiency.
Vision: The Visual System, the Eye, and Color Vision
However, in real life, contrast will never reach unity. Figure 7. Relationship between visual acuity of a dark object perceived on a background receiving increasing illumination for four contrast values. When video display units appeared on the market, letters or symbols were presented on the screen as light spots on a dark background. Later on, new screens were developed which displayed dark letters on a light background. Many studies were conducted in order to verify whether this presentation improved vision. The results of most experiments stress without any doubt that visual acuity is enhanced when reading dark letters on a light background; of course a dark screen favours reflections of glare sources.
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The functional visual field is defined by the relationship between the luminosity of the surfaces actually perceived by the eye at the workpost and those of the surrounding areas. Care must be taken not to create too great differences of luminosity in the visual field; according to the size of the surfaces involved, changes in general or local adaptation occur which cause discomfort in the execution of the task. Moreover, it is recognized that in order to achieve good performance, the contrasts in the field must be such that the task area is more illuminated than its immediate surroundings, and that the far areas are darker.
Time of presentation of the object. The capacity to detect an object depends directly on the quantity of light entering the eye, and this is linked with the luminous intensity of the object, its surface qualities and the time during which it appears this is known in tests of tachystocopic presentation.
Movements of the eye or of the target. Loss of performance occurs particularly when the eye jerks; nevertheless, total stability of the image is not required in order to attain maximum resolution. But it has been shown that vibrations such as those of construction site machines or tractors can adversely affect visual acuity. Visual acuity is higher in binocular than in monocular vision. Binocular vision requires optical axes that both meet at the object being looked at, so that the image falls into corresponding areas of the retina in each eye.
This is made possible by the activity of the external muscles. If the coordination of the external muscles is failing, more or less transitory images may appear, such as in excessive visual fatigue, and may cause annoying sensations Grandjean In short, the discriminating power of the eye depends on the type of object to be perceived and the luminous environment in which it is measured; in the medical consulting room, conditions are optimal: high object-background contrast, direct daylight adaptation, characters with sharp edges, presentation of the object without a time limit, and certain redundancy of signals e.
Moreover, visual acuity determined for diagnosis purposes is a maximal and unique operation in the absence of accommodative fatigue. Clinical acuity is thus a poor reference for the visual performance attained on the job.
What is more, good clinical acuity does not necessarily mean the absence of discomfort at work, where conditions of individual visual comfort are rarely attained. At most workplaces, as stressed by Krueger , objects to be perceived are blurred and of low contrast, background luminances are unequally scattered with many glare sources producing veiling and local adaptation effects and so on. According to our own calculations, clinical results do not carry much predictive value of the amount and nature of visual fatigue encountered, for example, in VDU work.
A more realistic laboratory set-up in which conditions of measurement were closer to task requirements did somewhat better Rey and Bousquet ; Meyer et al. Krueger is right when claiming that ophthalmological examination is not really appropriate in occupational health and ergonomics, that new testing procedures should be developed or extended, and that existing laboratory set-ups should be made available to the occupational practitioner.
Binocular vision allows a single image to be obtained by means of synthesis of the images received by the two eyes.