Ocular Disease and Ocular Fatigue Due to the Working Environment in ATC

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Ocular Disease and Ocular Fatigue Due to the Working Environment in ATC

17TH ANNUAL CONFERENCE, Copenhagen, Denmark, 24-28 April 1978

WP No. 85

Ocular Disease and Ocular Fatigue Due to the Working Environment in ATC

 

The problem of ocular diseases was included in the working programme of the Medical Sub- Committee during the Lyon Conference in 1976. Initial investigations revealed the importance and complexity of this subject. The importance of eyesight for an air traffic controller whether in the control tower or in front of a radar screen need not to be emphasised. The integrity of the visual perception is one of the principal information factors leading to the final decision of the controller. The complexity arises from the fact that the controllers’ eyesight is put to the test during the whole of his working life and under very different conditions. It is not unusual for him to pass from a sunlight control tower position to a semi-dark radar room. Radar work is a proven source of eye fatigue: The fact that a controllers’ eye is continually taking in the various pieces of information which are presented by diverse means (strips, cathode-ray screens, television screens, maps etc.) and which are of different luminous intensities, is also a cause of eye strain and can even lead to eye troubles. In various studies eye diseases are cited as having a probable or evident relation to the stress inherent in our profession.

The above considerations lead the Medical Sub-Committee to continue investigation among MA’s and a questionnaire was sent out in September 1976. MA’s reply and some significant results are published in the appendix (A1). It can be seen that very few cases of eye disease have been notified and it is because of this that we have concerned ourselves even more so with the eye strain problems common to our profession. We have been particularly interested in a study of this subject by Dr. J CHEVALLERAUD and Dr. G. PERDRIEL entitled: “Visual problems provoked by reading radar screens”. This paper was presented at the 5th Symposium on Ergophtalmology at Bordeaux, France (23/24 May ’74). Here we quote this study almost in its entirety except for certain points which would be evident for a controller and for certain medical considerations too technical for the profane.

“Visual problems provoked by reading radar screens.” J. Chevalleraud – G. Perdriel

The growth of air traffic and airspace control has rendered the establishment of electromagnetic detection necessary since a number of years. The radar operator has taken a permanent and necessary place among aeronautical technicians. It is he who guarantees air safety.

We propose to describe briefly:

  • working conditions
  • operator fatigue
  • methods chosen or which should be chosen to improve the working conditions as well as the criteria for selecting and looking after such personnel.

Working Conditions

Control rooms are usually well equipped with units which are more or less noisy. To this noise is added the necessary conversations between ground and air. The work is generally carried out in low light conditions. Ventilation is often deficient and the room temperature unsuitable. The small size of one spot on the screen together with its lack of intensity forces the operator to work close to the screen (about 30cm/1ft), which is close to the near accommodation limit for a normally sighted subject. The rotating beam provokes a rotation of the eyeballs which has been proven by cinematographic means. In addition, on the screens appear spots corresponding to moving objects, echoes of high luminous intensity from large surfaces together with meteorological conditions. These superimposed images often cause dazzling which the eye must eliminate.

Finally, the eye which has detected an echo must determine its type, fixed or mobile. To do this it must follow the trace on the screen, leading to horizontal, vertical and rotating movements which take place spasmodically. This work can be carried out only if the subject concentrates all his attention on the screen. The working conditions as described allow us to envisage easily the visual problems which arise:

  • adaption for low light level
  • repeated dazzling
  • extrinsic and intrinsic muscular effects

Operator’s Eye Fatigue

What is the origin of the eye fatigue of which operators complained? All authors agree in thinking that it cannot be of retinal origin. The real source of eye fatigue is situated at the level of the ciliary muscle without affecting visual acuity. This fatigue reveals itself in close vision by a sensation of fogging between the eye and the scope. If the eyes are not relaxed, e.g. by closing them or looking at distance objects, this discomfort continues to increase. Smartings, eyeburns and watering of the eye begins, followed by frontal headache accompanied by photophobia and a feeling of eyestrain. At the same time occurs a congestion in the free edge of the eyelid leading eventually to blepharitis. Sometimes hyperhemia pertaining the conjunctiva becomes evident. All these conditions diminish with a period of repose. They are accentuated or appear more rapidly when the subject is less resistant, either temporary or permanently. There would thus appear to be a link between eye fatigue and general fatigue.

What are the causes and the consequences of eye fatigue?


A. Intrinsic Causes

Ametropia are found in 49% of subjects who complain of eye fatigue.

  • Anomalies of oculo-motor equilibrium also frequently found.
  • Anomalies of night vision generate fatigue. Pour adaptation to ambient lighting conditions and frequent repetition of dazzling with poor recuperation make working conditions difficult.
  • Subjects show a dychromatipsy and seem to become tired more quickly.
  • General or muscular fatigue certainly affects vision.
  • Insomnia and the place which the work period occupies in the day – night cycle also intervenes.

B. Extrinsic Causes

These are physical and chemical.

  • The physical causes are due to dazzling and the contrast in luminosity between the echoes and the background illumination. Dazzling is produced each time the operator leaves the radar room passing from attenuated to very bright lighting. The same effect is produced when the contrast between the luminosity of the screen and the ambient lighting is too great.
  • Sound levels in the control room also play a role.
  • The chemical causes become evident if the climatisation is insufficient. The confined area modifies the oxygen and carbonic gas content. In addition, the fumes given off contribute to modify the atmosphere, creating an additional cause of irritation.

C. Consequences of Fatigue

The eye fatigue leads to headache and certainly is the cause of a diminution of attention which can lead to errors. Certain blips are not seen or falsely identified with regard to their speed or trajectory.

A study by Hans and Zetzmann has shown that the number of omissions is a function of the number of stimuli per hour: For 240 stimuli/h there are 2.9% of omissions and 7% of errors. For 24 stimuli/h there are 15.8 % of omissions and 34.5 % of errors.

These authors also have carried out a study of the risks of accidents with one or two observers on the same screen: For a single observer the risk of collision increases sharply after the 90th minute. For two observers the risk is considerably reduced.

Measures for improving the working conditions of radar operators

Such personnel must first of all be selected and supervised to check if they maintain their ability. Candidates with myopic astigmatism greater than 1,5 diopter as well as those with hypermetropia greater than 2 diopter should be eliminated. Also those with oculo-motor disequilibrium and poor binocular vision should not be admitted. Good night vision should be required as well as the integrity of the back of the eyeball so as to eliminate chorretinal anomalies. From the chromatic point of view anomalies can be tolerated but it would be preferable to eliminate them. Working conditions should be improved by perfect climatisation of the control rooms, taking care that the air is renewed and the temperature and moisture content are kept constant. Transition zones between the dim lighting of the control rooms and the exterior should be carefully organised. The ambient lighting should be attenuated and be of a wavelength approaching that of daylight. Care should also be taken of the acoustic ambience and the seating comfort. Scopes should be well regulated so as to obtain the best conditions for echo detection. Their luminance should be of the order of 0.07 nits. The ambient luminance should never be greater than that of the scopes. Finally, operators should have access to a calm, comfortable restroom, near the radar room, where they can relax.

Conclusions

The work of a radar operator, the details examined, are undoubtedly causes of fatigue. It can be carried out satisfactorily only if the working conditions as described are established. The work – rest relationship should also be controlled. A break every 90 minutes would appear necessary both for the comfort of the operator and for the safety of his mission.

Another study made in France reveals some interesting facts on eye problems. The paper is “Material Working Conditions, Hygiene and Security, Working Hours”. It forms part of the “Scientific Study on Physiological Conditions Encountered in an Air Traffic Control Environment” by Dr, Cooby and Nolland.

The adjustment of the working place to the subject should take into account a certain number of physiological conditions among which is visual adaptation. Visual adaptation is very important in control work. It calls for a strict selection on admission and a regular surveillance during the career. The integrity of the visual perception is one of the principal pieces of information, leading to the final decision of the controller.

Visual perception brings into play complex phenomena of treatment on the level of the central nervous system and its quality is thus closely connected, on the one hand to the quality and the limits of adaptation of the ocular receptor bound by the fundamental laws of physics and, on the other hand on the general physical and mental behaviour of the controller, on his adaptation to several professional factors and on his physiological ageing process.


Recommendations concerning general lighting conditions

  • It should be possible to adjust the lighting intensity by zone.
  • Direct lighting should be attenuated as much as possible to avoid reflections. Direct lighting sources should not be in the field of vision when reading information situated on the upper part of the console.
  • The attenuation of ambient lighting so as to render the radar scope symbols or information on television screens more visible may appear satisfying but the only effect of this is to make visual accommodation, already under strain, even more difficult.
  • The search for a better visual perception of information must find a solution in technical modifications to the actual means of visualisation.

Recommendations concerning the screen

  • The mirror effect, that means reflections either of images behind the controller by which he is unconsciously affected, or the luminous projections coming from the openings of the radar room create a dazzling condition which interferes with the perception of symbols. The use of mat screens is strongly recommended from a medical point of view, the background colour is an important factor in the quality of the screen. The blue range makes the eye slightly myopic especially in low light levels. By contrast the red range provokes hypermetropy calling for an extra effort of accommodation, and, interfering with normal activity especially on subjects about to develop presbyopia or showing signs of hypermetropic astigmatism.

Recommendations concerning television screens, cathode ray screens and other sources of information

  • The position of these sources of information is important. The greater a reading angle (70 degrees or more) is, the more the source of error due to parallax and visual and muscular fatigue becomes important.
  • Character size should be chosen in such a manner so as to be readable by all controllers including those with presbyopia and hypermetropia and corrected by bifocals. The latter are generally difficult to support due to the constant changes of accommodation provoked by different visual planes.
  • It also is recommended that the lighting level should be adjustable at each work place.

A meeting of doctors specialising in the medical aspects of working conditions held in Geneva, December 1977, under the auspices of the French-speaking Ergonomic Society, drew the attention of the public to the problems posed by cathode-ray (television) screens. These screens, used more and more in control centres for flight-planning information, cause excessive visual fatigue among users. This fatigue is even greater if the information is renewed frequently or if the operator is unable to adjust his screen, or if the operator’s eye has to change often from the screen to another document.

Specialists recommend a certain number of technical improvements (e.g. better contrast), and advise users to take care of lighting conditions in which the screens should be used (individually adjustable lighting). In addition, frequent rest periods would appear of great necessity in the interest of the enterprise. The doctors note that it is becoming urgent to modify the ophthalmic examination to which employees using these screens are subjected.

A recent article by Dr. A. Serati, head of the Medical Service of Swiss Administration, entitled “Ergonomic Aspects of Work with Visual Terminals”, analyses these same problems and proposes the following measures to avoid certain inconveniences inherent in the systems:

  • Respect of ergonomic rules in designing work tables and chairs.
  • Creation of favourable microclimatic conditions.
  • Adjustable lighting of work places.
  • Elimination of sources of reverberation and dazzling.
  • Suppression of high intensity noise sources.
  • Judicious organisation of working timetable (breaks included).
  • Maintenance of agreeable human relation.

The various studies which are quoted above lead all in the same direction and to the following propositions:

It is recommended that air traffic controllers undergo an annual ophthalmic examination which takes into account real working conditions. That the equipment used by controllers and the entirety of the working place be checked by oculists so that it is conform to the requirements necessary for the safety of air traffic and health of the controllers.

It is recommended that air traffic controllers working with visual terminal, radar operators take a break very 90 minutes.

It is recommended that the rules of ergonomics be respected in the design of work places and that optimum microclimatic conditions be obtained or maintained.

It is recommended that great attention has to be paid to the lighting conditions of the workstation. Indirect light, adjustable by zones, similar to daylight provoking neither reflections nor dazzling is very important. The luminosity should be adjustable at each working place and light sources be cleaned regularly and replaced if necessary.

The employer of air traffic controllers should take the necessary steps to protect the eyesight of employees and in particular provide sunglasses.

It is recommended that the Plenary Assembly accepts this WP as information material and the recommendations to form part of the professional policy of IFATCA.

ANNEX TO WP 85 of 1978

MEDICAL GLOSSARY

ASTIGMATISM: an anomaly of vision provoked by an inequality of the curvature of the cornea.

MYOPIA: excessive curvature of the lens, causing near sightedness.

PRESBYOPIA: diminution of the power of accommodation of the lens, preventing near vision (frequent in the elderly).

HYPERMETROPIA: the inverse of myopia, or farsightedness.

MESOPIC: state of vision at weak luminosity; ex: dawn or dusk without artificial lighting.

PUNCTUM PROXIMUM- OF ACCOMMODATION: the point closest to the eye where vision is in focus.

EMMETROPE: one having normal vision.

LUMINANCE: the intensity of light emitted by a surface.

EXTRINSIC: coming from outside an organism.

INTRINSIC: coming from within an organism.

CILIARY MUSCLE: muscles forming a ring around the cornea and controlling the size of the pupil.

ACCOMMODATION: the reflex modification of the curvature of the lens which permits the formation of sharp images in the eye.

PHOTOPHOBIA: intolerance to light which causes painful sensations.

BLEPHARITIS: inflammation of the eyelid.

CONJUNCTIVAL HYPERAEMIA: redness of the membrane covering the eye, caused by excessive blood in its vessels.

AMETROPIA: all the anomalies of vision due to faults in the eye.

DISCHROMATISM: partial colour vision in which only two colours are seen.

NYCTHEMERE: duration of 24 hours including day and night.

RETINA: the membrane lining the interior of the eye, sensitive to light.

STIMULUS: brief excitation of an organ.

DIOPTER: unit of measure for optical systems.

OCULOMOTOR: that which moves the eye.

CHORIORETINAL: the external (choroid) and internal (retina) membranes of eye taken together.

ANOPSIA: brief blindness.

BINOCULAR: having paired vision, two eyes.

HYGROMETRY: the measurement of atmospheric humidity.

NITS: units of luminosity = 1 candle/meter squared.

CATHODE SCREEN: luminescent screen excited by electrons and thereby producing a picture.

ERGONOMY: the study of the body relative to work or energy output.

Last Update: September 19, 2020  

November 23, 2019   240   Jean-Francois Lepage    1978    

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