MCA Research Project - RP 602 Dark Adaptation and Lookout Duties

The Health and Safety Laboratory undertook a study of visual tasks and lighting levels aboard two types of ship and these were related to the available literature on the process of dark adaptation in the eye and the minimum standards for lights on large and small vessels. This study was commissioned by MCA to investigate dark adaptation in the context of the practical aspects of night time lookout duties.

Lighting levels on most parts of the bridges studied were within the recommended standards. The highest light exposures arose where paper charts were used. These levels were in excess of recommendations and may have led to some temporary loss of function. This was probably less significant than the temporary loss that arose when those with lookout duties came back to these duties having previously been in lit areas of the ship. Recommended handover times were not regularly followed in this situation.

The lighting levels on the ships’ bridges studied were such that eyes were not fully dark adapted but functioning in the middle (mesopic) mode where both the precision and colour receptors (the cones) and the low light receptors (the rods) played a part in vision.

Individual variables, such as age and some forms of illness, are important determinants of both the rate at which dark adaptation is achieved and the threshold for perception.

The minimum required level of navigation lights on large ships is such that these can readily be seen at the distances specified by those who have adapted to the lighting levels found on the bridges studied. The levels of lighting required for small craft as well as their position in relation to the bridge as a vantage point are likely to be less reliably seen under the lighting levels found on the bridges studied. Perception of unlit objects will also be less than optimal.

The overall effectiveness of the Lookout will be determined by factors other than dark adaptation. Vision complements information on nearby vessel movements from radar and other electronic sources, although this too is less reliable for small craft. Night time lookout duties often rely on continuing alertness to seek rare events in an monotonous field of view. Vigilance is as important as visual thresholds in determining whether an object will be seen and correctly recognized as significant.

The authors of the report, based on the findings summarized above, have identified the following topics where further consideration is needed because they could contribute to improved effectiveness of night time lookout.

1. The scientific studies of dark adaptation can only be extrapolated to the conditions on a bridge at night. Studies that simulated the actual lighting levels encountered would provide more reliable information on which to base recommendations for good practice.

2. Three measures to retain dark adaptation levels should be evaluated. Speeding the move to ECDIS charts and related systems will avoid the temporary impairment to vision that follows exposure to the lighting levels needed to work on paper charts. The use of red light or red goggles are well tried methods of maintaining dark adaptation when entering lit areas: the main practical issues are the acceptability of such aids and the limitations they place on tasks such as work on coloured charts. Prolonged exposure to bright sunlight during the day will also impair dark adaptation: avoidance or use of heavily tinted sunglasses can reduce this.

3. Improvements to the working environment and practices on the bridge can contribute to better dark adaptation. The recommended 15 minute handover period for Lookouts coming on watch needs to be followed habitually. Exposure to light on the bridge could be reduced by better design of bridge layout and of patterns of work. Vigilance is essential; there is good evidence that this is reduced by long periods of monotonous work, especially if fatigued.

4. The specification of navigation lights for small craft is such that they may not always be seen. Changes to the form and levels of light need to be considered.

5. The vision tests used to determine fitness for lookout duties do not assess the visual requirements for night time lookout, beyond confirming adequate colour perception. Further work would be required to establish if there are practical tests for use, provided that the acceptability of selection for night time lookout duties based on such test is acceptable.

6. Unlike naval Lookouts, those in the merchant navy are not formally trained in how to scan for objects at night or how to use the different visual capabilities of different parts of their field of vision to optimize performance. The introduction of such training is being considered.

In conclusion, the project has identified some limitations to the effectiveness of night time visual lookout under current prevailing conditions and suggests a number of measures that could be considered to improve performance. Some of these are operational practices that ship operators could adopt, others are for MCA to consider, while a number would require international agreement to be effective.