In theory, searching at night is easier than searching in broad daylight. At night the torch-beam offers a limited area for the searcher’s attention, and there will be little or no ‘visual noise’. Paradoxically, in practice it is more difficult to search at night. The searcher is deprived of much of the visual information that would enable them to detect an object. Objects are generally reduced to uniformly dark shapes. Little is seen in the way of color. The distinction between an object and its background will disappear if they are of the same tonal value, and the object will in effect become invisible.

You can see much more in the dark than you realize. However, to take full advantage of this ability it is important to understand how your eyes are constructed and how to best use them in conditions of poor visibility.

Your eyes are like a simple camera:-

         • The lens focuses light entering the eye

         • The iris opens and closes to regulate the amount of light entering the eye through the pupil

         • The retina is like the camera film where images are formed. These are transmitted to the brain via the optic nerves. The brain tells us what we see.

 

The retina is made up of photo-receptors called cones and rods. The cones enable us to see color, shape and contrast. A great deal of light is needed to activate them and they are ‘blind’ during periods of low illumination. These are your ‘day’ eyes. These cone cells are concentrated in the centre of the retina, directly behind the lens.

Rod cells are found mainly around the outside of the cone cells and these become active in darkness or periods of low illumination – these are your ‘night’ eyes. They allow you to distinguish black and white and general outlines. They are not sensitive to color.

Principles of night vision:-

Dark adaptation – allow your eyes to become accustomed to low light. It can take about 30 minutes to achieve full night vision, though the eyes start to adapt (change from cones to rods) after about 10 minutes. The

rod cells produce a chemical substance or visual pigment (rhodopsin) which constantly renews itself. This makes them active and enables you to distinguish objects in dim light.

Off-centre vision – look at the object without looking directly at it. When you look directly at an object the image is formed in the cone region which is not sensitive at night. If you look slightly left, right, above or below an object the image will be formed in the area of rod cells. This is usually about 6 – 10 degrees away from the object.

Scanning – use off-centre vision to observe an area or object. When you use rod vision the visual pigment blacks out in about 5 to 10 seconds and the object observed disappears. As the visual pigment bleaches out in one area of the rod cells you must shift your eyes slightly so fresh rod cells are used. Move your eyes in short, abrupt, irregular movements over and around the target without looking directly at it. Pause a few seconds at each point of observation as your eyes cannot see while in motion.

A searcher needs two kinds of lighting:

       A personal light to enable them to see where they are putting their feet; this is typically a head-torch

    A searching light; this is typically a hand-held torch with a powerful beam

 

Lights should be used sparingly so as not to impair night vision. Also be aware of the obvious but often overlooked fact that the batteries in these do not last for ever; as the battery discharges then the light will diminish and less will be seen. Even the most powerful searching light is not much use by the end of a night spent searching.

Searcher discipline is important. If searchers are exposed to the light from a colleague’s searching light or head-torch then their night vision will be seriously impaired.

The problem for the searcher is made even worse because of the difficulties of moving across the terrain they find themselves in. That will take more effort and attention than it would do in daylight. The amount of searching that can be done at night will be less than could be done in the same area in daylight. Searching will be more difficult and be