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VISION
Whether driving a car or crewing a boat the importance of seeing and correctly interpreting the information provided by the eye cannot be understated.  Probably more that 80% of us have never been trained to use our eyes for the best results. PROPER LOOKING TECHNIQUES ARE NOT AUTOMATIC - THEY MUST BE LEARNED & PRACTISED.  Read on for a better understanding

           The sensory organs for vision - the eyes - are at the front of the head, but the actual visual sense is provided by areas of the brain at the back and sides. Nerve impulses generated by "rods" and "cones" in the retinas of the eyes travel along the optic nerves to the optic chiasma, where they partially cross over. "Mixed" impulses from both eyes pass through the optic tracts to the striate cortex at the back of the brain and end in the temporal lobe area so that right and left halves of the visual field merge. When light rays reach the retina (the film of the eye's camera), light energy is converted into electrical nerve signals. Crisscrossed with blood vessels, the retina has three layers of microscopically thin nerve cells. Nearest to the lens is a layer of ganglion cells, then a layer of bipolar cells and finally the photoreceptors. It is the photoreceptors that actually process the packets of light energy or photons that impact on the retina, so light must pass through the ganglions and bipolar cells to get to others.

           There are two types of photoreceptor cells which, because of their shapes, are called "rods" and "cones." Rods are sensitive enough to respond to a single photon, the basic unit of light, but together they create only one coarse, gray image, which is just adequate for seeing in poor light. Fine detail and color come from the cones, but they need a lot more light and work best in broad daylight. Inside the human eye, there are eighteen times more rods than cones. These are arranged in such a way as to produce the best possible combination of night and day vision.

       The bottom figure shows the distribution of rods and cones in the retina. This data was prepared from histological sections made on human eyes.
       In the top figure, you can relate visual angle to the position on the retina in the eye.
       Notice that the fovea is rod-free and has a very high density of cones. The density of cones falls of rapidly to a constant level at about 10-15 degrees from the fovea. Notice the blind spot which has no receptors.
       At about 15°-20° from the fovea, the density of the rods reaches a maximum. A longitudinal section would appear similar however there would be no blind spot. Remember this if you want to present peripheral stimuli and you want to avoid the blind spot.
       It follows that, at night or when light levels are very low and the rods are more effective, faint objects will be best seen by "looking" about 10-15 degrees away from the expected position.

Practical Visual Scanning Techniques
            Vision is vulnerable to just about everything : dust, fatigue, emotion, germs, fallen eyelashes, age, optical illusions, alcohol etc. In many activities our vision is altered by factors such as atmospheric conditions, dirty or distorted windows, acceleration, glare, heat & cold, light conditions and comfort. Most of all the eye is vulnerable to vagaries of the mind. We can 'see' and identify only what the mind lets us see. For example lack of concentration, daydreaming and staring into space does not allow the mind to register things that are in the field of vision - to say nothing of the things that might be in peripheral view. One function of the eye that is a constant problem (although a person is probably never aware of it) is the time required for accommodation. Our eyes automatically accommodate for or refocus on near and far objects. The change from something close like an instrument in a usually dark panel to an object much further away takes at least a second, and sometimes more, for the eye to accommodate.
            Another focusing problem usually occurs where there is little or nothing to focus on at infinity. Although this is more relevant to an aviator it still has some effect - say when controlling a motor vehicle or boat. If the eye doesn't focus properly or at all, a person stares without the mind registering. This is called 'empty-field myopia'.
            Another sight inhibiting factor is caused by 'binocular vision'. To accept what we see we need to get vision from both eyes. If an object is visible to one eye but is not visible to the other because of some obstruction the total image is blurred and not always acceptable to the mind.
           Another inherent problem is that of a narrow field of vision. Although our eyes accept light rays from an arc of nearly 200 degrees they are limited to a relatively small area of 10-15 degrees in which they can actually focus on an object. Though we can perceive movement in the periphery we cannot identify what is happening. We tend not to believe what we see in the corner of our eyes. Aided by the brain this leads to 'tunnel vision'. This means that without relative motion or good contrast an object becomes much more difficult to see and register.
            The eye is also limited by environment. Limited visibility actually means limited vision. In strong light glare can make objects hard to see. Furthermore some types of sunglasses can be useless in reducing glare and, in some cases, can obstruct clear vision. On the other hand on cloudy and / or hazy days a lack of contrast will make objects more difficult to see - especially when there is little or no relative movement and the background is of a similar colour to the object.
            Then there is the mind which through many factors can distract to the point of not seeing anything at all or lull us into myopia where we stare at something without seeing it. Perception is affected by many factors. We all tend to over estimate our visual abilities and misunderstand the limitations of our eyes. We need to appreciate that proper looking techniques do not happen automatically - they need to be consciously practiced.

            Visual scanning techniques are designed to employ the movement detection capabilities of the rods, and the clear, sharply focused ability of the cones, while recognizing the limitations of the time needed to adjust and refocus while switching from one view to another. The most effective scanning patterns are based on how your eyes function, and incorporate a series of short, regularly spaced eye movements bringing successive areas of the environment into your field of vision. Successive scans should move 10 degrees and focus on that segment for at least one second. The most effective scan pattern covers the entire field of vision in successive movements and focusing of this nature. This environmental scanning technique is a continuous and ongoing activity for the early detection of possible conflicts and weather phenomena, and a general high level awareness.
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