See­ing In The Dark

The Oban Times - - LEISURE - with John Wal­lace

It’s of­fi­cial. The clocks have gone back and win­ter is well on its way. You will prob­a­bly be trav­el­ling to and from work in the dark. The hu­man eye is amaz­ingly adap­tive and can cope with most light lev­els. How­ever, your eyes don’t work as well at night as they do in day­light.

Our reti­nas have two forms of light re­cep­tors …rods and cones. Rods work in low light sit­u­a­tions and cones, which give us colour vi­sion, in day­light. Cones tend to be densely packed to­gether at the fovea (the cen­tre of our retina). That’s why we have high def­i­ni­tion vi­sion in day­light. It’s also why we need the lights on at night if we want to read or see clearly when driv­ing. In low light lev­els cones switch off and the rods kick in. Rods are spread across the retina but are dens­est slightly off­set from the fovea. Have you ever no­ticed when look­ing at stars in the night sky you don’t see them if you look straight at them but if you look slightly off to one side they be­come vis­i­ble? That’s be­cause you are us­ing your rods which are very sen­si­tive in low light con­di­tions. Cones only func­tion in high light con­di­tions. If you drive you have prob­a­bly no­ticed at dusk you re­ally can­not see very well. That’s be­cause it’s too bright for rods and too dark for cones.

When we drive we use our cen­tral vi­sion (cones) for HD vi­sion and our pe­riph­eral retina is more for pe­riph­eral aware­ness and mo­tion de­tec­tion. As we age cataracts de­velop. These cause prob­lems with glare from car head­lights. Some of us will de­velop mac­u­lar de­gen­er­a­tion which re­duces the clar­ity of our vi­sion and our eyes take much longer to re­cover from be­ing daz­zled by on­com­ing bright lights.

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