What you need to know about night vision

What is night vision?
First things first, we�re talking about biological night vision, not the hi-tech equipment you see wildlife film makers using to watch animals frolicking in the dark.

Oh. Not special-ops binoculars then? Ok, so what�s biological night vision?
It's how the human eye sees in the dark. It�s a complex and clever device, the human eye, and the way it functions in the dark is different from how it functions in light. It�s all to do with rods and cones.

What have fishing and traffic management got to do with it?
Rods and cones are the two types of light detecting cells in the retina at the back of the eye that send signals to the brain and enable us to see. Cones require lots of light in order to be triggered, but are sensitive to both light and colour. It�s the eye�s six to seven million cone cells, concentrated toward the centre of the retina, we use mostly during the day. There are even more rod cells � approximately 125 million. These are far more light sensitive, requiring just a single photon to trigger them. They are almost entirely responsible for our night vision but they do not register colour.

Is that why we don�t see colour in the dark?
Exactly. You may also notice that it�s difficult to determine the colour of an object that�s right at the edge of your vision. That�s because, due to their denser concentration towards the outer edges of the retina, rods are also the primary enablers of our peripheral vision.  As a result, in low light it�s often easier to spot something if you�re not looking directly at it, but catch it out of the corner of your eye.

With you. Tell me more, but keep it simple.
We�ll try. Rods are full of a chemical called Rhodopsin. When light hits the Rhodopsin it�s broken down in a reaction that sends a message to the brain. The problem is that it takes time for the body to replace the Rhodopsin. In very bright light, all of the Rhodopsin in the rod cells is destroyed and it can take around 30 minutes for it to completely replenish. This is the time it takes for your night vision to reach maximum strength.

I�m not sure I understand...
Picture this: you�re in a bright room. The strong light means that all the Rhodopsin has been broken down and you�re just using your cone cells to see. You then step into a dark cupboard and shut the door. It�s too dark for the cones to work. You are, more or less, completely blind. However, gradually your night vision improves as the Rhodopsin in the cones is replaced until, after half an hour or so, you�re able to spot the extension lead you thought you�d lost sitting in the corner. But if the rods are exposed to light again, the Rhodopsin is depleted and the recovery process starts over.

Like if somebody shines a torch in your face?
Annoying, isn�t it? But it can be avoided. Many torches have a red-light function. The Rhodopsin in rod cells is not sensitive to, and therefore not broken down by, wavelengths in the red spectrum. As such it�s possible to use a dim red light (and it should be a dim red light) to read a map, or tie a bootlace without wrecking your night-vision.

What about the blue and green lights some torches have?
Great for discos, great for illuminating the dark (although white light is better), but also great for spoiling your night vision. Avoid.

And will eating carrots help?
The whole carrots being good for night vision thing was just Second World War propaganda. The UK didn�t want the Germans to know about a new technology they�d developed called �radar�, so put out the story that the increased number of German bombers being shot down was due to pilots� eyesight being improved by eating carrots. The fib caught on � the British public took to eating carrots to help them see during blackouts. Although carrots contain beta-carotene which the body converts to Vitamin A which aids eye health, a balanced diet gets enough Vitamin A from other sources � carrots aren�t essential.

So my mother lied to me?
We couldn�t possibly comment.