Friday, 7 December 2007

Why do we see better at night with our peripheral vision?

It all comes down to anatomy.

Daylight is a period in which there is an abundance of photons flying around. In this time of plenty, our eyes make use of a high-demand, high-resolution colour system to provide us with images. Basically, at the "centre" of our eyes (actually, not the centre, but where the photons from something in the centre of our visual fields fall) there is an area called the fovea centralis. At this location, the retina is packed with cones - colour sensitive cells - that are so densely packaged as to provide outstanding resolution to an image. The downside is that they require large amounts of photons (of the particular wavelength) hitting them to fire.

Further out to the periphery, our retinas are manned by rods. These cells aim to scavenge any photons (regardless of their wavelengths); as such, they can collectively form an image from much less light than the cones need to fire. Unfortunately, they are not densely packed in the retina, and so give a relatively low quality image. Furthermore, they are not nearly so wave-length specific, and so only provide black-and-white images.

At night, photons are scarce. Therefore, our vision has to switch from the high-demand, high-resolution colour system of the cones to the low-demand, low-resolution black-and-white system of the rods. Since there are virtually no rods in the fovea (at the centre of our visual fields), objects are best appreciated at night by our peripheral vision, which has the rods. Astronomers are keenly aware of this fact, and usually advise you to look slightly to the left or right of a star cluster in order to get the best image of it.

Incidentally, this bit of anatomy also explains why objects at night appear black-and-white. (Amazingly, many people are unaware of this occurrence!) Furthermore, rods are so sensitive to any light that they are temporarily "blown" out of action by daylight conditions. This is why it takes time for our eyes to adjust to the dark conditions of a cinema if we walk in from the daylight outside.

Fascinating, don't you think?

No comments:

Post a Comment