Vision and perception
The human eye: how we see a screen
An LED screen exists only thanks to the eye that looks at it. Anyone who wants to understand screens therefore starts with the eye: a surprisingly camera-like organ with two kinds of light cells for day and night.
Your eye actually works like a camera. At the front sits a little lens that brings the light into focus. At the back, on the retina, sit millions of tiny light cells that capture the image and pass it on to your brain.
There are two kinds of light cell. One kind (the cones) works in daylight and sees colour. The other kind (the rods) works in the dark, but sees only shades of grey. That is why we see almost no colour at night!
Optically, the eye closely resembles a camera: it gathers reflected light and brings the image into focus. The light passes in turn through the cornea, the pupil (the opening whose size the iris controls), and the lens, which changes shape via small muscles to focus. The image falls on the retina, which converts light into electrochemical signals and sends them to the brain via the optic nerve.
Cones and rods
The retina contains two kinds of photoreceptor. The cones provide colour vision and sharp vision when there is enough light (photopic conditions); there are about 8 million of them, largely concentrated in the fovea. The rods work only in low light (scotopic conditions), see no colour, and number about 120 million.
The fovea: small but decisive
The fovea is a minuscule area at the centre of the retina, packed with cones. It covers only about 2% of the field of view, yet delivers the sharpest vision and the most colour information. Beyond it the number of cones drops sharply; the number of rods falls off drastically past a field angle of about 20°. That explains why you only really see a screen sharply when you look straight at it, and why the placement of a sign along the road is so important.
The retina contains ~120 million rods and ~8 million cones. Remarkably, both photoreceptor types face away from the incoming light. Cone density is maximal in the fovea (~2% of the field of view) and remains low but fairly constant beyond it; rod density falls sharply past ~20°. Information from the left eye goes to the right hemisphere and vice versa; the occipital lobe is the primary processing area and receives the lion’s share of its input from the fovea.
Photopic versus scotopic vision
At daylight levels (photopic), the cones dominate; sensitivity peaks around 555 nm and colour vision is full. At very low light levels (scotopic), the rods take over; the sensitivity peak shifts to ~507 nm (the Purkinje shift) and colour perception disappears. A display designer counts on photopic conditions, and therefore on the cone response and V(λ), when determining brightness and colour.
Consequence for display design
Because acuity and colour are carried by the fovea, the angular size of characters on the retina is decisive for legibility: it depends on both character height and viewing distance. That is the physiological basis underlying the viewing-distance calculation and the importance of placing a screen within the target audience’s cone of vision.