MEASUREMENT OF COLOUR

Colour is perception and sensation experienced caused by light reflected from or transmitted through objects. If you have some coloured object, it means it has certain reflectance characteristics—the patterns of light wavelengths that are reflected and absorbed—that are physical properties of object. However, colour is our subjective perception of the wavelengths of light that end up bouncing off the object and onto our retina. Photoreceptors in the retina begin the process by selectively responding to different wavelengths. A single type of photoreceptor alone cannot accomplish color vision since it cannot distinguish between enough wavelengths. We overcome this difficulty by using three different types of cone photoreceptors to code for color, a concept known as trichromacy. One of the key observations in developing the trichromatic theory of color vision was that lights can be added together to form mixtures that look identical to other, single light wavelengths. Another activity illustrates additive and other forms of color mixing.

Human Visual System

The human visual system consists of two functional parts, the eye and (part of the) brain. The brain does all of the complex image processing, while the eye functions as the biological equivalent of a camera.

What our eyes perceive of a scene is determined by the light rays emitted or reflected from that scene. When these light rays are strong enough (have enough energy), and are within the right range of the electromagnetic spectrum (about 300 to 700 nm), the healthy eye will react to such a ray by sending an electric signal to the brain through the optic nerve. When a light ray hits the eye, it will first pass through the cornea, then subsequently through the aqueous humor, the iris, the lens, and the vitreous humor before finally reaching the retina. The cornea is a transparent protective layer, which acts as a lens and refracts the light. The iris forms a round aperture that can vary in size and so determines the amount of light that can pass through. Under dark circumstances the iris is wide open, letting through as much light as possible. In normal daylight, the iris constricts to a small hole. The lens can vary its shape to focus the perceived image onto the retina. In the retina, the light rays are detected and converted to electrical signals by photoreceptors. The eye has two types of photoreceptors: rods and cones, named after their approximate shape. The rods are abundant, about 100 million in a human eye, and spread evenly about the retina, except at the fovea, where there are almost none. The fovea is the area of the retina where our vision is sharpest. There are much fewer cones, about 6 to 7 million, which are mainly located around the fovea, but can be found in a low density in the entire retina. No photoreceptors are found at the point where the optic nerve attaches to the eye (the so-called blind spot), so we cannot perceive anything there. Since rods are more responsive to light than cones we can identify three types of vision, depending on the amount of light that reaches the eye. Under dark circumstances, practically only the rods are active. Since rods cannot discriminate colors, we perceive only shades of grey. We call this scotopic or night vision. Under daylight circumstances, the cones are most active, and we experience photopic or day vision. In dimly lighted circumstances there is an intermediate stage where both rods and cones are active called mesopic vision. We are able to distinguish colors because there are three distinct types of cones, each sensitive to a different band of the electromagnetic spectrum.