Introduction - Feature Extraction and Image Processing - page 5

Choroid

Ciliary muscle

Lens

Fovea

Blind spot

Retina

Optic nerve

Figure 1.3 Human eye

these sensors to stimulate photochemical transmissions, which results in nerve impulses
that are collected to form the signal transmitted by the eye. There are two types of sensor:
first, the rods − these are used for black and white ( scotopic ) vision; and secondly, the
cones -these are used for colour ( photopic ) vision. There are approximately 10 million
cones and nearly all are found within 5° of the fovea. The remaining 100 million rods are
distributed around the retina, with the majority between 20°

of the fovea. Acuity is
actually expressed in terms of spatial resolution (sharpness) and brightness/colour resolution,
and is greatest within 1° of the fovea.
There is only one type of rod, but there are three types of cones. These types are:

and 5°

α − these sense light towards the blue end of the visual spectrum;

β − these sense green light;

− these sense light in the red region of the spectrum.

The total response of the cones arises from summing the response of these three types
of cones, this gives a response covering the whole of the visual spectrum. The rods are
sensitive to light within the entire visual spectrum, and are more sensitive than the cones.
Accordingly, when the light level is low, images are formed away from the fovea, to use the
superior sensitivity of the rods, but without the colour vision of the cones. Note that there
are actually very few of the α cones, and there are many more β and γ cones. But we can
still see a lot of blue (especially given ubiquitous denim!). So, somehow, the human vision
system compensates for the lack of blue sensors, to enable us to perceive it. The world
would be a funny place with red water! The vision response is actually logarithmic and
depends on brightness adaption from dark conditions where the image is formed on the
rods, to brighter conditions where images are formed on the cones.
One inherent property of the eye, known as Mach bands , affects the way we perceive