Photoreceptors: rods and cones

As mentioned above, there are two distinct kinds of photoreceptive
cells in the retina: rods and cones. Rod cells are extremely sensitive
to light, with the ability to send a signal to the brain if a single
photon (unit of light) is absorbed. On the other hand, rod cells are
slower to fire than cones, with a response time of about 100ms.
Because of the high sensitivity of rods to light, they are the primary

CC227 Creative Computing II Perception and Information Retrieval
photoreceptors used in dark conditions, such as at night. In
addition, the rods are largely absent from the centre of the retina
(completely absent from the fovea) and are common on the outside;
thus they are used for peripheral vision. The rods are sensitive to
motion, but have poor spatial discriminating power, which explains
why peripheral vision often gives the information that something is
moving, but not any detail of what that something is.
The rods have a single kind of pigment within them, which absorbs
light in the wavelength range of about 380–640nm. In particular,
they are completely insensitive to light of wavelengths above 640nm
(a shade of red). This is the cause of the Purkinje2 effect, the name 2Jan Evangelista Purkyne
(1787–1869), Czech anatomist and
physiologist.
given to the observation that blue objects appear brighter than red
ones under darker illumination conditions. Apart from this, rods
play little part in colour vision, as they cannot discriminate between
different wavelengths of light.
Cone cells, on the other hand, come in three different kinds, each
with a distinct pigment; one most sensitive to longwavelength
light,
one more towards the medium wavelengths, and one to
shortwavelength
light: respectively, their peak sensitivies are at
wavelengths around 570nm, 540nm and 430nm. The cones are able
to respond quickly to a stimulus, and have high concentration in the
fovea, giving highdetail
central vision; however, they only operate
under conditions of bright illumination.
The three different pigments present in cone cells permits colour
vision: the differentlypigmented
cells will fire in different
proportions when viewing light of particular frequencies, which
allows the brain to attribute a colour to a stimulus. The next section
discusses some details of colour vision in more depth.