Image Processing Reference
In-Depth Information
o
90
P'
ε
O'
P
α
Eccentricity
0
o
O'
ε
O
O''
Optic axis
P'
Fig. 1.2. Given the diameter
O
O
, the eccentricty
(
left
), and the azimuth
α
, one can deter-
mine the position of a point
P
on the retina (
right
)
of the reasons why the
spatial resolution
, also called
acuity
, which determines the
picture quality for details that can be represented, is not very high in night vision. The
peak resolution is reserved for day vision, during which there is more light available
to those photoreceptors that can sense such data. The density of cones decreases with
high eccentricity, whereas that of rods increases rapidly. Accordingly, in many night-
active species, the decrease in rod concentration towards the fovea is not as dramatic
as day-active animals, e.g. in owl monkey [171]. In fovea there are approximately
150,000 cones per mm
2
[176]. The concentration decreases sharply with increased
eccentricity. To switch to night vision requires time, which is called
adaptation
, and
takes a few minutes in humans. In human retinae there are three types of cones,
sensitive to long, medium, and short wavelengths of the received photons. These are
also known as “red”, “green”, and “blue” cones. We will come back to the discussion
of color sensitivity of cones in Chap. 2.
The retina consists of six layers, of which the photoreceptor layer containing
cones and rods is the first, counted from the eye wall towards the lens. This is an-
other remarkable difference between natural and human-made imaging systems. In
a camera, the light-sensitive surface is turned towards the lens to be exposed to the
light directly, whereas the light-sensitive rods and cones of the retina are turned away
from the lens, towards the wall of the eye. The light rays pass first the other five lay-
ers of the retina before they excite the photoreceptors! This is presumably because
the photoreceptors bleach under the light stimuli, but they can quickly regain their
light-sensitive operational state by intaking organic and chemical substances. By be-
ing turned towards the eye walls, their supply of such materials is facilitated while
their direct exposure to the light is reduced (Fig. 1.3). The light stimulus is translated
to electrical pulses by a photoreceptor, rod, or cone, thanks to an impressive chain of
electrochemical process that involves hyperpolarization [109]. The signal intensity
of the photoreceptors increases with increased light intensity, provided that the light
is within the operational range of the photoreceptor in terms of its photon amount
(intensity) as well as photon wavelength range (color).
