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In-Depth Information
Maximum light threshold
Cones
Rods
5
25
Time in minutes
Figure 3.15 Dark adaptation curve
3.2.3.1 Light sensitivity
The quantity of light received by the retina depends on the quantity of light emitted
by a source and the pupil diameter. That is why we express light intensity, in terms of
retinal illumination, in Trolands (Td): Td
L where S is the surface of the pupil
expressed in mm 2 and L is the luminance expressed in cd/m 2 . However, sensitivity at a
given instant depends on the level of light adaptation of the retina at that instant. When
the illumination is high, only the cones can function (rods are saturated). It is known as
photopic vision. When cones and rods are both active, the vision is mesopic. When only
rods are active, however, the vision is scotopic (illumination is insufficient to activate
the cones). When we stay in strong light for some time, it is necessary to regenerate the
pigments to activate the rods. This duration corresponds to the time taken to adjust
with darkness. We feel dazzled (saturation of cones) if a light of extremely high intensity
is produced in photopic vision and we need some recovery time so that the cones can
regain their sensitivity. After experiencing this blinding sensation, it is possible to trace
the curve of adaptation to the dark (Figure 3.15).
In addition to the quantity of light, sensitivity also depends on the duration and the
surface area of stimuli which explain the existence of frequency sensitivities in addition
to this absolute sensitivity.
=
S
×
3.2.3.2 Frequency sensitivities
These correspond to the stimuli whose luminance varies sinusoidally in time and space.
There are three types of sensitivities - temporal, spatial and spatio-temporal. Temporal
frequency is a periodic variation of intensity, characterised by the amplitude of intensity
modulation. It is described in terms of Michelson contrast (C m ) which depends on
maximum luminance (L max ) and minimum luminance (L min ):
L max
L min
C m
=
+
L max
L min
For a given light source (size, average luminance and given spectrum), its flickering has
a temporal frequency which leads to a stable perception of the illumination. It is called
 
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