
64 Virtual Reality: Concepts and Technologies
Rods
525
Cones
Maximum light threshold
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 = S × 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