DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

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Figure 19.1 Luminance
range of vision
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Figure 19.2 Adaptation
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Adaptation
Vision operates over a remarkable range of luminance
levels – about eight orders of magnitude (decades)
sketched in Figure 19.1. For about four decades at the
low end of the range, the rods are active; vision at these
light levels is called scotopic. For the top five or six
decades, the cones are active; vision at these light levels
is called photopic.
Mesopic vision takes place in the range of luminance
levels where there is some overlap between rods and
cones. Considered from the bottom of the photopic
region, this is called rod intrusion. It is a research topic
whether the rods have significance to color image
reproduction at usual luminance levels (such as in the
cinema). Today, for engineering purposes, the effect of
rod intrusion is discounted.
Vision adapts throughout this luminance range, as
sketched in Figure 19.2. From sunlight to moonlight,
illuminance changes by a factor of about 200000; adaptation
causes the sensitivity of the visual system to
increase by about a factor of 1000. About one decade
of adaptation is effected by the eye’s iris – that is, by
changes in pupil diameter. (Pupil diameter varies from
about 2 mm to 8 mm.) Adaptation involves
a photochemical process involving the visual pigment
substance contained in the rods and the cones; it also
involves neural mechanisms in the visual pathway.
Dark adaptation, to low luminance, is slow: Adaptation
from a bright sunlit day to the darkness of a cinema can
take a few minutes. Adaptation to higher luminance is
rapid but can be painful, as you may have experienced
when walking out of the cinema back into daylight.
Adaptation is a low-level phenomenon within the visual
system; it is mainly controlled by total retinal illumination.
Your adaptation state is closely related to the
mean luminance in your field of view. In a dark viewing
environment, such as a cinema, the image itself controls
adaptation.
196 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES