Civil Engineering Reference
In-Depth Information
One way to overcome difficulties with dark adaptation through design is being suggested by the fact
that, while rods are highly sensitive to light, their sensitivity to color is very limited. In particular, they are
insensitive to long wavelengths (which, as we will discuss later, lead to the perception of a red hue). We
can turn this apparent limitation into an advantage by illuminating objects in red light and thus mini-
mize the stimulation of rods. By doing so, the need for dark adaptation is minimized, which can be very
useful, for example, when briefing pilots before a night mission.
Rods do not only exhibit high sensitivity to light; they are also highly effective for perceiving orientation
and motion (e.g., Leibowitz, 1988). One often cited illustration of how to utilize this affordance of photo-
pic vision is the so-called MalcolmHorizon that helps pilots notice changes in an airplane's roll and pitch
without having to look directly at a foveal visual display (Stokes et al., 1990). TheMalcolmHorizon is a line
of red laser light that is projected onto the instrument panel to the left and right of the traditional attitude
indicator. The Malcolm Horizon moves along
with the artificial horizon relative to the earth's
surface as the plane moves through space.
Unlike the attitude indicator, the Malcolm
Horizon utilizes the peripheral visual system,
which is represented almost exclusively by rods
and, which is highly effective for processing
motion and orientation cues efficiently and
effortlessly.
In contrast to rods, which are highly sensitive
to light and highly effective for perceiving orien-
tation and motion, the 4 to 6 million cones,
which are concentrated in the fovea (a 0.3 mm
diameter area in the retina that covers approxi-
mately 2
(a)
Lightness or
brightness
of visual angle), support high acuity
and color vision. The human eye can distinguish
three properties of color: hue, saturation, and
brightness (Figure 23.2).
If we think of visual perception as the
transformation of light, that is, waves of electro-
magnetic energy into electrochemical neural
energy, then the amplitude of the electromagnetic
energy translates into the perceived brightness of
a stimulus. Brightness can be further subdivided
into illuminance, that is, the amount of light
falling on an object, and luminance, that is, the
amount of light that is reflected from a surface.
Illuminance is measured in lumens per square
meter (lm
8
(b)
Grey
m
2
) or lux whereas luminance is
expressed in candela per meter squared (cd
/
m
2
).
Saturation — the second distinguishable attri-
bute of color — refers to the extent to which a
stimulus is considered “diluted” or “pure,” that
is, the extent to which a stimulus represents a
mixture of various wavelengths. Finally, the
wavelength of light determines the perceived
hue. To most humans, wavelengths from
400 nm (perceived as blue-violet) to 700 nm
(perceived as red) are visible. Color perception
is accomplished by three types of cones that
/
FIGURE 23.2 The human color space with its
dimensions of hue, saturation, and brightness. (a) and
(b) are different views of the overall organization of
subjective color experience ellicited by spectral
distribution of light. (Adapted from Purves, D. and
Lotto, R.B., Why We See What we Do: An Empirical
Theory of Vision, Sunderland, MA, Sinauer Associates
Inc., 2003.)
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