Environmental Engineering Reference
In-Depth Information
M
otion
s
MeAr
A bird approaching a moving blade under high wind conditions may be unable to
see the blade due to motion smear—reduced visibility of the blades, especially at the
tips—and may not hear the blade until it is very close—if it is able at all to hear it at
all (Dooling, 2002). As an object moves across the retina with increasing speed, it
becomes progressively blurred; this phenomenon is known as
motion smear
,
motion
blur
, or
motion transparency
and is well known in human psychophysical research.
It results because the human visual system is sluggish in its response to temporal
stimulation; that is, the visual system in humans summates signals over periods of
about 120 ms in daylight.
The phenomenon of motion smear is apparent at the tips of wind turbine rotor
blades as the observer (bird, human, or camera) approaches the turbine. Motion
smear is not apparent in the central regions of the rotors. Even though the cen-
tral regions and the tips are rotating at the same number of revolutions per minute
(RPM), the absolute velocity of the blades is much higher at the peripheral regions.
The higher velocity of the bade tip has placed it in the temporal-summation zone,
where the retina is sluggish in its ability to resolve temporarily separated stimuli. In
contrast, the lower velocities of the more central portions are below the transition
point between blur and non-blur, so the individual blades can be seen more or less
clearly. Moreover, the absolute velocity of the blade in the visual world is not criti-
cal; rather, it is the absolute velocity of the image of the blade that sweeps across the
retina that is the critical variable.
For reasons that will be explained later, as the observer approaches the turbine,
the retinal image of the blades increases in velocity until the retina can no longer
process the information. This results in motion smear or motion transparency—the
blade becomes transparent to the view. A solution to avian collisions with wind tur-
bines must take into account the causes of motion smear and consider whether blade
patterns could minimize this effect.
Theory of Motion Smear
One of the characteristics of motion smear is that it eliminates the high spatial
frequencies from visual patterns, which is why they appear to go out of focus and
become virtually transparent. High spatial frequencies are those Fourier compo-
nents of a visual object that are found at edges and corners and in the fine details.
The print on this page, for example, is made up mainly of high spatial frequencies.
If they are removed by optical blur or refractive error, the text becomes transparent
and, in the worst case, virtually disappears. Motion smear causing bird collisions
with seemingly slow-moving turbines seems paradoxical given the acute vision that
most birds, especially raptors, possess; however, as the eye approaches the rotating
blades, the retinal image of the blade increases in velocity until it is moving so fast
that the retina cannot keep up with it. At this point the retinal image becomes a
transparent blur that the bird probably interprets as a safe area to fly through, with
disastrous consequences (NREL, 2000).
