frequencies. At high frequencies the ring-lights were replaced 
with a single, centrally mounted strobe light. The strobe 
frequency was set to produce a full range of the wing 
movement over 1-2 seconds. The longest sequences of images 
captured were 15 seconds in duration, producing several 
hundred epochs of target tracking. 
Coordinates for the targets for each epoch of measurement were 
computed from simple intersections. The estimated mean target 
coordinate precisions were approximately 20 micrometres. 
Target locations were tracked in object space using a three 
dimensional trajectory model. The predicted position in any 
new epoch was based on the previous three epochs of 
measurement to allow a non-linear extrapolation. The new 
object space position was then used to locate the target centroid 
window within the left and right images. Target coordinate 
data from the sequences were used to produce visualisations of 
the cyclic movement and deflection of the wing surfaces 
(Woodhouse et al, 1999). An example is shown in figure 3. 
(91) 
S N 
8 Y 
WS 
N à Y 
ÿ à 
V 
Range of wing movement over 55 Wing shape at Epoch 0 
image sets (exaggeration = 5x) 
  
Figure 3. Visualisation of the target movement for the wing 
surfaces of the micro-flight vehicle. 
3. SOLAR COLLECTOR FRESNEL LENS 
The second application discussed is the measurement of a one 
metre long Fresnel lens membrane used to concentrate light on 
solar collectors (Pappa et al, 2002). The lens is composed of 
silicone-rubber and is stretched between the end support arches 
(see inset figure 4). The lens produces a thin line of light 
directed at the centre of a rectangular solar collector in order to 
improve the efficiency of the energy conversion and reduce the 
overall weight. 
The 250 mm high lens elements will be assembled in banks (see 
inset figure 4) of 35 on panels with a dimension of three metres 
by one metre. The panels support the flexible concentrator 
lenses and the solar cells, and also serve as heat radiators. The 
overall weight of the solar lens array is just 1.6 kilograms per 
square metre and requires only 12% of the area of conventional 
solar cells for the same power output. A self-deployed structure 
composed of a series of hinged panels and containing 280 of the 
solar collectors may be tested in the cargo bay of the space 
shuttle during a future mission (figure 4). Once deployed, the 
solar lens array will be capable of 360 degrees of rotation to 
track the sun. 
The experimental set-up to characterise the surface shape and 
vibration modes of the Fresnel lens is shown in figure 5. The 
lens is mounted vertically with the base connected to an exciter 
unit used to simulate the vibrations from the shuttle reaction 
292. 
  
Figure 4. Artist impression of the deployed solar lens array — 
inset is a section of an assembled bank of solar cells 
and concentrator lenses. 
  
Figure 5. Experimental set-up for the surface measurement 
and tracking of the Fresnel lens — inset is detail of the top 
camera and viewing slot. 
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