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2) Detection System
•
The following diagram shows the spectral reflectivity of the (yellow) EDH-1 pres-
sure pad and the spectral transmissitivity of the original detecting filter.
Spectral Reflectivity/
Transmissivity
(%)
100
80
60
40
20
0
•
The reflectivity of the yellow pressure pad, indicated by the bold line (
averages above 80% in the wavelength range 530nm to 700nm, but below 26% in
the range 400nm and 490nm.
•
The filter's transmissitivity, indicated by the thin line (
87% in the wavelength range 400nm to 460nm, but below 5% in the range 530nm
and 700nm.
•
When detection is operating, therefore, the yellow reflected light from the pad is
cut off by the filter before it reaches the CCD sensor. The CCD sensor therefore
receives light only from the original.
•
Note, however, that the yellow pad does in fact reflect some light (an average of
below 26%) within the 400nm to 490nm range. A threshold is therefore intro-
duced with respect to reflectivity within this wavelength range, to prevent the sys-
tem from interpreting this light as coming from the original.
•
The CPU sets this threshold value based on the maximum reflectivity of the yel-
low light from the pad.
Transmitting Area
Filter
Transmissivity
400
Cutoff Area
Threshold
Average Yellow Reflectivity
500
Wavelength (Visible Range)
M-34
Yellow Reflectivity
600
1155M108CA
), averages above
700
(nm)
),
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