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handbook of modern sensors

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242 6 Occupancy and Motion Detectors<br />

(A)<br />

(B)<br />

Fig. 6.11. A facet lens creates multiple images near the sensing element (A); sensitive zones<br />

created by a complex facet lens (B).<br />

within its clearings. The mask operation is analogous to the complex sensor’s shape<br />

as described in Section 6.5.1.2.<br />

6.5.1.4 Facet Focusing Element<br />

Another way <strong>of</strong> broadening the field <strong>of</strong> view while employing a small-area detector is<br />

to use multiple focusing devices. A focusing mirror or a lens may be divided into an<br />

array <strong>of</strong> smaller mirrors or lenses called facets. Each facet creates its own image, resulting<br />

in multiple images as shown in Fig. 6.11A. When the object moves, the images<br />

also move across the element, resulting in an alternate signal. By combining multiple<br />

facets, it is possible to create any desirable detecting pattern in the field <strong>of</strong> view, in<br />

both horizontal and vertical planes. Positioning <strong>of</strong> the facet lens, focal distances, number,<br />

and the pitch <strong>of</strong> the facets (the distance between the optical axes <strong>of</strong> two adjacent<br />

facets) may by calculated in every case by applying rules <strong>of</strong> geometrical optics. The<br />

following practical formulas may be applied to find the focal length <strong>of</strong> a facet:<br />

f = Ld<br />

, (6.10)

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