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handbook of modern sensors

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136 4 Optical Components <strong>of</strong> Sensors<br />

(A)<br />

(B)<br />

Fig. 4.11. Spherical (A) and parabolic (B) first surface mirrors.<br />

higher absorption and reflectance loss, (2) absence <strong>of</strong> distortions incurred by refracting<br />

surfaces due to dispersion (chromatic aberrations), and (3) lower size and weight as<br />

compared with many types <strong>of</strong> lenses. Spherical mirrors are used whenever light must<br />

be collected and focused (focus is from the Latin meaning fireplace—a gathering<br />

place in a house). However, spherical mirrors are good only for the parallel or nearparallel<br />

beams <strong>of</strong> light that strike a mirror close to normal. These mirrors suffer from<br />

imaging defects called aberrations. Figure 4.11A shows a spherical mirror with the<br />

center <strong>of</strong> curvature in point C. A focal point is located at a distance <strong>of</strong> one-half <strong>of</strong><br />

the radius from the mirror surface. A spherical mirror is astigmatic, which means that<br />

the <strong>of</strong>f-axis rays are focused away from its focal point. Nevertheless, such mirrors<br />

prove very useful in detectors where no quality imaging is required—for instance, in<br />

infrared motion detectors, which are covered in detail in Section 6.5 <strong>of</strong> Chapter 6.<br />

A parabolic mirror is quite useful for focusing light <strong>of</strong>f-axis. When it is used in<br />

this way, there is complete access to the focal region without shadowing, as shown<br />

in Fig. 4.11B.<br />

4.5 Lenses<br />

Lenses 1 are useful in <strong>sensors</strong> and detectors to divert the direction <strong>of</strong> light rays and<br />

arrange them in a desirable fashion. Figure 4.12 shows a plano-convex lens, which<br />

has one surface spherical and the other flat. The lens has two focuses at both sides: F<br />

and F’, which are positioned at equal distances −f and f from the lens. When light<br />

rays from object G enters the lens, their directions change according to Snell’s law.<br />

1 The word lens is from the Latin name for lentils. A lentil seed is flat and round, and its sides<br />

bulge outward—just like a convex lens.

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