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handbook of modern sensors

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4<br />

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

“Where the telescope ends, the microscope begins.<br />

Which <strong>of</strong> the two has the grander view”<br />

—Victor Hugo<br />

Light phenomena, such as reflection, refraction, absorption, interference, polarization,<br />

and speed, are the powerful utensils in a sensor designer’s toolbox. Optical<br />

components help to manipulate light in many ways. In this chapter, we discuss these<br />

components from a standpoint <strong>of</strong> geometrical optics. When using geometrical optics,<br />

we omit properties <strong>of</strong> light which are better described by quantum mechanics and<br />

quantum electrodynamics. We will ignore not only quantum properties <strong>of</strong> light but<br />

the wave properties as well. We consider light as a moving front or a ray which is<br />

perpendicular (normal) to that front. To do so, we should not discuss any optical elements<br />

whose dimensions are too small compared to the wavelength. For example,<br />

if a glass window is impregnated with small particles <strong>of</strong> submicron size, we should<br />

completely ignore them for any geometrical calculations from the near-infrared to<br />

longer wavelengths. Another example is a diffractive grating. Its operation cannot be<br />

described by methods <strong>of</strong> geometrical optics. In such cases, the methods <strong>of</strong> quantum<br />

electrodynamics (QED) need to be used. Here, we summarize those optical elements<br />

most applicable for the sensor design. For more detailed discussions <strong>of</strong> geometrical<br />

optics, we refer the reader to special texts, (e.g., Ref. [1]).<br />

Before light can be manipulated, first we need to have light generated. There are<br />

several ways to produce light. Some sources <strong>of</strong> light are natural and exist without<br />

our will or effort; some must be incorporated into a measurement device. The natural<br />

sources <strong>of</strong> light include celestial objects, such as the Sun, Moon, stars, and so forth.<br />

Also, natural sources <strong>of</strong> light include all material objects that radiate thermal energy<br />

depending on their temperatures, as it was covered in Chapter 3. These include fire,<br />

exothermic chemical reactions, living organisms, and other natural sources whose<br />

temperatures are different from their surroundings and whose thermal radiation can<br />

be selectively detected by the optical devices. The man-made sources <strong>of</strong> light include

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