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handbook of modern sensors

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134 4 Optical Components <strong>of</strong> Sensors<br />

Fig. 4.9. Second surface mirror.<br />

absorptive (varies at different wavelengths), and transmitted, which is whatever is<br />

left after reflection and absorption. Because all windows are characterized by specific<br />

spectral transmissions, <strong>of</strong>ten they are called filters.<br />

4.4 Mirrors<br />

A mirror is the oldest optical instrument ever used or designed. Whenever light passes<br />

from one medium to another, there is some reflection. To enhance a reflectivity, a single<br />

or multilayer reflecting coating is applied either on the front (first surface) or the rear<br />

(second surface) <strong>of</strong> a plane-parallel plate or other substrate <strong>of</strong> any desirable shape.<br />

The first surface mirrors are the most accurate. In the second surface mirror, light<br />

must enter a plate having, generally, a different index <strong>of</strong> refraction than the outside<br />

medium.<br />

Several effects in the second surface mirror must be taken into consideration.<br />

First, due to the refractive index n <strong>of</strong> a plate, a reflective surface appears closer (Fig.<br />

4.9). The virtual thickness d <strong>of</strong> the carrier for smaller angles 1 may be found from<br />

a simple formula:<br />

d ≈ L n . (4.22)<br />

The front surface <strong>of</strong> the second surface mirror may also reflect a substantial<br />

amount <strong>of</strong> light, creating the so-called ghost reflection. For instance, a glass plate<br />

reflects about 4% <strong>of</strong> visible light. Further, a carrier material may have a substantial<br />

absorption in the wavelength <strong>of</strong> interest. For instance, if a mirror operates in a farinfrared<br />

spectral range, it should use either first surface metallization or a second

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