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handbook of modern sensors

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4.2 Photometry 129<br />

Fig. 4.6. Radiant energy distribution at an optical<br />

plate.<br />

absorbed portion <strong>of</strong> light is converted into heat, a portion <strong>of</strong> which P is lost to a<br />

supporting structure and surroundings through thermal conduction and convection.<br />

The rest <strong>of</strong> the absorbed light raises the temperature <strong>of</strong> the material. The temperature<br />

increase may be <strong>of</strong> concern when the material is used as a window in a powerful<br />

laser. Another application where temperature increase may cause problems is in farinfrared<br />

detectors. The problem is associated with the flux ε = α − P , which<br />

is radiated by the material due to its temperature change. This is called a secondary<br />

radiation. Naturally, a radiated spectrum relates to a temperature <strong>of</strong> the material and<br />

is situated in the far-infrared region <strong>of</strong> the spectrum. The spectral distribution <strong>of</strong> the<br />

secondary radiation corresponds to the absorption distribution <strong>of</strong> the material because<br />

absorptivity and emissivity are the same thing.<br />

For materials with low absorption, the absorption coefficient can be determined<br />

through a temperature rise in the material:<br />

α = mc 2n<br />

γ n 2 + 1<br />

( dTg<br />

dt<br />

+ dT )<br />

L<br />

T 0 , (4.11)<br />

dt<br />

where m and c are the mass and the specific heat <strong>of</strong> the optical material, respectively,<br />

and T g and T L are the slopes <strong>of</strong> the rising and lowering parts <strong>of</strong> the temperature<br />

curve <strong>of</strong> the material, respectively, at test temperature T 0 . Strictly speaking, light in<br />

the material is lost not only due to absorption but to scattering as well. A combined<br />

loss within material depends on its thickness and can be expressed through the socalled<br />

attenuation coefficient g and the thickness <strong>of</strong> the sample h. The transmission<br />

coefficient can be determined from Eq. (4.10), which is modified to account for the<br />

attenuation:<br />

γ ≈ (1 − ρ 2 )e −gh . (4.12)<br />

The attenuation (or extinction) coefficient g is usually specified by manufacturers <strong>of</strong><br />

optical materials.<br />

4.2 Photometry<br />

When using light-sensitive devices (photodetectors), it is critical to take into a consideration<br />

both the sensor and the light source. In some applications, light is received<br />

from independent sources; in others, the light source is a part <strong>of</strong> the measurements

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