Incident hght

The main limitation of photoelectric detectors is the noise cansed by thermal excitation of the carriers from the valence band or from the impnrity levels. If there is a large dark current (a cnrrent generated by the detector in the absence of incident hght), the sensitivity of the photoelectric detector becomes poor (only very intense beams will indnce an appreciable change in the detector condnctivity). In order to rednce the dark cnrrent, photoelectric detectors are nsnally cooled dnring operation. 

Extension ratio relative to isotropic state of reference Wavelength of the incident hght Poisson s ratio... 

The subscripts s and i denote scattered and incident hght, respectively, and refer to the appropriate ophcal polarizahon directions of the hght waves here [3]. Omitting, for simphcity, the tensor properties of Sa we see the spectral power density Is i(q, cu) to be proportional to the mean square fluctuation component a(q) at frequency co [Eq. (3)]. 

According to the law of conservation of energy, the energy balance for the incident hght may be written as... 

The Fresnel equations describe the reflection and transmission coefficients at the interface of two optical media. The polarisation of the incident Hght affects the magnitude of these coefficients. It is possible to derive expressions for the intensities of the reflected and refracted rays. These differ for the TE and TM polarisations as follows ... 

Here Ig, g represents the absorbed light. If the rate is expressed in terms of the incident hght, we must write... 

Fig. 1.4 Light transmission through polyciystalhne translucent ceramics (e.g. coarse AI2O3). Loss in intensity of the incident hght due to absorption, reflection on the two surfaces, diffuse scattering at pores and scattering caused by the birefringent splitting (specifically for noncubic materials). Reproduced with permission from [7]. Copyright 2009, Elsevier...

The optical measmements of diffuse reflectance are dependent on the composition of the system. Several theoretical models have been proposed for diffuse reflectance, which are based on the radiative transfer theory, and all models consider that the incident hght is scattered by particles within the medium. The most widely used theory in photometric sensors is the Kubelka-Munk theory, in which it is assumed that the scattering layer is infinitively thick, which may, in practice, be the case with the chemical transducers utilized in photometric sensors. The absolute value of the reflectance R is related to the absorption coefficient K and the scattering coefficient S by the equation... 

If Iq is the illumination when the screen is normal to the incident hght, then I is the illumination when the screen is at an angle 9. Thus,... 

Light transmittance (luminous transmittance, light transmissivity). The ability of a material to pass incident hght through it, whether specular or diffuse. ASTM (www.astm.org) prescribes several tests of this property in plastics. Transmissivity is... 

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