Lambertian scatterer

A final practical note involves instrument intensity measurement calibrations. The intensity measurement is self-calibrating relative to the incident beam from the source. However, measurements typically have a dynamic range of 10 -10 , and care must be taken to insure the detection system is linear. A method of calibrating the scatterometer is to characterize a diffuse reflector having a known scattering characteristic. For example, a surface coated with BaS04 makes a nearly Lambertian scatterer, which has a BRDF of 1/Jt at all angles. 

Another swept approach is shown in Figure 16.21 [85]. In the center, a semi-transparent bidirectional Lambertian scattering projection screen rotates around its center axis at... 

Reflectance spectroscopy concerns the measurement of four distinct types of materials and their interaction with light. Specular materials reflect the predominant amount of radiation at an angle equal and opposite to the incident radiation. Diffusely reflective materials scatter light over a wide range of angles, with the perfectly diffuse (or Lambertian) scatterer exhibiting a cosine response to the incident radiation. Gonioapparent... 

Packed PTFE exhibits extremely high reflectance and is close to a perfect Lambertian scatterer over a very wide reflectance range (190-2500 nm/0.19-2.5 /on). The material is nonhydroscopic and is very easily prepared from commercially available sources. The powdered PTFE is also reasonably inexpensive—currently about 8.00/pound in bulk. 

Under the typical summertime conditions, the thinner cloud shows an increase of 65% in the actinic flux above the cloud whereas the thicker cloud shows an increase of almost a factor of three, the maximum theoretically possible. This is due to scattering of diffuse light from the top of the cloud, as well as from the ground. As expected, below the thicker cloud, the total actinic flux is reduced, in this calculation, to 19% of the clear-sky value. However, for the thinner cloud of optical density 8, the actinic flux below the cloud is actually calculated to be greater than for the cloudless case. This occurs in the case of a small solar zenith angle and direct (rather than diffuse) incident light because the direct incident light is diffused as it traverses the cloud as discussed earlier for the case of the actinic flux above a Lambertian surface, conversion of a direct to diffuse source leads to an enhancement in the actinic flux. 

From a theoretical point-of view, significantly higher current densities are feasible, but require further improved front TCO films and perfect mirrors as back reflectors. This is illustrated by the dotted curve in Fig. 8.28, which shows simulations of quantum efficiency for a 1 pm thick pc-Si H solar cell. These simulations reveal a current potential of 29.2 mA cm-2 by improved optical components like reduced parasitic absorption in the front TCO, ideal Lambertian light scattering, dielectric back reflectors, and antireflection coatings on the front side [147]. However, this still has to be achieved experimentally. 

Initially, we will focus on the mesoscopic description associated with the radiative transfer equation. Then, we will introduce the single-scattering approximation and two macroscopic approximations the PI approximation and two-flux approximation. AH of these discussions are based on the configuration shown in Fig. 6. Collimated emission and Lambertian emission wiU also be considered in the discussion later they correspond to the direct component and the diffuse component of solar radiation, respectively. Throughout our study, the biomass concentration Cx is homogeneous in the reaction volume V (assumption of perfect mixing), and the emission phenomena in V are negligible. The concentration Cx is selected close to the optimum for the operation of the photobioreactor the local photon absorption rate. 4 at the rear of the photobioreactor is close to the compensation point A.C (see Section 5 and chapter Industrial Photobioreactors and Scale-up Concepts by Pruvost et al.). 

Real reflectance can be described analytically or empirically [261]. There are several analytic reflectance models that can be used to describe various types of surfaces. The simplest diffuse source is Lambertian. The Mie theory can compute light scattering by spherical particles, as well as some other simple shapes like elongated ellipsoids [266]. 

On the contrary, if there are clouds, the discrepancy between flie observation and the model is large in general, and the cause of uncertainty is thought to be the contribution of albedo of clouds. When the actinic flux f tot is divided by direct radiation component Fq, and downward and upward diffusive radiation component Fj, and F, respectively, assuming a Lambertian surface i.e. a virtual completely diffusive surface for which radiance is constant being independent of the direction of observation (isotropic scattering),... 

The Voyager spacecraft has a large plate mounted on the main bus so that the instruments on the scan platform can view the plate almost normally. The surface of the aluminum plate is chemically etched and scatters light in all directions when illuminated by the Sun. From the visible to 20 /tm, the scattering characteristic is that of a near perfect Lambertian diffuser, as verified by laboratory measurements one or two years before launch. Exposure of the calibration plate to sunlight requires a complex maneuver of the spacecraft Since the telemetry antenna does not point in the Earth direction at that time, such calibration sequences cannot be performed frequently. Cahbrations have been carried out a month or so before and after each planetary encounter, except following the Saturn encounter by Voyager 2, where... 

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