Illumination direction

Diode-array spectrophotometry exploits a different principle to conventional spectrophotometry (Figure 7-7). In this, the cuvette is illuminated directly by the light source, and the monochromator is positioned between the cuvette and the detector. In place of an exit slit on the monochromator, the detector consists of a linear diode array which enables light at all wavelengths to be detected simultaneously. The advantage of this construction is that rapid measurements can be made of the complete spectrum the disadvantage is that the sample is continuously exposed to high intensity illumination. 

Figure 17. Effect of front side and back side illumination of semiconductor electrodes coated with phthalocyanine thin films. The direction of the photocurrent could be controlled by illumination direction and the redox couple in the solution.

One of the simplest tristimulus colorimeters is the Lovibond Tintometer. The apparatus consists essentially of a viewing tube and light reflected from a standard white surface. One half of the field is illuminated by the light reflected by, or transmitted through, the specimen under examination. The other half is illuminated directly by a beam of standard light. Into the path of the latter beam coloured glass filters of the subtractive primaries are inserted and moved by mechanical means. 

If, on the other hand, the spin is made to rotate either by an applied external field or by an internal magnetic field, the direction of the decay e emission evolves. It may be compared to a light house whose illumination direction is rotating. The probability of decay p detection is then. 

The Case of Diffuse Illumination Direct Solution of the Diffusion Equation... 

A lamp of luminous intensity 1000 cd is suspended 2m above a laboratory bench. Calculate the illuminance directly below the lamp ... 

A street lantern suspends a 2000 cd light source 4 m above the ground. Determine the Illuminance directly below the lamp and 3 m to one side of the lamp base. 

Figure 3.67 The J-V characteristics of (a) n-DSSC, (b) p-DSSC and (c) tandem DSSC, irradiated at 100 mW cm with a light source simulating AM 1.5 global solar radiation n-side and p-side indicates the illumination direction. Reprinted from Nakasa etal., 2005 with permission from the Chemical Society of Japan...

In integrated photoelasticity it is impossible to achieve a complete reconstruction of stresses in samples by only illuminating a system of parallel planes and using equilibrium equations of the elasticity theory. Theory of the fictitious temperature field allows one to formulate a boundary-value problem which permits to determine all components of the stress tensor field in some cases. If the stress gradient in the axial direction is smooth enough, then perturbation method can be used for the solution of the inverse problem. As an example, distribution of stresses in a bow tie type fiber preforms is shown in Fig. 2 [2]. 

There are many ways of increasing tlie equilibrium carrier population of a semiconductor. Most often tliis is done by generating electron-hole pairs as, for instance, in tlie process of absorjition of a photon witli h E. Under reasonable levels of illumination and doping, tlie generation of electron-hole pairs affects primarily the minority carrier density. However, tlie excess population of minority carriers is not stable it gradually disappears tlirough a variety of recombination processes in which an electron in tlie CB fills a hole in a VB. The excess energy E is released as a photon or phonons. The foniier case corresponds to a radiative recombination process, tlie latter to a non-radiative one. The radiative processes only rarely involve direct recombination across tlie gap. Usually, tliis type of process is assisted by shallow defects (impurities). Non-radiative recombination involves a defect-related deep level at which a carrier is trapped first, and a second transition is needed to complete tlie process. 

Selenium exhibits both photovoltaic action, where light is converted directly into electricity, and photoconductive action, where the electrical resistance decreases with increased illumination. These properties make selenium useful in the production of photocells and exposure meters for photographic use, as well as solar cells. Selenium is also able to convert a.c. electricity to d.c., and is extensively used in rectifiers. Below its melting point selenium is a p-type semiconductor and is finding many uses in electronic and solid-state applications. 

Figure 10.6 Two-dimensional representation of i and i (broken lines) and their resultant ifotai (solid line) for scattering by a molecule situated at the origin and illuminated by unpolarized light along the x axis. The intensity in any direction is proportional to the length of the radius vector at that angle. (Reprinted from Ref, 2, p. 168.)...

The contrast for specimen detail in the field of view is gready enhanced by darkfield illumination (10). The degree of contrast and sensitivity of detection of smaH-object details depend on the relative indices of the specimen and the mounting Hquid and on the intensity of the illumination. Darkfield illumination is not, however, a satisfactory solution for biologists who need direct transmitted light in order to observe specimens, especially stained specimens. It is, however, very usefiil in detecting asbestos fibrils that often exist in door tiles or water and air samples as 20-nm fibers (10 times finer than the resolution of an asbestos analyst s usual microscope) (11). Darkfield illumination yields an uimatural appearance and difficulties in interpretation hence, a need for better contrast methods stiU exists. 

The schlieren microscope is able to detect refractive index variations to six decimal places. Any small difference in optical path (index difference, film thickness, etc) is very precisely detected by the schlieren microscope, especially in the Dodd modification. It is, in effect, a darkfield method. The specimen is illuminated with light in a portion of the illuminating cone and that direct light is masked in the conjugate back focal plane of the objective (Fig. 3). The only light to pass through this plane is refracted, reflected, or diffracted by the specimen. 

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