Optical concentrator

Two-stage optical concentration — Over ONE HUNDRED AND FIFTY SUNS ... 

The particle size distribution of each powder was determined using a Sympatec Helos/ Rodos laser diffraction particle size analyzer (Sympatec Inc., Princeton, New Jersey, U.S.A.) with dry powder dispersion capability. The powder dispersion pressure was varied between 0.5 and 2.0 bar (depending on the tendency for agglomeration) with direct feed into the dispersion funnel. The optical concentration was maintained in the range of 5% to 20%. The mean value of duplicate determinations is reported. 

Optical concentration of API in the carrier liquid For particles less than 10 pm, the optical concentration will fall within the 5-15% range for obscuration (85-95% transmittance). Concentrations that greatly exceed this range will result in spurious data as multiple scattering will occur within the sample, causing the material to appear smaller than it actually is. 

With the same numerical values as above for the irradiance of the Sun at 700 nm, the scattering of the radiation over 47t, or (which is strictly equivalent) its absorption in a photochemical reaction, cause its effective temperature to decrease from Tr = 5500 K to Trs = 1297 K. The maximum efficiency calculated from Eqs. (11) and (12) then drops from = 0.95 to = 0.77. The solid angle does no longer appears in Eq. (13). It should be borne in mind, however, that if the solid angle of collection is increased at the absorber by use of an optical concentrator, the measured irradiance, and hence the temperature, potential, and efficiency increase in proportion. In the same conditions as in the example above, concentration of the incident radiation by a factor of 10, for instance, translates into a rise of the radiation temperature from 1297 to 1517 K and an augmentation of the maximum conversion efficiency from 0.77 to 0.80. 

Effective concentration ratio. This value permits comparison of the performance of LSC with the conventional optical concentrators. This ratio expresses the number of equivalent suns under AM 1 condition concentrated by the LSC. 

E. 50 mm Binoculars and Other Optics. Soldiers are most vulnerable when using powered optics and staring at weapons systems using lasers or at distant objects where lasers could be employed. Optics concentrates the light increasing vulnerability and staring increases the opportunity for exposure. Many weapons systems have laser protection built into the optics. 

Imino IH NMR. Imino NMR spectroscopy is a powerful direct probe for the presence and nature of the base pairing in nucleic acids. As shown in Figure 6, the imino spectra for the control and the four stable a-containing duplexes exhibit resonances for the five chemically distinct imino protons in each self-complementary decamer, indicative of stable base pair formation. In addition, temperature dependent experiments showed no evidence for pre-melting of the base pairs comprising the a-nucleotides. Virtually identical NMR spectra have been obtained for the a-duplexes at optical concentrations (3 to 6 pM duplex), demonstrating that the duplex form exists under the conditions used in the thermodynamic studies. 

Beam restrictors and concentrators using capillary optics concentrate the beam on a small spot without the loss of signal from masks. Commercial units can achieve spot sizes of 25 pm with suitable intensity. Masks and beam restrictors are used to observe microscopic regions within a sample and, using raster techniques, can provide elemental image maps of samples that are heterogeneous. 

Obscuration percentage or fraction of incident light that is attenuated due to scattering and/or absorption, also known as optical concentration also known as Optical Concentration. 

In the case when the surface of the detector receiving the incident optical flux is not equal to the detector active area, i.e., when some kind of concentrator is used to collect radiation from a larger area and direct it to the active area, a factor of optical concentration [8] may be formally introduced into the expression for the specific detectivity. This factor is equal to the square root of the ratio between the optical and active ( electrical ) detector area. 

There are different methods of light management in a photodetector that can be divided into four groups. Fig. 2.1 optical concentration, use of antireflection stmctures, optical path increase, and light localization. 

Optical concentration methods actually collect incident radiation from a larger area (denoted as optical area) and concentrate it (focus) to the smaller active area of photodetector (the electrical area). The concentration efficiency can be then defined as the ratio between the optical and the electrical area, minus absorption and scattering losses. This method of light management is typically done by utilizing stmctures which are not themselves a part of the detector, but can be integrated with it. The simplest case of a concentrator would be an immersion lens, but there are a number of different other stmctures to serve the same purpose. Roughly, one could... 

Fig. 2.1 Methods of photon management use of optical concentrator, antireflection structure, stmctures for optical path increase (cavity enhancement) and light localization structures...

One of the obvious methods to increase the optical input to the detector is to collect optical signal from a larger area and to concentrate it to the smaller electrically active area. This can be done by some kind of optical concentrator, i.e., focusing optics integrated with the detector unit. 

A schematic presentation of an optical concentrator is given in Fig. 2.3. The shape of the system can be arbitrary, as well as its mechanism of focusing. The important factor of the system is its concentration ratio, defined as a ratio between the input (collector) area and the exit area. In an ideal concentration system, this ratio should be equal to the ratio of incident optical flux to the exit flux. In a real system, possible absorption losses (for instance in metal parts) will have to be deducted. Thus the concentration ratio of a light concentrator can be defined either as the geometric concentration ratio C, the ratio of the entry aperture area (AcoUector) to the exit aperture area (Aexit), or as the irradiance gain C the ratio of the irra-diance on the collector (fr ) to that on the entry aperture (Ir ). The two concentration terms are related by the fraction of total incident power entering the module that reaches the concentrator exit aperture. 

Different strategies can be used to concentrate the incident optical power. There are three main groups of optical concentrators. One of them is based on refractive optics (conventional lenses). The second one is reflective concentrators (mirrors), while the third group is diffractive optical elements. Obviously, a system may simultaneously incorporate two or even aU three of the mentioned structures. 

From the point of view of coupling efficiency (minimization of reflection losses), the best solution is monolithic or hybrid integration however, in practical situations one can encounter all of the mentioned approaches. Two main types of focusing lenses may be used—either refractive lenses fabricated in material with high real part of refractive index and low absorption coefficient at IR wavelengths, or diffractive lenses. Any of them may be either discrete or arrayed. Reflective optical concentrators may be used, also reflective holographic optical elements or any of their combinations. 

Fig. 2.4 Some spherical and aspheric discrete microlenses for optical concentrators in MWIR and LWIR range that may be fabricated by microsystem technologies, a calotte b hemisphere c hyperhemisphere d ball lens e truncated sphere f bulb g hemi-cylinder h cylinder i curvilinear cone j concave concentrator k gradient-index lens (GRIN) I complex two-element lens (sphere/GRIN). The grid corresponds to the homogeneity of refractive index, i.e., describes its gradient...

A very important type of optical concentrators, both in optical telecommunications and for MWIR and LWIR photodetectors are hyperhemispheric microlenses (combinations of a hemisphere and a cylinder with joined bases, i.e., the rod lens )... 

As mentioned before, most of the stmctures for infrared detection can be fabricated by microsystem technologies (MST) and the microlenses for optical concentration are not an exception, regardless of the type of the optical element in question... 

A point of special importance is the choice of proper material for MWIR and LWIR optical concentrators. A number of materials are customarily used to fabricate infrared optics. Besides these, optical concentrators may be fabricated directly in the photodetector material and indeed be monolithicaUy integrated with it. 

A number of procedures are used to fabricate microlenses for telecommunications and some of them can be modified and adapted to fabricate optical concentrators for the MWIR and LWIR range. Some of these procedures already became standard. 

Chemical micromachining is one of the methods of choice for fabrication of microlenses [123]. Silicon and germanium were used as lens materials. For spherical and aspheric geometries solutions for anisotropic etching are used, while nonmonotonous surfaces can be fabricated by combining masks and anisotropic etching. This method was used to fabricate various types of optical concentrators, among them cones and inflected surfaces [107]. 

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