Fresnel mirrors

Natural weathering (see Section 5.2.5) can be accelerated directly by exposure in a climate more severe than that expected in service. There are established test sites for this purpose in Australia and in the hotter states of the USA. The severity of exposure can also be maximised by arranging for the plane of the test pieces to automatically follow the sun. Another approach is to use a Fresnel mirror concentrating device that increases the intensity of sunlight falling on the test piece. These procedures are standardised in ISO 877 [25]. 

ISO 877 [205] is an accepted standard for outdoor weathering which also describes the procedure for irradiation through window glass and for accelerated weathering by solar radiation with the aid of Fresnel mirrors. Further important standards include ISO 9845 [199], DIN EN 61725 [200] and ISO 9370 [206]. 

Whereas the XSW technique takes advantage of the standing wave established on the total reflection of X-rays from a mirror surface, a conceptually more straightforward approach is that of simply specularly reflecting an X-ray beam from an electrode coated with the film of interest, measuring the ratio of the intensities of the incident and reflected rays, and fitting the data, using the Fresnel equations, to a suitable model an approach similar to optical ellipsometry. 

The necessary pump powers can be achieved either by other lasers (e.g. nitrogen lasers, solid-state lasers or even focussed He-Ne- or Ar+-gas lasers) or by flash-lamps. The simplest practical arrangement is a square spectrophotometer cell, polished on all sides, containing the dye solution which is pumped by a nitrogen laser whose beam is focussed into a line parallel to and directly behind one of the cell windows. Then the Fresnel reflection from the two adjacent windows gives enough feedback in most cases, so that no additional resonator mirrors are needed and the dye laser oscillation starts. 

Focusing collectors are usually cast acrylic Fresnel lenses, or mirrors of aluminized polyester film in frames of aluminum. These reflectors are either enclosed in a bubble of poly(vinyl fluoride) film, or under polycarbonate glazing, which may be covered with a fluorocarbon film to reduce the reflectivity. The absorbers for active systems are copper or aluminum since the temperatures are too high (325—370°C) for plastics. The frames, however, can be molded ABS, high density polyethylene or polyurethane, either solid or structural foam. Polybutylene or chlorinated PVC can be used for piping hot water, and tanks can be made of either reinforced polyester or blow- or rotational-molded, high density polyethylene (12—15). 

As in any other laser, the lasing threshold in a semiconductor laser diode is reached when the gain of the active material overcomes the losses of the laser cavity. These losses have two basic origins, namely the finite reflectivity of the mirrors mid distributed losses due to scattering and parasitic absorption in the active medium. In contrast to other lasers, the mirrors in typical semiconductor lasers are simply formed by cleaved or etched crystal facets. Therefore, the reflectivity (Fresnel reflectivity) is rather low, about 20% in the case of the nitrides. 

Figure 3.6-8 Experimental set-up for CARS spectroscopy in the condensed phase. A = aperture AL = achromatic lens BS = beam. splitter CL = calibration lamp D = diffuser Dl, D2 = diodes FR = double Fresnel rhombus L = lens M = mirror P = Glan-Thompson polarizer PA = preamplifier PHS = Prism Harmonic Separator PM = photomultiplier PR = linear dispersing prism arrangement S = shutter (Materny et al., 1992a).

Ellipsometry grants the independent determination of two results per resolution element without changing the experimental geometry this technique is described in Sec. 6.4.4.2. More often however, the information on the phase shift is abandoned and just the reflectance is determined. Usually it is measured by comparing the signal caused by the reflected intensity with the one obtained when the sample is replaced by a mirror. Even without correcting for its reflectance such results can often be used for further mathematical evaluation. The Fresnel reflection coefficient r in the form given by Eq. 

This practice describes the outdoor-accelerated-exposure testing of plastics and plastic-made products using Fresnel reflecting concentrator. The latter uses the sun as a source of UV and longer wavelength radiation and involves a system of plane mirrors focused on an air-cooled target board on which the test specimens are mounted. The three basic exposure methods are as follows ... 

If a mirror is to be an effective optical element then the reflectivity must be as close to 100% as possible. Parratt (1954) gives an expression, derived from the Fresnel coefficient for reflection (Compton and Allison 1935), for the reflectivity, R, for an incidence angle, 6, and critical angle, 0C, for a perfectly smooth homogeneous mirror surface... 

To produce micrometer sized focused beams, one employs highly demagnifying optics to image the source onto the sample. Such optics can include Kirkpatrick-Baez mirrors, Fresnel zone plates, tapered capillaries, and compound refractive lenses, all of which have been used to produce submicron focal spots at third generation storage rings. 

Reflective devices include capillaries, Bragg-Fresnel lens, and mirrors. Capillaries, typically tapered, rely on internal reflection to concentrate photons into small spots defined by the downstream aperture (Bilderback and Thiel 1995 Heald et al. 1997 Dhez et al. 1999 Bilderback and Huang 2001). Sub-micrometer beams have been achieved whereas the small working distances tend to be the main limitation. Bragg-Fresnel lens (Hayakawa et al. 1989 Chevalier et al. 1995 Snigirev et al. 1995 Dhez et al. 1999) are similar to FZPs but operate in reflection mode. 

We have used a femtosecond-written Nd YAG ceramic optical waveguide as an active media to achieve continuous wave 1.06 pm laser operation. We have obtained output laser power of 40 mW and with a laser slope efficiency in excess of 40%. Single mode and stable laser oscillation have been achieved by using the natural Fresnel reflection for optical feedback without the requirement of any kind of mirror or reflective component. 

In absence of laser mirrors, optical feedback at the both faces was only provided by the Fresnel reflection. Taking into account the refractive index of Nd YAG ceramics (no=1.8), and using Fresnel equations. 

At mirror surfaces, incident radiation can be reflected regularly. This reflectivity (the ratio of the reflected to the incident radiation power) depends on the refractive indices of the two layers forming the interface [96,98]. According to the Fresnel relationships, the reflectivity is given by... 

Polymers have many potential applications In solar technologies that can help achieve total system cost-effectiveness. For this potential to be realized, three major parameters must be optimized cost, performance, and durability. Optimization must be achieved despite operational stresses, some of which are unique to solar technologies. This paper Identifies performance of optical elements as critical to solar system performance and summarizes the status of several optical elements flat-plate collector glazings, mirror glazings, dome enclosures, photovoltaic encapsulation, luminescent solar concentrators, and Fresnel lenses. Research and development efforts are needed to realize the full potential of polymers to reduce life-cycle solar energy conversion costs. 

---