Let light of well defined specific intensity I < Imax enter a passive optical system from a medium of refractive index n = 1. Then wherever n = 1 and the specific intensity I' is well defined, I' < 'max- This inequality is obtainable from the Second Law of thermody-namics. If geometrical optics holds everywhere, it is a theorem of geometrical optics. In this paper it is shown that, even if diffraction and interference effects occur within the optical device, the inequality remains valid. A quantum description of the electromagnetic field is employed, and the inequality is obtained from the unitarity of the time evolution operator in quantum mechanics. Only "one photon" or weak-field theory is employed; many-photon effects are not included. Thus, despite the quantum language, the domain of validity of the derivation is that of classical electromagnetic theory. An analogous inequality, subject to analogous limitations, is obtained for the density in phase space, for an ordinary scalar particle undergoing elastic scattering.
The design of nonimaging systems has been based so far on a few principles without an elaborate superstructure of theory such as is found in conventional optical design. The relationships between the two disciplines are discussed.
A generalized algorithm for tracing rays through both imaging and non-imaging radiation collectors is presented. A computer program based on the algorithm is then applied to analyzing various two-stage Winston concentrators.
In this paper, wide angular band Bragg holoconcentrators are considered. These complex holographic structures (transmission or reflection) are of the type of holographic sandwich which consist of a set of sub-holograms. Each sub-hologram is angular narrow-band concentrator, with quasi-rectangular angular diffraction efficiency characteristics, having concentration which is free of aberrations. Therefore, due to selectivity of Bragg effect, the global system reaches paraxial limit of the concentration ratio, even for wide angular spectrum of quasi-monochromatic illumination.
We present here a review of the theoretical analysis of the limits of concentration under extended sources of arbitrary radiance based solely on the Fermat principle and its derivation, the theorem of the conservation of the etendue, and not on the specific shape of the concentrators. It is concluded that for casting increasingly high values of energy on the collector, which in photovoltaic. cases would be a bifacial solar cell, it is necessary to collect a lower portion of the total sky energy. This theory is applied to several types of static concentrators indicating to which extent their performance approaches the established limits.
This paper is concerned with the new field of nonimaging optics. Roughly this may be defined as the collection and redirection of light (or, more generally, electromagnetic radiation) by means of optical systems which do not make use of image formation concepts in their design. A non-trivial example is the compound parabolic concentrator (CPC) invented in 1965 for collecting Cerenkov radiation from large volumes of gas and concentrating it onto the relatively small area of a photomultiplier cathode. This task would, according to conventional optical practice, be performed by a lens or mirror image-forming system of high numerical aperture, but much greater concentration was achieved by a comparatively simple de-vice, the CPC. The key was to abandon the principle of imaging with high numerical aperture and instead to get the collected rays into as small an area as possible without attempting to produce an image.
The construction of airborne observatories, high mountain-top observatories, and space observatories designed especially for infrared and submillimeter astronomy has opened new fields of research requiring new optical techniques. A typical far-IR photometric study involves measurement of a continuum spectrum in several passbands between - 30pm and 1000pm and diffraction-limited mapping of the source. At these wavelengths, diffraction effects strongly influence the design of the field optics systems which couple the incoming flux to the radiation sensors (cold bolometers). The Airy diffraction disk for a typical telescope at submillimeter wavelengths (- 100pm - 100011m) is many millimeters in diameter; the size of the field stop must be comparable. The dilute radiation at the stop is fed through a Winston nonimaging concentrator to a small cavity containing the bolometer. The purpose of this paper is to review the principles and techniques of infrared field optics systems, including spectral filters, concentrators, cavities, and bolometers (as optical elements), with emphasis on photometric systems for wavelengths longer than 60 pm.
Criteria for evaluating the quality of optical concentrators are first discussed. Since nearly any optical system can behave as a concentrator, the suitability of many imaging and nonimaging optical systems for this purpose is examined and their relative merits are evaluated. An example is given in which it is shown how the concentrator system of choice can be selected nearly exclusively on the basis of the absolute and relative values of the required concentration ratios.
This paper describes the advantages of operating a non-imaging collector in a mode where steam is generated directly in the collector, rather than using a heat transfer oil and a secondary heat exchanger. The predicted performance advantages from generating steam directly in the collectors are significant, and that performance was verified using a collector built and tested at Argonne National Laboratory. A description of the collec-tor and the test method used to test the collector while operating in a steam generating mode is also presented by this paper. Test results are included for a 6.4 m2 array of evacuated tube collectors with an advanced absorber coating, silver reflectors, and tubes oriented in a N-S configuration.
The theory of non-imaging concentrators when used to pump cylindrical laser rods from flashlamps was developed for several different types of cavities. The theory was used to generate several cavity shapes which were constructed in the laboratory. These cavities were tested along with a diffuse cavity and a semi-eliptical cavity. The threshold and slope efficiency of each set is compared. The lasers were passively Q-switched and the output used as a measure of the pump uniformity of the rods for each cavity. In each case when rod and lamp were matched the non-imaging cavities have the best overall performance.
A discussion of curved conical horns used for the purpose of absorbing unwanted optical radiation is given. Several examples are presented, along with performance data from a horn made of pyrex glass and painted with gloss black paint.
A computer simulation study of the performance of occasionally tilt-adjusted, low concentration parabolic trough collectors using evacuated tube absorbers is presented. Using manual adjustment of 20 times per year, 3-4X concentrators appear to be most suitable for moderate temperature applications (100-200°C). Annual output is comparable to that of stationary concentrators in spite of a higher operating temperature, and relative cost-effectiveness is extremely promising compared to stationary designs because of a low absorber tube requirement.
The method of tracking used by a solar concentrator has an effect both on the cost of the system and on the size of concentrator necessary to collect a given amount of energy. A simple computer model was used to compare a mirror utilization factor, defined as the energy collected per unit mirror area, for five types of collectors: a fully tracking paraboloid, a North-South tracking parabolic trough, the General Atomics-Russell collector, a fixed spherical bowl with a tracking receiver, and a fixed mirror collector such as a CPC or a cusp. Two cases were considered: sea horizon and low-sun-angle shading. As might be expected, the mirror utilization factor for a given concentrator was found to be roughly proportional to the complexity of the tracking system, and therefore its cost, however, there are some seasonal variations which might make one type of concentrator more desirable than another in a given situation.
Two fixed, evacuated, glass solar thermal collectors have been designed. The incorporation of nonimaging concentration, selective absorption and vacuum insulation into their design is essential for obtaining high efficiency through low heat loss, while operating at high temperatures. Nonimaging, approximately ideal concentration with wide acceptance angle permits solar radiation collection without tracking the sun, and insures collection of much of the diffuse radiation. It also minimizes the area of the absorbing surface, thereby reducing the radiation heat loss. Functional integration, where different parts of these two collectors serve more than one function, is also important in achieving high efficiency, and it reduces cost.
The geometric concentration factor A/B for an optimally designed trough type solar collector with flat mirrors and an acceptance angle d is shown to equal sin(d + (2n+l)a)/sin(d + a), where a = mirror angle and n = number of reflections. The number n is given by n = Int((90-d)/(2a)) and the mirror width R by R/B = (A/B - 1)/(2 sin a). It is shown that The theoretic maximum concentration factor of such a trough equals 1/sin d, i. e. the theoretic maximum concentration factor of any trough. The formulae have been used to design different "cornet" type solar concentrators, and one of these is described. Collector performance taking mirror reflectivity and absorber glazing transmittivity into account has been calculated for rays incident perpendicular to the aperture. Preliminary results from a study of concave absorber glazing transmittivity are presented and suggest lower losses than flat glazing in several instances.