Nonimaging optics (also called anidolic optics) is a branch of optics that is concerned with the optimal transfer of light radiation between a source and a target. Unlike traditional imaging optics, the techniques involved do not attempt to form an image of the source; instead an optimized optical system for optimal radiative transfer from a source to a target is desired.
Applications The two design problems that nonimaging optics solves better than imaging optics are:
solar energy concentration: maximizing the amount of energy applied to a receiver, typically a solar cell or a thermal receiver illumination: controlling the distribution of light, typically so it is "evenly" spread over some areas and completely blocked from other areas Typical variables to be optimized at the target include the total radiant flux, the angular distribution of optical radiation, and the spatial distribution of optical radiation. These variables on the target side of the optical system often must be optimized while simultaneously considering the collection efficiency of the optical system at the source.
Solar energy concentration For a given concentration, nonimaging optics provide the widest possible acceptance angles and, therefore, are the most appropriate for use in solar concentration as, for example, in concentrated photovoltaics. When compared to "traditional" imaging optics (such as parabolic reflectors or fresnel lenses), the main advantages of nonimaging optics for concentrating solar energy are:
wider acceptance angles resulting in higher tolerances (and therefore higher efficiencies) for: less precise tracking imperfectly manufactured optics imperfectly assembled components movements of the system due to wind finite stiffness of the supporting structure deformation due to aging capture of circumsolar radiation other imperfections in the system higher solar concentrations smaller solar cells (in concentrated photovoltaics) higher temperatures (in concentrated solar thermal) lower thermal losses (in concentrated solar thermal) widen the applications of concentrated solar power, for example to solar lasers possibility of a uniform illumination of the receiver improve reliability and efficiency of the solar cells (in concentrated photovoltaics) improve heat transfer (in concentrated solar thermal) design flexibility: different kinds of optics with different geometries can be tailored for different applications Also, for low concentrations, the very wide acceptance angles of nonimaging optics can avoid solar tracking altogether or limit it to a few positions a year. The main disadvantage of nonimaging optics when compared to parabolic reflectors or Fresnel lenses is that, for high concentrations, they typically have one more optical surface, slightly decreasing efficiency. That, however, is only noticeable when the optics are aiming perfectly towards the Sun, which is typically not the case because of imperfections in practical systems.
Illumination optics Examples of nonimaging optical devices include optical light guides, nonimaging reflectors, nonimaging lenses or a combination of these devices. Common applications of nonimaging optics include many areas of illumination engineering (lighting). Examples of modern implementations of nonimaging optical designs include automotive headlamps, LCD backlights, illuminated instrument panel displays, fiber optic illumination devices, LED lights, projection display systems and luminaires. When compared to "traditional" design techniques, nonimaging optics has the following advantages for illumination:
better handling of extended sources more compact optics color mixing capabilities combination of light sources and light distribution to different places well suited to be used with increasingly popular LED light sources tolerance to variations in the relative position of light source and optic Examples of nonimaging illumination optics using solar energy are anidolic lighting or solar pipes.
Other applications Modern portable and wearable optical devices, and systems of small sizes and low weights may require nanotechnology. This issue may be addressed by nonimaging metaoptics, which uses metalenses and metamirrors to deal with the optimal transfer of light energy. Collecting light emitted by high-energy particle collisions with a scintillator using the fewest photomultiplier tubes. Collecting luminescent radiation in photon upconversion devices with the compound parabolic concentrator being to-date the most promising geometrical optics collector. Some of the design methods for nonimaging optics are also finding application in imaging devices, for example some with ultra-high numerical aperture.
Theory Early academic research in nonimaging optical mathematics seeking closed form solutions was first published in textbook form in a 1978 book. A modern textbook illustrating the depth and breadth of research and engineering in this area was published in 2004. A thorough introduction to this field was published in 2008. Special applications of nonimaging optics such as Fresnel lenses for solar concentration or solar concentration in general have also been published, although this last reference by O'Gallagher describes mostly the work developed some decades ago. Other publications include book chapters. Imaging optics can concentrate sunlight to, at most, the same flux found at the surface of the Sun. Nonimaging optics have been demonstrated to concentrate sunlight to 84,000 times the ambient intensity of sunlight, exceeding the flux found at the surface of the Sun, and approaching the theoretical (2nd law of thermodynamics) limit of heating objects to the temperature of the Sun's surface. The simplest way to design nonimaging optics is called "the method of strings", based on the edge ray principle. Other more advanced methods were developed starting in the early 1990s that can better handle extended light sources than the edge-ray method. These were developed primarily to solve the design problems related to solid state automobile headlamps and complex illumination systems. One of these advanced design methods is the simultaneous multiple surface design method (SMS). The 2D SMS design method (U.S. patent 6,639,733) is described in detail in the aforementioned textbooks. The 3D SMS design method (U.S. patent 7,460,985) was developed in 2003 by a team of optical scientists at Light Prescriptions Innovators.
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