Nonimaging optics (also called anidolic optics)Roland Winston et al., Nonimaging Optics, Academic Press, 2004 R. John Koshel (Editor), Illumination Engineering: Design with Nonimaging Optics, Wiley, 2013 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 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:William J. Cassarly, Taming light using nonimaging optics, SPIE Proceedings Vol. 5185, Nonimaging Optics: Maximum Efficiency Light Transfer VII, pp.1–5, 2004 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.I. Moreno, M. Avendaño-Alejo, and C. P. Castañeda-Almanza, "Nonimaging metaoptics," Opt. Lett. 45, 2744-2747 (2020). https://doi.org/10.1364/OL.391357
Collecting light emitted by high-energy particle collisions with a scintillator using the fewest photomultiplier tubes.
Collecting luminescent radiation in photon upconversion devicesG. E. Arnaoutakis, J. Marques-Hueso, A. Ivaturi, S. Fischer, J. C. Goldschmidt, K. W. Krämer, and B. S. Richards, Enhanced energy conversion of up-conversion solar cells by the integration of compound parabolic concentrating optics, Solar Energy Materials and Solar Cells 140, 217–223 (2015). https://doi.org/10.1016/j.solmat.2015.04.020.G. E. Arnaoutakis, E. Favilla, M. Tonelli, and B. S. Richards, Single crystal monolithic upconverter solar cell device tandems with integrated optics, J. Opt. Soc. Am. B, JOSAB, vol. 39, no. 1, pp. 239–247, Jan. 2022, doi: https://doi.org/10.1364/JOSAB.437892. with the compound parabolic concentrator being to-date the most promising geometrical optics collector.G. E. Arnaoutakis and B. S. Richards, Geometrical concentration for enhanced up-conversion: A review of recent results in energy and biomedical applications, Optical Materials 83, 47–54 (2018). https://doi.org/10.1016/j.optmat.2018.05.064.
Some of the design methods for nonimaging optics are also finding application in imaging devices, for example some with ultra-high numerical aperture.Pablo Benítez and Juan C. Miñano, Ultrahigh-numerical-aperture imaging concentrator, J. Opt. Soc. Am. A, Vol. 14, No. 8, 1997
Theory Early academic research in nonimaging optical mathematics seeking closed form solutions was first published in textbook form in a 1978 book.W.T. Welford and Roland Winston, The Optics of Nonimaging Concentrators: Light and Solar Energy, Academic Press, 1978 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 concentrationRalf Leutz and Akio Suzuki, Nonimaging Fresnel Lenses: Design and Performance of Solar Concentrators, Springer, 2001 or solar concentration in generalJoseph J. O'Gallagher, Nonimaging Optics in Solar Energy, Morgan and Claypool Publishers, 2008 have also been published, although this last reference by O'Gallagher describes mostly the work developed some decades ago. Other publications include book chapters.William Cassarly, Nonimaging Optics: Concentration and Illumination in Michael Bass, Handbook of optics, Third edition, Vol. II, Chapter 39, McGraw Hill (Sponsored by the Optical Society of America), 2010
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.Concentration of sunlight to solar-surface levels using non-imaging optics Nature
The simplest way to design nonimaging optics is called "the method of strings",Solid-state lighting requires specialized optical design for optimal performance SPIE 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 () is described in detail in the aforementioned textbooks. The 3D SMS design method () was developed in 2003 by a team of optical scientists at Light Prescriptions Innovators.Pablo Benítez et al., Simultaneous multiple surface optical design method in three dimensions, Optical Engineering, July 2004, Volume 43, Issue 7, pp. 1489–1502
Edge ray principle In simple terms, the edge ray principle states that if the light rays coming from the edges of the source are redirected towards the edges of the receiver, this will ensure that all light rays coming from the inner points in the source will end up on the receiver. There is no condition on image formation, the only goal is to transfer the light from the source to the target.
Figure Edge ray principle on the right illustrates this principle. A lens collects light from a source S1S2 and redirects it towards a receiver R1R2.
The lens has two optical surfaces and, therefore, it is possible to design it (using the SMS design method) so that the light rays coming from the edge S1 of the source are redirected towards edge R1 of the receiver, as indicated by the blue rays. By symmetry, the rays coming from edge S2 of the source are redirected towards edge R2 of the receiver, as indicated by the red rays. The rays coming from an inner point S in the source are redirected towards the target, but they are not concentrated onto a point and, therefore, no image is formed.
Actually, if we consider a point P on the top surface of the lens, a ray coming from S1 through P will be redirected towards R1. Also a ray coming from S2 through P will be redirected towards R2. A ray coming through P from an inner point S in the source will be redirected towards an inner point of the receiver. This lens then guarantees that all light from the source crossing it will be redirected towards the receiver. However, no image of the source is formed on the target. Imposing the condition of image formation on the receiver would imply using more optical surfaces, making the optic more complicated, but would not improve light transfer between source and target (since all light is already transferred). For that reason nonimaging optics are simpler and more efficient than imaging optics in transferring radiation from a source to a target.
Design methods Nonimaging optics devices are obtained using different methods. The most important are: the flow-line or Winston-Welford design method, the SMS or Miñano-Benitez design method and the Miñano design method using Poisson brackets. The first (flow-line) is probably the most used, although the second (SMS) has proven very versatile, resulting in a wide variety of optics. The third has remained in the realm of theoretical optics and has not found real world application to date. Often optimization is also used.
Typically optics have refractive and reflective surfaces and light travels through media of different refractive indices as it crosses the optic. In those cases a quantity called optical path length (OPL) may be defined as where index i indicates different ray sections between successive deflections (refractions or reflections), ni is the refractive index and di the distance in each section i of the ray path.