Non-imaging optics is the study of reflecting or lensing light for a desired purpose without preserving an image of the source. Fresnel lenses used in light houses and car headlights, or for diffusing light through windows, are non imaging lenses. The use of Fresenl reflectors (arrays of flat or curved mirrors) is a non-imagining strategy that greatly decreases the cost of solar collectors without necessarily sacrificing concentrating capacity. Where concentrating power and not imaging is the desired result, multistage non-imaging optics are ideal, and have concentrated sunlight at the thermodynamic limit.
Imaging optics used for telescopes, projectors and other visual applications are non-ideal for solar energy concentration. This is because the sun is not a point in the sky, and thus perfect imaging collectors are limited in their capacity to concentrate sunlight to an area the size of the image of the sun. Perfect parabolic collectors fall well short of the theoretical limit of concentration.
A non-imaging collector usually consists of a primary array of fresnel reflectors or lenses focusing on a secondary CPC (compound parabolic collector). The recent advance of tailored edge-ray secondary reflectors increases the capacity of CPCs (or relieves some of the unideal constraints their use imposes), and makes the system less susceptible to optical errors (double reflections of mirrors, slightly inaccurate solar tracking, and human errors).
Fortunately, this relieves individuals of the need for forming huge and precise parabolic dishes to achieve high temperatures in solar collectors. Instead, the burden of precision is pushed to the design and construction of CPCs and tailored edge-ray secondaries. The primary may be an array of flat mirrors or imperfect Fresnel lenses, which are much cheaper to produce and to track with.
Tailored edge-ray concentrators as ideal second stages for Fresnel reflectors by Jeffery M Gordon and H. Ries.