Nonimaging concentrators are called "non-imaging" because they do not produce any optical image of the source, as opposed to imaging concentrators, which produce an image of the sun by reflecting it on the receiver. The non-imaging concentrators are able to reflect to the receiver all of the incident radiation, either beam or diffused, intercepted over a wide range of incidence angles. These systems are not precise, but they are more flexible. For example, if the right hand side of the sun is reflected to the left hand side of the receiver, the image of the sun is not preserved while the total energy incident on the aperture of the concentrator is additive. The most commonly used technology that leverages nonimaging optics is the compound parabolic concentrator (CPC).
The typical concentration ratios of CPCs are in the single digits. Despite the low concentration ratios of nonimaging systems, nonimaging systems can be very useful for increasing the performance of systems are relatively low costs, particularly in regions where the solar resource is less than ideal for concentrating systems to begin with (such as most of Europe and the northeast of the US).
The following reading describes in detail the optical principles and energy paths in the CPC collectors.
Book chapter: Duffie, J.A. Beckman, W., Solar Engineering of Thermal Processes, Chapter 7: Sections 7.6-7.8.