The physics of linear receivers is analysed, including the effects of the natural aperture of the solar radiation (32' conical angle) on the radiation concentration achievable with a given geometry. This analysis is oriented to identify features of the thermal performance of the receiver in different radiation conditions. It will be seen that two thermal regimes can be distinguished in that dependence, with a step-wise separation from the first one, at low levels of radiation intensity, to the second one, at high levels. The step appears at a value of the concentrated radiation intensity which depends on the temperature that must be achieved at the outlet of the receiver. This fact points out the need of studying at depth the features of systems with concentration factors around the threshold to the second regime, where thermal efficiencies reach a plateau. It is shown that some Fresnel structures with adequate concentrating geometries are able to go beyond the threshold for reaching final temperatures of 400 degrees C, which opens new possibilities for these structures. (C) 2011 Elsevier Ltd. All rights reserved.