TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Components: The emission layer includes a luminescent material of formula (AaBbX1xX2yX3z).A = elements of group IA;B = elements of group IVB;X1-X3 = elements of group VIIB;a, (x+y+z) = 1-9; andb = 1-5.    TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred Components: The resonant cavity waveguide further includes: a substrate having a tubular shape and including an outer surface and an inner surface, where the outer surface of the substrate faces the incident solar radiation, and the outer reflector is adjacent to the inner surface of the substrate; includes a substrate having a rod shape and including an outer surface, and the inner reflector is adjacent to the outer surface of the substrate; and a spacer layer disposed between the emission layer and the inner reflector having broadband reflectivity, where the spacer layer has a refractive index of not greater than 1.5. The outer reflector, the emission layer, and the inner reflector correspond to an inner luminescent stack, and the resonant cavity waveguide further includes an outer luminescent stack adjacent to the outer surface of the substrate. The resonant cavity waveguide includes a cylindrical portion and a tapered end portion, and the photovoltaic cell is adjacent to the tapered end portion. The resonant cavity waveguide defines a groove that extends through at least a portion of the emission layer and is sized so as to accommodate the photovoltaic cell. The emission layer is disposed between the outer reflector and the inner reflector so as to be substantially centered at the anti-node position. The peak emission wavelength is in the near infrared range. Emitted radiation includes the energy band having a spectral width of not greater than 80, preferably not greater than 50 nm at Full Width at Half Maximum. The outer reflector includes a dielectric stack having narrowband reflectivity with respect to the emitted radiation. The spacer layer includes at least one of oxide and fluoride, and the inner reflector includes at least one of metal and metal alloy. The inner reflector is a first inner reflector, and the resonant cavity waveguide further includes a second inner reflector disposed between the emission layer and the first inner reflector, where one of the first inner reflector and the second inner reflector has narrowband reflectivity with respect to the emitted radiation, and another one of the first inner reflector and the second inner reflector has broadband reflectivity. The emission layer is an outer emission layer, and the resonant cavity waveguide further includes: an inner emission layer disposed between the outer emission layer and the inner reflector with respect to a second anti-node position within the resonant cavity waveguide; and a spacer layer disposed between the outer emission layer and the inner emission layer. The spacer layer is configured to guide at least a fraction of the emitted radiation towards the photovoltaic cell via optical mode transfer. The spectral concentrator further includes a first reflector and a second reflector, and the luminescent material is disposed between the first reflector and the second reflector and is configured to absorb the incident solar radiation and emit radiation that is guided towards the photovoltaic cell, and at least one of the first reflector and the second reflector has narrowband reflectivity with respect to the emitted radiation. The first reflector has narrowband reflectivity with respect to the emitted radiation, and the second reflector has broadband reflectivity.