A device for applying radiation to a target is provided. The device includes a radiation emitter to emit electromagnetic radiation having a peak emission wavelength in the range from 10 nm-1 mm, and a first reflector that extends in a length direction with a concave cross section. The first reflector defines a cavity area having a perimeter, and includes an inward facing reflective border for at least 50% of the perimeter of the cavity area. Radiation is provided to the cavity area with an intensity distribution I and a maximum intensity Imax. The cavity area includes a focal area defined by all points at which a normalized intensity I/Imax is greater than 0.2. A width of the focal area is 0.0001-0.5 times a width of the cavity area.