Energy transport by radiation is an important contribution to the energy budget in stellar atmospheric structures. In this paper, radiative relaxation of small-scale structures is investigated. The authors show in a linear analysis: (1) Already at structural lengths of 10 opacity scale heights, horizontal photon exchange is important for the energy budget. (2) In atmospheric layers near continuum optical depth τc = 1 and below, the continuum absorption and emission processes dominate the radiative relaxation. (3) Weak spectral lines or lines with σlɛ ≤ 1 have little influence on the energy exchange. (4) At large heights, transport in few spectral lines with σlɛ very large 1 can compete with continuum processes.