Effect of electromagnetic radiation on the Lamb shift

If there is an external radiation field surrounding the atom, it influences the value of the Lamb shift. It is shown that for black-body radiation at temperature comparable to 105 degrees, the change in the Lamb shift due to the influence of radiation is of the same order as the Lamb shift itself. Whereas the transitions to the continuum largely contribute to the Lamb shift (in the absence of radiation), the change in the Lamb shift is largely due to transitions to discrete states.