Abstract: A method and apparatus is disclosed for predicting an effective and safe laser light energy range for sub-epidermal laser surgery. The method is especially useful in controlling beam energy during the treatment of PWS. The method is accomplished by first impinging a measurement laser pulse on a predetermined treatment area, wherein the measurement laser pulse has an energy below a predetermined threshold of coagulation and below a predetermined threshold of skin damage. The thermal emission caused by the measurement laser pulse emanating from the treatment area is then detected and the delay time from the measurement laser pulse to the detection of the thermal emission is measured. The rise rate of the thermal emission is then measured. The layer thickness is then calculated based upon the delay time, wherein the layer thickness is substantially the epidermal thickness. An internal measurement temperature rise is calculated based upon the layer thickness and the rise rate.