Measurements of the intensity of light transmitted between two points on the surface of tissue are not only relatively straightforward and inexpensive to obtain, but also contain a remarkable amount of useful information, as demonstrated by the clinical successes of NIRS (Obrig and Villringer 2003). So-called continuous wave (CW) systems require a source that either emits at a constant intensity, or is modulated at a low (a few kHz) frequency in order to exploit the significant improvements in sensitivity available from phase-locked detection techniques. CW sources have been used to investigate the head, testes and breast by viewing light which has been transmitted through the body since at least as far back as the early nineteenth century (Bright 1831, Curling 1843, Cutler 1929). CW trans