A therapeutic light source, for example for photodynamic therapy (PDT), comprises an air-cooled array of LEDs (Lx,y), the air being vented in the vicinity of the array. The array may be mounted at the distal end of a hand piece suitable for invasive therapy. The LEDs may be coupled to a light guide (W, L). The emission spectra of the LEDs may be substantially limited to the range 550 to 660 nm, and preferably to one of the ranges 590 to 640 nm, 560 to 644 nm, 650 to 660 nm, and 550 to 570 nm. The therapeutic light source may comprise a non-planar array of light-emitting diodes L conforming with the shape of an external area to be treated or diagnosed. The therapeutic light source may comprise a non-planar array of independently switchable red and blue light-emitting diodes LR, LB, mounted on a flexible backing.
