Abstract: An LED array and a method for controlling such an array is described. The array includes one or more LED-optical sensor pairs in which the optical sensor measures the light output of the associated LED. In some forms of the invention, the optical sensors are provided with filters to attenuate light at a wavelength different to that of the LED associated with that optical sensor. In some forms of the invention, the LED-optical sensor pairs are at least partially shielded from extraneous light. In one form of the invention a plurality of sets of LEDs is provided, each set outputting light having a different frequency and each set being controllable to provide a desired light output.