A system, display apparatus and method for providing illumination uses internal reflection to monitor and control the illumination, which is produced by light from a number of light source devices, such as light emitting diodes. Some of the light generated by the light source devices is transmitted into an optically transparent region between the light source devices and a substrate, where the light is internally reflected within the optically transparent region. The optical properties of the light within the optically transparent region are measured using an optical sensor positioned within the optically transparent region so that the light generated by the light source devices can be selectively adjusted to control the illumination.