This document discusses different light sources including natural sunlight and various artificial light sources such as incandescent, fluorescent, and discharge lamps. Incandescent lamps emit light due to the filament heating up, while discharge lamps emit light from an electric current passing through gas. Incandescent lamps are very inefficient as most of the energy is wasted as heat rather than visible light. Fluorescent lamps contain mercury vapor and phosphors that convert UV light to visible light, providing high efficacy. Other discharge lamps discussed include high and low pressure sodium, metal halide, and halogen incandescent lamps.