An illumination apparatus includes a liquid crystal device (L) disposed in the optical path of light projected from a light source (P) and a driving circuit (D) connected to the liquid crystal device for applying a special driving voltage to a composite film (1) of the device (L). The applied driving voltage alternately causes the composite film (1) to attain an opaque state and a transparent state. The time periods of the alternating voltage pulses are varied, thereby changing the ratio of the duration of the opaque state to the duration of the transparent state per unit time period, and thus controlling the power of light transmitted through the composite film (1) without undesirably altering the color spectrum of the transmitted light.