A flash fusing system for fusing toner images onto support material in which the support material is transported in a path past one or more flash fusing lamps. The flash fusing lamps are coupled to a circuit including a power supply voltage which is adjusted by an input from a sensing circuit including, a photodiode which measures the density of the unfused toner images on support material and converts the information into electrical signals. The electrical signals are supplied to an integrator which in turn is coupled to the power supply voltage of the circuit for energizing the flash lamps.