A flash tube has an internal gas pressure above 600 millimeters of mercury. It is energized by a capacitor having an operating voltage such that the ratio of the discharge gap of the tube to said operating voltage is no greater than 0.5 centimeters per 100 volts and having a capacitance such that the voltage will not drop by more than 15 percent upon reaching peak current through the tube. In one embodiment the tube has two electrode chambers connected by a duct and the chambers, but not the duct, are cooled. In a second embodiment there are two radiators, one for ultraviolet radiation and the other for infrared radiation. In one form these two radiators are flash tubes operated in a timed manner so that the maximum of the infrared radiation from one coincides with the maximum of the ultraviolet radiation from the other.