Blackbody color change as temperature increases

A variac is connected to an incandescent light bulb. As the voltage is slowly increased and the temperature of the filament is increased the light bulb initially glows (within the visible spectrum) almost entirely in red and as the voltage is increased even further the filament begins to glow brighter and brighter in blue. You can also demonstrate that red becomes continuously brighter even as the blue light becomes visible. This can be shown to a classroom using a camera with switchable red and blue filters placed in front of camera lens.