Incandescent bulbs do flicker like all other artificial light sources. However, this occurrence is not noticeable on a camera. That's because these bulbs work on a different principle than the LED bulbs.   The incandescent bulbs operate with a filament that becomes white-hot to give off heat and light. This filament does not cool down even during the off state. As a result, the residual heat from the glowing hot filament gives an impression that it's on. Hence, no flickering effect is seen on the camera.