A popular misconception about x-ray film is that it is primarily sensitive to x-rays. That's not the case. The film holders contain phosphor coated screens which emit blue or green light when exposed to x-rays. The film is held in close contact with the screen, and it is the light from the glowing phosphors that make the bulk of the exposure. Green sensitive films are used in conjuntion with screens using green emitting phosphors and blue sensitive films get used with blue emitting screens. X-ray film comes in different sensitivities for different applications. Some are faster than others. It does not surprise me that under incandescent light, which is sorely lacking in the blue/green spectrum, would be very slow. The dramatic speed increase under daylight is also not surprising. It is good to know that there are some films that are coated only on one side. I imagine these would be best for pictorial use. The support of these films is thicker than pictorial films because they must be sturdy enough to withstand rough handling. Double coated films would have one side considerably off register, and consequently out of focus.