Proponents of both the incendiary paint hypothesis and the hydrogen hypothesis agree that the fabric coatings were probably responsible for the rapid spread of the fire. The combustion of hydrogen is not usually visible to the human eye in daylight, because most of its radiation is not in the visible portion of the spectrum but rather ultraviolet. Thus what can be seen burning in the photographs cannot be hydrogen. However, black-and-white photographic film of the era had a different light sensitivity spectrum than the human eye, and was sensitive farther out into the infrared and ultraviolet regions than the human eye. While hydrogen tends to burn invisibly, the materials around it, if combustible, would change the color of the fire.