Photograph stability refers to the ability of prints and film to remain visibly unchanged over periods of time. Different photographic processes yield varying degrees of stability. In addition, different materials may have dark-storage stability which differs from their stability in light. An extreme case with slides was stability under the intense light of projection. For example, when stored in darkness, Kodachrome's long-term stability under suitable conditions is superior to other types of color film. Images on Kodachrome slides over fifty years old retain accurate color and density. Kodachrome film stored in darkness is largely responsible for excellent color footage of World War II, for example. It has been calculated that the yellow dye in Kodachrome, the least stable, would suffer a 20% loss of dye in 185 years. This is because developed Kodachrome does not retain unused color couplers.