Sensitivity of media
The sensitivity of media to light and the subsequent fading of the colours in a collection item is one of the factors considered when establishing the acceptable amount of light exposure for a collection item.
The use of the ISO rating, or Blue Wool (BW) Standard, enables a comparison of the fading that occurs to various types of media. BW 1 is the most responsive to light (meaning it fades the quickest); BW 2 is approximately half as responsive as BW1; and so on to BW 8 which is the least responsive. The media used in collections can be broadly grouped using their likely responsiveness to light. For example pristine condition watercolours are categorised as BW1 items. They will fade very quickly when exposed to light.