Detection and assessment of photochromism internal basis

This part of ISO 105 specifies a method intended for detecting and assessing change in colour, after brief exposure to light, of coloured textiles which change in colour on exposure to light but which virtually return to their original shade when stored in the dark.
What Is Photochromism?
Principle
A specimen of the textile is exposed to light of high intensity for a time much shorter than that necessary to cause a permanent change. The change in colour of the specimen is assessed immediately after exposure, using the grey scale. The specimen is then stored in the dark and assessed again.
click here for detail: Detailed method
or
http://www.dfmg.com.tw/member/standard/bs/BS-b05.HTM