Patent ID: 8403558

Claim:
A time temperature indicator, comprising: (a) a first layer comprising (a1) a photochromic colorant introduced into, atop, or into and atop (a2) a matrix substrate, and charged by exposure to UV light; (b) a transparent light absorbing protecting layer adhered to the first layer (a), wherein the transparent light absorbing protecting layer (b) comprises 1-50 wt % of (b1) a UV light absorber or a UV light absorber mixture, wherein the transparent light absorbing protecting layer (b) comprises 0.1-10 wt % of (b2) an optical brightener, and wherein a weight ratio of the UV light absorber (b1) and optical brightener (b2) is 0-10, wherein a color of the indicator, after activation, fades depending on time and temperature.