Indicator operating by color change.

摘要

The invention relates to an indicator operating by color change, including a liquid (15) having a predetermined index of refraction and rate of evaportion; a reservoir (14) for holding the liquid (15); and an opacifying layer (12) of microporous material overlying the reservoir (14). The opacifying layer (12) has an index of refraction approximately the same as that of the liquid (15), and has an open-cell network of pores for absorbing the liquid (15) from the reservoir (14). The opacifying layer (12) is in a first radiation scattering condition when the liquid (15) occupies the opacifying layer (12) and is in a second scattering condition when the liquid (15) is depleted from the opacifying layer (12).