St. Denis Cathedral - some of the oldest stained glass
Until the middle ages, church windows were little more than narrow slits carved in the stone high above the people's heads. Thus early buildings were physically dark and cold places.
The process of making colored glass developed around 1100; at about the same time as Gothic architectural design; which using flying buttresses and arched supports allowed for entire walls of glass. First used about 1140 in the church of St. Denis Paris, God's house was suddenly flooded with the full spectrum of light. [see photo above]
continued in section, Stained Glass ...