article link:
https://www.kqed.org/news/11791667/what-are-those-grids-of-glass-in-the-sidewalk-and-why-are-they-purple
All over the city you can see glass squares and circles in big grid patterns covering a few feet of sidewalk. Sometimes the glass is clear, but often it’s a purple hue.
What are they?
They're vault lights, which are essentially skylights for below ground. The little glass pieces are projecting sunlight down into subterranean spaces.
While the top of the glass is flat, so it doesn't trip anyone, underneath the glass can take a variety of shapes. Some are shaped into prism forms that diffuse and bend the light so it reaches the inner areas of underground spaces.
