One type comes from the way window glass was first made. Glass was blown into a crown or hollow globe that is then flattened and cut. It is flattenned by reheating and spinning out the bowl-shaped piece of glass (bullion) into a flat disk by centrifugal force, up to 5 or 6 feet in diameter. The glass is then cut into the size required, because of the manufacturing process the best, thinnest, glass is in a band at the edge of the disk, with the glass becoming thicker and more distorting towards the centre. Due to the distribution of the best glass, in order to fill large window spaces many small diamond shapes would be cut from the edge of the disk and these would be mounted into a lead lattice work and fitted in the window.
Crown glass was one of the two most common processes for making glass for windows up until the 19th century, the other was blown plate. The process was first perfected by French glassmakers in the 1320s, notably around Rouen. The process was kept a careful trade secret, for example the first crown glass made in London was not until 1678. Compare to polished plate, cylinder sheet, rolled plate, and float glass.
The second use of the phrase is for a type of optical glass used in compound lenses. This Crown glass is produced from a special alkali-lime silicate incorporating a high amount of phosphorus pentoxide, it has a low index of refraction and low dispersion.