Quartz glass hyper hemispherical dome lens with flange

Dome is composed of two parallel optical surfaces. Among optics domes are unique because, unlike any other optical components, the key attribute of the dome is to have no optical effect. Generally a dome is often exposed to the environment. Accordingly, domes made from hard ceramic materials are preferred due to their ability to withstand wind and rain erosion. 

Why use Dome ports for Underwater Photography?
Domes make it better – Dome lens help correct aberrations that occur when light travels thru water as opposed to air. Domes can deliver better results compared to flat ports, and also make it easier or possible some results that flat ports would not capture. 

Size matters 
Bigger domes are better than smaller domes, for a number of reasons. The tradeoffs are; heavier, bulkier, pricier than smaller domes. 

Glass is optically superior
Glass domes are optically superior to plastic or acrylic, and they do not scratch as easily.