A photolith is an element of city infrastructure that may serve several purposes. Semantically the word is a combination of the two words 'photo' which is used merely as a prefix and is intended to mean light or regarding light, and 'monolith' which is traditionally a large stone or obelisk. When combined this term is used to represent an independent structure that is specifically designed to convert, concentrate, capture, direct, or utilize light in a variety of ways.
The most common type of photolith is one that merely absorbs and then redistributes the available light in any given area. The main functionality of this is to provide even and reliable illumination for an entire area. Normally due to architectural complexity there are areas that fall into shadow, additionally there are subterranean areas that would normally be pitch black; the photoliths solve both of these problems by collecting light in overly bright areas and then projecting or beaming it to areas that would normally be underexposed.
Obviously the placement of photoliths is remarkably sophisticated due to the fact that the light from the sun is always coming in from a different angle during the day / night cycle. Naturally the city is planned in such a way that all the calculations have been made in advance, so that no matter what level or direction of light, there is always a photolith available to utilize it. In this way the entire city is constantly illuminated, not only the surface, but also the underground areas.
In addition to the common types of photoliths there are also far less common types, rare types, and finally uniques. I will try to give a few examples of each. Less common photoliths include lenses that create projections of far away locations, there are also ones that focus light into high powered lasers that heat water, surfaces, and form natural boundaries. Rare photoliths most frequently collate dozens, hundreds or thousands of beams of light into single beam which is then used to power large-scale kinetic functions. Unique photoliths typically create a visual spectacle, holograms, maps, and aesthetically pleasing displays.