In the early days of 3D printing, stereolithography (SL) was one of the main methods of building parts layer by layer. It was invented by Chuck Hull, who built a company around this technology, 3D Systems. Stereolithography is now considered to be one of the technologies under the category vat photopolymerization. The others include Digital Light Synthesis (DLS) and digital light projector (DLP).

SL typically uses a UV light to harden and cure liquid photo-sensitive resin into a three-dimensional shape. Early versions had the build plate dip into the resin and a laser would then cure that layer. By today’s standards, this process is considered slow.

Video from Formlabs.

Efforts to speed up the curing of the liquid resin resulted in several technical advances. One was the use of a type of digital light projector (DLP), which could cover more area in one pass. Think of it this way, UV light is used to essentially draw one layer at a time while DLP exposes one cross section of material, speeding the curing/building process. The DLP process exposes light in pixels. The brightness of these pixels can be controlled individually, enabling different depths of curing, which enabled greater accuracy and surface finish.
A major developer of DLP 3D printing is EnvisionTEC and the company has since introduced variations of DLP to speed the build process even more. Continuous Digital Light Manufacturing (CDLM) is one variation. The build plate undergoes continuous motion in the Z direction for faster build speeds.

Video from EnvisionTEC

EnvisionTEC also offers 3SP (Scan, Spin, and Selectively Photocure), a technology that uses a UV laser to rapidly scan, spin and selectively photocure a variety of resins. While 3SP looks similar to SL, it is not the same.

The Digital Light Synthesis technology from Carbon helped ignite the race to turn vat photopolymerization into a faster technology. This innovation sped up the build process considerably. Carbon researchers found a way to inject oxygen into the resin at a precise point in the build to inhibit the resin from curing too quickly.