What is Fresnel Lens?

In the 19th century, a Frenchman named Fresnel invented an optical lens;  this lens from front looks like a dart board, but each ring is tilted at a different angle, Each ring is an independent lens, which together is a lens group,through different structural design, the direction of light can be adjusted to produce different optical effects. It is an optical structure, which widely applied in projection, solar energy, virtual display; The difficulty is in the processing, need ultra-precision processing equipment to achieve the ideal optical effect!