Fresnel lens (Fresnel lens), also known as thread lens, by the French physicist Augustin Fresnel (Augustin Fresnel) invented, he first used in 1822 this lens designed for Build a glass Fresnel lens system - lighthouse lens.
Another understanding is that the continuous surface of the lens part of the "collapse" to a plane. Figure:
From the profile, the surface consists of a series of serrated grooves, the central part of the oval arc. Each groove is different from the adjacent groove angle, but will focus on one light, the formation of the central focus, that is, the focus of the lens. Each groove can be seen as a separate small lens, the light is adjusted to parallel light or condenser. This lens also eliminates part of the spherical aberration.
Simply put, Fresnel lens side is flat, the other side is raised. The first use of the Fresnel lens was in the early 18th century, when it was used in the lighthouse's searchlight, focusing on the shot out of the beam. When people need a thin and light lens, the plastic Fresnel lens will come in handy. Although the image quality is not as good as glass lenses, we do not need perfect image quality in many applications.
The Fresnel lens principle is based on the Fresnel zone plate, which has a similar lens effect that allows the incident light to converge and produce great light intensity.
