What is a Fresnel mirror? What does it do?

Fresnel reflector spotlight is widely used in film photography lighting. The optical system of this spotlight consists of Fresnel lens (commonly known as screw lens) and spherical reflector. The center of the luminous body of the light source is located at the spherical center of the spherical reflector. Through the focusing mechanism, the light source and the reflector move back and forth along the optical axis of the Fresnel lens, thus obtaining a beam with continuously changing angles. On the non-threaded surface of Fresnel lens, various patterns such as "turtle pattern" or "honeycomb" are usually pressed, so that the light can be scattered appropriately, the lighting effect is soft and uniform, and the irradiated light field has no obvious boundary, which is convenient for receiving light. The beam angle of the lamp can be adjusted in the range of15 ~ 60. There is a shutter outside the light box, and there is usually a frame in front of the lens that can hold light paper. Fresnel lens spotlights for film photography lighting mostly use tungsten halogen lamps as light sources, and a series of lamps ranging from 200 watts to 20 kilowatts are formed according to the different power of the light sources. In addition to tungsten halogen lamps, there are Fresnel lens spotlights with carbon arc as light source, which is commonly called carbon essence lamps. Carbon lamp has complex mechanical structure, heavy weight and large volume, and is polluted by cyanide gas and smoke. However, due to its high luminous intensity and wide beam projection area, carbon lamps can form high illumination, which is still difficult to be replaced by other lamps. In 1980s, Fresnel lens spotlight with metal halide lamp as light source entered the practical stage and gradually improved.