TIR  Fresnel Lens - working principle, optical properties and applications

Fresnel lens, succession of concentric rings, is used particularly in lighthouses and searchlights to concentrate the light into relatively narrow beam. This optical structure save lots of materials and reduce the weight significantly.

(Fresnel lens and the equivalent convex lens, picture from zhihu.com)

(Fresnel lens from a Spanish Maritime Museum, picture taken by TOPLITE)

Most of the traditional light sources emit 360 degrees light. Although some light can be reflected to the front side by some optics such as a reflector, there is still a lot of light lost. With the development of solid state light sources(typically such as LED and laser), it has become a reality that light is emitted only from one side. Because the light source has a fixed emitting area and emitting angle, how to make more use of light has become a problem that optical R&D engineers try to solve. Generally, the emitting angle of LED is about 120 degrees, and the focal length of Fresnel lens is relatively long, since the traditional manufacturing process can not achievev a very short focal length, although Fresnel lens can collimate and concentrate the light, light still has a large loss, especially in narrow angle lighting. In order to solve this problem, on the one hand, we can optimize the light source to narrow its emitting angle, for example, make the light source into the form of LED module, the cost of this solution is relatively high. On the other hand, try to find one optical lens having short enough focal length.

(Long focal length of the Fresnel lens and the LED module, picture from TOPLITE)

A TIR lens takes advantage of "total internal reflection" to change the path of light and collimate light. This kind of lens looks like a cone with a hole in its bottom. This hole fits over one solid state light source and even covers it which means that the light doesn't escape and is basically used by lens. When the power is not high, light source is generally relatively small. TIR lens looks like a fat child and the narrow space of its bottom hole may be OK for small light source, for example, low power LED. Now, high power LED is widely used, high power always means hot and large, it is necessary to consider whether the narrow space(hole) limits the use of high-power LED. We need to make big TIR lens to fit with large-size LED, even make it by glass instead of plastic to resist high temperature. Big TIR plastic lens usually needs a long time of pressure holding during injection molding, however, it is still easy to shrink, resulting in poor consistency of the finished lens, and, using glass to make the lens means expensive.

In order to solve the above difficulties, TOPLITE's R&D team designed a new optics called "total internal reflection Fresnel lens". TIR Fresnel lens fits with the solid state lighting source, e.g. LED, laser, IR and UV.

(TIR Fresnel lens and its equivalent TIR lens, picture from TOPLITE)

The TIR Fresnel lens also has the rings structure like Fresnel lens, it can save optical materials, reduce lens volume and weight significantly. Different from the Fresnel lens, light is shot into lens from the side of rings structure, refracted on inner interface of each ring, totally reflected on the outer interface of each ring, and then emitted on the lens plane side. The difficulty of this TIR Fresnel optics design is that it is hard to modeling and tracing rays by existing optical design software, because there is no ready-made library or mathematical model to rely on. The R&D team of TOPLITE established a set of special calculation methods which can successfully solves the modeling of complex and irregular optics. TIR Fresnel lens based on this calculation method has won the invention patent granted by CNIPA.

Special light path, looks to be folded, that is calculated by the new methods makes TIR Fresnel lens have much shorter focal length than Fresnel lens, for example, the focal length of a Φ126mm TRI Fresnel lens is just 39mm. At the mean time, the space between light source and lens is larger than TIR lens, TIR Fresnel lens fit very well with the solid state light source which is high-power, large emitting surface and high illuminance. TIR Fresnel lens is thin, high-efficiency and convenient for the miniaturization of new lighting fixture. Under the same light path, beam angle of the lens changes with emitting surface of light source. When the emitting surface is large, beam angle will be wide, when emitting surface is small, beam angle will be narrow, with a minimum of 1 degree.

(Changes in beam angles, picture form TOPLITE)

(Same lamp same light path, different emitting surfaces, different beam angle, picture from TOPLITE)

Nowadays the demand of high-power lighting becomes more and more, common solution is to use a single high-power light source, a COB for instance, high power usually means that larger emitting surface of lighting source,   and heat dissipation is also becoming a problem in various engineering applications. Because large emitting surface means wide beam angle, adopting TIR Fresnel lenses matrix is a reliable solution to solve several key problems like high power, narrow angle, heat dissipation and efficiency. The beam angle of LED-lens matrix is almost consistent with that of single LED-lens, each LED in the matrix keeps a certain and safe spacing which is conducive to heat dissipation, good heat dissipation ensures LED works well.

(TIR Fresnel lens matrix and the pencil beam, picture from TOPLITE)

TOPLITE provides three kinds of TIR Fresnel lens, named PEGG, KULLEN and KULLEN-GLASS in a wide diameter range from Φ45mm to Φ200mm.

(TIR Fresnel lenses, picture from TOPLITE)

KULLEN-GLASS TIR Fresnel lens is made by special glass molding process, its rings structure has sharper inclination angle and better surface quality. Optical performance of the lens is better consequently. At present, TIR Fresnel lens has been applied to outdoor searchlights, fire search and rescue lights, ship searchlights, helicopter searchlights, auditorium audience lights, stadium lighting, commercial lighting, museum lighting, film and television shoot lighting and other fields.

(TIR Fresnel lens in theatre lighting, picture from BB&S)

TOPLITE will spare no effort to explore new optic designs, continuously providing customers with high-quality optics and reliable optical solutions.