Fresnel Lenses

Fresnel lenses are a special type of microstructured lenses designed for wavefront correction and removing aberrations, most commonly, spherical aberration (Fig1). The aberration-free Fresnel lenses made by Luminit perfectly match with Luminit diffusers to blend light in laser diode and laser assemblies, and can be used as light condensers in overhead projection and rear-projection systems, collimators, collectors, magnifiers, such as range finders for sensors and single-lens reflex cameras, and more. The benefits of Fresnel optics are more efficient use of light, almost unlimited design flexibility, easiness in making large areas and high apertures, and thickness/weight reduction. However, the Fresnel lenses are typically not well suited for imaging applications in VIS spectrum.

Fig 1. Illustration of spherical aberration from regular lens

Luminit LLC has in-house capability to mold the custom designed Fresnel lenses to satisfy the most demanding specifications. Typical materials offered are acrylic, polycarbonate, rigid vinyl, and others such as zeonex on various substrate applications in a wide wavelength range (see Diagram 1). Please call Luminit Sales team to discuss your requirements in details: (310) 320-1088.

Diagram: Applications of typical plastics to various wavelength bands

The history of this peculiar-shape lens goes deep into XIX century. The French pioneer of optics, A.J. Fresnel, known for Hyugence-Fresnel principle of light propagation introduces a special class of focusing elements (Fresnel zone plates). He first realizes that the optical fields, diffracted from a lens, are not always in phase. There are even and uneven phases (shifted by p) that contribute differently to the total field E-vector, and, being off-shifted, may blur the image rather then focus it. Figure 1b shows the Fresnel zone plate, which acts like a focusing lens with focus distance f. The n-th Fresnel zone is described by n-th radius, related to wavelength and focal distance f, which is an area of consecutive even (in phase) and odd (out-of-phase) contributing fields.

Fig. 2. Fresnel zones and Fresnel zone plate. White areas are even zones (where there is constructive interference of secondary waves occurs: Summed up in-phase wave vectors many times exceed the field, which would be without the zone plate, in observation point P), black areas are out-of-phase (odd) zones.

Note that in Fig.2 the zone plate has a flat surface. Thus removal of a large amount of bulk material in a standard lens under certain conditions would not affect optical performance. The trick is that the contour of a lens does the most of the work to bend the rays of light regardless of the bulk material. Thus, the bulk material, which only adds to signal attenuation and weight, can be safely removed as shown in Fig.3 The removal of the bulk material is just the first step. One needs to ensure that the odd and even zones are in phase. This could be simply achieved by increase of odd surface thickness on the amount of Δd=λ/2 to provide extra phase shift between the odd and even zone, equaled to p.

Fig, 3. Intermediate Fresnel lens construction or enhanced Fresnel zone plate. Notice the even and odd zones differ by thickness to provide extra p-shift. As a result all the vectors of secondary (diffracted) fields are in phase, and focusing is sharper. Final polish comes when the original lens structure is replaced by a central curved portion and concentric grooves. The inclination angle of each groove is the same as the corresponding portion of the original surface, translated toward the plane surface of the lens; with the angle of the groove been slightly modified to meet this translation. This is done in order to fight second-order effect and ensure maximum focusing intensity.

Fig.4. Final form of Fresnel lens, which is a smoothed surface of Fig. 3 profile.