Fresnel lenses are specialized optical components with grooved surfaces instead of traditional curved ones. Owing to their unique design, Fresnel lenses are thin and lightweight, typically have short focal lengths, and are more efficient than conventional optical lenses.

Augustin-Jean Fresnel — an early 19th century pioneer in the field of wave optics — is credited with the invention and application of grooved lenses. While working as a commissioner of lighthouses in France, Fresnel discovered that grooved lenses captured more oblique light than conventional ones, allowing lighthouse lights to operate more efficiently, use less material, and be visible over a greater distance.

A large (6-feet high) Fresnel lens for use in a lighthouse.

Image credit: University of Arizona

The grooves in a Fresnel lens act as individual refracting surfaces, much like a group of tiny prisms. The grooved surfaces replace the continuous surface of a conventional lens into a set of surfaces with the same curvature. The primary advantage of Fresnel lenses is that they can be manufactured much thinner than a spherical lens with equivalent optical power, saving space, materials, and cost. Additionally, the thinner lens results in reduced light lost to absorption. The image below compares a Fresnel lens and a conventional lens of equivalent power.

A Fresnel (left) and conventional lens. Image credit: Wikipedia

Fresnel lenses typically sacrifice image quality for efficiency and are therefore infrequently used in precision imaging applications. While the original lighthouse lenses are obviously very large, small- and medium-sized Fresnel lenses are used in projectors and TLR/SLR camera screens, respectively.

Applications

When used for projection applications, Fresnel lenses reduce image quality as noted above, so that their use is most appropriate in non-critical uses or those prohibiting the bulk of a conventional lens. General purpose Fresnel lens applications include:

• Projection (overhead projectors and projection televisions)
• Magnification
• Traffic lights
• Automobile headlamps
• Rear-view mirrors
• Optical landing systems
• Camera viewing systems
• Specialized lighting effects

Due to their favorable optical power to thickness ratio, Fresnel lenses are often used to concentrate sunlight for heating and solar power applications. By using Fresnel lenses to concentrate sunlight onto a photovoltaic cell, the surface of the cell can be reduced by several hundred times, resulting in significant cost savings. Other uses involving concentrated sunlight include solar water heating, cooking, melting metals, and steam generation.

A Fresnel magnifier.  Image credit: DHgate

Specifications

As a specific types of lens, Fresnel lenses share many specifications — including important optical points, polarity, and wavelength ranges — with the broader group of optical lenses. For more information on these figures, please visit Engineering360's Optical Lenses Selection Guide. The specifications below are unique to Fresnel lenses.

Groove Density

A lens's groove density is measured in grooves per millimeter. Higher groove density (more grooves per millimeter) results in better quality images with lower optical efficiency, while lower density lenses yield better efficiency and poorer images. Less-dense lenses are most suitable for light gathering applications, while high density ones are more suited for imaging and projection.

A close-up view of Fresnel grooves.  Image credit: faq.org

Standards

Fresnel lenses may be designed, manufactured, or used according to one or various standards. Common standards include:

ISO 877-1 (Plastics - methods of exposure to solar radiation)

MIL-DTL-24560 (Fresnel lens lights; globe type)

References

Wiley VCH - Fresnel Lenses (pdf)

The Fresnel Lens: A 19th Century Innovation with 21st Century Uses

Image credit:

Knight Optical