© 2011 - Chris J LeBlanc Photographer

A **Fresnel lens** (pronounced /fre?'n?l/ fray-NELL) is a type of lens originally developed by French physicist Augustin-Jean Fresnel for lighthouses. The design enables the construction of lenses of large aperture and short focal length without the weight and volume of material that would be required in conventional lens design. Compared to conventional bulky lenses, the Fresnel lens is much thinner, larger, and flatter, capturing more oblique light from a light source, thus allowing lighthouses to be visible over much longer distances.

Sizes of lighthouse lenses Fresnel's lighthouse lenses ordinarily fell into six orders based on their focal length. The order of a Fresnel lens is approximately the Dioptre or optical power of the lens. The dioptre or diopter is the reciprocal of the focal length of the lens in meters. A Fresnel lens with a focal length of 50 cm or 0.5 m would be classified as a second order lens. However, there are a few hyperradiant Fresnel lens -- the largest size ever made -- 'off the chart' of 6th through 1st Order. One such lens was on hand when it was decided to build and outfit the Makapuu Point Light. Rather than order a new lens, the huge optic construction -- twelve feet tall and with over a thousand prisms -- was used there.

The largest (first order) lens has a focal length of 36 in and an optical area 8.5 ft high. The complete assembly is about 12 ft tall and 6 ft wide. The smallest (sixth order) has a focal length of 5.9 in and an optical area 17 in high. Subsequent development extended this to seventh and eighth orders, an intermediate three-and-one-half order, and two orders even larger than first: mesoradial and hyperradial.