Why Fresnel Lenses?
Fresnel lenses, invented in the 1800’s for lighthouses, are used for the same reason today; namely as equivalents to simple lenses without thickness and bulk. In essence, a fresnel lens is a realisation of the optics theory that tells us that the ray bending (refractive) power of a simple lens is determined by the curvature of its surfaces and is independant of its thickness.
In P.I.R (passive infrared) applications, most materials are highly absorbent. The reduced thickness of fresnel lenses allows radiation to be transmitted. Polyethylene, the most commonly used material, is far from ideal because it has many absorbtion bands. Even so, injection moulded fresnel lenses with good performance can be produced. Fresnel lenses usually have one plane side and one side grooved. Since one surface does all the optical “work”, the optical facets and steps have to have a precise form. Our in-house raytracing facilities has allowed us to generate groove profiles that are optimised for one set of conjugates (object and image positions). In practice, such lenses are effective over a very wide range of object positions.
The main fresnel lens types are:
- Constant groove depth.
- Contant groove pitch types.
The optically active facets may be flat, spherical or aspheric, in progressive order of theoretical efficiency. However, the P.I.R. sensor, housing and electronics have to be of the highest quality for any optical benefits to be realised practically.
Some exotic variants include lenses which use phase effects to add diffractive energy to the main refractive radiation collection. Again, practical gains are difficult to realise unless the lens mouldings have near perfect form and are distortion-free.
Like any photographic objective, the collecting power of a fresnel lens is set by its F#-number, the ratio of focal length f, to diameter d, ( f/d = F# ). Although it is possible to make fresnels of large dimensions compared to their focal length, transmission of energy becomes increasingly low beyond a deviation angle of around 30 degrees. Edge rays from a F#0.87 lens are deviated by 30 degrees. Increasing the lens diameter beyond F#0.87 will not significantly increase radiation collection. When fresnel lens segments are used off-axis, similar effects occur and a segment having an average deviation much greater than 30 degrees, will be be poor collector of radiation.
Apart from these limitations, radiation collection by a fresnel lens is not ideal because of several loss mechanisms:
- Shadow losses at the groove steps/walls.
- Diffraction losses at surface discontinuity.
- Scattering losses at the surfaces and in the material.
- Absorption by the material.
Losses will also occur if a lens is badly mounted or distorted (focus errors and optical aberrations). For best optical performance fresnel lenses should be mounted with grooves facing the object (long conjugate), however, it is common practice to face grooves internally so as to minimise dust build-up in the groove steps. Choice of polyethylene material is important, as are pigments and additives (for example U.V. stabilising agent-recommended for external mounted P.I.R. units) as they have a direct impact on overall lens transmission.
We have a range of diamond turned, lens masters (constant groove-depth type) which are optimised point by point over their entire active surface. Focal lengths of 25, 26, 27, 29, 30.5 and 32mm are available for client applications.
Our Volumetric Box lens, shown above, has been designed in line with the technical principles briefly outlined above. It’s unique features are:
- Segments angled towards line of sight.
- Integral support frame.
- Integral location pegs (may be heat-welded).
- Effective creep zone coverage.