Fresnel Lens

An embodiment of the present invention will now be more particularly described by way of example and with reference to the accompanying drawings, in which:

FIG. 1 is a light ray trace for half of a fresnel lens;

FIG. 2 is a schematic view of facets of a lens drawn to a larger scale and

FIG. 3 is an enlarged, schematic cross sectional view of a lens incorporating features of the present invention.

A preferred form of lens embodying the invention is substantially planar and is provided with a plurality of fresnel facets on the face directed toward a detector for the radiation focussed by the lens. In fact, the lens is not entirely planar since it is preferred for ease of moulding that the lens is very slightly thicker at its central point than it is at the edges. For example, the center may be a maximum of 0.5 mm thick and a maximum of 0.45 mm thick at the edges. Again, it is preferred that the additional thickness at the center is taken up on the non-faceted side of the lens, which, in use, faces outwardly. The lens could be used with facets facing outwardly, on the side remote from the detector, but this will lead to a certain loss of light and will also make the exterior surface of the lens more difficult to clean.

The fresnel grooves of the lens are of substantially even depth, with a minimum lens thickness of 0.25 mm under the grooves. With the even depth of the grooves, the spacing between them becomes narrower towards the edge of the lens. This can be seen in the Figure.

The form of each facet is an aspheric curve determined by the formula: ##EQU1## where z is the height of curve,

y is the radial distance from the axis,

p is the eccentricity of the base curve,

R is the radius at the apex of the aspheric surface, and

A.sub.4,A.sub.6,A.sub.8. . . A.sub.N are correction coefficients.

These coefficients may be used for fine curve adjustment to provide the required phase optimalisation. In addition, adjustment of the highest appropriate coefficient can also be used to delay the onset of offense against the critical angle at the outer edge of the lens. This can provide one or two extra facets over and above that which can be achieved using a pure conic equation.

In designing the lens, assuming that it is a concentric fresnel lens, each facet is given a thickness and aspheric curvature so that light passing through that facet and received at the detector will be in phase with light passing through the immediately preceding adjacent facet. This can be seen from FIG. 2.

As each facet is phase correlated with the preceding facet, the lens thickness will change. In order to maintain the overall thinness of the lens, and with reference to FIG. 3 the surface of the lens is divided into zones such as those designated by the numerals 1, 2, 3 and 4, each zone having a number of facets, and with the light path between that zone and the detector increasing from one zone to the immediately adjacent zone by an integral number of wavelengths of the light passing through the lens. When the change in lens thickness is about to exceed 0.1 mm as at 5 in FIG. 3, the value of n will be changed, preferably by a unitary integral value, in order to control the thickness of the lens. This ensures that all light received in the detector is substantially in phase.

A preferred material for the lens is polyethylene, although other materials may be used. In an intruder detector lens, it is generally desirable to be able to focus on several zones in the area being scanned. To this end, the entire lens is preferably an assembly of sections of individual fresnels. These are generally of rectangular shape but the exact form of each section is adapted to focus radiation from a different area of the space being scanned. Once the desired form of the composite lens has been determined, the entire lens may be integrally moulded from polyethylene.