It is well know that optically refracting Fresnel lenses with prismatic, spherical or aspheric profiles, either as a single lens or in arrays of multiple lenses, are used as a means of focusing passive infrared radiation for example, for intruder detection within security devices, switching of lighting for domestic and industrial applications and other applications that require sensing by passive infrared radiation. Arrays consisting of one or more Fresnel lenses are used to provide a plurality of sensing zones from a fewer number of electronic infrared sensing components.

U. S. Patent No. 4,787,722 discloses Fresnel lenses with prismatic, spherical or aspheric groove profiles of the type mentioned above.

It is also known that such Fresnel lenses, either as single lenses or in arrays can be used in infrared detector devices. In such devices, a suitable light source emits infrared radiation and one or more sensors detect the infrared radiation reflected back from a target object to be detected. The lenses, or arrays of lenses, are used to refract the infrared radiation in both the emitting and sensing parts of the device.

Typically, when produced in large volume, the Fresnel lens arrays are made by moulding, embossing, stamping or any similar process to accurately replicate the Fresnel lens forms in an in-expensive thin plastic

component. Polyethylene is a convenient plastic material to use because it has reasonable transmission properties in the infrared spectrum.

Nevertheless transmission losses are suffered through thick sections, so the arrays have to be made as thin as possible to maintain adequate transmission of infrared radiation. This means that the Fresnel lens grooves have to be made as fine as possible to enable thinner lens array sections to be made. A problem with this is that, as the Fresnel grooves are made finer, it becomes more difficult to achieve accuracy in the form of the Fresnel lens and, as accuracy decreases, the performance of the lens array suffers.

The Fresnel lens forms used for infrared detection devices are typically made by single-point diamond machining to enable Fresnel lens groove profiles of the order of approximately 0.2 millimetres depth to be made with sufficient accuracy and an optical surface finish. The Fresnel lens forms can either be directly machined in the final component or, more usually, into tooling surfaces used for moulding, embossing or stamping, lenses or arrays in large volume. Arrays of lenses are usually formed by assembling together a number of individual lenses.

There are a number of other problems with known infrared detection lens arrays made with prismatic, spherical or aspheric Fresnel lens elements.

Firstly, each lens element in an array of more than one lens has to be individually made or replicated from a master lens form and a different

master Fresnel lens form has to be made for each different focal length, or other property, of the lens array.

Furthermore, the single-point diamond machining typically used can be expensive, especially when used to produce a large number of individual Fresnel lens forms of different properties to be later assembled together into an array.

A further problem arises due to the fact that only Fresnel lens forms that are rotationally symmetric about their optical axis can be made in accordance with the methods mentioned above which limits the optical properties of the Fresnel lens or arrays.

A still further problem arises in that a Fresnel lens can be corrected only for spherical aberrations in its focus properties, but cannot be corrected for other lens aberrations such as astigmatism, coma or chromatic aberrations.

An object of this invention is to provide focusing optical element for use in infrared detector devices, and a method of manufacturing the same which overcomes, or at least minimises, the above mentioned problems.

According to a first aspect of the present invention there is provided a focusing element for an infrared detector device said element comprising a diffractive optical element.

According to a second aspect of the present invention there is provided an infrared detector device comprising an infrared source, an

infrared detector and a focusing element, said element comprising a diffractive optical element.

According to a third aspect of the present invention there is provided a method of detecting an object comprising the step of: emitting infrared radiation and focusing said emitted radiation by diffracting said radiation in order to direct said radiation towards an object to be detected.

One advantage of this invention is that the size of the groove pattern typically used for the diffraction optical element is much smaller than the grooves used in a Fresnel lens, typically by a factor of 10 or more.

Therefore, the overall section thickness of the diffractive optical element which could be moulded, embossed or stamped in a typical polyethylene plastic material, can be smaller than that needed for an array of Fresnel lenses to perform the same function. The smaller section thickness allows better infrared transmission to be achieved, as compared to a conventional Fresnel lens.

The relatively small groove depth to overall section thickness ratio this invention allows gives improved characteristics for the large volume manufacture of components in a plastic material such as polyethylene. In order to replicate the very fine groove detail of the diffractive optical element by the preferred method of injection moulding, unusually high pressures and plastics melt temperatures have to be used than would be

typically used for conventional devices. This also allows the final injection moulded component design to be more complex, for example with additional features of frames, holes, protrusions, snap-fit features, push-fit features and a wide variety of other features for securing the diffractive optical element array into the detector device, to be produced at the same time from the same moulding cavity.

The diffractive optical element preferably comprises a segmented surface microrelief structure where the active planar surface is divided into individual segments. A wavefront incident on the surface is split into secondary wavelets by each of the segments where each segment is characterised by its surface relief profile and its segment boundary. The surface relief structure is designed in such a way that the desired optical function in the far field is performed by the superposition of all the secondary wavelets produced by the surface segments. The required constructive or destructive interference of the secondary wavelets is achieved by choosing the segment boundaries, depths and shapes to ensure that the optical phase difference of two rays crossing neighbouring segments and meeting at the desired far field position is an integer multiple of 2n. Various computerised mathematical methods can be used to determine and optimise the surface microrelief structure which are well documented.

Preferably, the focusing element comprises an array of diffractive

optical elements. Preferably, the array is formed from a single piece of material.

Preferably the method includes the additional steps of: receiving infrared radiation reflected from the object, focusing the reflected radiation by diffraction and detecting said focused radiation.

Preferably, the or each diffractive optical element comprises a holographic optical element, but it could comprise other types of diffractive optical element, for example binary optics, kinoforms or diffraction gratings.

The or each diffractive optical element preferably comprises a plurality of fine grooves, for example of approximately 20 micrometers depth, disposed on the optical surface of a lens or mirror. The grooves on the optical surface impose a change in phase of the wavefront passing through, or reflecting from, the surface, which can be designed to focus the light transmitted through the surface by diffraction.

It will be appreciated that the grooves can be formed by a wide variety of different means, for example recording in photo-sensitive media, single-point diamond machining, ion beam etching, chemical etching, laser machining, laser writing, electron beam writing or photo-masking. Some of the various means of producing the grooves of the diffractive optical element allow the whole lens area of a closely packed array of diffractive optical elements to be made in one single piece without the need for assembling together the individual lens elements of the array. This allows

better positional accuracy to be achieved between each element in the array and removes the cost of assembling together each element of the array to produce the final components or tooling for moulding, embossing, stamping or the like of large volumes of lens arrays.

Since the suggested means of making the fine diffractive optical element element groove structures can produce features as small as in the order of 0.5 micrometers, it is possible to encrypt a unique symbol or some other marking strategically placed within the diffractive optical element's structure which identifies the diffractive optical element structure uniquely, for example to a particular design or manufacturer, or to designate some other property of the element. This"anti-counterfeiting"device may be made so small that it does not adversely affect the optical properties of the diffractive optical element element and cannot be seen with the naked human eye, but can be seen under high powered magnification. Any attempt to copy the diffractive optical element element design by a replication method would similarly replicate the anti-counterfeiting device allowing the origins of the original to be traced.

In one embodiment of the invention, at least a part of the diffractive optical element is formed from groove patterns that are non-rotationally symmetric. In particular, this means that the grooves can be something other than concentric rings centred on the optical axis of the element.

Using groove patterns that are non-rotationally symmetric allows the optical

power of the element to be different in each axis across the surface of the element passing through the optical centre. This enables a range of geometric optical aberration including astigmatism and coma to be corrected, or introduced into the optical elements in a controller manner.

This may be used to provide a different vertical sensing zone size in relation to the horizontal sensing zone size, or to change the shape of the sensing zone, of each element in an array for an infrared detection device. This enables the focused image shape to match the actual shape of the detector surface which are quite often not circular and so improve the signal to noise ratio of the detected signal.

In another embodiment, the or each diffractive optical element in the array has spatial filtering properties to provide some initial processing of the detected image for each of a number of detection zones. This may be arranged to enable each optical element of the array to provide the infrared detector with a different detecting sensitivity to large objects than small objects, thus enabling some distinction in the detection between, say, a human being and a small animal and therefore reducing or removing the need for this processing to be done electronically. For instance, in the simplest form, the detection zones could be shaped to be tall narrow rectangular zones so that a human target would fill most of the zone and give a strong signal at the detector, whereas a small animal target will only fill a very small part of the zone and so give a weak signal. A more

elaborate method may be to shape the detection zones to be trapezoidal in shape, so the detection zones are wider at the top of the zone and narrow at the bottom. In this way, a human target will fill more of the wider part of the zone at the top and so produce a large signal at the detector, whereas a small animal target will fill only a small part of the zone at the narrow bottom of the zone and so produce a small signal. Additionally, these trapezoidal zones will give some spatial filtering effect in that a small animal target will be more likely to move between the narrow, wider spaced areas of the zones nearer the floor level.

In a further embodiment, other optical corrections are performed to correct chromatic aberrations typically in the passive infrared wavelength detection range of 7 to 14 micrometers but possibly at other wavelength ranges to suit other applications in infrared sensing. The chromatic properties of the diffractive optical element or elements may be selected to diffusely scatter light in the visible to near infrared wavelength range while maintaining good optical performance in the required mid-infrared wavelength range. This property can be used to improve an infrared detector system's immunity to false sensing of light sources outside the wavelength range of the intended infrared sources. As an example, this can reduce the susceptibility to false alarms by extraneous radiation sources in passive infrared intruder alarm detectors.

A further possible embodiment may be an optical device having only

one diffractive optical element in the form of a holographic optical element or hologram which is capable of reconstructing multiple detection zones to an electronic detector when infrared radiation passes through the diffractive optical element element.

The diffractive optical element may be comprised in a flat or curved transmission optical device. The diffractive optical element may be applied to a curved surface to help the mechanical construction of the device or to produce a combined effect in which both the diffraction and transmission device provides some optical effect in the transmitted beam.

In order that the invention may be more clearly understood embodiments thereof will now described, by way of example, with reference to the accompanying drawings in which: Figure 1 shows a plan view of a typical prior art aspheric Fresnel lens form; Figure 2 illustrates a cross-sectional view of the lens of Figure 1 taken along the line ll-ll; Figure 3 shows a perspective view of an array of prior art Fresnel lens elements as typically used in a passive infrared detector device.

Figure 4 illustrates cross-sectional view of an embodiment of a diffractive optical element according to the invention; Figure 5 illustrates a perspective view of another embodiment of a

diffractive optical element according to the present invention with non-rotationally symmetric grooves and representing the different optical powers in each axis across the surface; Figure 6 illustrates a perspective view of an embodiment of an array of diffractive optical elements according to the invention; and Figure 7 illustrates a schematic exploded perspective view of an infrared detector including the array shown in Figure 6.

Figures 1 to 3 show prior art. Referring to Figures 1 and 2 a Fresnel lens 10 has a thin circular body 11. One face surface 12 of the body 11 is flat. The other face 13 has a plurality of concentric grooves 14. Other types of prior art lenses have grooves on both faces. Each groove 14 may have a triangular, spherical or aspherical cross section 17. The grooves 14 may be of constant depth and a varying width. The width of each groove 14 is the radial distance between each pair of successive vertical sides 15.

The width of each groove 14 decreases as the distance between the centre 16 and the groove 14 increases. Thus the grooves 14 nearer the edge of the lens 10 are narrower than the grooves 14 nearer the centre 16. Some prior art designs may instead have grooves of constant width and therefore varying depth.

Figure 3 shows a perspective view of an embodiment of an array 20 of prior art Fresnel lens elements 10 as typically used in passive infrared detector devices. The lenses 10 provide a plurality of detection zones in the

detector device. The array 20 is typically made by moulding an infrared transmitting plastic, such as polyethylene. The array has a frame 21 with a flat surface 22 for mounting the array 20 in to a detector device.

Figure 4 illustrates one embodiment of a cross-sectional view of a diffractive optical element 50. Referring to this figure the element 50 has a thin, substantially rectangular in plan, body 51. It will be appreciated, however, that the body could be circular, rectangular or some other shape in plan as desired or as appropriate and depending on the means used to produce the body 51. One surface 52 of the body 51 is flat. The opposite surface 53 has a plurality of grooves 54. The element 50 could, however, have grooves 54 on both surfaces. Each groove 54 is formed by a substantially vertical surface 55 (when viewed in the orientation shown) and a curved surface 56. The height of the vertical surfaces 55 and the width and curvature of the curved surfaces 56 will depend on the change in phase they are to impart on a wavefront transmitted through the diffractive element 50. Therefore, the heights of the faces 55 and the widths of the curved surfaces 56 will vary and may not follow a uniform trend across the entire diffractive element structure surface 53. The grooves 54 may be rotationally symmetric and concentric about the centre 57. However, it may be preferential to have grooves 54 that are not rotationally symmetric and are not placed concentric about the centre 57 to provide a different optical power in each axis across the surface of the element 50 in order to change

the shape of the field of view of a detector in which the lens is fitted.

Referring to Figure 5 this is shown a perspective view of an embodiment of a diffractive optical lens 60 consisting of one element 50 formed from an infrared transmitting plastic, such as polyethylene. The element 50 in this case has grooves 54 which are non-rotationally symmetric about the centre 61. In this embodiment the non-rotationally symmetric grooves 54 of the element 50 provide a different optical power in each axis across the surface of the element 50. Light paths 62 passing through the element 50 from the focal point 63 to form the projected beam cross-section 64 are re-directed by a different angular amount dependant on the radial position of the groove 54 from the centre 61 they intercept with.

The non-rotationally symmetric grooves 54 therefore transform a circular bundle of light paths 62 projected from the focal point 63 into a non-circular beam cross-section 64. Similarly, if the focal point 63 is considered to be a detector the non-circular beam cross-section 64 defines the field of view the detector would achieve through the element 50.

Referring to Figure 6 there is shown a perspective view of an array 70 of diffractive optical elements 50 as typically may be used in a passive infrared detector device. The elements 50 provide for a plurality of detection zones in the detector device. The array 70 may be made in one piece regardless of the number of elements 50 in the array 70, provided, of course, that the array 70 is within the size constraints of the particular

means used to produce it. Alternatively, the elements 50 could be assembled together to form the array 70 the array 70 is formed from an infrared transmitting plastic, such as polyethylene, or the array 70 can be etched or replicated by some method on to tooling surfaces for moulding, embossing, stamping etc. The array 70 may have a frame 21 with a flat surface 22 for mounting the array 70 in a detector or sensor device, similar to the prior art Fresnel lens arrays.

Referring to Figure 7 there is shown an exploded view of an infrared detector 80 comprising a housing formed from two portions 81 and 82.

The first portion 81 houses electronics, particularly a printed circuit board 85 on which is mounted an infrared emitter 86 and sensor 84. The second portion 82 having an array 83 of diffractive optical elements. In use the emitter 86 produces infrared radiation. This is focused by the array 83 in a desired manner to produce a detection zone. Any radiation reflected back by objects in the detection zone is re-focused by the array 83 onto the sensor 84. The sensor 84 generates a signal in response to received radiation. This signal is subsequently processed by the electronics to determine the state of objects in the detection field.

A number of methods by which diffractive optical elements according to the invention may be produced will now be described EXAMPLE 1: Laser writing A substrate sample is coated in a photoresist, light sensitive, material

to a controlled thickness. This is then placed on a XY scanning motion stage under a focused laser beam, whose intensity is synchronously modulated as the photoresist coated sample is scanned, to write a continuous exposure in the photoresist layer. This enables the fine relief pattern of the diffractive optical element structure to be written with a wide range of feature sizes and is produced in one continuous writing operation.

The area size that can be patterned in this way is only limited by the length of travel of the XY axes, the accuracy of the XY motions over longer travels, and the duration of the total writing time. The typical focused beam size used is 1.5 microns, although by overlapping the Gaussian beam profiles of each successive pass of the writing beam, features of the order of 1 micron can be made with good accuracy. The depth of the structures that can be made is limited by the thickness of the photoresist layer and the depth of focus properties of the laser beam, however, diffractive optical element structures of up to 20 microns in depth can be made for use at mid- infrared wavelengths.

EXAMPLE 2: ELECTRON BEAM WRITING A substrate sample is coated in photoresist and scanned under the exposing beam of focused electrons. Electron beam writing has to be carried out under vacuum. The electron beam can be focused to a spot of 50-100 nanometers to produce very small spatial features in the diffractive optical element structures. However, the resolution of the process is quite

often limited by the scattering of the electrons in the photoresist layer, therefore as the depth of the diffractive optical element structure written increases, the lateral resolution of the structure degrades. Typically, areas of only 1 mm x 1mm are written, so larger area diffractive optical element structures are made by"tiling"of a number of scanned areas.

EXAMPLE 3: PHOTO-MASKING Again, a photoresist coated substrate sample is prepared. The diffractive optical element structure is exposed by using a mask placed in an optical projection system and the mask image projected onto the sample.

The whole area of the diffractive optical element structure may be exposed at the same time, depending on its size. By using a series of masks, each aligned in accurate registration to the last, the diffractive optical element structure depth and profile is exposed in a series of exposure steps. The mask can have grey scale gradations within them to smooth the steps between successive exposure levels. The overall resolution of the process is limited by the accuracy of the masks used which can be as good as of the order of 50 nanometers, the alignment accuracy of each mask and the performance of the optical imaging system used.

Once one of the above methods has been employed to produce a diffractive optical element structure in a photoresist material this master surface is transferred on to a mould tool surface to provide a robust surface that will resist the harsh moulding conditions. This is done by using an

electroforming method. The surface of the photoresist material is made electrically conductive by depositing a thin layer of gold, nickel or some other metal under vacuum. The vacuum coated component is then electroplated with a thick layer of nickel to produce a robust tooling surface for the mould tooling.

The tool may then be used to mould large number of lenses. The lenses are moulded for high density polyethylenes (HDPE), a typical grade used is Rigidex 6070. These are used either in their natural form or pigmented with zinc sulphide, zinc selenide, titanium oxide or some mixture of these to provide a white coloured material or iron oxides or carbon to provide a black coloured material. Other pigment materials and mixtures can possibly be used to produce other colours. The pigments and their relative concentrations are chosen to provide the required visible colour and to, as much as possible, maintain the mid-infrared transmission of the base HDPE. UV inhibitor additives are also typically added to the formulations to reduce the UV degradation of the polymers and so enable the lenses to be used out-doors.

The typical moulding conditions used for the diffractive optical element lens components are as follows: Melt temperature-250 to 270°C Tool temperature-40°C Injection pressure-120bar Hold pressure-40bar Cycle time-"8 seconds The above embodiments are described by way of example only, many variations are possible without deporting of the invention.