The present invention relates to a segmented Fresnel lens, to be used as concentrator of infrared rays on a light-sensitive element (optical sensor), particularly for application in anti-intrusion systems or in occupancy detection systems in general.
As is known, infrared occupancy detection devices, or sensors, comprise an element able to collect and concentrate rays to maximise the signal gain and an electro-optical sensor able to receive the radiation signal.
Associated with the sensor is electronic circuitry able to detect fluctuations in the output signal from the sensor which are the result of the variations in the radiation signal.
The principle on which these passive infrared sensors are based is the following.
Since people and objects in general normally have temperature differentials with respect to the surrounding environment, they emit levels of infrared radiation that generally differ from the background levels. Consequently, they produce fluctuations in the intensity of radiation when they pass through the optical field which, suitably amplified, can be detected by the electronic circuitry associated with the sensor.
Using said radiation concentrators, temperature differentials of less than one degree Celsius are sufficient for the presence of a moving person or object to be detected.
Fresnel lens or segmented Fresnel lens, that is lens consisting of a plurality of bands or segments of Fresnel lens, each of which is able to cover a certain zone of the room to be monitored, are usually used as radiation concentrating elements.
US 4 930 864 describes a domed system of segmented Fresnel lens associated with an infrared occupancy sensor.
The system of lens comprises a plurality of segments of Fresnel lens distributed on the surface of a spherical hood, each able to receive the radiation from a distinct zone centered around a corresponding optical axis and focus it in the focal region.
Italian utility model No. 218696 filed on 10 August 1987 describes a similar sensor with a segmented Fresnel lens, able to be inserted in a standardized wall-mounted boxes for holding electrical modules.
EP-0484293 describes an infrared occupancy sensor comprising a hood lens with a plurality of segments of Fresnel lens disposed in three bands one on top of the other to cause the radiation coming from the upper part, the central part and the lower part, respectively, of the room being monitored to converge on the infrared sensitive element. In the solutions of the prior art a visual correspondence normally exists between the segments of the Fresnel lens formed on the plastic hood and the corresponding segments or ray beams that are directed onto the sensitive element by said plastic segments. This allows easy identification of the direction of the rays even at a distance.
Furthermore, the arrangement of the bands determined by the Fresnel lens segments does not always offer an absolute barrier to intercept any attempt at intrusion.
The object of the invention is to eliminate the drawbacks of the solutions of the prior art.
In particular, one object of the invention is to provide a segmented Fresnel lens in which there is no visual correspondence between the beams of radiation and the plastic segments, so as to hide the direction of the beams.
Another object of the invention is to provide such a segmented Fresnel lens with an orientation of the beams that allows the creation of a complete barrier able to intercept any attempt at intrusion into the room monitored by the sensor combined with the lens.
The above objects are achieved by the segmented Fresnel lens according to the invention, which has the characteristics of appended independent claim 1.
Advantageous embodiments of the invention are set forth in the dependent claims.
Essentially, the lens according to the invention comprises two horizontal bands of plastic segments, to which four horizontal bands of radiation segments or radiation beams correspond. There is therefore no correspondence between the plastic segments and the radiation segments or intercept beams.
The four bands of radiation or intercept segments are of different vertical sizes and are divided into a different number of segments.
In particular, starting from the bottom, the first band has three segments, the second five, the third six and the fourth three.
The angular horizontal extension of the field of coverage of the lens is about 112 degrees, whilst the angular vertical extension is about 36°, plus 6° on the horizontal and -30° beneath the horizontal.
The first and lowest band has a coverage limited to a maximum of 1.5 metres, whilst the others create a barrier at about 7 metres.
The third band from the bottom with six segments has practically no inclination with respect to the horizontal.
Further characteristics of the invention will be made clearer by the detailed description that follows, referring to a purely exemplary and therefore non-limiting embodiment thereof, illustrated in the appended drawings, in which:
With reference to said figures, and for now in particular to Figures 1 and 2, the lens according to the invention has been designated as a whole with reference numeral 1.
The lens 1 according to the invention is called a segmented Fresnel lens because it consists of a plurality of segments or lenghts of Fresnel lens, as will be described better hereunder.
The lens 1 proper, as shown in Figure 1, has a spherical surface and is supported by a box-shaped body 2 consisting of pairs of flat opposite facing walls.
The lens 1 and the supporting body 2 are normally formed in a single body by molding of plastic material.
As can be seen schematically in Figure 1, guides 3 and coupling means 4 for fixing of the lens, for example to an electrical module that can be inserted in a wall-mounted box, are provided on the body 2.
As shown diagrammatically in Figure 1, the lens 1 is geometrically divided by a line L into two horizontal bands 5, 6, each comprising nine plastic segments, seen better in the development of Figure 2. The plastic segments, diagrammatically indicated with S in Figures 1 and 2, related to the external geometry of the lens, do not correspond to the same number of optical segments, that is, radiation segments.
On the contrary, as can be seen in Figure 2, and better in Figures 3 and 4 which illustrate the angles of coverage of the lens, the two bands 5 and 6 of plastic segments S coincide with four bands A, B, C, D of radiation segments, of which the corresponding foci FA, FB, FC and FD are shown in Figure 2.
In particular, proceeding from bottom to top, band C comprises three radiation segments, band B five segments, band A six segments and band D three segments again.
The lack of visual correspondence between the plastic segments S and the radiation segments makes it possible to hide the direction of the radiation beams. This is especially useful in anti-theft applications.
The orientation of the radiation beams which are directed into the focal region 11 of the lens (shown hatched in Figure 2), where an optical sensor 10 is disposed, can be seen from Figures 3 and 4.
In particular, considering that the sensor, in normal wall-mounted box installations, is disposed at a height of about 1.2 metres from the ground, the lower band C is directed downward and has an extension of about 1.5 metres (Figure 4) with an angular coverage on the horizontal plane of about 75°.
The remaining three bands B, A and D create a barrier at about 7.5 metres and overall determine an angular coverage, on the horizontal plane, of about 112 degrees.
Figure 3 shows the radiation beams corresponding to the segments of the different bands A, B, C and D.
As can be seen from Figure 4, the angular extension on a vertical plane is about 36°, with an inclination of+6° upward and -30° downward. Band A with six segments, the third from the bottom, has no inclination with respect to the horizontal, so as to determine the greatest concentration of rays at the typical intercept height for a person, corresponding to the level of installation of the sensor, that is, about 1.2 metres.
The division of the lens into four radiation bands, with a total of 17 segments, makes it possible, compared with conventional solutions, to have a greater angular coverage both on the vertical and on the horizontal, the geometrical size of the lens remaining the same.
The creation of a barrier able to intercept anyone who approaches the sensor is in any case ensured.
The advantages of the segmented Fresnel lens according to the invention are obvious from the above description, but its construction is nevertheless not limited to what has been described and illustrated in the annexed drawings, but is liable to numerous modifications of detail within the reach of a person skilled in the art, without departing from the scope of the invention, as set forth in the claims that follow.