VISION ACCESSORY IN SUB-CEILING LAYER FOR AN INFRARED DETECTOR

Abstract

An optical device for arrangement on a detector provided with an infrared sensor in order to modify the visual field of the detector. The device includes primary and secondary conical mirrors. The primary mirror collects the infrared radiation coming from a sub-ceiling layer around the device for returning it onto the secondary mirror which, itself, reflects it to the infrared sensor.

Claims

1. An optical device, intended to be arranged on a detector equipped with an infrared sensor in order to modify a field of view of the detector, comprising: a primary mirror of frustoconical overall shape, comprising a circular opening at its center, a secondary mirror of conical overall shape, at least one connecting means for connecting the primary mirror and the secondary mirror, so that a reflective surface of the primary mirror is arranged facing a reflective surface of the secondary mirror, the primary and secondary mirrors being designed to reflect radiation in the infrared; and the primary and secondary mirrors being configured to define a field of view of the device, to form an afocal system and to form a continuous image of a periphery of the device, a center of the image being hidden by the secondary mirror.

2. The optical device as claimed in claim 1, wherein the angle α of the field of view of the device is comprised between 5° and 10°.

3. The optical device as claimed in claim 1, wherein the optical device consists of a single piece of injection-molded plastic, at least the surfaces of the primary mirror and of the secondary mirror being metalized.

4. The optical device as claimed in claim 1, wherein a maximum diameter of the primary mirror is less than 100 mm, and a height of the device in a direction of an axis of a cone defining the reflective surface of the primary mirror is less than 40 mm.

5. An infrared detector comprising an optical device as claimed in claim 1, arranged in such a way as to form the image on the infrared sensor of the detector.

6. The use of the infrared detector as claimed in claim 5 to detect a buildup of heat or a cold spot in a layer just below a ceiling of a room.

7. The use of the infrared detector as claimed in claim 5 to detect a buildup of hot smoke in a layer just below a ceiling or an abnormal change in a temperature of walls of a room in a region of the layer just below the ceiling.

8. An optical accessory intended to be arranged on an infrared detector, comprising an optical device as claimed in claim 1 and a mechanism configured to attach the optical device to the infrared detector.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0052] FIG. 1 is a schematic side view of an infrared detector according to the prior art;

[0053] FIG. 2 is a schematic view of an optical device according to the invention;

[0054] FIG. 3 is a view in longitudinal section of an optical device according to the invention, viewed in section;

[0055] FIG. 4 is a schematic view of an optical device according to the invention arranged on an infrared detector according to the prior art;

[0056] FIG. 5 is a side view of an optical device according to the invention arranged on an infrared detector according to the prior art;

[0057] FIG. 6 is an image obtained by an infrared detector according to the prior art;

[0058] FIG. 7 is a simulation of the image obtained by an infrared detector according to the prior art on which an optical device according to the invention is arranged.

DETAILED DESCRIPTION

[0059] Throughout the present application, the terms “vertical”, “lower”, “upper”, “low”, “high”, “bottom” and “top” are to be understood with reference to an infrared detector in the configuration of operation mounted on a ceiling and facing toward the ground. Thus, in an operating configuration, the sensor of the infrared detector faces the ground in the vertical direction.

[0060] FIG. 1 has already been described in the preamble, and is therefore not commented upon hereinafter.

[0061] An optical device according to the invention is now described with reference to FIGS. 2 to 5.

[0062] The optical device 10 comprises a primary mirror 11, a secondary mirror 12 and connecting means 13 for connecting the primary mirror and the mirror.

[0063] In the embodiment illustrated, the connecting means 13 are rigid connecting means consisting of four supports of elongate shape distributed at equal angles, each attached by one of its ends to the primary mirror and by the other end to the secondary mirror.

[0064] The height of the optical device, namely its dimension in the vertical direction, may typically be of the order of 25 mm.

[0065] The primary mirror comprises a central opening 14 and a reflective surface 15.

[0066] Typically, the diameter of the primary mirror may be of the order of 60 mm.

[0067] The secondary mirror has a reflective surface 16. The reflective surface 16 of the secondary mirror is arranged facing the central opening 14 of the primary mirror.

[0068] Thus, from the viewpoint of the sensor, the secondary mirror 12 hides the floor, and only the radiation reflected by the secondary mirror enters the central opening 14 to reach the optical system 2 of the detector 1.

[0069] As is more clearly apparent in FIG. 3, which shows the optical lines of the infrared rays coming from the layer just below the ceiling, the reflective surfaces 15, 16 of the primary and secondary mirrors are of frustoconical and conical shape, respectively, and are configured to transmit to the optical system 2 of the detector the rays coming from the layer just below the ceiling.

[0070] The frustoconical profile of the primary mirror is such that the incident rays coming from the layer just below the ceiling are passed on to the secondary mirror, of which the profile is designed to reflect the rays onto the optical system 2 of the detector 1.

[0071] The field of view of the device extends continuously over 360° about the vertical and has a field angle α on the layer just below the ceiling of between 5° and 10°, as is more particularly visible in FIG. 5.

[0072] The mirrors are configured in such a way that the image formed on the sensor is sharp, with aspherical corrections.

[0073] Advantageously, the two mirrors form an afocal device.

[0074] The optical device 10 is preferably made as a single piece of injection-molded plastic, such as polymethyl methacrylate (PMMA) or polycarbonate (PC). The entire component, or at the very least the reflective surfaces of the mirrors, are then metalized so as to be able to reflect incident infrared radiation.

[0075] FIGS. 4 and 5 schematically depict an optical device 10 according to the invention arranged on an infrared detector 1.

[0076] Advantageously, the optical system 2 of the detector requires no modification and no electrical connections are necessary in order to arrange the optical device 10 on the detector 1.

[0077] In order to fix the optical device 10 to the detector 1, an attachment mechanism may be provided. This mechanism may for example comprise a semitransparent hemispherical dome made of polyethylene (PE) with a small thickness, typically close to 0.5 mm so as to effectively transmit the infrared radiation. The secondary mirror is secured to the internal face of the dome and this dome is attached to the base of the detector, thus covering the device.

[0078] FIG. 6 is an image obtained by an infrared detector according to the prior art. The field of view of the detector is aimed toward the floor of the room, and its angle α does not exceed 90°.

[0079] FIG. 7 shows, for comparison, the result of a computer simulation reproducing the effect obtained by fitting an optical device according to the invention on the infrared detector used for obtaining the image of FIG. 6.

[0080] It may be seen that the field of view of the detector is modified and allows observation of the layer just below the ceiling. The floor in the center of the image is completely hidden because the secondary mirror blocks the field of view of the detector in the direction of the ground.

[0081] In the case of a fire-prevention detector, this hidden center makes it possible to avoid any risk of inappropriate triggering of the alarm by a false signal brought about for example by an occupant of the room or an object (for example a cup of coffee) that this occupant is handling.

[0082] Thus, by virtue of the invention, a simple, compact optical device containing no lenses can be used to modify the field of view of an infrared detector so as to observe the layer just below the ceiling.

[0083] This may notably be an accessory that is installed on an existing detector. Installing an optical device on an existing detector is easy: specifically, the original lens of the detector can still be used, no electrical connections are needed, and the positioning of the optical device does not require great precision.

[0084] The invention can be used to act as a fire alarm. Specifically, the infrared monitoring of the layer just below the ceiling allows the detection of hot smoke. It also enables observation of the thermovelocimetry of the walls of a room, namely the rate of increase in wall temperature, which is liable to indicate a pre-fire situation.

[0085] The invention may also be implemented with a view to improving thermal comfort in a room: infrared monitoring of the layer just below the ceiling may indicate a buildup of heat or enables detection of a window that has been left open when the outdoor weather conditions are wintry.

[0086] Other variants and advantages of the invention may be realized without thereby departing from the scope of the invention. The invention is thus not limited to the examples described hereinabove.

[0087] Although described with reference to the main target application, namely that of modifying the field of view of an infrared detector mounted on the ceiling of a room, the invention also applies to any field in which it is advantageous to modify the field of view of infrared viewing apparatus using a simple and inexpensive optical device in order to obtain a periscopic field of view.

[0088] Thus, the optical device described can also be used in the automotive and transport field.