Passive infrared alarm device

A passive infrared alarm device evaluates the changes in infrared radiation level of a monitored room or a monitored portion of a room for alarm purposes. In accordance with the invention, an infrared sensing element is employed toward which an optical reflector directs infrared radiation emanating from one or more angles in a room to be monitored. An infrared filter is mounted in the path of the reflected infrared radiation before such radiation reaches the sensing element. The infrared filter is designed to absorb infrared radiation below a predetermined minimum wavelength, and additional means is provided for partially absorbing infrared radiation below the predetermined minimum at the reflector.

BACKGROUND OF THE INVENTION 
The present invention relates to a passive infrared alarm device comprising 
an infrared radiation detector, an optical device which directs the 
infrared radiation from one or more than one angle which is to be 
monitored towards the detector, an analysis device which processes an 
output signal supplied by the detector and possibly triggers an alarm 
signal, and a radiation filter which is located in front of the detector 
and withholds undesired radiation from the detector up to a limit 
wavelength. 
A passive infrared alarm device of this kind is known and described, for 
example, in British Pat. No. 1,335,410 (German As No. 21 03 909). As a 
passive motion alarm, it is based on the principle of recording the 
characteristic radiation of a room or monitored sections of a room and 
analyzing any changes in the recorded value as significant of the entry of 
a person into the monitored room for alarm purposes. For example, an 
analysis device of this type responds to a change in the infrared 
radiation with the frequency 0.2 to 5 Hz. This selected frequency range 
allows differentiation between a change in radiation produced by a person 
entering or leaving the monitored room and a change in radiation caused, 
for example, by temperature changes in the room or in the environment. 
The advantage of a passive alarm device consists mainly in that no active 
signal from a monitoring device is present and, therefore, does not 
produce the possibility of discovery. 
The radiation of the room or of monitored sections of the room is focused 
via an optical device onto an element--the infrared radiation 
detector--which responds to infrared radiation. This can be carried out 
via a lens system or via appropriately shaped reflectors or via a 
combination of the two. A radiation filter is arranged prior to the 
detector. The radiation filter is to prevent false alarms triggered by 
reflected sunlight or by other light sources such as, for example, 
incandescent lamps and fluorescent lamps. It transmits radiation with a 
wavelength from approximately 4.5 .mu.m to 20 .mu.m and screens other 
radiation from the detector. As window panes are only permeable to beams 
of up to a maximum of 4 .mu.m wavelength, beams entering through the 
window panes from outside of the enclosed, monitored room are withheld 
from the detector. The radiation filter is constructed by providing a 
detector input window with a germanium layer upon which a thin layer of 
dielectric material is vapor deposited. This thin layer reflects radiation 
up to a limit wavelength of approximately 7 .mu.m for the main part. 
However, a part thereof is absorbed in the dielectric itself and 
transmitted to the germanium where it is absorbed. Germanium itself 
absorbs electromagnetic radiation of up to 1.8 .mu.wavelength. As a result 
of the absorption, the detector input window is heated and can itself 
trigger an alarm as a result of characteristic radiation. Therefore, an 
obviously unintended alarm can be triggered by strong light sources such 
as car headlights and sunshine passing through the window panes of the 
monitored room. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide a passive infrared alarm 
device which is substantially free of false alarms and in particular 
responds only to processes within the monitored room. Possible alarm 
sources outside the monitored room are entirely eliminated. 
For the realization of this aim, it is proposed in accordance with the 
present invention that in order to prevent heating of the radiation filter 
which could trigger a false alarm, a further filter device be provided in 
the beam path prior to the radiation filter, which device filters out 
beams up to the wavelength limit of the radiation filter--at least up to 
1.8 .mu.m as the wavelength limit of germanium--and withholds these from 
the radiation filter. 
In a preferred embodiment of the present invention, the optical focusing 
device contains reflective elements such as, for example, concave 
reflectors. In this case, it is proposed that the reflective elements be 
provided with a coating which largely absorbs the beams below the 
wavelength limit of the radiation filter and transmits beams having a 
wavelength above the wavelength limit to the associated reflective 
surface. The coating can also be contrived to be such that beams having a 
wavelength above the wavelength limit are partially reflected and 
partially transmitted to the reflective surface bearing the coating. The 
transmitted component then passes twice through the coating and unites 
with the component reflected by the coating surface to form an overall 
useful radiation. The coating can consist of germanium or lead sulphide. 
Fundamentally, this forms one possibility of withholding the undesired 
beams from the actual radiation filter by means of selective absorption 
prior to a focusing reflection. 
A different embodiment, employing focusing with reflective elements, 
includes first carrying out a selective reflection and then either 
absorbing the undesired beams or forwarding these so that they are 
harmless to the actual radiation filter. For this purpose, in another 
embodiment, it is proposed that the reflective elements consist of a 
carrier having a selectively reflective layer, which layer reflects beams 
above the wavelength limit of the radiation filter, and transmits beams 
below into the carrier, and that in the carrier these transmitted beams 
are either absorbed or transmitted onwards. 
In a still further embodiment, the carrier possesses, in addition to the 
selectively reflective layer, an absorption layer in which the beams below 
the wavelength limit which are transmitted by the selectively reflective 
layer are absorbed. 
A passive infrared alarm device constructed in accordance with the present 
invention ensures that no radiation lying below the wavelength range 
provided for the infrared detector can trigger an alarm. This applies not 
only to light sources within the monitored room, but also, in particular, 
to those outside of the monitored room, such as sun or headlights which 
constitute strong light sources which influence the light conditions 
inside the room through the window panes.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The fundamental sectional diagram in FIG. 1 illustrates an infrared 
radiation detector 1 as receiver for thermal radiation, comprising an 
input window which serves as radiation filter 2, and consists, for 
example, of germanium having a coating of a dielectric material. This 
material has a filter characteristic which is such that light beams having 
wavelengths of above 4 .mu.m are transmitted and those having a wavelength 
of below 4 .mu.m are reflected and absorbed. The material consists of a 
layer of combined dielectrics having different coefficients of refraction, 
composed, for example of Mg0 and ZnS. In dependence upon the strength of 
the oncoming infrared radiation, the detector 1 supplies an electric 
output signal which is processed in an analysis device 3 to form a 
possible alarm signal. The analysis is carried out, for example, as 
disclosed in British Pat. No. 1,335,410 and corresponding German AS No. 21 
03 909. 
The radiation filter 2 faces away from the beams incoming into the alarm 
device. These beams are focused by a concave reflector 4 arranged behind 
the detector 1 onto this radiation filter 2. Reflectors 5 arranged in 
front of the detector 1 ensure that not only the beams 6 incoming in 
parallel form from the angle of the room around the optical axis are 
focused onto the detector 1, but also the beams 7 incoming in parallel 
from a different angle of the room depending upon the positioning of the 
reflector. In this way, any angle of the room can be monitored. 
The concave reflector 4 consists of a carrier 8 consisting of metal, glass 
or ceramic, of a reflective, metallic layer 9 arranged thereupon, and of a 
selectively absorbent coating 10 arranged thereupon. In the drawing the 
carrier 8 has been shown as consisting, for example, of glass. 
The coating 10 absorbs oncoming beams 11, 12--11 in parallel with 6 and 12 
in parallel with 7--with a wavelength of below 4 .mu.m, and consists, for 
example, of lead sulphide. As a result, only beams having a wavelength of 
above 4 .mu.m reach the reflective layer 9 and are directed towards the 
detector 1. Thus, the radiation filter 2 is unable to receive beams from 
light sources which could trigger false alarms as a result of the heating 
of the radiation filter 2. Advantageously, the limit wavelength of the 
overall radiation filter--in this case 4 .mu.m--is taken into account. It 
would also be possible to disregard the possible heating of the dielectric 
layer of the radiation filter 2 and to aim only to prevent heating due to 
absorption in the germanium layer of the radiation filter 2. Then the 
coating 10 would only have to be aligned to 1.8 .mu.m as the critical 
wavelength of germanium and could consist, for example, of germanium. 
A variant is shown in broken lines in FIG. 1. The coating 10 can be 
contrived to be such, when it consists, for example, of quartz, that the 
beams above 4 .mu.m are already partially reflected. The other part is 
then transmitted through the coating 10 and reflected by the reflective 
layer 9. 
FIG. 2 illustrates a design in which a so-called folded optics is used in 
order to shorten the structural length As in FIG. 1, a concave reflector 4 
consists of a carrier 8, this time composed of metal such as, for example, 
A1, and of a layer 9 bearing a coating 10. The beams 6 above 4 .mu.m which 
are incoming in parallel in this example from only one single angle of the 
room are reflected by the layer 9 of the concave reflector 4 and thrown 
back onto a plane reflector 13. This again carries out a reflection and 
directs the beams 6 onto the detector 1, of which the input window, acting 
as radiation filter 2, in this case faces towards the direction of the 
originally incoming beams 6. The plane reflector 13 advantageously 
likewise bears a selective absorption layer 14 on the reflective layer 15 
which, for example, itself forms the metallic carrier. This ensures the 
complete absorption of beams 11 above 4 .mu.m which are not entirely 
absorbed in the selectively absorbent layer 10 of the concave reflector 4 
and have been reflected on the layer 9. 
FIGS. 3 and 4 illustrate a fundamental design in which it is not a question 
of carrying out selective absorption followed by reflection, but of 
carrying out selective reflection followed by an optional absorption. 
FIG. 3 again illustrates a concave reflector 16 wherein a carrier 7 bears a 
selectively reflective layer 18 consisting, for example, of quartz. Beams 
6 above 4 .mu.m are reflected on the reflective layer 18 and directed 
towards a detector 1, whereas beams 19, 20 below 4 .mu.m are transmitted 
into the carrier 17. This carrier 17 consists, for example, of glass and 
forwards the beams 20, or else consists of a material such as, for 
example, PVC or another synthetic and absorbs the beams 19. 
FIG. 4 illustrates a variant of the principle of selective reflection which 
is suitable for the metallic concave reflector carrier 21 of a concave 
reflector 22. The detector 1 is as in FIG. 3. A carrier 21 bears on 
absorption layer 23 followed by a selectively reflective layer 24 again 
consisting, for example, of quartz. The beams below 4 .mu.m transmitted 
from this layer 24 are absorbed in the absorption layer 23 and rendered 
harmless. 
It will be apparent to those skilled in the art that many variations and 
modifications may be made without departing from the spirit and scope of 
the novel concepts of the present invention.