Presence and absence detection

1. A method for detecting presence and absence of a creature, the method comprising:

receiving an input signal from a radiation sensor;

applying a first low-pass filter to the input signal with a first time constant and generating a first filter output;

applying a second low-pass filter to the received input signal with a second time constant and generating a second filter output;

determining a presence of a creature by comparing a slope of the first filter output to a predetermined value;

setting a first threshold based on the second filter output;

decreasing the second time constant and thereafter waiting until the second filter output becomes equal to the first filter output within a predetermined tolerance;

increasing the second time constant;

calculating a difference between the first threshold and the second filter output;

adjusting the first threshold based on the second filter output; and

determining an absence when the first filter output falls below the first threshold.

2. The method according to claim 1 , wherein the presence is determined by comparing the slope of the first filter output to a sum of the second filter output and a predetermined offset.

3. The method according to claim 2 , further comprising:

after decreasing the second time constant, one of the following steps:

waiting for a predetermined time; and

waiting until the second filter output becomes equal to the first filter output within a predetermined tolerance;

wherein adjusting the first threshold based on the second filter output is for the first filter output to follow the second filter output within a predetermined tolerance;

wherein adjusting the first threshold based on the second filter output includes limiting the threshold to a maximum value;

wherein determining a presence includes determining a start of the presence of the creature;

wherein determining an absence includes determining an end of the presence of the creature; and

further comprising: counting creatures by considering one or more determined presences and/or one or more determined absences.

4. The method according to claim 1 , further comprising, after decreasing the second time constant:

waiting for a predetermined time.

5. The method according to claim 1 , wherein adjusting the first threshold based on the second filter output is for the first filter output to follow the second filter output within a predetermined tolerance.

6. The method according to claim 1 , wherein adjusting the first threshold based on the second filter output includes limiting the threshold to a maximum value.

7. The method according to claim 1 , wherein determining a presence includes determining a start of the presence of the creature.

8. The method according to claim 1 , wherein determining an absence includes determining an end of the presence of the creature.

9. The method according to claim 1 , further comprising:

counting creatures by considering one or more determined presences and/or one or more determined absences.

10. A device for detecting presence and absence of a creature, the device comprising processing resources that are adapted to:

receive an input signal from a radiation sensor;

apply a first low-pass filter to the input signal with a first time constant and to generate a first filter output;

apply a second low-pass filter to the received input signal with a second time constant and to generate a second filter output;

determine a presence of a creature by comparing a slope of the first filter output to a predetermined value;

set a first threshold based on the second filter output;

decrease the second time constant;

increase the second time constant;

calculate a difference between said first threshold and the second filter output;

adjust said first threshold based on the second filter output; and to determine an absence when the first filter output falls below said first threshold,

wherein said processing resources are adapted to also:

after decreasing the second time constant, perform one of the following steps:

waiting for a predetermined time; and

waiting until the second filter output becomes equal to the first filter output within a predetermined tolerance.

11. The device according to claim 10 , wherein said radiation sensor is selected as one of: an infrared sensor, a thermopile sensor, a camera, an optical sensor, and a temperature sensor.

12. The device according to claim 11 , wherein said radiation sensor is a thermopile with a field of view of 5° and below, and further comprising said radiation sensor.

13. The device according to claim 10 , wherein said radiation sensor is a thermopile with a field of view of 5° and below.

14. The device according to claim 10 , further comprising said radiation sensor.

15. The device according to claim 10 , wherein said processing resources are adapted to also:

determine the presence by comparing the slope of the first filter output to a sum of the second filter output and a predetermined offset.

16. The device according to claim 10 , wherein said processing resources are adapted to also:

determine a start of the presence of the creature.

17. The device according to claim 10 , wherein said processing resources are adapted to also:

determine an end of the presence of the creature.

18. A method for detecting presence and absence of a creature, the method comprising:

receiving an input signal from a radiation sensor;

applying a first low-pass filter to the input signal with a first time constant and generating a first filter output;

applying a second low-pass filter to the received input signal with a second time constant and generating a second filter output;

determining a presence of a creature by comparing a slope of the first filter output to a predetermined value, wherein the presence is determined by comparing the slope of the first filter output to a sum of the second filter output and a predetermined offset;

setting a first threshold based on the second filter output;

decreasing the second time constant and thereafter waiting until the second filter output becomes equal to the first filter output within a predetermined tolerance;

increasing the second time constant;

calculating a difference between the first threshold and the second filter output;

adjusting the first threshold based on the second filter output;

determining an absence when the first filter output falls below the first; and

after decreasing the second time constant, performing one of the following steps:

waiting for a predetermined time; and

waiting until the second filter output becomes equal to the first filter output within a predetermined tolerance,

wherein adjusting the first threshold based on the second filter output is for the first filter output to follow the second filter output within a predetermined tolerance,

wherein adjusting the first threshold based on the second filter output includes limiting the threshold to a maximum value,

wherein determining a presence includes determining a start of the presence of the creature,

wherein determining an absence includes determining an end of the presence of the creature; and

further comprising: counting creatures by considering one or more determined presences and/or one or more determined absences.