ACTIVE ELECTRO-OPTICAL INTRUSION ALARM SYSTEM HAVING AUTOMATIC BALANCING MEANS

• US 3,597,755 A
• Filed: 05/28/1968
• Issued: 08/03/1971
• Est. Priority Date: 05/28/1968
• Status: Expired due to Term

• Current Assignee: William J. Parkin
• Original Assignee: William J. Parkin
• Inventors: Parkin, William J.
• Application Number:
  US04/732,757
• Time in Patent Office: 1,162 Days
• US Class Current: 340/555
• CPC Class Codes: G08B 13/187 by interference of a radiation field

Abstract

An active reflected energy optical intruder detection system wherein changes in reflected energy received by the system indicate the entry, exit or change of position of an object within the field-of-view. An automatic balancing means is included in the receiving portion of the apparatus whereby large background signals are balanced out such as to improve the sensitivity of the apparatus.

Claims

1. An intrusion detection system comprising means for illuminating a preselected volume of space with amplitude-modulated radiant energy of a preselected carrier frequency such that said energy will be incident upon and reflected by objects entering, moving within and exiting said volume;

means for receiving said reflected radiant energy having a field-of-view which is at least partially coincident with said illuminated volume of space, including means for collecting and focusing radiant energy reflected from said objects within said coincident portion of said field-of-view, and a single photodetector disposed in the focal plane of said collecting and focusing means, said photodetector producing an electrical output signal which is representative of the level of radiant energy received thereby; and

signal processing means coupled to said photodetector, including an electrical band-pass filter coupled to said photodetector and tuned to said preselected frequency, a phase-sensitive demodulator coupled to the output of said band-pass filter, a low pass filter coupled to the output of said demodulator, a signal comparator coupled to the output of said low pass filter, and an automatic balancing means coupled between the output of said demodulator and the input of said band-pass filter whereby an inverted signal at said carrier frequency and of an amplitude controlled by the output level of said demodulator is coupled to the input of said band-pass filter to thereby substantially cancel time invariant background signals, and producing an output signal in response only to changes in the level of said received radiant energy.

2. Apparatus as recited in claim 1 wherein said illuminating means includes a variable focus lens whereby the dimensions of said illuminated volume of space may be selected from a range of values, and said collecting and focusing means comprises a variable focus lens whereby the dimensions of said receiving means field-of-view may be selected from a range of values.

3. Apparatus as recited in claim 1 wherein said radiant energy is infrared radiant energy.

4. Apparatus as recited in claim 1 wherein said illuminating means includes a thermal energy source comprising an insulated container having an exit aperture, and being substantially filled with a highly emissive metallic wool material, and a gas-fired burner for heating said metallic wool whereby thermal energy from said heating means is absorbed by said metallic wool and reradiated as infrared radiant energy through said exit aperture, and a rotating chopper disposed adjacent said exit aperture.

5. Apparatus as recited in claim 1 wherein said illuminating means includes a solid-state laser device, and an electronic modulator coupled to said laser device.

6. Apparatus as recited in claim 5 wherein said solid-state laser device is a gallium arsenide diode.

7. Apparatus as recited in claim 1 further including substantially identical infrared band-pass optical filters;

one disposed adjacent said illuminating means in the path of said illuminating radiant energy, and one disposed adjacent said collecting and focusing means in the path of said reflected radiant energy.

8. Apparatus as recited in claim 1 wherein said photodetector is selected from the group of photoconductors consisting of lead sulfide lead selenide, and lead telluride.