Advanced Detection Methods for Perimeter Photo Beams

Perimeter protection and intrusion detection can be unwieldy, requiring special consideration to prevent the occurrence of false alarms, especially in light of potentially noisy environments.

Recent studies and programs have called attention to issues with regards to being able to have the confidence to dispatch personnel on alarm notifications. There is an alarming (no pun intended) trend toward a verified response approach—not to dispatch emergency personnel until and unless a confirmation of the alarm condition is obtained. There are even organizations such as the False Alarm Reduction Association (FARA) with programs designed to exchange information, provide a clearinghouse for agencies and to foster cooperation between law enforcement agencies, alarm industry and consumers.

Other studies indicate there is a need not only for false alarm reduction programs of the FARA type but also for improved technology, which is what this article addresses.

Photo beam perimeters continue to be the first line of protection for an alarm system and detection at the protected premises. This article is designed to take a look at the technology behind detectors and several different beam approaches, which include:

Detection of light--There are detectors devised for virtually every wavelength of light known to exist, deep UV through far reaches of the infrared spectrum. Detectors simply receive photons in the region of interest (outside the region in the form of noise) and convert the photons to electrons. The electrons fill up a bucket which is based upon an R/C time constant (Resistance/Capacitance), which then dumps the bucket at a fixed interval.

There are many sources of noise that can affect the detected ratio of Signal to Noise (S/N), ranging from Shot and Johnson noise of the circuit itself, through external noise sources such as ambient light (DC) and RFI/electronic (other than DC). The trick of any detection scheme is to maximize the signal and minimize the noise (S/N ratio).

Technique 1-Double Modulated Phase Lock Loop (DMPLL)

Phase Lock Loop is a technique where, when the phase matches there is a resultant 2X multiplier of the signal in phase, there is an additional benefit of effectively ignoring noise sources that are not in phase. It employs Dynamic Circuitry that constantly monitors and adjusts to assure no phase offset (see Illustration 1 Phase offset & 2 DMPLL). The aggregate benefit of Double Modulation further sets the technique apart in that it adds an additional second layer of modulation. The results of the combination of the frequencies is essentially an immunity to naturally occurring noise sources.


  • Ambient light immunity (50K lux)

RFI (electrical noise) immunity

  • Inclement weather reserve (to 1000X)

Highest S/N technique available

Technique 2-Synchronized Beam

Synchronization techniques are a Static Approach, which attempts to derive some of the benefit of Phase by synchronizing the frequency. The issue is if there is no facility to adjust the frequency to match and maintain phase, then only partial benefit is attained on only the portion of the beam that is in phase. The portion of the detection R/C time constant which is not in phase is free to integrate noise (see Illustration 2 Synchronization). If a single frequency is employed, additional noise sources can also be integrated (see Illustration 1).

Technique 3-Continuous Wave (CW) Detection

In this approach, no attempt is made to synchronize the detector to a generated pulse of light; thus each R/C time constant integrates everything it sees (including all sources of noise). Going back to the bucket approach, if the noise fills the bucket to the rim, there is no room for a signal above the noise; only the signal in excess of the integrated noise will be detected (see Illustration 2 Continuous Wave).

Whether it is a service call, an unnecessary dispatch of personnel or potentially even a delay in dispatching, there are and can be real costs associated with less capable technology. It is incumbent on the security industry as a whole to develop and employ technology which results in:

  • Lessened cost of ownership
  • Reduced service and support requirements
  • Alleviated strain on overburdened law enforcement
    and fire agencies
  • Instills a confidence to dispatch

We all have a vested interest in working in a coordinated fashion to preserve the relationship with law enforcement agencies, conserve operating costs and maintain customer confidence. Everyone benefits from technically advanced robust, mature and reliable technology.


James F. Babb is the director of Integrated Solutions for Takex America Inc., Santa Clara, Calif., and a former director of Optical Engineering with 24 years of background in Electro-Optics.