Black mirrors and pet alleys

Passive infrared detectors are by FAR the most popular choice for intruder detection, and rightly so, as the best units of this type are small, reliable, cost-effective and easy to use. However, not all PIR detectors are created equal. It’s important for installers to know about the differences so they can choose the best products for their applications.

PIR basics

Let’s start by looking at the basics of PIR detection. All warm-blooded creatures are heat sources that radiate infrared energy, simply because their bodies are at a higher temperature than the surroundings. Many non-living things, such as computers, light bulbs and, of course, space heaters, also radiate infrared energy.

Living things can, for our purposes, be considered as mobile sources of infrared energy, since an intruder who doesn’t move is unlikely to be much a of a threat! In contrast, the other infrared sources mentioned are fixed. PIR detectors are, therefore, always designed to respond only to moving sources.

This is achieved by providing them with an optical system which conveys the infrared energy present in the area under surveillance to a pyroelectric sensor, but does so in such a way that the area is broken up into a number of surveillance zones (see overpage).

The sensor continuously monitors the infrared energy it receives and, when a heat source crosses one or more of the zones, it sees the change of temperature and produces an output signal.

False and real alarms

This signal is analysed electronically and, by this process, the detector decides whether or not it represents an alarm condition. This decision-making process is critical. If the detector gets it wrong, the result is either a false alarm or failure to detect a genuine intruder.

Choosing PIR detectors typically boils down, therefore, to selecting those products that are best at discriminating between real and false alarm conditions. Naturally, manufacturers work hard to optimise this process, and the methods they use depend on the sensor’s optical systems, and how the infrared signal is filtered.

Filtration is necessary because PIR detectors not only have to cope with infrared radiation, but also with visible light from sources such as the sun and artificial lighting.

In practice, a visible light filter is normally included in front of the sensor. Such filters are very effective at blocking visible light, but further filtration is needed to remove spurious infrared radiation and ensure that only radiation which is typical of human beings reaches the sensor.

Lenses and mirrors

Detectors without mirrors make use of the Fresnel lens principle. The surface of the filter is divided or faceted so that it acts as a lens which focuses infrared radiation on the sensor. The coverage of the detector, as well as the size and arrangement of the surveillance zones, depends on the shape of the lens facets, and the overall curvature of the lens.

An alternative approach to PIR detector design is to use a mirror rather than a lens to focus the infrared radiation on the sensor. The mirror is segmented so as to produce the required arrangement of surveillance zones and, in conventional mirror-based detectors, the radiation passes through a filter before it reaches the mirror.

Black mirror technology

This optical system is based on the use of so-called ‘black mirrors’. These operate very differently from standard mirrors. As the name suggests, the mirror appears black to the naked eye, since they use a special coating which allows the residual visible light and the unwanted short-wavelength infrared rays to pass straight through, where they are absorbed by the black base material of the mirror.

Only infrared radiation, such as the type produced by human beings, are reflected by the mirror, which also serves to focus it on the pyroelectric sensor.

Mirror-based designs allow optical systems to be produced which offer well-defined surveillance zones, and which have focal lengths ideally adapted to surveillance applications. In addition, in well-designed units, the signal produced by the sensor is virtually independent of the distance to the target, making subsequent evaluation easier and more reliable.

Processing the signal

As we have already seen, no matter how sophisticated the optical system, the raw signal from the sensor needs to be processed and evaluated so that the detector can decide between real and false alarms. This is a complex process, and a large number of factors have to be taken into account to produce reliable results. In modern units, this task is therefore performed by microprocessor-based digital systems.

The firsts step is for the signal to be converted from analogue to digital form. This in itself brings benefits compared with earlier types of detector that worked purely with analogue signals. In particular, digital signal processing is much less affected by electromagnetic interference and temperature-related problems, both of which can easily cause sensitive analogue systems to produce anomalous information.

Key evaluation factors

Modern PIR intruder detectors suitable for commercial and the more demanding domestic applications can be expected to consider all of these factors when evaluating signals from the sensor:

• Shape – what type of waveform does the signal have?
• Amplitude – how large is the signal from the sensor?
• Energy – how powerful is the source of the signal?
• Time – for how long are the signals present?
• Frequency – what is the frequency of the infrared radiation?
• Multiple events – are signals constant or intermittent?

When all of these factors are considered in conjunction with each other, exceptionally reliable discrimination between real and false alarms is achieved. There are, however, two relatively common concerns that need special consideration.

Insects and pets

These are insect infestation and pet animals. The first of these is easy to tackle. Most good quality PIR detectors now have sealed optical chambers which prevent the entry of insects. To avoid problems, therefore, installers need do no more than confirm that the detectors that they choose definitely do have sealed chambers of this type. As an added safeguard, any apertures created in the detector casing during installation should be sealed with an appropriate sealing compound.

The generation of spurious alarms by pets moving through the surveillance area is a more difficult problem, and manufacturers have adopted various techniques to tackle the issues associated with having animals inside premises with an active alarm system. It is fair to say that prevention is better than cure, with the most reliable solution being one of animal containment.

One solution is to use a detector with a mirror that has many facets. This means that each of the surveillance zones is effectively divided into a number of sub-zones. An animal is too small to register simultaneously in all of the sub-zones which make up a complete zone, so it does not produce an alarm.

Another method, in lens-based detectors, is to change the standard Fresnel lens for a special ‘pet alley’ lens, and then to mount the detector at exactly the height specified by the manufacturer for this type of application.

With this arrangement, the detector is blind below the mounting height, and so an animal walking around the room is ignored. There is an obvious problem, however, if the pet can jump above the mounting height.

Manufacturers also use masking techniques that have the effect of making detectors less sensitive to the infrared signatures of the animals concerned. Detectors with this type of technology need to be matched to the size of animal, and are usually broken down into weight-based categories – for example, 10kg, 20kg, 40kg, etc.

Dual-technology detectors

While the latest PIR detectors, properly fitted, provide excellent results on their own, legislation has stressed the need to reduce the industry system false alarm rate, resulting in the combination of two detection technologies in one detector. One of these technologies is invariably PIR, and the other, in most cases, is either microwave or ultrasonic detection.

While of little use on its own, since its coverage penetrates certain materials (such as glass and wood), microwave detection is a good partner for PIR. Unaffected by thermal convection effects, and very effective at confirming motion (Doppler Effect), it provides a useful double check on signals produced by the PIR detector.

And although adding microwave functionality to a detector necessarily increases the cost, dual-technology detectors of this type remain affordable for everyday commercial and domestic applications.

In contrast, dual-technology detectors that combine PIR with ultrasonics are typically four to five times the price of standard detectors. A transmitter beams continuous sound waves via an electro-acoustic converter in the inaudible frequency range. A receiver adsorbs the sound energy reflected by the environment via a microphone (Doppler Effect). The change of frequency is proportional to the speed of movement, which is measured radially from the detector.

Ultrasonic motion detectors operate as active systems and are suitable for complete room protection, and when used in conjunction with PIR technology, make a very stable detector. However, this is usually too small a benefit to justify the high cost of fitting these units, except in the most sensitive of applications, such as banks, museums and data centres.

Installation

All these types of intrusion detectors discussed in this article can provide very effective protection. To do so, however, they must be properly installed, with particular consideration given to these points:

• operational range
• mounting height
• direction of any possible movement
• possible sources of interference
• physical obstacles

Passive infrared intruder detectors are basically simple devices, but manufacturers have now developed the technology to the point where the best of the modern units, particularly those which use black-mirror optical systems, provide excellent performance and reliability.

Choosing detectors is, therefore, relatively straightforward. Unless there are very special considerations, specification of good quality PIR devices from a reputable manufacturer is recommended.

Always check that the products chosen are suitable for the environment in which they are going to be installed, and fit them carefully, following the recommendations and instructions provided by the manufacturer. The result will be effective intruder protection and an installation that offers a long, reliable working life.