PIR detectors with anti-masking.

PIR detectors with anti-masking.

Passive infrared (PIR) detectors, which have become widespread in the last 10 years, are one of the main technical means of security alarms for protecting premises for various purposes. This is due to a number of undoubted advantages that they have compared to microwave or ultrasonic sensors of similar purposes. First of all, this is a passive principle of operation, due to which there is no impact on a person or, for example, on valuable exhibits located in its coverage area. Also, an unlimited number of detectors can operate in one room without interfering with other detectors. PIR detectors are easy to set up and operate, consume little power from a power source, have small dimensions and fit well into the interior.
Passive infrared detectors react to a slight positive or negative difference in the temperature of an object relative to the ambient background temperature. They detect a person moving in the detector's coverage area with a high degree of reliability. Any detector of this type includes a sensitive element — a pyro receiver and an optical system that forms sensitive zones in space.
However, despite good detection capabilities and resistance to false alarms, security detectors with a passive infrared detection channel are among the most vulnerable to neutralization prepared by intruders. This is due to their «passive» operating principle: the detector must register a change in the IR radiation flow falling on the pyroelement. It is clear that this flow can be quite easily screened («masked»), which can be used by an intruder. Installing screens that are opaque to IR radiation obscures the detection zone, and the sensor stops responding to human movement. To counteract blocking of the detector in this way, a special anti-masking circuit is used in modern highly reliable detectors. This circuit operates continuously, even when the system is disarmed. And the alarm signal can be issued only when trying to arm the system by opening the alarm relay or immediately after detection by opening a separate relay. In the first case, if the masking was accidental (for example, the detector was covered by a curtain or an open window, and by the time the alarm was set, the masking had resolved itself), no alarm signal is issued, but you can only learn about the masking when you set the alarm. Intentional masking will become known almost immediately, but false masking messages are more likely when the premises are not under protection.
Anti-masking is the ability of a detector to detect attempts to neutralize it by an intruder by shielding (masking) it with a material that blocks the passage of infrared radiation.

Methods of masking detectors
The main methods of masking detectors are: shielding, stickers, and aerosol sprays.
Shielding is one of the simplest ways to mask detectors. To do this, an object is placed in front of the detector that does not allow IR radiation to pass through, thereby blocking part or all of the detection zone (Fig. 1). This can be a cardboard box, a rag, or a hat that is hung on the detector. It is clear that this type of masking is done during the day (working) time, when the premises are disarmed. When the premises with the masked detector are armed, the intruder returns to penetrate the facility, knowing that the detector has been neutralized.
Of course, such shielding can be easily detected when inspecting the premises before arming, but in practice such inspections are rarely carried out.
A more sophisticated method of masking can be to cover the optical system (Fresnel lens) with a self-adhesive film that is transparent to visible light and opaque to IR radiation (for example, vinyl film), or by applying transparent varnish to the lens with a brush. Of course, this will require more time for preparation and implementation, but the result is worth it: the masking will be less noticeable to personnel.
And finally, the fastest way to mask can be to apply paint, aerosol plaster or white polystyrene foam to the lens using aerosol sprayers. Moreover, masking of the detector with a sprayer can be done even in a room that is armed, if the intruder manages to get to the detector unnoticed.

Methods for detecting masking
To detect masking, leading manufacturers of security detectors have detectors with an anti-masking function. The anti-masking channel is active and consists of LEDs and photodiodes emitting in the infrared spectrum, acting as radiation receivers. Detectors with an anti-masking channel differ from each other in the number and location of LEDs and photodiodes, methods of illuminating the detector lens, and methods of signal processing. To prevent deception of the detector by illuminating the photodiode with an external source of IR radiation, the IR LED signal is modulated, and the signal arriving at the photodiodes must be synchronized with the IR LED signal.
The most common method for detecting detector shielding is the reflected light method (Fig. 2). The detector has an IR LED on its surface that emits IR energy into space. If a foreign object (screen) is placed near the detector, the radiation is reflected from it back through the lens into the detector and is detected there by special IR photodiodes. The reflected light method is also suitable for detecting masking directly on the lens surface, for which one of the LEDs is located on the board under the lens, and the signal reflected from the masked lens goes directly to the photodiodes.
A more sophisticated version is used in Through the Lens technology. These detectors have two prisms at the base of the lens, which are optical waveguides for illuminating the lens with an IR LED located on a printed circuit board inside the detector housing. The IR radiation passes through the lens and is detected by an IR photodiode. By masking the detector by placing a sticker that blocks the passage of IR radiation into the detector, the signal from the photodiode will be reduced and the detector will generate an appropriate alarm.
In detectors manufactured using the Retroreflector technology, there is an optical waveguide (light guide) with a multiprismatic (ribbed) structure of the outer surface. Reflection of the IR signal in the optical waveguide is possible due to different refractive indices of the waveguide and the environment, as well as a sufficiently acute angle of reflection. Thus, at the output of the receiving photodiode in the standby mode, there is a signal equivalent to the amount of light reflected from the surface of the optical waveguide. If a foreign substance is sprayed onto the surface of the detector, the refractive index of the environment at the boundary of the waveguide will change, and therefore some of the IR radiation begins to be absorbed or released. As a result, the amount of reflected light will decrease, which is detected by the photodiode at the other end of the waveguide and a masking notification is issued. It should be noted that, in essence, in this case, masking is recorded not of the lens, but of the optical waveguide, which, generally speaking, is not the same thing.
An important point is the issue of transmitting a masking notification to the control panel. There are three possible options: transmission via alarm relay contacts, transmission via relay contacts combined with a case opening sensor, or transmission via contacts of a separate relay. In the second or third cases, when signals from detectors are output to 24-hour loops, masking will be detected faster, but false masking signals are more likely (presence of personnel in the protected premises, masking from curtains, open windows, moving cabinets, etc.). When transmitting a masking notification via alarm relay contacts, when arming with a masked detector, an «arming error» will be issued, and the security officer will be forced to identify the masked detector by its indication.

Using PIR detectors with anti-masking
Abroad, regulatory requirements for security alarm systems clearly define the categories of premises where detectors with an anti-masking channel must be used. Therefore, such detectors are becoming more and more widespread.
The new European standard EN 50131 sets requirements for detectors with anti-masking functionality, according to which such detectors must detect attempts to mask with the following materials:
a sheet of black paper;
a sheet of aluminium 2 mm thick;
an acrylic sheet 3 mm thick;
white polystyrene foam;
self-adhesive transparent vinyl;
aerosol plaster;
clear varnish applied with a brush.
In this case, the masking message must be generated within 180 seconds of applying the masking material and continue as long as the masking material remains in place. 80% of the materials must pass the test successfully.
In the new version of the Russian GOST R 50777 «Requirements for security alarm systems» in paragraph 6.2.14 «Protection against masking» it is specified that in order to obtain the masking effect, a screen made of a sheet of paper is installed or a layer of an IR-opaque aerosol or varnish is applied to the lens of the detector. After 1 min. the detector must issue a masking message.
It is clear that detectors with anti-masking are classified as high-security detectors, corresponding to the cost, and should be used at facilities with increased security requirements. Such facilities include banks, museums, jewelry stores, warehouses, energy facilities, etc. However, where cost does not play a significant role, such detectors can also be used in country houses, premium apartments and offices.

Optical-electronic security detector «Photon-16» (RIELTA)
It has three versions, differing in different detection zones (volumetric — 12 m, linear — 20 m and surface — 15 m). The spherical Fresnel lens and microprocessor signal processing provide uniform sensitivity throughout the detection zone and high noise immunity. To provide anti-masking, an active IR channel is used based on three emitting IR LEDs and two receiving photodiodes, which ensures detection of masking of any part of the lens. Unique circuitry and algorithmic solutions provide detection of masking by various materials in accordance with European standards EN 50131. The detector also has supply voltage control, «alarm memory», thermal compensation, tamper contact and a swivel bracket included.

PRESTIGE AMQD PLUS (TEXECOM)
Prestige AMQD Plus is a passive IR detector for indoor use with a range of 15 m. The detector uses a quadruple pyroelectric element, which increases the accuracy of detection — for the detector to be triggered, the intruder must cross 4 subzones simultaneously. Thanks to the microprocessor signal processing system, digital temperature compensation and pulse counter, as well as reliable protection against backlighting, the detector ensures stable operation and a high level of protection against false alarms.
The operating principle of the anti-masking system in the Prestige AMQD Plus detector is based on the use of two additional infrared LEDs, which are located in a protected compartment of the optical system. The radiation of these LEDs penetrates into the surrounding space through the lens, and upon receiving a return signal, the system determines that the detector has been covered with something or the lens has been painted over. In this case, a special alarm signal is generated, which is transmitted via a separate relay output.

HX-40AM (OPTEX)
The HX-40AM is a passive outdoor IR detector that features a unique optical system consisting of two independent pyroelectric elements that form a high-density detection area consisting of 94 cross zones.
Processing the signal from both pyroelements when operating in the «I» mode allows for high accuracy in determining the temperature difference of moving objects and avoiding false alarms that can be caused by small animals or birds. Thanks to the digital signal processing system, the detector fully controls the surrounding space and can detect masking attempts. In addition, the detector constantly monitors changes in the environment and, depending on the degree of these changes, automatically adjusts the sensitivity of the anti-masking system, ensuring reliability and stability of operation.

Tower 20AM /MCW (VISONIC)
All-weather outdoor PIR detector with mirror optics and anti-masking function in wired and wireless versions. It has a high level of noise immunity in open outdoor conditions, a two-level active anti-masking function, and vandal-proof protection of the optical system. Detection area — 90°; 12 m. Immune to animals weighing up to 18 kg. It has two levels of protection against masking. Immunity to illumination — more than 25,000 lux. Operating temperature range: -35 °C to +60 °C. Protection class — IP55. European classification — Grade 3.The detector has 8 independent dual IR sensors, each of which acts as a separate detector with its own detection zone. Tower 20AM distinguishes signals from sources that are at different angles in the horizontal and vertical plane. The «double» anti-masking mode provides reliable protection against masking the detector with sprays or blocking objects.

Tower 10AM (VISONIC)
Professional wired PIR detector with mirror optics and anti-masking function. Long-range detection zone, two-level active anti-masking function, vandal-proof protection of the optical system, replaceable mirrors: «curtain» or «wide angle». Volumetric detection area — 90°; 25 m. «Curtain» detection area — 35 x 2.5 m; 5°. Immune to animals weighing up to 18 kg. Has two levels of protection against masking. Immunity to illumination — more than 15,000 lux. Operating temperature range: -20 °C. +50 °C. Protection class — IP41. European classification — Grade 3.
The special shape of the mirror increases the collecting capacity of a parabolic or spherical shape, which allows to dramatically increase the detection zone. The double anti-masking mode provides reliable protection against masking of the detector by sprays or blocking objects. The impact-resistant, ultra-miniature protective screen made of hard plastic reliably protects the optical system from vandalism.