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Laser break, infrared break or passive infrared trigger?

With all the various triggering methods based on laser and infrared it is hardly surprising that there is some confusion over, not just how they work, but also, which one is best.

Here, we will describe the different methods and explain the benefits of each one.

Visible light spectrum

Light is measured in very small wavelengths ranging from about 700 nanometres (red) up to about 380 nanometres (violet),  to give some idea, BBC radio 4 long wave broadcasts at 1515 metres.  This plot shows the range of light which is visible to the naked eye.

Laser break

Probably the most common method of "beam break", it will use a laser transmitter (laser pen or rifle sight) and a receiver (sensor).  The laser transmitters used for this purpose are based around a laser diode and can be either red (650nm), green (532nm) or blue/purple (405nm).

The laser receiver (sensor) will typically respond to anything from 800nm to 440nm, this means that any of the three laser colours should function.

The advantage of using a laser beam break system is that you can see the laser beam, this makes it easier to line up the receiver and transmitter.

The disadvantage is that, unless the laser beam is disabled once it has been broken, it is always visible and must therefore be positioned such that it does not appear in the image.  It is possible to disable the laser beam but this means that another cable is needed which will then be either too long or too short ...

Infrared break

Another method of "beam break", it uses active infrared (940nm) and again has a receiver and a transmitter.

The advantage of this sytem is that it is invisible to the naked eye and so should not appear in the image (unless they have been specifically modified, all cameras have an infrared filter in front of the sensor element).

The disadvantage is how to line up the transmitter and receiver, the usual method is to have an indicator on the control unit which shows if the beam has been detected.  The problem is that there is no way to know when the beam is centred (it may be lined up on the periphery of the beam which means that a slight movement will throw out the alignment).

This system is best used in a fixed application where the receiver and transmitter are both secured in a common frame.

Passive infrared (PIR)

This is not a "beam break" system, it is a receiver which relies on living objects to radiate body heat (800nm to 1,400nm) and thus act as a transmitter.  A typical passive infrared receiver will have a conical detection pattern of about 120°, think of a line passing out from the centre of the sensor, now move this line 60° away (any direction) and rotate.  The range is typically a metre or more but is best optimised at 2 metres.

The disadvantage of this system is that the detection range may be greater than we need.

The advantage is that we can modify the detection range by placing either a baffle on one or more sides or by fitting a tube (collimator) to give a parallel detection pattern.