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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) may typically respond to anything from 800nm to 440nm, this means that any of the three laser colours could function.

The advantage of using a laser beam break system over an infrared 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 between the laser transmitter and the controller (along with additional electronics), our Laser Trap and X Trap systems do just this.

A lesser known problem with most laser beam break systems is that the sensors react to daylight and will detect the ambient light even when the laser beam has been broken, the advanced design of the Laser Trap and X Trap systems solve this problem and photographers who have purchased them all state that they outperform other systems which they have used.

Infrared break

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

The advantage of this system 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).

One 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).

The biggest disadvantage though is that it is very prone to infrared blocking caused by sunlight, this means that even though the beam has been broken the sensor has not registered it due to the blocking effect.

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 (beamwidth) of about 120°, think of a line passing out from the centre of the sensor, now raise the end of this line by 60° and rotate through a full circle, range is typically a metre or two.

The disadvantage of this system is that the detection range beamwidth may be greater than we need but this may be overcome by placing either a baffle on one or more sides or by fitting a tube (collimator) to give a parallel detection pattern.

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