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28/02/2018 - 01/03/2018

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Why choose a laser sensor?

Why choose a laser sensor?

Modern photoelectric sensors typically employ one of two light technologies, either LED or laser. And the choice of light source can be as important for a reliable application as the choice of sensing mode, as the experts at Turck-Banner explain.

In a choice between an LED or laser sensor, it all comes down to the key property of coherence. Both laser and LED use a diode to generate the light, the laser then bounces this light back and forth within its housing to generate more of the same wavelength of light (monochromatic, single colour), and more critically all the waves are in line with each other (coherent). LEDs also generate near monochromatic light, they have a broader spectrum of wavelengths than lasers but the critical feature is that the light is incoherent, the wavelengths are randomly produced.

All natural light and most man-made light is incoherent and has many wavelengths which is why it spreads out from its point source as it travels. Laser light remains in a tightly focused beam over great distances. It is this coherence of laser light that gives laser sensors their unique properties of very narrow beams that can travel great distances without spreading out, making them especially suited for certain types of application and a very poor choice for others.

In order for an object to be detected the light from the emitter must reach the receiver or the light from the emitter must be blocked from reaching the receiver. This is true for all optical sensing modes. Initial set-ups are often fairly simple but maintaining reliability can be difficult, especially if the environment is dirty or vibration is present. 

The emitted light pattern on an LED based sensor spreads out very much like a simple torch light, and the receiver has a similar field of view. As long as the beam pattern of the emitter falls on the lens of the receiver and the field of view of the receiver covers the lens of the emitter a light beam will be established between the two. With a wide beam pattern and field of view the emitter and receiver can move and rotate a considerable amount without the beam being broken, as often occurs if the units are knocked or through vibration. A laser emitter has a very narrow beam pattern and although this beam can travel a very long way small amounts of vibration will move the beam off the receiver and break the beam. Laser based sensors are restricted in the amount of light power they can produce as the risk of blinding someone would make them too dangerous to use in a normal working environment. 

Whilst conditions are good this low power does not cause any problems as it remains concentrated in a small area but dirt, oil, snow and other contaminants can disperse, block or divert the beam. This can also be an advantage when detecting clear objects and semi opaque objects: where an LED beam could pass straight through, the laser beam will be affected.

The small spot size of the laser sensor make it ideal for detecting small objects or small features on an object. It also allows objects to be detected in holes or recesses where the reflections from the side walls would make it impossible for LED sensors. Further, the single wavelength of the laser light produces a precise colour that allows the detection of small colour differences that an LED sensor would struggle with.

Where laser sensors really excel is in distance sensing. The narrow beam is ideal for triangulation, where the position of the beam reflected from an object will change with the distance that the object is away from the sensor, the intensity of the beam will remain relatively unchanged with distance. 

The opposite is true for an LED sensor, intensity changes with distance but the reflected beam is too wide to gain any accurate position information. As the intensity also changes with object colour and reflectivity attempting to use a standard LED sensor for distance measurement is normally a poor choice. The laser sensor can detect colour or reflectivity with a degree of immunity from distance changes.

The ability to detect the object distance allows the sensor to overcome a number of common sensing problems. A background suppression  or fixed field sensor will ignore a shiny object that is beyond its cut-off point and still be able to detect a dull object just a few millimetres in front, whilst a foreground suppression sensor will ignore objects closer than its cut off point. A combination of the two can create a window of detection between its near and far cut-off points. This window of detection is commonly employed together with an analogue output to monitor the position of an object within the window.

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