We’ve heard there are 2 different types of Fresnel lens, one being better than the other in alarm sensor applications. Has SEN heard anything about this?

A: There are 2 types of ‘fresnel’ lens – one type has a design based on lighthouse lenses and can be manufactured anywhere in the world. The second type is manufactured by a US-based company called Fresnel.

PIR lenses manufactured by Fresnel Inc. are based on the same technology as the others with their own variations. The key different is that Fresnel Inc. stamps its lenses, it doesn’t mould them. What the better optics of the stamping process mean is that PIR gain doesn’t need to be turned up as far when using these lenses, enhancing performance and reliability.

If overall sensor design is good and there’s a reasonable signal-to-noise ratio, sensor stability is improved, in part because amplification raises noise levels significantly because a PIR’s pyroelectric signal is miniscule. Given typical PIR signal is down at microvolt levels, PIR gain needs to amplify the signal 1000s of times – not surprising, this means any induced noise will be amplified, too. Better lenses mean lower amplification, mean better performance.

Fresnel offers 7 series of PIR Fresnel lens arrays, along with assorted types – each series contains different lens arrays, each of which has the same mechanical size and shape but offers a different detection pattern. Fresnel also collaborates on custom lens designs at no charge, which is awesome.

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