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We are seeking after a motion sensor, probably PIR , that can be covered with regular plastic or ABS ( that means you will have a regular product enclosure where the sensor is inside it ) .

I could see that most of them have this white dome covers them , and i wonder if there is an option to cover them with any material and still let them work as usual .

After all , they are working with temperature so why cant you cover them ?

Is there other physics that i miss here ?

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    \$\begingroup\$ Would you expect to be able to put a visible-light camera inside an opaque box and still have it work? The enclosure needs to be IR-transparent for IR sensors. \$\endgroup\$ – pjc50 Aug 21 '15 at 9:08
  • \$\begingroup\$ Its a very wrong example. Temperature is also involved here. \$\endgroup\$ – Curnelious Aug 21 '15 at 9:12
  • \$\begingroup\$ No it's an accurate example. Optical sensors look through a window. The lens of a camera, or the lens of a PIR, is the window. They're different at different wavelengths, but you still need a way for the radiation to pass through the wall of the box. \$\endgroup\$ – tomnexus Aug 21 '15 at 9:26
  • \$\begingroup\$ How about radar? That passes through a much thicker plastic wall than light / LWIR. Think of a Doppler radar for a door opener etc. \$\endgroup\$ – tomnexus Aug 21 '15 at 9:28
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I'd say you are missing the concept of remote temperature sensing versus direct contact temperature sensing.

They are not just working with temperature (as in body heat) but with infrared radiation (the other form of temperature). So the covering material must be transparent to this wavelength. Depending on the sensor type used the wavelength will be different, but as humans are roughly 30°C on the outside, their main wavelength is somewhere in the long wavelength infrared region (~10µm), so sensors are used which are very sensitive in that region.

A typical sensor uses the pyroelectricity of materials. As those sensors are only able to detect changes, they often have special fresnel lenses built in such way, that you get stripes of detection areas. A moving object which crosses those stripes will then generate several pulses which can be measured.

So you have to use a material which is transparent at 10µm wavelength. Typical lenses are made out of polyethylene which gives it that whiteish/milky look. There are coloured lenses around to make them look better (more fitting to the case), though some have reduced transmission.

If you want to get through ABS you have to change your wavelength. Radar sensors work fine through ABS as demonstrated by TURCK for example.

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The White Dome is actually a Fresnel lens which expands the field of vision for PIR motion sensor. You could directly use the PIR sensor ( the 3 pin sensor only ). You can either make a DIY Fresnel lens or get it manufactured according to your needs

More on Fresnel Lens here : https://en.wikipedia.org/wiki/Fresnel_lens

More here : http://www.glolab.com/focusdevices/focus.html

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