Red fresnel lens for a PIR sensor?

Hi all,

Can anyone think of a reason I would not be able to use red plastic to create a custom fresnel lens? I have seen them in black and imagine that red couldn’t be any worse than that.

Thanks :slight_smile:

Use a working PIR sensor, and keep the red plastic in front of it to test if it works (I doubt it).
Making a good lens is not easy. How are you going to make it ? With a gouge ?

Hi Koepel,

I was thinking I could create a 3D computer model and have it printed on a 3D printer. I have seen different designs for the fresnel lens. Some look like a golf ball and others have concentric rings. The more complex designs probably have better functionality but I just need something that can detect someone a couple of meters away.

To further muddle the question:

I am considering using conductive plastic so that my enclosure can be touch activated. Depending on the density of the plastic, this would probably be a bigger issue than the color.

Thanks for the help Koepel.

As far as I know the lens should have sharp edges and flat in between. Using a 3D printer is the opposite. Conductive plastic as well ? Then I have even more doubts. Polymere plastic ? Maybe I have not more doubts, but less doubts (that it is not going to work).

From my (limited) research, the important aspect of the fresnel lens is that the different densities in the plastic create clear points of reference for the sensor to detect motion. That is to say, the first time a person is detected the pir sensor does not actually detect motion. It is only when the object then "disappears" behind a thicker segment of the lens and then reappears inside another thin segment that object's motion becomes clear to the sensor and it triggers the signal.

I've got other options regarding plastic that I choose to use. I might even use conductive paint and leave the area with the PIR sensor bare.

Thanks again for indulging me :slight_smile:

Your understanding of how a Fresnel lens works could be improved. There are two important considerations:

A Fresnel lens has carefully shaped ridges that bend the light to a focus. The shape depends on the refractive index of the lens material at the wavelength of interest.

If made out of plastic, the plastic must be transparent to IR. Red plastic appears red because it absorbs other wavelengths of light.

You can use a fresnel lens whereever a lens would work, its just a collection of lens fragments arranged
to save weight and thickness.

However PIR sensors use far infrared (10um wavelength range) so you will need to make it out
of a material transparent to heat radiation (polythene is one example I think). Most plastics and
glasses have strong absorptions across the IR spectrum (IR spectroscopy is routinely used to
identify organic chemicals). For quality optics germanium is a common material for PIR lenses
and not cheap.

Just to clarify the infra-red section of the spectrum is far larger than the visible spectrum and covers 0.8um to
over 10um - its important to know exactly which parts of this spectrum your sensor is sensitive to and ensure
your lens works across the whole range, rather than focussing some of it and scattering the rest.

There is a reason PIR imaging systems are expensive.

Great info guys. I will start researching appropriate plastics to use for a lens and how the fresnel lens helps the sensor to do it's job. Thank you for the info. :slight_smile:

The fresnel lens is essential to create zones.
The pyroelectric sensor does not measure absolute value but responds to a change in value.

The lens creates zones and as a person moves from zone to zone a change is detected.

Ah, you don't mean lens at all then, but prism array?

yes , i think.

The prisms are arranged to form many separate lenses focussed on the same spot.

Lens array ?

The plastic used has a passband around 10 -13 uM

jshields:
I've got other options regarding plastic that I choose to use. I might even use conductive paint and leave the area with the PIR sensor bare.

That is unlikley to work.

The pir depends on change to see a signal.

Normally two elements and the heat 'pulse' moving across them generates the output.

These people do generic ones.

http://www.kube.ch/downloads/pdf/kube_multizone_lenses.pdf

MUTR used to do them singly but i cannot find the site now.

Edwards may also do them but are probably pricey.

The link mentions cone optics, but i did not have much joy with that.
It may depend on a different sensor.
Cheap pir i have used all seem to have the two or four element detectors.

EDIT

This is what i was looking for

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