Products > Thermal Imaging

Plastic lenses for LWIR?

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Ultrapurple:
Has anyone tried using plastic Fresnel lenses for thermal imaging?

For a long time I've been mulling over the multi-zone lenses on the front of PIR security sensors but then found that a few manufacturers produce lightweight Fresnel lenses that are supposed to be able to "replace expensive germanium lenses in systems requiring moderate imaging quality".
See http://www.kube.ch/downloads/pdf/kube_single_fresnel_lenses.pdf from Kube Electronics AG, Switzerland.

On the face of it, a 13.5mm FL f/0.5 (!) lens made from 0.5mm thick HDPE with ~70% transmission at 10µm looks quite attractive - and a lot cheaper than germanium. But their idea of 'moderate imaging quality' may be different from mine.

I asked about prices of a couple of the lenses and the answers were in the 10 to 20 euro range, plus 24 euro for P&P. (All very reasonable for one-off sample sales, I reckon).

Thoughts?

Fraser:
The use of thin fresnel and diffractive lenses made from LWIR transmissive material for thermal cameras has been considered over the years. There is at least one company that took the idea of using plastic fresnel lenses into production for relatively low resolution tasking. From what I know of the plastic fresnel lens product, it is very 'course' and created a poor thermal image on a 320 x 240 pixel core. Diffractive lenses using ZnSe and Silicon have produced affordable and good quality imaging lenses. The plastic lenses have some way to go in terms of technology before they can be considered a serous alternative to conventional silicon, ZnSe, ZnS and Germanium optics. There are diffractive Silicon and ZnSe lenses that work very well at limited resoloutions. The TAU2 has a diffractive ZnS rear lens element and the LEPTON cores use Silicon diffractive lenses.

I was looking at using the plain (non fresnel) PIR windows as lens protector material. It's transmission losses seemed quite high compared to the thin shrink film that I normally use. A PIR is looking for a 36C target against a approx 22C background. The two or four pyroelectric detector elements are huge compared to a cameras microbolometer pixels. Transmission losses can be quite high in such a design and it will still work.

I would like to track down more affordable ZnSe lenses of greater diameter than 20mm and greater FL than 100mm. They would be useful for supplementary lenses. For me, plastic fresnel lenses is not the way forward at the moment.

Fraser

Vipitis:
Is there any source where I could get such a cheap lens?

My Lepton isn't high resolution anyways, but I want a telephoto lens to shoot the moon, the stock the fov is really wide.

Ultrapurple:
I have some of these plastic lenses on order and will test them with my Therm-App, which has easy access to the sensor. I will report what I find, though I strongly suspect Fraser is right and the results will be at best mediocre.

Fraser:
Ultrapurple,

I will be very interested to hear what you think of the lenses.

A source of these lenses is decent quality IR Thermometers where they are often used to focus the energy onto the thermopile sensor.

Thank you for spending your money investigating these lenses. It is a pity the company does not offer a free sample as they are not expensive to produce once the press mounds are made.

Fraser

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