Follow-up to the IR thread 1

[cluelessone]

I am trying to do temperature measurement through a sheet plastic window.
I have the electronics portion working well in my lab but all the "IR" windows I put in front of it pass so little energy that the unit is useless. ("windows glass" and clear polycarb)
The supplier of the sensor says that I am looking to pass 5 to 15 um. These windows seem pitched to near IR and my range extends deep into mid IR.
I have been trolling the net and found recommendations to use polyethylene so I grabbed some clear poly parts bags and they work very well.
While these work from an optical point to view I need a rigid window on the production machine.
I tried some sheet HDPE from a local plastic shop and that did pass some signal but the losses were unacceptable high.
Any suggestions on what I should be looking for?
Thanks in advance.

 

[IRstuff]

Saran wrap works pretty well. Sometimes you can go thinner, rather than more stiff.

Neither window glass nor PC are particularly useful in the IR. Bear in mind that the near-infrared than passes through glass or PC is at least 2x lower in wavelength than your band of interest.

TTFN

FAQ731-376

 

[IRstuff]

Germanium, silicon, zinc selenide, zinc sulphide are the more commonly used IR window materials. However, they are not even remotely cheap, and generally require expensive coatings to get the full benefit of their use.

TTFN

FAQ731-376

 

[Pud]

COC is one of the better polymers for IR transmission, but probably not good enough:

http://www.topas.com/optical_en_080603.pdf

Page 9 for transmittance graph.

H

http://www.tynevalleyplastics.co.uk

 

[cluelessone]

Thanks for the link to Topas.

The transmission chart only goes to 800 um and does not tell me anything about 800 to 1500 um range, with the 800 um to 1200 um being the areas of highest interest. It is worrisome that the copy that goes with the chart mention near IR by name.

I don't really know much about polyethylene so maybe this is a really stupid question - is there a grade or type that has the same optical properties as the soft clear bags but is rigid in a 0.03" sheet?
I have hope that this is true as most motion sensors are working in the same optical range and the ones I have looked at have a grayish translucent rigid plastic with a slight bluish cast.

 

[patprimmer]

Define rigid.

at 0.030" section thickness PE is not all that rigid.

HDPE is the most rigid type.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[cluelessone]

Hmm, excellent point.

I am thinking in terms of a part that once die cut would keep a flat shape while the assembler positions it over the machine and presses it in place using a PSA film.

I would consider a overhead write-on sheet as rigid.

I would consider poly-sheeting used to enclose a construction site as flexible to the point of being a impediment to easy assembly.

 

[patprimmer]

HDPE at 0.030" is probably OK

Builders film is MDPE or LDPE and is more like 0.004" thick.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[Pud]

Try a Tupperware container wall - thin PE.

H

http://www.tynevalleyplastics.co.uk

 

[IRstuff]

Is this supposed to be a production product? Why not simply install the Saran wrap onto a frame that is in turn installed into its site?

TTFN

FAQ731-376

 

[cluelessone]

In this application the sensor bar will be mounted on a vehicle with road dust and dirt hitting the sensor at highway speeds.

Saran wrap my not be enough to keep gravel from getting into the sensor package.

This particular build is a prototype so I can use a poly parts bag as it is pretty tough (already tested and works well) but for production I need a better solution.

I will try the tupperware as soon as I can get one in the lab.

 

[IRstuff]

OK, I'm confused. Is the window for looking in, or for looking out? If looking out, you need a hard window, with hard carbon coating, i.e., germanium or silicon, with AR coatings and hard coating. Anything less will not meet all the environmental requirements, particularly if your sensor is doing any imaging of any sort.

TTFN

FAQ731-376

 

[cluelessone]

The sensor is looking out up the housing, with a 7 degree beam angle so it can be thought of as imaging.

The poly bag I have been working with seems to be working well; the sensing area does not seem to be markedly changed and the drop in output is manageable and can be calibrated out.

It is my hope that a hard sheet of plastic will work as will and keep the envirionment out up my box well enough to do some limited field testing. In a perfect world this material will be durable enough for a production product.

 

[IRstuff]

Plastic, outdoors, is also subject to UV damage, and needs to be coated for protection, which may prevent IR transmission.

Additionally, most plastics will concede when met with gravel, and the resultant dings and scratches will not promote good imaging, and will reduce the effective transmission of the window.

TTFN

FAQ731-376

 

[patprimmer]

Dishwasher safe Tupperware is more likely PP.

The PP Tupperware is smooth and shiny. HDPE Tupperware has grainy surface and is translucent, not transparent, at least to visible light.

PE is quite poor to UV and will be badly damaged by one summers exposure in any temperate or tropical climate. Stabilisers improve it, but as IR said, they may filter the bandwidth you are trying to measure. Surface coatings do not go easy with PE.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[cluelessone]

After considerable surfing I found a few interesting links:

http://www.kube.ch/optics/index.php

http://www.acrylite.net/sites/dc/Do...uct/ACRYLITE/3648 IRT Tech Bulletin GP FF.pdf

http://www.fresneltech.com/materials.html

http://www.infraotica.com/

And this raises almost as many questions as it answers:

http://people.csail.mit.edu/jaffer/FreeSnell/polyethylene.html

 

[Demon3]

Why does it raise questions? For all polymers you can measure or find the IR spectrum showing exactly what wavelengths are absorbed and which are not. Thus you know whether it's transparent at your wavelength.

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry

 

[Demon3]

Just go to

http://www.sigmaaldrich.com

find the polymer you are interested in and push the IR Spectrum button. For example, here is the spectrum for PMMA

http://www.sigmaaldrich.com/spectra/ftir/FTIR002075.PDF

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry

 

[cluelessone]

The problem is that this "find the polymer you are interested in" is my original question:
"Any suggestions on what I should be looking for?"

When I said "this raises almost as many questions as it answers" it is because it describes a few properties in:
"thin sheets of polyolefin plastics, and high density polyethylene in particular." but does not nail down what material I should be asking for.

As mentioned above I tested several materials that meet this basic definition but there have been huge differences in performance.

Some variables that I have no clue on: extruded vs cast, density, degree of crystallinity, and whatever else it is that I should know about specifying plastics but don't.

As can be seen from the links referenced some manufactures are offering products with plastic windows so it must be possible but they are cleverly not providing a material description that I can specify in my own products. (Ok - Kobe will sell me the material directly but still really don't tell me what it is.)

I have samples coming of the Evonik product. I have been talking to my local plasic suppliers and I get the same basic thing - tell me the resin and other details and I can get you samples; I don't know what to ask for.

 

[Demon3]

You can get some material and have it analyzed. Should be simple to tell you what polymer it is. Ironically, it's the infrared spectrum of the polymer that tells what it is. If you want contact to a lab, the one I use is

http://www.stabilization-technologies.com

Thanks for explaining your uncertainty. It can be a minefield.

Extruded versus cast, should be no difference.

Amorphous materials like polystyrene, SAN, PMMA and polycabonate should be better than semi-crystalline materials like PE, PP and nylons because crystals can scatter light.

Density makes no difference in itself.

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry