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I am currently working on a project that requires a section of "release liner" to be removed and a clear plastic to be put in its place after a few holes are punched out of the paper. What I am trying to figure out is how to detect if the plastic did indeed cover all of the glue or if there is glue present. An addition to this would be to check and make sure that the release liner is also removed in the current section.

It would be preferred to do this without contact since there is very little room to test if there is glue present (on the surface) before it goes into the next stage of the process.

The materials used are as follows: light-brown chipboard (type of cardboard/heavy paper) that is backed with glue on 1 side White (semi-glossy) release liner that sits on top of the glue Clear (PET) thin plastic backing which is very reflective

Thoughts that I have had, but do not know what avenues to pursue are bouncing a laser off of the surface and measuring the amount reflected back and also ambient reflection to detect if it is bouncing off of a white surface.... Any and all avenues, tips, and/or help will be greatly appreciated.

Friendlyghost89
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    Suggest you investigate if you can get better contrast using other than visible spectrum. IR is sometimes convenient. – Spehro Pefhany Feb 04 '14 at 14:37
  • Not one mention of EE - is it an EE question? – Andy aka Feb 04 '14 at 14:48
  • White light source and a photodiode may be all you need. Or you might have to go all the way to a reflectance spectrometer: https://www.google.com/search?q=reflectance+spectrometer&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a – Wayfaring Stranger Feb 04 '14 at 14:51
  • @Andyaka I didn't post as an EE question because I figured that it was more of a Materials question than that of an EE, but if you have any input on how to distinguish the different surfaces, I would greatly appreciate it. – Friendlyghost89 Feb 04 '14 at 15:06
  • @WayfaringStranger do you know of any prebuilt diffused solutions, where the light source and photo diode are enclosed in some sort of fashion? I know that there are Diffused and reflective laser solutions for such, but not sure for the white light source... – Friendlyghost89 Feb 04 '14 at 15:09
  • @Friendlyghost89 Don't know, but someone else on here might. – Wayfaring Stranger Feb 04 '14 at 16:02
  • You might get surprisingly distinct readings using a [TCRT5000](http://www.vishay.com/docs/83760/tcrt5000.pdf) infrared reflective optical sensor: These contain an IR LED and an IR sensor in a single unit, and are ridiculously cheap, at [$1.40 for 10](http://www.ebay.com/itm/10X-TCRT5000L-TCRT5000-Reflective-Optical-Sensor-Infrared-IR-950mm-5V3A-12mm-TR-/181297125419?pt=LH_DefaultDomain_0&hash=item2a3626982b) or perhaps even cheaper elsewhere - inexpensive enough to experiment. The sensitivity to reflectivity is extremely sharp, permitting a clear distinction between different surface types. – Anindo Ghosh Feb 05 '14 at 07:52
  • @AnindoGhosh I actually had a couple similar, smaller IR sensors laying around and tried those and there was a difference between the 3 types but a lot of the time there was not enough of a difference between the plastic and glue to get a good reading to distinguish them apart.... – Friendlyghost89 Feb 06 '14 at 18:20
  • I did however purchase a few different laser diodes and optical lenses that I am going to try and bounce the beam back at an angle and measure the ambient light reflected as well as the intensity of the beam with a photovoltaic cell and a photodiode.... Any extra tips/tricks with that approach would be appreciated.... I did some visual tests with a basic red laser and there was a distinct difference on the reflected beam (the paper was reflected with alot of the beam scattered as ambient light, plastic was a good reflection and the glue was in the middle).... – Friendlyghost89 Feb 06 '14 at 18:25
  • Does it show up when illuminated by near UV ie a "UV" LED flashlight? –  Feb 26 '14 at 15:20
  • @DirkBruere I have not used a UV light in order to check it, However i have noticed that when reflecting a laser off of the surface the bounced beam has specific patterns that are repeatable for each surface type and when there is minimal of that type showing or if that is all that is being reflected. – Friendlyghost89 Feb 26 '14 at 19:14

2 Answers2

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An alternative to the laser could be to use ultrasonics measure device, there are special US devices (usually with a US array) to mess the thickness of fluid-film and its distribution.

Here an example to mess the thickness of oil-films.

Alf
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Is your plastic opaque? Perhaps you could put a UV reactant in the glue and shine UV on it and detect the fluoresce with a photo diode/resistor?

EkriirkE
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