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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.

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    \$\begingroup\$ Suggest you investigate if you can get better contrast using other than visible spectrum. IR is sometimes convenient. \$\endgroup\$ – Spehro Pefhany Feb 4 '14 at 14:37
  • \$\begingroup\$ Not one mention of EE - is it an EE question? \$\endgroup\$ – Andy aka Feb 4 '14 at 14:48
  • \$\begingroup\$ White light source and a photodiode may be all you need. Or you might have to go all the way to a reflectance spectrometer: google.com/… \$\endgroup\$ – Wayfaring Stranger Feb 4 '14 at 14:51
  • \$\begingroup\$ @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. \$\endgroup\$ – Friendlyghost89 Feb 4 '14 at 15:06
  • \$\begingroup\$ @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... \$\endgroup\$ – Friendlyghost89 Feb 4 '14 at 15:09
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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.

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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?

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