Detecting metals would be easier than distinguishing paper and plastic.
You'll need to provide details if the application. Is this for sorting or separating recyclables from waste streams? If so, that's a mostly solved problem based on what I've seen in recycling facilities.
I saw a documentary on TV that showed a waste sorting facility in the North East USA where the town folk didn't need to sort their waste; the facility did that for them. It was not based on sensors distinguishing. It was based mostly on the physical properties (density), as well as magnets and eddy currents. They did have sensors to sort the different colors of glass. Everything else was sorted mechanically, blowing air, magnets, or eddy currents for the non-ferrous metals. I think that they did have some humans as well. It was impressive because the residual waste stream to be land-filled or composted was tiny.
Unfortunately for some areas waste sorting facilities are not available. Can I ask you another question? Do you know how optical sensor works? Do you think its possible to detect paper and plastic with that?
A vision system could be programmed to recognize certain common objects. The variability of paper vs plastic probably overlaps too much.
I'm not sure how a sensor could distinguish between a sheet of paper and a similar sheet of plastic. The Mars Rover has a laser that can fry a sample many meters away and then read the spectrum to determine chemical composition. Obviously anything is possible, but practical?
Some places shed everything, soak the paper waste to make pulp, and then skim off the plastic bits that got through.
With sufficient complications, SOME plastics can be separable from paper, if thin enough and transmissive enough. Many plastics are highly transmissive in the near and short wavelength infrared (NIR/SWIR). Polycarbonates, for example are nearly transparent above 800 nm
TTFN
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Terahertz (T-Ray) sensors are excellent for characterizing non-conductive (i.e. dielectric) material properties such as thickness, basis weight, density, etc. A bit overkill for sorting though.
