What sensor is used to judge tansparency?

I want to create the system to separate between aluminum can and plastic bottle

both are light weight so, I think of using the transparency, meaning that the can is not transparent but plastic bottle is transparent

Will the light sensor works out?

or Would you suggest me which type of sensors I should use?

Thanks in advance

Will the light sensor works out?

Probably.

You will need a source of illumination, a light sensor and perhaps shielding to prevent interference by reflected room lighting.

There would be a squillion garbage separators that have already solved this problem - if it is reliably solvable. The amount of video around showing low-wage humans toiling over high-capital conveyor belts suggests that it isn’t. I’m guessing these installations are not actually devoid of sensors, they probably have more than one type, and none of them use light. But I could be wrong, and humans do it all.

I've seen some thing that could separate non-ferrous metals from non-metal trash (after ferrous was removed by a magnet) using the Lenz effect. Some kind of big spinning magnet wheel and as the stuff flowed over the end of it the aluminum would veer off to one side a little.

See this page. Watch the video in the middle of the page. https://terpconnect.umd.edu/~wbreslyn/magnets/is-aluminium-magnetic.html

Aluminium (and other conductive but non-magnetic metals) can be separated by using eddy currents. Basically a high frequency electrical field creating electrical currents and with it a magnetic field in the metal, which shoots the material away from the rest. That's the common way of sorting trash.

On the other hand, sorting plastics by type (can be done reasonably reliably by looking at the product) is still very much a human task. For example bottles come in four kinds of plastic. HDPE, commonly used for milk bottles (both the semi-transparent ones and the white ones), PP (often used for shampoo bottles), PET (the crystal clear water and soda bottles) and PVC (still used for water - also crystal clear - the difference is that PVC will turn milky white when creased and PET bottles have a little teat at the bottom because of the different way they're produced).

OP didn't specify the source and quality (cleanliness, etc) of the bottles, not sure if they're looking at sorting trash or just distinguishing between two bottles which is a very different thing.

Place a bottle between a light source and a light sensor and you'll be able to tell the difference between a transparent (PET or PVC) and a non-transparent bottle (aluminium, HDPE, PP), but you can't say whether the non-transparent bottle is aluminium or plastic that way.

Anyway, OP, please clearly define your problem. What exact kind of bottles? In what form? (neatly placed or trash on a conveyor) How many/how fast? How reliable?

And whether the plastic bottles have labels....

And of course many soda bottles aren't clear: ginger beer tends to be dark brown, and some brands of lemon type drinks are in yellowy green bottles.

But that said, the OP mentioned transparency and he presumably knows what he's dealing with.

Simple enough to test: hook up an ldr as explained here and get your torch out ;)

kenwood120s: But that said, the OP mentioned transparency and he presumably knows what he's dealing with.

After joining this forum I very quickly learnt not to make that assumption, especially when talking about poorly defined objects such as "bottles" which come in so many different shapes, sizes, types of material (PET is transparent to IR, PVC not, while both are transparent to our eyes), etc.

sorasiri: or Would you suggest me which type of sensors I should use?

An inductive proximity sensor

Come to think of it, capacitive should work as well then.