Which optical sensor to detect polypropylene wipes pass by?
A carefully calibrated noise-rejecting transmission setup probably could detect the attenuation of a typical wipe between the emitter and detector, but there's likely a better way.
Instead of transmission, use reflection.  Put the light source and detector on the same side, and detect the illuminated wipe.  Figure out a non-reflecting background to put behind it; something flat black and maybe a surface tilted at an angle such that any reflection would not return to the detector anyway.
If you cannot fully shield the setup from outside light, then you should make it reject interference.  A simple method it so modulate the light source, and use a bandpass filter on the receiver (eg, how TV remote controls do it).  But you can get a better analog result by using synchronous detection, where you alternately turn the light source on and off at a high rate and subtract the reading with the source off from the reading with it on.  Depending on needed dynamic range you can do this either in circuitry or in software after A/D conversion.