Sensor capable of detecting small seeds passing through

I need a sensor capable of detecting and counting any kind of seed, including smaller ones like canola seeds which are in the range of 1mm of diameter. The seeds will be falling through a tube, and the sensor should be at the bottom of it. I dont know if normal IR sensors have beams that are narrower than 1mm, or if there is any way in which I could adapt the receiver so that it could detect such small seed.

If there is any other kind of sensor capable of this that I am missing, let me know. Also it si very important that the trajectory of the seed ramains untouched after detecting it.

Bounce them off a piezo, and count the pulses. That's the most reliable way. You will of course affect the trajectory so maybe you should move the counting to before they go into the tube (that way you still know exactly how many come out the other end). Another way would be a high speed camera, possibly combined with a strobe for lighting, and high end image processing to follow the seeds as they fall through the image to avoid double counting.

Buy one of these. Claimed to be able to count canola (rapeseed) seeds.

Nice one.

But it raises the question: what is the actual count rate OP is looking for? That optical sensor can do no more than 122 seeds per second, and presumably they must pass the optical field one by one with sufficient separation between two seeds. A bounce plate can do higher rates with less separation.