What Non-Contact Displacement Sensor?

I am looking for a sensor that I can buy or make from scratch to sense displacement of a very light object. The sensor must be non-contact, and the item cannot be weighted down. Ideally the technology would be optical with a sensor positioned between 1.0 and 2.0 mm away from the object that is moving past the sensor. The environment is indoor ambient, relatively clean. The motion is about 20 mm., and it need not be tracked for direction. I need high bandwidth - 50,000 samples per second. Absolute position accuracy is not needed - I just need to measure acceleration. And the moving part cannot be weighed down by an accelerometer.

Robert Scott
Hopkins, MN
Thanks for the suggestion about the laser interferometer. This particular one looks like I would violate the specs on maximum travel velocity ( 4m/sec ). And I really don't need that much accuracy.

Here is the application: I am developing a system to measure the inertia of piano keys, as referenced from the front of the keyboard. A competing system (Key Force One) does this by moving an actuator at a know acceleration profile and measuring the force exerted with a strain gauge.

I intend to do the opposite: Exert a known force with a spring-driven actuator and measure the acceleration profile. The key movement is about 13 mm. But with 400 grams of spring force suddenly exerted, the key will accelerate quite rapidly.

Since we are measuring inertia, any mass that we attach to the key will add to this inertia and have to be systematically subtracted to the get inertia of the key alone. This is OK if the added fixture inertia is not too big compared to the inertia of the key. And the smaller the fixture inertia the better.

So the portion of the spring that contacts the key is unavoidably part of the inertia, but that is OK because we can figure out how much it adds and then subtract that off from the reading. I don't want to increase it even more by a contact-based displacement sensor. But as you can see I don't really need a displacement sensor.

I need a velocity sensor, or just an acceleration sensor - but not an accelerometer because that requires attachment to the key under test.