Lasers can help you visually see the vibration that your motors and props are creating. By reflecting a laser off of a mirror that's attached to your plane, tricopter, quadcopter, or multi-rotor you can easily see and and correct balance and vibration issues!
The effects of an unbalanced prop or motor can be seen with your FPV with the onboard camera causing a jello-like, wavy, warped video playback. You can also sometimes simply feel the vibration by touch as well.
The idea is simple, point a laser towards a mirror which is attached to your multirotor arm, plane or where your onboard camera is setup. Reflect that laser off of the mirror and onto a wall.
The dot that is formed on the wall while the engine is off is what you want to match when your props are running. Unbalanced props will result in wavy or elongated deformation of that laser dot reflection.
Add or remove tape from your prop and try again until the vibration is to your satisfaction.
This is a great way to test the vibration of any area of your RC plane or multi-rotor, attaching the mirror to any place you want to 'see' the vibration.
We got the idea from this youtube video: