Fiber optic ballistic chronometer

From PDI

Ballistic chronometers are used to determine the velocity of an object (sporting item or any projectile) by detection of the object usually at 2 points, at a fixed distance, and determining the time required for the projectile to pass the 2 points.

The detection can be made via thin wires (the screen is broken, cutting conduction), electromagnetic induction (metal detection), or optically (ambient light, incandescent source, LED, laser).

Each method has advantages and disadvantages...wire screens need replacement after each use. Electromagnetic doesnt work with nonferrous items. Optic methods can have detection problems due to the cone shaped area of detection, which can create poor sensitivity and small area of detection.

I propose a fiber-optic based detection to create a long yet narrow area of sensitivity for the photodetectors. The fiber optic method can use a bundle of fiber optics, connected at one end to a photodiode or phototransistor, and at the other end is spread out in a linear array or row to detect the passage of the shadow of the projectile. The bundled, photodetector end would be light-shielded so only that light entering the linear array is detected.

A one-piece, molded plastic "light pipe" can be used in place of a bundle of fiber optics but accomplishes the same objective.

The light source above the linear photodetector array could be ambient, incandescent, LED, or laser with a line-forming lens attached so that the light is optimally covering the linear array only.