The Model 57 Ballistic Screens detect projectile passage through a reference plane. An infrared light source is mounted at the top of the screen and multiple photodetectors are mounted in the base. The screens will normally detect projectiles as small as .17 caliber at velocities as low as 250 fps. Sensitivity improves with larger projectile and higher velocities. Automatic gain control is included to compensate for effects such as dust in the light path.
The screens will operate under reasonable conditions of ambient light from darkness to sunlight. The housing protects the photodetectors from external fluorescent lights, but the screens may be triggered by abnormally bright muzzle flash or ignited tracers. Special precautions may be required for operation in adverse environments.
The screen provides a nominal +12 volt pulse of approximately 2 ms duration from a panel mounted BNC connector. The output can be approximated by 50 ohms to +12v in the high state and by 10K ohms to ground in the low state. Rise time is approximately 0.1 microsecond. More than one pulse may occur for the passage of a single projectile.
For any projectile velocity less than 1200 fps or for automatic fire, you must place a wooden blast shield midway between the muzzle and the first screen. The shooting hole through the center of the baffle should be as small as is convenient. The hole need not be square or round; a vertical rectangle is often convenient and effective.
The units are normally shipped set for operation with 120 volt ac power. Either 50 or 60 Hz is acceptable. Each unit can be readily changed to 240 volt operation. The included line cord has the standard US three-conductor grounding plug; user must supply line cord for other local standards.
There is a line of 72 infrared emitting diodes in the top of the unit. These infrared diodes are arranged in 12 series strings. Each string includes six IR diodes plus a red diode visible from the front of the unit. The illumination of a red diode assures that the associated infrared diodes are operating.
Oehler can provide Model 57 Screens modified for better operation at lower velocities with larger projectiles such as sporting balls. Such modification is required for use in testing coefficient of restitution.