Optical choppers are mechanical or electronic devices that pass then interrupt a beam of light for a known brief interval. Optical shutters, by contrast, are mechanical or electronic devices used to control the amount of time that a light sensitive material is exposed to radiation. Technologies used for optical choppers and optical shutters include liquid crystals, rotating shutters, tuning forks, etc.
Optical choppers and optical shutters are in several configurations: variable frequency rotating disks, fixed frequency tuning fork choppers, and optical shutters.
Rotating Disk Optical Choppers
Rotating disk optical choppers are used in situations where frequency, aperture size, or mark/space ratios (duty cycle) are variable. The variable frequency rotating disks resemble small fans. They have a slotted disk mounted on a motor head, and can be used at a variety of frequencies by adjusting the motor speed. An optical switch senses disk position and provides a reference output to automatically adjust the chopping frequency. The entire device is often connected to an external signal so that the optical chopper maintains the same speed and frequency as the system in which it is placed.
Tuning Fork Choppers
Tuning fork choppers, also called resonant choppers, are appropriate for optical chopping when a single known fixed frequency is desired and small size is require, or long life needed. Tuning fork optical choppers resemble musical tuning forks with small vanes mounted on the tines. They vibrate in response to an AC signal at a specific resonant frequency. Their vibration occurs at a frequency fixed by the mechanics of the structure. Fork optical choppers are extremely durable and have no wearing parts. As a result they are often used in high acceleration and vibration applications, in which they provide a long service life. Tuning fork optical choppers are versatile and are usable at extreme temperature or in a vacuum.
Optical shutters differ from optical choppers in that they are not limited to a simple periodic on-off cycle. They follow an arbitrary, varying pattern of openings and closings. Optical shutters are useful for low frequency chopping, particularly when slow behavior is desired. Optical shutters respond to a TTL input waveform. When the single vane mounted behind the aperture receives a signal, it moves; either opening or closing the aperture. The vane can be stopped in either the open or closed position.