piCell Nematic Liquid Crystal Devices
- Microsecond rise times, 3 millisecond fall
- OEM prototyping
- Welding shutters fast response times
- Liquid crystal variable retarder
- Liquid crystal variable attenuator
- Two stage device for NIR bandwidths
- Two stage visible device for 3D
- Small volume precision LC optics
- Large volume inexpensive LC optics
- Large aperture LC optics
- Low threshold voltages
- Very low current devices (milli amps)
- 0-30 Volts RMS, AC
- 2 KHz carrier frequency, square wave
- DC Driving-Zero DC bias between +/- voltages required
- Low capacitance (nano farads)
- 90% internal transmittance
pi-Cell Shutter | Summary
pi-Cells are used in applications requiring high speed switching. They are commonly used in 3D applications to avoid flicker effects, ultra fast shutters and switching components in tunable filters. The name comes from the twist of molecules, which is 180° formed by the parallel rubbing direction on each glass substrate. The fast speed is the result of fluid dynamics. When voltage is removed from a pi-cell the molecules feel very little torque to return to their low voltage state in a parallel configuration. A “holding” voltage at the desired low voltage state is required to maintain pure polarization switching. Pi-cells therefore operate between a high voltage (30V) and a low holding voltage (approx.-2V).
Parallel aligned cells also have naturally higher viewing angles due to the symmetrical molecular alignment at glass surface boundaries. High contrast is easily achieved with a film compensation layer to subtract residual birefringence at the surfaces.
pi-Cell Device Applications
Fast Liquid Crystal Shutters
pi-Cell LC shutters are constructed with crossed linear polarizers and optional compensator film (high contrast). A push-pull configuration can be manufactured incorporating two thin cavity pi-Cells creating an ultra fast, self compensating optical shutter.
Fast Liquid Crystal Variable Retarder – Wavelength Specific
Fast tuning to unique retardance for each specific wavelength. pi-Cells are a good solution for tunable filters requiring rapid response times.
Pi-Cell Shutter | Precision LC Devices
OCB (pi-Cell) precision components are handmade devices which often incorporate difficult specifications. High uniformity, faster response times or simple patterned apertures are typical upgrade requests. An example of a common precision pi-Cell LC device would have the following specifications.
Glass Fused Silica Thickness 6-8mm (no compensator) standard, optional alternatives Outer Dimensions Up to 75mm OD Clear Aperture (CA) cell outer diameter – 6mm (gaskets and landings) = CA Buffing Parallel Transmitted Wavefront Distortion .25 waves / 25mm or better over clear aperture BBAR Coatings BBAR VIS (400-900nm),BBAR NIR (900-1700nm), V-Coat Internal Transmittance 400-450nm – 85% (ITO absorption), 450-1000nm – 92% Contrast Ratio 500:1 standard , 1000:1 compensated Switching Speed Low as 3mSec, 10-90% full cycle (function of cell gap thickness) Bandwidth Visible or Near infrared Temperature Range 10˚ C to 50˚ C Damage Threshold 500 Watts/centimeter2 CW
.5 Joules/ centimeter2 squared 10nsec pulses @ 1064nm typical
Pi-Cell Shutter | OEM Volume LC Devices
OEM volume components can leverage the power of numbers to decrease cost. BVO builds proof of concept prototypes and then rolls into large volume production using overseas vendors. Result is components for much less.Optically compensated bend (pi-Cell) liquid crystal cells can be delivered as raw cells or post laminated with linear polarizers. Custom AR coated glass caps can be added to protect and increase transmission.Large volume liquid crystal components easily approach the cost of passive components making electronically tunable solid state devices appropriate for OEM systems.