Vertically Aligned Nematic Liquid Crystal Devices
- Ultra high contrast ratio devices
- Thermally stable dark state
- Variable attenuator – gray scale control
- Two stage device for NIR bandwidths
- OEM prototyping
- Small volume precision LC optics
- Large volume inexpensive LC optics
- Very low current devices (milli amps)
- 0-25 Volts RMS, AC 2 KHz carrier frequency, square wave or
- 0-25 Volts DC Driving, Zero DC bias required between + and – voltages
- Low capacitance (nano farads)
- 90% internal transmittance
VA Mode Nematics | Summary
Vertically aligned LCD’s are best known as homeotropic liquid crystals with the LC molecules aligned perpendicular to the glass substrate surfaces. The black mode has crossed linear polarizers on the outside substrates to achieve extremely high contrast ratios with no voltage applied and is the most common device configuration. As voltage is applied, the molecules rotate from vertical to horizontal in relation to the glass substrates achieving grey scale light amplitudes. The normal black mode is highly achromatic as the LC essentially has zero birefringence and homeotropic LC devices are considered to have the highest contrast of any LC mode. This configuration’s dark state has the additional benefit of having high temperature stability. The final contrast level is defined by the quality of the linear polarizers.
Liquid crystal fluids that exhibit negative dielectric anisotropy are required for this mode and special alignment fluids. VA cells can be manufactured uniformly in mass volumes with thin glass. If low transmitted wavefront distortion is required, precision cells can be manufactured with polished glass substrates.
VA Mode Device Applications
Ultra High Contrast Variable Attenuators
5,000:1 contrast ratio possible with high performance polarizers. No compensation film required
VA Mode Nematics | Precision LC Devices
VA mode 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 VA mode 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 Anti-parallel Transmitted Wavefront Distortion .25 waves / 25mm or better over clear aperture BBAR Coatings BBAR VIS (400-900nm),BBAR NIR (900-1700nm) Internal Transmittance 400-450nm – 85% (ITO absorption), 450-1000nm – 92% Contrast Ratio 5,000-10,000:1, no compensator required Switching Speed Low as 30mSec, 10-90% full cycle (limited by LC rotational viscosity) 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
VA Mode Nematics | 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.VA mode 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.