Twisted Nematics Liquid Crystal Devices
- Most common display LC mode
- Most achromatic variable attenuator
- Analog grey scale control
- Visible / NIR bandwidths
- OEM prototyping, pre-volume production
- Large volume – inexpensive LC optics
- Small volume precision LC optics
- Very low current devices (milli amps)
- 0-10 Volts RMS, AC 2 KHz carrier frequency, square wave or
- 0-10 Volts DC Driving, Zero DC bias required between + and – voltages
- Low capacitance (nano farads)
- 90% total internal transmittance
Twisted Nematic | Summary
Twisted Nematic (TN) liquid crystals are the most common type of LC’s used in the display industry. TN cells are basically 90° polarization rotators. As linear polarized light enters the cell it rotates along the LC helical structure formed from the front glass substrate to back substrate. TN cells can be made in either a normally black mode or normally white mode. Normally black mode is a dark state with zero voltage applied with polarizers mounted to the outside of the substrates in parallel to each other. As voltage is applied light transmission increases as a function of voltage. Normally white mode has maximum transmission with zero voltage applied with the polarizers orthogonally mounted on the glass substrates. The light is attenuated as voltage increases. This mode achieves higher contrast as the dark state takes advantage of the crossed polarizer configuration.
TN’s advantages are low voltage requirements and they operate over a large wavelength range. TN 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.
TN Device Applications
Achromatic Variable Attenuator
Normal white mode 1000:1 contrast ratio capable over 200nm band (450-650nm). Greater than 500:1 contrast maintained over wide wavelength range. TN devices can be optimized for NIR applications.
Twisted Nematic | Precision LC Devices
TN 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 TN 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 Orthogonal, front 90˚ to back 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 500:1 uncompensated Switching Speed Low as 5mSec, 10-90% full cycle (visible bandwidth) 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
Twisted Nematic | 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.Twisted nematic (TN) 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.