VA Mode Nematics

VA mode – Vertical Aligned Nematics

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

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Ultra Dark Variable Attenuators

A thermally stable dark state with 9,000:1 contrast ratio. A zero volt dark state tunable to maximum retardance. A good analog device requiring no compensation films.

Microscopy Imaging

VA Mode Liquid Crystal Devices
ultra dark variable attenuator

BVO VA mode components for ultra dark contrast in cellular biology imaging Sub Heading

Precision VA Mode LC Devices
BVO Developmental Liquid Crystal Lab

Glass Eagle XG Display Glass or Polished Fused Silica
Thickness 1-2mm thick: Thin devices made with standard display glass
6-8mm thick: Low wavefront distortion devices made with fused silica
Outer Dimensions Maximum Size – 90mm | Minimum size – 12mm
Clear Aperture (CA) 2mm internal gasket & 2mm landing
Buffing Anti-parallel
Transmitted Wavefront Distortion Polished glass devices: .25 waves over clear aperture
BBAR Coatings Optional coatings
Contrast Ratio 9000:1 uncompensated
Switching Speed Low as 20-40mSec, 10-90% full cycle (function of cell gap thickness)
Bandwidth Visible or Near infrared
Temperature Range 10˚ C to 60˚ C
Damage Threshold 500 Watts/cm2 CW
.5 Joules/cm2 10nsec pulses @ 1064nm typical
VA mode precision components are the highest contrast ratio devices available. The benefits of VA Mode LC devices are a thermally stable dark state and an ultra extinction ratio over wavelength bandwidth. Liquid crystal attenuators provided with appropriate visible linear polarizers. NIR devices require two cells and optional NIR linear polarizers.

“off the shelf” Products – VA Mode Attenuator

Ultra Dark Variable Attenuator

25.4mm x 26.2mm x 1.75mm

Active area: 22.8mm x 21.8mm

Ultra dark attenuator delivers exceptional visible light contrast

  • 9,000:1 contrast (polarizer limited)
  • supplied with crossed polarizers
  • Polarizer transmission axis 0˚ to edge
  • Thin device, 1.75mm thick with polarizers
  • Rise Time 18 mSec
  • Fall Time 23 mSec
  • ON state, 30 volts
  • DARK state, 0 volts
  • Halfwave retardance for visible bandwidth 
  • Double cell for NIR bandwidth
  • No compensator films required

Voltage vs Transmission

Standard transmission vs voltage curve of a VA Mode ultra contrast attenuator. Click to enlarge.

volume manufacturing

   • 1mm thick cells, custom design mass production 

   • large panels up to 450mm x 350mm

   • aoi , contrast enhancement films value add

   • AR coated caps optional

   • high speed shutters

   • custom flex cable

   • lowest cost option

OEM volume components leverage the power of numbers to decrease cost. BVO builds proof of concept prototypes, verifies cell integration and after approval moves into production. BVO and customer work together to meet cost target.

Liquid Crystal Specifications & Drivers

Liquid Crystal Cell Drive Specs

  • 2 KHz carrier frequency, square wave
  • AC Drive, 0-30 Volts RMS, 50/50 duty cycle
  • DC Drive, Bipolar DC-balanced square wave
    Zero DC bias between +/- voltages required
  • Very low current devices, 15 milli amps Typical
  • Low capacitance, 35 nano farads Typical
Don’t Let This Happen To You – Zero DC Bias is Essential !
Optical Finesse LC Drivers


Click to learn more about Optical Finesse Liquid Crystal Drivers. BVO’s recommended driver manufacturer.

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