The Modes In Liquid Crystal On Silicon (LCOS)

The Modes In Liquid Crystal On Silicon (LCOS)

Nov 25, 2017

1.pattern classification

LCOS The patterns of selection can be divided into two categories,The NW and NB display modes include NW:

Mixed twisted nematic model (MTN)

Self compensated twisted nematic mode (SCTN)

Twisted electron controlled birefringence (TN- ECB)

Electrically controlled birefringence constant white mode (ECB- NW);

NB display mode

Includes: mixed field effect mode (HFE)

Reflective twisted nematic mode (RTN)

Vertical array mode (VA)

Electrically controlled birefringence normal black mode (ECB- NB)

It is worth, no matter what the mode of liquid crystal display mode design parameters are the same, namely liquid crystal twist angle psi, the direction of the incident polarized light and near the liquid crystal layer orientation angle beta and optical retardation of delta lambda ngd/. This model does not apply to the phase compensation sheet, parameters we (PSI, beta, delta lambda ngd/) form, calculated in accordance with the 550nm wavelength. We will introduce them according to the display sequence of off state normal and normal black.

The similarities between the MTN mode and the conventional TN mode are high contrast, low dispersion and low driving voltage. The difference lies in the low optical delay, the beta angle and the wide viewing angle. The distortion angle of the MTN mode is below 90 degrees, and the driving voltage and reflectivity decrease with the increase of twist angle.

2.Introduction of display mode

The monolithic LCOS color display, and the driving voltage is changed at the same time, in order to maintain the appropriate color, at the same time in order to make the RGB three primary colors can be used in the same LCOS, we must make the color as far as possible is less than 90 DEG MTN (90 DEG, 20 DEG, 0.25) is a good choice, although polarization state 90 conversion rate is the lowest, but the boundary layer perpendicular to each other, open state phase compensate each other, so the black state driving voltage is relatively low, can reach good contrast Figure 1 shows the test curve of 90 DEG MTN, the light source is quasi white light, through the photoelectric conversion is given the overall electro-optical properties the three primary colors RGB wavelength here driving voltage of the electro-optical characteristic curve is the basis of gamma corrected LCOS control circuit, while the polarization conversion efficiency is low, but the reflectivity difference off state RGB three primary color is not poor, this It is very beneficial to single chip display.


SCTN (60°, 30°, 0.35) self compensation characteristics of twisted nematic mode is off state on state voltage low dispersion is serious, figure 2 shows the simulation for the three chip electro-optic curve projection display, this model is very suitable for RGB three primary colors to choose different optical anisotropy, red light, blue light minimum you can get off state, good reflectivity compared with influence on state pretilt angle on SCTN substrates is larger, the friction angle deviation of high impact on the MTN on the basis of this, (70°, 30°, 0.31) is proposed, this model is the optimization of the SCTN dispersion will be slightly better than the 60°.


TN- ECB (63.6°, 0°, 0.192) has high off state reflectivity, low dispersion, wide viewing angle of thin box and good time response characteristics are the point of this mode.

Because there is no self phase compensation, the liquid crystal molecules on the boundary is difficult to be driven, so that the on state voltage high ECB- NW (0°, 45°, 0.14) simply using electrically controlled birefringence principle, is characterized by high reflectivity and quick response, but because of the driving voltage is very high and very low contrast 5V as shown in Figure 3.


3.Introduction of display mode of normal black

HFE (45°, 0°, 0.53) mode is close to RTN (54°, 0°, 0.52). The reflectivity of RTN to HFE is about 7% higher, color cast, thick box width range, the contrast is very similar, the two display mode has the advantages of low driving voltage, but because of the delay amount is too large, slightly longer response time. It can be seen from Figure 4 that the reflectivity of RGB three primary colors is not synchronized with the voltage variation curve, which is not suitable for single chip display.


VA (0°, 45°, 0.25λ-0.5λ ) liquid crystal molecules perpendicular to the substrate direction, anti parallel friction this mode has thick box width range of high contrast and low dispersion characteristics of good, but compared to HFE and RTN two modes, the driving voltage was increased, as shown in figure 5.


Figure 6 shows the normally black type electrically controlled birefringence mode ECB- NB (0°, 45°, 0.27). The reflectivity is very high, the driving voltage is lower than that of HFE and RTN, but the dispersion is serious, the contrast is low, and the photoelectric curve is greatly affected by the box thickness.