Dichroic displays

Other dichroic display types to improve the viewing angle of standard TN-cells can be found in the literature. The most popular are dichroic phase change effect displays or White-Taylor mode displays [21 ] and Heil-meier type dichroic LCDs [20, 22, 23]. 

Figure 1. Operational principle of a dichroic display (Heilmeier display).

With regard to the dye-related parameter measurements, the dye concentration is usually kept at about 0.5-2%, depending on its absorption coefficient. For azo dyes the dye concentration is about 0.25-1% for dye doped TN, about 2-4% for Heilmeier displays, and about 2-5% for phase change displays. The cell thickness is usually about 5 pm for dye-doped TN at the first Gooch Tarry minimum [ 15,16,88], about 7-10 pm for dye-doped TN at the second Gooch Tarry minimum [15, 16, 88], about 5-15 pm for Heilmeier, and about 10-20 pm for phase change dichroic displays. Usually, the dye concentration is increased by a factor of about 1.5-2 when anthraquinone dyes are used instead of azo dyes, as anthraquinone dyes have less absorbance than azo dyes. Sometimes combinations of azo and anthraquinone dyes are used [16, 54]. 

Polarizers, color filters, and reflectors are usually mounted outside the cell, as dichroic displays do not have very high line reso-... 

As dichroic displays provide a man-machine interface through human eyes, their features and parameters must be optimized for human vision requirements [75]. For a clear understanding and detailed description of human vision, color and its measurement, visual requirements, and reliability issues of LCDs, the reader is referred to the article by Bahadur [75]. Some of the issues... 

Figure 16. Threshold characteristic of a phase change dichroic display ...

Dichroic displays presently used in avionics have a reflective contrast ratio of >25 1 and a transmissive contrast of about 6 1 with a brightness of over 18% at a 45° viewing angle. These displays operate from -30° to +85 °C. The low-temperature operation is assisted by a heater. 

The contrast ratio of the phase change dichroic display is found to decrease with an increase in temperature (see Fig. 22). This decrease is basically caused by the reduced absorption in the quiescent state that results from the decrease in the order parameter and pitch of the dichroic mixture at higher temperatures. The cells show faster switching speed and lower memory at higher temperature. The switching becomes slow at lower temperature due to an increase in the viscosity of the mixture, and a heater is usually required for low temperature operation. 

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