Cell Heilmeier displays

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]. 

The 180° twisted Heilmeier displays show switch-off behavior similar to r-cells. It takes a long relaxation time to return to the quiescent state after a very fast decay to an intermediate level. This long relaxation is undesirable, and a 180° Heilmeier shutter can function reasonably only after using a holding voltage similar to that of r-cells. 

Figure 10. Effect of cell gap on the threshold characteristic and contrast of a Heilmeier display. The trans-mittances of cells with 5, 8, and 11 im cell gaps are 23.6%, 20.0%, and 16.0%, respectively. Operating voltage, 10 V.

For monochrome applications, one can either put a monochrome color filter inside the cell or use monochromatic back-lighting. For Heilmeier displays, there is another way to get a monochrome display by using monochromatic pleochroic dyes [26]. For full-color display, color filters must be used. Heilmeier displays exhibit better color performance than TN displays, as the chroma-ticity of the pixels does not change much at large viewing angles. 

Recently, Ong [126] proposed a Heilmei-er display having two asymmetric cells. This display has a high contrast and wide viewing angles, but low transmission. One can also use two separate Heilmeier cells with different dyes to make a composite cell, and by controlling voltage differently a set of colors can be obtained. Two cells with different dyes and one polarizer, can also be used. 

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