1992–present: Microdisplays

 

Although the successful production of active-matrix displays using thin-film transistors (TFTs) beginning in 1989 revolutionized direct-view display technology, LCOS devices provide a higher-performance solution for projection and near-eye applications. Figure 13 shows an early Displaytech 256 × 256 SLM operated as a color-sequential microdisplay in a 1993 company-internal demonstration. Figure 14 (Handschy, 1996 SID Digest; Banas, J. SID) compares the same device, illuminated by red, green, and blue LEDs, to a conventional transmissive triad-color LCD.

Figure 13

Figure 14

In 1997 JVC reported development of a 1365 × 1024 d-ILA (for "direct-drive image light amplifier") device (Figure 15, Nakano Proc. SPIE 3296). It utilized an analog, one-transistor pixel, with a metal light-shield layer below the pixel mirrors to prevent photo-induced discharge of the pixel capacitor. The d-ILA device achieved a high contrast ratio through use of the vertically aligned nematic (VAN) mode, marking the first instance of what has become essentially the standard nematic LCOS mode.

Figure 15

At about the same time IBM reported development of a 2048 × 2048 LCOS panel, which they used in a three-panel engine to demonstrate a 28-inch diagonal rear-projection computer-monitor prototype, as seen in Figure 16 (Melcher, IBM J. of Res. and Dev. vol. 42). Their LCOS device featured a 17 μm pixel pitch, used the 45°-twist normally-white nematic LC mode, and notably relied on spacer posts photolithographically defined between the pixel mirrors, as shown in Figure 17, to maintain the LC cell gap (Colgan, IBM J. R&D).

Figures 16 and 17

Sony reported in 2000 the development of a very high resolution color-sequential LED-illuminated 1920 × 1200 FLCOS microdisplay. Figure 18 (Akimoto, 2000 SID Digest) shows a displayed image (a) along with the "bitplane" digital grayscale technique used to achieve a gray-scale depth of 10 bits/color.

Figure 18

The next year Displaytech began commercial shipments of a color-sequential 320 × 240 (quarter-VGA) FLCOS electronic viewfinder (EVF) product (see, for example, dpreview.com).

While the Displaytech and Sony FLCOS devices used digital gray scale, the above JVC and IBM nematic LCOS devices used analog gray scale. In 2004 JVC changed tack, and reported an all-digital nematic LCOS device, for rear-projection televisions, as shown in Figure 19 (Shimizu 2004 SID Digest, JSID), which also shows the bitplane gray-scale scheme. JVC began offering RPTVs that same year.

Figure 19

The following year (2005) Sony reported its version of LCOS, "SXRD," with 1920 × 1080 and 4096 × 2160 resolution panels (Hashimoto, 2005 SID Digest). Again, this panel used the VAN mode, with analog gray scale.

 

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