Version 1.0 July 09 1999
by
Salman Saeed
Reflective displays rely on ambient light for information display and hence are ideal devices for portable electronic equipment as they do not require the power hungry backlight illuminator. However reflective displays suffer from inherent difficulties in producing high contrast and high color images with adequate resolution. Some of the newer reflective technologies however have made substantial headway into overcoming these problems, especially the full color issue. Japanese companies such as Sharp introduced the 260,000 color high resolution TFT display (HR-TFT) in 97, and Matsushita the full color (8 bits of grayscale) TFT polysilicon display.
There are a number of reflective display technologies, incorporating modes such as ECB, STN (double and single polarizer), BTN, HAST (Casio), TFT, LTP, HR-TFT, OCB, HAN, Heilmeier type GH, Double layer GH, PCGH, PDLC(GH), PDLC, PSC, HCR. Contrast ratios for most of these devices range between 5-15 with colors ranging between 4 and 260,000.
In my opinion the attractiveness of reflective displays for portable computers is not as great as it was a few years ago mainly due to the fact that the electronics/datastorage devices eat more power than the display now. Through 1995 power consumption of portable computers was nearly equally divided between the electronics/data storage devices and the display. In the years since power consumed by high speed CPU’s, DRAMS and extra capacity HDD has increased rapidly such that now power consumed by the display accounts for less than a quarter of the total power usage. Under these conditions the use of reflective displays reduces the total power consumption by ~ 15%, still a significant amount but not of do or die proportion 54.
However in the case of bistable displays or small displays such as handheld PDAs and cellphones etc, power is only needed when driving the display and for displays which don’t require frequent updates the power savings can be substantial.
Some of the more popular modes currently being used in reflective devices are: