Introduction
This page provides details on the various laptop display technologies used with DOS laptops, portable and portable computers. For completeness this should be read in conjunction with the graphics cards page as well as my CRT Monitors page.
These days we take it for granted that our modern laptops have color high-resolution screens. When manufacturers were trying to make an IBM PC or compatible more transportable and lightweight, installing a cathode ray tube (CRT) miniature monitor in a case made it prohibitively heavy. CRTs were relatively cheap despite their weight, so using smaller CRTs in their portable machines was an obvious choice. The first portable PC – the compaq portable range – also used a CRT and IBM itself used one in its first portable ibm portable,
After only a few of these giants, the PC market turned to liquid crystal display (LCD) technology, and while large LCD screens were extremely expensive to make, as the popularity of mobile computing grew, prices fell, and so LCD use stuck more or less in some form throughout the DOS era.
Liquid Crystal Display (LCD)
LCD technology was another technology that first appeared in the 1960s. It was highly energy-efficient and required little space, so it was suitable for mobile devices. Since LCDs don’t produce any light themselves (unlike gas plasma), they actually need a backlight (literally, light shining from the back of the screen) to make the screen more readable. Backlit LCD displays didn’t really start appearing until 1988 – before then your laptop computer’s LCD display was like a big calculator screen, using nothing more than a reflective layer behind the liquid crystal to reflect ambient light! There was also the concept of ‘sidelit’ displays where instead of shining light from behind the screen, there were lights on each side of the display.
Unfortunately, even with backlight, LCD displays have poor contrast and slow refresh rates, which produces a “ghosting” effect whenever the displayed content scrolls or moves.
Toshiba T1100 with its monochrome LCD display (1985)
gas plasma
Gas plasma displays were first used in the 1960s. In a gas plasma display, each pixel is illuminated by a small portion of plasma (charged gas). Gas plasma displays are thinner than CRTs, and brighter than LCD displays due to not requiring an electron gun at a certain distance from the back of the screen. The gases used in gas plasma displays are neon and xenon, both inert, and these are sandwiched between two plates that have been coated in a conductive print – one of which has vertical lines, and the other of which has horizontal lines. So together they form a grid. When an electric current is passed through the conductive prints on these two plates, the gas at the point where they meet glows, which is seen by the user as the burning of a single pixel. Despite being particularly bright and producing a good clear image, gas plasma displays use a lot of power, making them quite unsuitable for portable computing, but several years later it made a comeback with flat-panel TVs!
Orange gas plasma display from the Toshiba T3200SX (1989)
Supertwist Nematic (STN) Display
Invented in 1983, the STN is a type of monochrome passive-matrix LCD display. The words “passive” here mean that each pixel must maintain its state (off or on) without active driving circuitry until it can be refreshed again. In twisted nematic (TN) displays an electric field is applied to the liquid crystal molecules to cause the molecules to be either turned off (twisted 90 degrees, electric field off), or on (untwisted, electric field on). Sadly, due to the limitations/limitations of passive-matrix addressing, TN displays could only be so large. In Supertwist nematic displays, the molecules are rotated 180 to 270 degrees allowing more rows and columns, hence a higher resolution display.
All STN displays were monochrome until NEC launched the first color laptop Prospeed CSXIn October 1989. Many other laptop manufacturers were working on color screen technology at the same time, so many more color laptops came to market in the 1990s.
One advantage of STN displays is that they do not require a backlight – they are still readable under direct sunlight with a reflective layer behind the display. Unfortunately, because they are still passive matrices, the ghosting effect seen with LCD monochrome displays is still present.
Compaq LTE Elite 4/40c with color STN display
Double Layer Supertwist Nematic (DSTN) Display
To overcome the slow refresh rate of STN screens, DSTN technology divides the screen into two halves, where each half refreshes simultaneously, making the overall refresh rate much faster.
Sadly, DSTN still suffers from the same inherent problems of any passive-matrix display, such as low contrast (typically DSTN displays have a contrast ratio of 15:1 compared to TFT displays which are 40:1 or better), and washed out colors.
The Toshiba Satellite Pro 400CDS with its color DSTN display
Thin Film Transistor (TFT) Display
TFT, also called “active-matrix” displays, arrived in mid-1992, and was available in both monochrome and color, although most laptop manufacturers stopped selling monochrome TFT laptops by 1994 due to lower prices for color displays.
TFT displays work based on each individual pixel being turned on or off by four transistors. The “active” in the name is because each pixel also has a capacitor that actively maintains pixel state. Despite being much more expensive than STN/DSTN displays, they overcome all the problems of passive-matrix displays. They have very high contrast ratios and fast refresh rates.
A 1996 Compaq LTE 5200 with color TFT display
The maximum display resolution of early TFT displays was 640 x 480 – if you wanted to run Windows in higher resolution you could do so by attaching an external monitor to the laptop. By 1996 SVGA resolution of 800 x 600 was becoming common for TFT laptops, and top-end ones even supported 1,024 x 768.
TFT technology is still widely used on modern laptops.