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A monochrome monitor is a type of CRT computer monitor which was very common in the early days of computing, from the 1960s through the 1980s, before color monitors became popular. They are still widely used in applications such as computerized cash register systems, owing to the age of many registers. Green screen was the common name for a monochrome monitor using a green "P1" phosphor screen.
Unlike color monitors, which display text and graphics in multiple colors through the use of alternating-intensity red, green, and blue phosphors, monochrome monitors have only one color of phosphor (mono means "one", and chrome means "color"). All text and graphics are displayed in that color. Some monitors have the ability to vary the brightness of individual pixels, thereby creating the illusion of depth and color, exactly like a black-and-white television.
But typically only a limited set of brightness levels was provided to save display memory which was very expensive in the '70s and '80s. Either normal/bright or normal/dim (1 bit) per character as in the VT100 or black/white per pixel in the Macintosh 128K or black, dark gray, light gray, white (2bit) per pixel like the NeXT MegaPixel Display.
Monochrome monitors are commonly available in three colors: if the P1 phosphor is used, the screen is green monochrome. If the P3 phosphor is used, the screen is amber monochrome. If the P4 phosphor is used, the screen is white monochrome (known as "page white"); this is the same phosphor as used in early television sets. An amber screen was claimed to give improved ergonomics, specifically by reducing eye strain; this claim appears to have little scientific basis.
The 5151 was designed to work with the PC's Monochrome Display Adapter (MDA) text-only graphics card, but the third-party Hercules Graphics Card became a popular companion to the 5151 screen because of the Hercules' comparatively high-resolution bitmapped 720×348 pixel monochrome graphics capability, much used for business presentation graphics generated from spreadsheets like Lotus 1-2-3. This was much higher resolution than the alternative IBM Color Graphics Adapter 320×200 pixel, or 640×200 pixel graphic standard. It could also run most programs written for the CGA card's standard graphics modes. Monochrome monitors continued to be used, even after the introduction of higher resolution color IBM Enhanced Graphics Adapter and Video Graphics Array standards in the late 1980s, for dual-monitor applications.
Pixel for pixel, monochrome monitors produce sharper text and images than color CRT monitors. This is because a monochrome monitor is made up of a continous coating of phosphor and the sharpness can be controlled by focusing the electron beam; whereas on a color monitor, each pixel is made up of three phosphor dots (one red, one blue, one green) separated by a mask. Monochrome monitors were used in almost all dumb terminals and are still widely used in text-based applications such as computerized cash registers and point of sale systems because of their superior sharpness and enhanced readability.
Some green screen displays were furnished with a particularly full/intense phosphor coating, making the characters very clear and sharply defined (thus easy to read) but generating an afterglow-effect (sometimes called a "ghost image") when the text scrolled down the screen or when a screenful of information was quickly replaced with another as in word processing page up/down operations. Other green screens avoided the heavy afterglow-effects, but at the cost of much more pixelated character images. The 5151, amongst others, had brightness and contrast controls to allow the user to set their own compromise.
The ghosting effects of the now-obsolete green screens have become an eye-catching visual shorthand for computer-generated text, frequently (and ironically) in "futuristic" settings. The opening titles of the first Ghost in the Shell film and the Matrix source code of the Matrix trilogy science fiction films prominently feature computer displays with ghosting green text. Green text is also featured in the Swan's computer in Lost series.
Monochrome monitors are particularly susceptible to screen burn (hence the advent, and name, of the screensaver), because the phosphors used are very high intensity. Another effect of the high-intensity phosphors is an effect known as "ghosting", wherein a dim afterglow of the screen's contents is briefly visible after the screen has been blanked. This has a certain place in pop culture, as evidenced in movies such as The Matrix.
This ghosting effect is deliberate on some monitors, known as "long persistence" monitors. These use the relatively long decay period of the phosphor glow to reduce flickering (and, therefore, eye-strain).