Theory
How a screen actually shows an image
Before drawing pixels, it helps to know what the display device is doing physically. Screen technology has evolved a lot, and each type has trade-offs that shaped how graphics was done.
This lesson surveys the main video display devices: the refresh CRT and colour CRT (the old glass tubes), the DVST (a storage tube), and the LCD (the flat panel you probably use now). Knowing how each forms an image, and in particular the idea of refreshing, sets up the frame buffer and scanning ideas that follow.
At a glance
| Device | How it forms the image |
|---|---|
| Refresh CRT | An electron beam hits glowing phosphor; redrawn many times per second (refresh) |
| Color CRT | A CRT that adds colour, commonly via a shadow mask and red/green/blue phosphors |
| DVST | Direct View Storage Tube: stores the image on the tube, so no refresh needed |
| LCD | Liquid Crystal Display: a flat panel; crystals block or pass backlight; dominant today |
Theory
The CRT and refreshing
A refresh CRT works by firing an electron beam from a gun at the back of the tube onto a phosphor-coated screen; the phosphor glows where the beam strikes. But that glow fades quickly, so to keep a steady picture the whole image must be redrawn many times per second, this is refreshing, and the rate (say 60 times a second, 60 Hz) must be fast enough that your eye sees a stable image rather than flicker.
A colour CRT produces colour, typically using a shadow mask and tiny red, green, and blue phosphor dots: mixing their brightness makes any colour. The need to constantly refresh is the CRT's defining behaviour, and it is why a frame buffer (next unit) exists to hold the image being refreshed.
Theory
DVST and LCD
The DVST (Direct View Storage Tube) takes a different approach: it stores the drawn image on the tube itself (on an internal storage grid), so it does not need refreshing. That gives flicker-free display of very complex images. The downside: you cannot easily erase part of the image or do smooth animation, changing anything means redrawing the whole screen, so DVSTs suited static, detailed drawings, not interactive graphics.
The LCD (Liquid Crystal Display) is the modern flat panel. It uses liquid crystals that, guided by electric signals, either block or let through light from a backlight, forming the image. LCDs are thin, low-power, and free of the bulky tube, which is why they replaced CRTs almost everywhere.
Quiz
What defines a refresh CRT's operation, and why is it needed?
- It stores the image permanently, so no redrawing is needed
- The phosphor glow fades, so the image must be redrawn (refreshed) many times per second to stay steady and avoid flicker
- It uses liquid crystals and a backlight
- It only shows black and white
Show the answer
The phosphor glow fades, so the image must be redrawn (refreshed) many times per second to stay steady and avoid flicker
In a refresh CRT, the electron beam makes the phosphor glow, but that glow fades quickly, so the image must be redrawn (refreshed) many times per second (e.g. 60 Hz) to keep a steady, flicker-free picture, that constant refreshing is its defining behaviour. Option A describes the DVST, which STORES the image and needs no refresh, the opposite of a refresh CRT. Option C describes an LCD (liquid crystals and a backlight), not a CRT. Option D is wrong: CRTs can be colour (via a shadow mask and RGB phosphors); the refresh CRT is not limited to black and white. The key idea is fading phosphor requiring repeated refresh, which is why a frame buffer holds the image to redraw.
Think first
Why could the DVST avoid refreshing when the CRT could not?
Both are tubes with electron beams. Why does the DVST need no refresh while the refresh CRT does? Then tap.
Show the answer
Because the DVST STORES the image on the tube itself, so the picture persists without being continually redrawn, whereas a refresh CRT relies on fast-fading phosphor that must be re-hit constantly. In a refresh CRT, the beam lights up phosphor that glows only briefly and then fades, so if you stopped redrawing, the image would vanish within a fraction of a second; the only way to keep it visible is to sweep the beam over the whole screen again and again, many times per second, which is the refresh. The DVST adds a clever mechanism: it has a storage grid inside the tube that HOLDS the pattern of the drawn image (using a flood of electrons that keeps the stored areas illuminated), so once you draw something it STAYS lit without the beam having to keep re-tracing it. This means the DVST can display very complex images completely flicker-free, and it does not need the frame-buffer-and-refresh machinery that a refresh CRT requires, an advantage when memory and refresh bandwidth were expensive. The catch is the flip side of 'stored': because the image is held on the tube, you cannot selectively ERASE or change part of it, to alter anything you must erase the ENTIRE stored image and redraw it all, which makes smooth animation and interactive editing impractical. So the DVST traded interactivity for stable, refresh-free display of static complex drawings. Storing the image removes the need to refresh, but also removes the ability to change it easily, a classic trade-off. Persistence versus interactivity is the heart of the difference.
Summary
Key takeaways
- Video display devices have evolved through several technologies, each with trade-offs.
- A refresh CRT fires an electron beam at phosphor that glows then fades, so the image must be refreshed many times per second to avoid flicker.
- A colour CRT adds colour, commonly via a shadow mask and red, green, and blue phosphor dots.
- A DVST (Direct View Storage Tube) stores the image on the tube, so it needs no refresh, but cannot easily erase part of it or animate.
- An LCD (Liquid Crystal Display) is a flat panel using liquid crystals to block or pass backlight; thin, low-power, dominant today.
- The CRT's need to refresh is why a frame buffer holds the image to redraw (next unit).
- Memory hook: CRT refreshes fading phosphor, DVST stores the image, LCD is the modern flat panel.