How SED-TV Works

For years, the picture on every television set came from a cathode ray tube (CRT). CRTs can make a high-quality picture with great colors, which is why many TVs still use them. Unfortunately, they're also bulky and heavy, and they can't support the big screens that people want today.

Most of the new TV types on the market have improved on CRT's size and weight, but some have a down side when it comes to the picture. Narrow viewing angles, poor black level, burn-in and various visual artifacts can plague newer TVs.

The surface-conduction electron-emitter display (SED) is yet another new TV. But unlike the others, it combines the picture quality of a CRT with the compact design of a flat-screen plasma display. An SED-TV can have a huge screen with a great picture, and it's only a few centimeters thick.

This article will investigate exactly how the SED-TV creates a picture. We'll start with reviewing how CRTs work, and we'll also look at the manufacturing process behind SED-TV's microscopic components.

CRTs: A Review

Some people think of CRTs as old or outdated, especially compared to newer display types like LCD, plasma, DLP and LCoS. But CRT technology is still superior in some ways, and understanding the CRT is central to understanding SED-TV.

Basically, a CRT fires electrons at a phosphorescent screen. When an electron hits the screen, that point, or pixel, glows. The CRT usually uses three streams of electrons, which each strike different phosphors for red, green and blue. Your eye and brain combine the glowing points to create the image you see.

The electrons in the CRT come from a heated filament called a cathode. A cathode is simply a negative electrode, and in a CRT it's similar to a light bulb filament. When electrical current reaches the cathode, electrons stream from it toward the positively-charged anode. The anode accelerates the electrons toward the screen. Electromagnetic steering coils direct the streams of electrons, causing them to paint the image one line at a time, from the top of the screen to the bottom.

Commonly called an electron gun, this collection of cathodes, anodes and electromagnets is the heart of a CRT television. You can learn more about the electron gun and how it creates a picture in How Television Works.

An SED-TV creates a picture in much the same way. It's essentially a flat-panel television that uses millions of CRTs instead of one electron gun. These miniature CRTs are called surface-conducting electron emitters (SCEs). A set has three SCEs for every pixel -- one each for red, green and blue. A widescreen, high-definition set can have more than 6 million SCEs.

Let's take a look at how the SCEs create electrons and the picture.

Creating the Picture

The heart of an SED-TV is the millions of miniature CRTs, called surface-conduction electron emitters (SCEs). An SCE is microscopic, and it consists of a layer of carbon with a gap down the center. One half of the carbon layer connects to a negative electrode, and the other connects to a positive electrode. When the circuitry delivers about 10 volts of current to the SCE, electrons appear at one side of the gap.

An SED-TV has millions of these SCEs arranged in a matrix, and each one controls the red, green or blue aspect of one pixel of the picture. Rather than directing electrons to create the image one row at a time, the matrix activates all the SCEs needed to create the picture virtually simultaneously.

As with a CRT set, the inside of an SED-TV is a vacuum. All of the SCEs are on one side of the vacuum, and the phosphor-coated screen is on the other. The screen has a positive electrical charge, so it attracts the electrons from the SCEs.

When they reach the screen, the electrons pass through a very thin layer of aluminum. They hit the phosphors, which then emit red, green or blue light. Your eyes and brain combine these glowing dots to create a picture.

Any part of the screen that's not used to create pixels is black, which gives the picture better contrast. There's also a color filter between the phosphors and the glass to improve color accuracy and cut down on reflected light.

To tie it all together, when the SED-TV receives a signal, it:

  1. Decodes the signal
  2. Decides what to do with the red, green and blue aspect of each pixel
  3. Activates the necessary SCEs, which generate electrons that fly through the vacuum to the screen

When the electrons hit the phosphors, those pixels glow, and your brain combines them to form a cohesive picture. The pictures change at a rate that allows you to perceive them as moving.

This process happens almost instantaneously, and the set can create a picture sixty times per second. Unlike a CRT, it doesn't have to interlace the picture by painting only every other line. It creates the entire picture every time.

The idea of a big-screen picture with CRT quality in a package that's about a quarter of an inch thick is pretty amazing. We'll look at the pros and cons of this TV technology next.

SED-TV Pros & Cons

SED-TVs were scheduled to hit the market sometime in 2006, and people who've seen them at CES and other electronics shows say that they have a remarkably good picture. SED-TVs have all of the best features of CRT televisions, like good color quality and black level, without a CRT's bulk or weight. SED-TVs have a wide viewing angle, and the SCE structure gets rid of the blurring that can happen around the edges of some CRT sets. The sets are compact and lightweight, and they consume less power than other flat-panel sets.

Figuring out the downsides of owning an SED-TV will be a little tricky until more people actually own and use them. But many people suspect that the sets will be too expensive for most people to afford -- rumors place the starting price of a 55-inch set at around $10,000.

SED-TV's release has also been delayed repeatedly. The original date was spring 2006. Then, Toshiba and Canon announced that it had been pushed to late 2007 for cost reasons. In the spring of 2007, the companies announced that the set would come out on an unspecified future date, citing patent, supply and production difficulties [source: Reuters]. 

For lots more information about SED-TVs and other television technologies, check out the links in the next section.

Related HowStuffWorks Articles

More Great Links


  • Kallender, Paul. "Toshiba to start SED-TV production this August." Computerworld, Vol. 11 Issue 13, April-May 2005.
  • Katzmaier, David. "Toshiba Challenges Plasma with LCD and SED Panel." CNet, January 12, 2005.
  • "SED Manufacturing Methods Revealed." Nikkei Electronics Asia, September 2005.
  • "Surface-conduction Electron-emitter Displays." Clarycon.
  • "Tunnel Effect." Japan Aerospace Exploration Agency, 2003.
  • "Toshiba's SED TV Makes Long-Awaited Debut at SEC 2006." Society for Information Display, January 2006.
  • Toshiba Microfilter Color Display Tubes. ToshibaMicrolfilterColorDisplayTubes.pdf
  • Yamamoto, K. et al. "Fabrication and characterization of surface-conduction electron emitters for SED." Journal of the Society for Information Display. Vol. 14, no. 1, 2006.