Answer to Question #2488 Submitted to "Ask the Experts"

Q

An article on your Web site indicated that there was less hazard from flat-screen displays than from CRTs. What about what the computer industry now refers to as a "true flat screen" CRT?

A

Flat screens can be CRTs, LCDs, or Plasma Displays

Cathode Ray Tubes, commonly referred to as CRTs, use high-voltage tubes to accelerate electrons and generate the traditional television image we have been used to over the years. In order to generate x rays, electrons have to reach high energies so you need both a high voltage and a high vacuum. In a CRT you have an electron beam accelerated by a high voltage in a high vacuum so it is clearly a potential source of x rays. These have been used in television receivers and computer displays. The surface of these displays is curved, although today there are "flat screen" CRT displays. A CRT display, whether curved or flat, still poses similar concerns regarding potential x-ray emissions. Lead and other heavy elements, added to the glass in making today's CRTs, absorb the x radiation so very little is emitted under any operating conditions.


Liquid Crystal Displays (LCDs) and Plasma Display Systems do not use high-voltage tubes. These display systems are also sometimes referred to as flat screens but, unlike the CRT flat screen, these are relatively thinner than CRT display systems and are used in laptop computers and wall-mounted screens. The voltages used in plasma displays are high enough to ionize the gas to generate and sustain the plasma. But in the plasma tube you don't have a high vacuum so the electrons cannot reach such high energies. They are pretty much limited to the ionization potential of the gas used to make the plasma, which is well below the energy of even soft x rays. LCD displays have neither high-voltage nor high-vacuum components. Therefore, neither of these two have the potential for x rays.

X-ray emissions associated with television receivers

All television receivers, regardless of type, must meet a mandatory federal performance standard so any x-ray emissions, if they exist at all, must be at very low levels. The actual limit is stated as "shall not exceed 0.5 milliroentgens per hour at a distance of five (5) centimeters from any point on the external surface of the receiver . . ."

The 20,000-25,000 volt x rays which were associated with television receivers in the 1960s were easily eliminated with small amounts of shielding material. Potential x-ray emissions are usually not associated with newer displays which no longer use high-voltage tubes, which were the potential source of these low-level x rays. On the back of your television receiver is a label which states compliance with the Code of Federal Regulations (CFR), Department of Health and Human Services (DHHS), 21 CFR 1020.10. This is the federal x-ray emission standard and certifies the safety of the product.

Orhan Suleiman, MS, PhD
Edward F. Dawson, PhD, Captain USPHS