Answer to Question #1433 Submitted to "Ask the Experts"

Q

I've learned that flat-screen computer monitors give off less radiation than nonflat-screen monitors. Would this also hold true for television monitors?

A

Yes, there would be less radiation from a flat-screen television monitor as well. The primary "radiations" of concern from video products are x radiation, electric fields, and magnetic fields. The only possible source of x radiation from television receivers, TV monitors, or computer monitors is the picture tube, also known as the cathode ray tube or CRT. The x radiation is generated when an electron beam, after acceleration through an applied high voltage, strikes the inside of the face of the CRT. This results in rapid deceleration of the electrons and therefore emission of electromagnetic radiation. The energy of this electromagnetic radiation depends on the high voltage that is applied to the CRT. The higher the high voltage, the more the electrons will be accelerated, the more kinetic energy they will have when they strike the CRT face, and the more energy the x radiation will have. Keep in mind that this x radiation is always generated inside a CRT. A properly designed and operated CRT has glass of sufficient density and thickness to highly filter the x radiation so levels outside the CRT meet 0.5 mR/hr exposure-rate limits. Typical high-voltage values in CRTs vary from 10,000 to 35,000 volts with larger-size CRTs having higher high voltages. Flat-screen monitors and flat-screen TVs have no CRT. They use a totally different display technology. They operate with maximum voltages of tens or hundreds of volts, nowhere near the thousands of volts needed to create x-ray energy emissions. They are inherently incapable of producing x radiation. For that reason, the Food and Drug Administration has ruled that they are not subject to the Federal Performance Standard for x-radiation emission from television receivers. TVs and computer screens can have fairly high static electric fields near the face of the CRT. This field is created by the presence of high voltage on the inside of the CRT. On computer CRTs, this is often filtered out with a grounded conductive coating. External filters commercially available to eliminate the glare on the CRT image often incorporate an electric field filter as well. Without high voltage, flat-screen displays do not generate this static electric field. Time-varying electric and magnetic fields are present around CRT-based computer monitors and TVs. The visible image is created by scanning the electron beam horizontally and vertically so it strikes all parts of the phosphor on the inside of the CRT face. The electron beam is modulated with picture information to illuminate some phosphor dots and not others to create the image. In a broadcast TV image, the screen is scanned from top to bottom with 525 horizontal lines of information 30 times a second to create a dynamic image. The 525 lines of image are created in two vertical scans with the even-numbered lines in the first pass and the odd-numbered lines in the second. This "interlacing" helps to eliminate perception of flicker in the image. The result is that the electron beam must be scanned from top to bottom 60 times per second and from side to side 525 x 30 = 15,750 times per second. The scanning of the electron beam is effected by the horizontal and vertical deflection yokes. These yokes are coils wrapped around the neck of the CRT. Current passes through the yokes to create magnetic fields. The magnetic fields deflect the electron beam as needed. These magnetic fields are present around the outside of the CRT as well. ELF magnetic fields at 60 Hz and VLF magnetic fields at 15.750 kHz can easily be measured around any TV receiver. Computer displays use higher scanning frequencies in order to create more lines on the screen resulting in a higher-resolution image. Magnetic fields of 64 kHz or even higher may be present around CRT-based computer displays. Electric fields can also be generated at these scanning frequencies but they are easily filtered out by a conductive coating applied to the inside of the video product cabinet. Flat-screen displays do not use scanning electron beams. The image is created by digitally addressing columns and rows of image dots sequentially. This does not require any deflection yoke so no yoke-related magnetic fields are present. Another source of magnetic fields would be 60 Hz fields generated by power supplies. Higher-frequency fields can be present from switching power supplies. Flat-screen displays typically consume a lot less power than CRT-based displays. Therefore, power supply-related magnetic fields from flat-screen displays would also be less than those from CRT-based displays. William S. Boivin, physicist Food and Drug Administration