Re: Βιντεοπροβολέας CRT
Μερικα προχειρα αρθρα που βρηκα για CRT
CRT monitors may have been around for a while, but they aren't without their hazards. CRT monitors generate and hold a dangerous electrical charge even after they're turned off. This charge can actually last up to several years and be hazardous and even deadly to anyone who opens a CRT monitor casing. Then there is the better know danger of radiation. Inside each CRT monitor is a metal shield behind the glass which protects users from a flood of radiation. Then there's the fact that CRT monitors have also been known to cause fires due to electrical malfunctions and excessive heat if the ventilation holes are accidentally covered up. Given the treatment monitors receive at the hands of novice users, it's surprising that this doesn't happen more often.
Televisions and computer monitors are in the process of being superceded by LCD technology as the quality and cost of these new devices improves. Nonetheless many consumers still use cathode ray tube (CRT) televisions and monitors in their homes and at work. A CRT contains a high voltage power supply and electron gun that beams electromagnetic radiation and x-rays into the surrounding environment. Some manufacturers have managed to reduce emissions in their CRT products, though many models still spray large amounts of electromagnetic radiation. Here are measurements from three examples of CRT type TV and monitors.
In Figure 3 the 27" TV has a high electromagnetic radiation emission in the first 12 to 15" from the front of the screen. The safe watching distance is beyond the lime green line at 43".
Figure 3
Electromagnetic
It has been claimed that the electromagnetic fields emitted by CRT monitors constitute a health hazard, and can affect the functioning of living cells.[7][8] Exposure to these fields diminishes considerably at distances of 85 cm or farther according to the inverse cube law, which describes the propagation of all magnetic radiation. As the coils in a CRT monitor are extremely inefficient antennas, there is no broadcast electromagnetic field.
See also: Magnetic dipole
See also: VLF
[edit] Ionizing radiation
CRTs can emit a small amount of X-ray radiation as a result of the electron beam's bombardment of the shadow mask/aperture grille and phosphors. The amount of radiation escaping the front of the monitor is widely considered unharmful. The Food and Drug Administration regulations in 21 C.F.R. 1020.10 are used to strictly limit, for instance, television receivers to 0.5 milliroentgens per hour (mR/h) (0.13 µC/(kg·h) or 36 pA/kg) at a distance of 5 cm from any external surface; since 2007, most CRTs have emissions that fall well below this limit.[9] This is one of the reasons CRT equipment sold in the United States is required to have the month and year of manufacture stamped on the back of the set.
Early color television receivers (many of which are now highly collectible, see CT-100) were especially vulnerable due to primitive high-voltage regulation systems. X-ray production is generally negligible in black-and-white sets (due to low acceleration voltage and beam current), and in virtually every color display since the late 1960s, when systems were added to shut down the horizontal deflection system (and therefore high voltage supply) should regulation of the acceleration voltage fail.
All television receivers and CRT displays equipped with a vacuum tube based high-voltage rectifier or high-voltage regulator tube also generate X-rays in these stages. These stages are universally housed in a metal enclosure called the "high-voltage cage" made from lead to substantially reduce (and effectively eliminate) exposure. As examples, a 1B3 and a 6BK4 vacuum tube would be installed inside this metal enclosure. For both X-ray and electrical safety reasons, the set should never be operated with the cover of the high voltage cage opened.
[edit] Toxicity
CRTs may contain toxic phosphors within the glass envelope. The glass envelopes of modern CRTs may be made from heavily leaded glass, which represent an environmental hazard. Indirectly heated vacuum tubes (including CRTs) use barium compounds and other reactive materials in the construction of the cathode and getter assemblies; normally this material will be converted into oxides upon exposure to the air, but care should be taken to avoid contact with the inside of all broken tubes.
In some jurisdictions, discarded CRTs are regarded as toxic waste. In October 2001, the United States Environmental Protection Agency created rules stating that CRTs must be brought to special recycling places. In November 2002, the EPA began fining companies that disposed of CRTs through landfills or incineration. Regulatory agencies, local and statewide, monitor the disposal of CRTs and other computer equipment.
In Europe, disposal of CRT televisions and monitors is covered by the WEEE Directive.
[edit] Flicker
The constant refreshing of a CRT can cause headaches in migraine sufferers and seizures in epileptics, if they are photosensitive. Screen filters are available to reduce these effects. A high refresh rate (above 72 Hz) also helps to negate these effects.
[edit] High voltage
CRTs operate at very high voltages, which can persist long after the device containing the CRT has been switched off and/or unplugged, sometimes for years. Residual charges of hundreds of volts can also remain in large capacitors in the power supply circuits of the device containing the CRT; these charges may persist. Modern circuits contain bleeder resistors, to ensure that the high-voltage supply is discharged to safe levels within a couple of minutes at most. These discharge devices can fail even on a modern unit and leave these high voltage charges present. The final anode connector on the bulb of the tube carries this high voltage.
[edit] Implosion
A high vacuum exists within all CRT monitors. If the outer glass envelope is damaged, a dangerous implosion may occur. Due to the power of the implosion, glass may explode outwards. This shrapnel can travel at dangerous and potentially fatal velocities. While modern CRT used in televisions and computer displays have epoxy-bonded face-plates or other measures to prevent shattering of the envelope, CRTs removed from equipment must be handled carefully to avoid personal injury.