Editing Cathode-ray tube (section)

==Health concerns==

===Ionizing radiation===
CRTs can emit a small amount of [[X-ray]] radiation; this is a result of the electron beam's bombardment of the shadow mask/aperture grille and phosphors, which produces [[bremsstrahlung]] (braking radiation) as the high-energy electrons are decelerated. The amount of radiation escaping the front of the monitor is widely considered to be not harmful. The [[Food and Drug Administration]] regulations in {{CodeFedReg|21|1020|10}} are used to strictly limit, for instance, TV receivers to 0.5 [[Roentgen (unit)|milliroentgens]] per hour at a distance of {{convert|5|cm|0|abbr=on}} from any external surface; since 2007, most CRTs have emissions that fall well below this limit.<ref>{{cite web |url=http://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfcfr/CFRSearch.cfm?fr=1020.10 |title=Subchapter J, Radiological Health (21CFR1020.10) |publisher=U.S. Food and Drug Administration |date=1 April 2006 |access-date=13 August 2007}}</ref> Note that the roentgen is an outdated unit and does not account for dose absorption. The conversion rate is about .877 roentgen per [[Roentgen equivalent man|rem]].<ref>{{Cite web |title=Rad Pro Calculator: Frequently Asked Questions (FAQ) |url=http://www.radprocalculator.com/FAQ.aspx#:~:text=Since%201%20Roentgen%20equals%200.877,00877%20Sieverts. |access-date=2022-03-06 |website=www.radprocalculator.com}}</ref> Assuming that the viewer absorbed the entire dose (which is unlikely), and that they watched TV for 2 hours a day, a .5 milliroentgen hourly dose would increase the viewers yearly dose by 320 [[millirem]]. For comparison, the average background radiation in the United States is 310 millirem a year. Negative effects of chronic radiation are not generally noticeable until doses over 20,000 millirem.<ref>{{Cite web |title=US Department of Energy, Dose Ranges Rem/Sievert Chart |url=https://www.nrc.gov/docs/ML1209/ML120970113.pdf}}</ref>

The density of the x-rays that would be generated by a CRT is low because the raster scan of a typical CRT distributes the energy of the electron beam across the entire screen. Voltages above 15,000 volts are enough to generate "soft" x-rays. However, since CRTs may stay on for several hours at a time, the amount of x-rays generated by the CRT may become significant, hence the importance of using materials to shield against x-rays, such as the thick leaded glass and barium-strontium glass used in CRTs.<ref name="auto35"/>

Concerns about x-rays emitted by CRTs began in 1967 when it was found that TV sets made by General Electric were emitting "X-radiation in excess of desirable levels". It was later found that TV sets from all manufacturers were also emitting radiation. This caused TV industry representatives to be brought before a U.S. congressional committee, which later proposed a federal radiation regulation bill, which became the 1968 Radiation Control for Health and Safety Act. It was recommended to TV set owners to always be at a distance of at least 6 feet from the screen of the TV set, and to avoid "prolonged exposure" at the sides, rear or underneath a TV set. It was discovered that most of the radiation was directed downwards. Owners were also told to not modify their set's internals to avoid exposure to radiation. Headlines about "radioactive" TV sets continued until the end of the 1960s. There once was a proposal by two New York congressmen that would have forced TV set manufacturers to "go into homes to test all of the nation's 15 million color sets and to install radiation devices in them". The FDA eventually began regulating radiation emissions from all electronic products in the US.<ref>{{cite web | last=Murray | first=Susan | title=When Televisions Were Radioactive | website=The Atlantic | date=2018-09-23 | url=https://www.theatlantic.com/technology/archive/2018/09/when-televisions-were-radioactive/570916/ | access-date=2020-12-11}}</ref>

===Toxicity===
Older color and monochrome CRTs may have been manufactured with toxic substances, such as [[cadmium]], in the phosphors.<ref name="auto93"/><ref>{{cite web |url=http://takebackmytv.com/pages/toxic_tvs |title=Toxic TVs |access-date=13 April 2010 |publisher=Electronics TakeBack Coalition |url-status=dead |archive-url=https://web.archive.org/web/20090227041627/http://takebackmytv.com/pages/toxic_tvs |archive-date=27 February 2009}}</ref><ref>{{cite web |url=http://e-stewards.org/wp-content/uploads/2010/03/OccupationalRisksCRTGlassProcessing1.pdf |title=Occupational Risks Associated with Electronics Demanufacturing and CRT Glass Processing Operations and the Impact of Mitigation Activities on Employee Safety and Health |access-date=20 January 2011 |author1=Peters-Michaud, Neil |author2=Katers, John |author3=Barry, Jim |work=Cascade Asset Management, LLC |publisher=Basel Action Network |url-status=dead |archive-url=https://web.archive.org/web/20110726012352/http://e-stewards.org/wp-content/uploads/2010/03/OccupationalRisksCRTGlassProcessing1.pdf |archive-date=26 July 2011}}</ref><ref>{{cite web |url=http://www.americanelements.com/cdm.html |title=Cadmium |access-date=13 April 2010 |publisher=[[American Elements]]}}</ref> The rear glass tube of modern CRTs may be made from [[lead glass|leaded glass]], which represent an environmental hazard if disposed of improperly.<ref>{{cite web |url=http://www.hinkleycenter.com/publications/lead_leachability_99-5.pdf |title=Characterization of Lead Leachability from Cathode Ray Tubes Using the Toxicity Characteristic Leaching Procedure |access-date=4 October 2009 |url-status=dead |archive-url=https://web.archive.org/web/20140222012228/http://www.hinkleycenter.com/publications/lead_leachability_99-5.pdf |archive-date=22 February 2014}}</ref> Since 1970, glass in the front panel (the viewable portion of the CRT) used strontium oxide rather than lead, though the rear of the CRT was still produced from leaded glass. Monochrome CRTs typically do not contain enough leaded glass to fail EPA TCLP tests. While the TCLP process grinds the glass into fine particles in order to expose them to weak acids to test for leachate, intact CRT glass does not leach (The lead is [[vitrified]], contained inside the glass itself, similar to [[Lead glass|leaded glass]] crystalware).

===Flicker===
{{Main|Flicker (screen)}}
At low [[refresh rate]]s (60&nbsp;[[Hz]] and below), the periodic scanning of the display may produce a flicker that some people perceive more easily than others, especially when viewed with [[peripheral vision]]. Flicker is commonly associated with CRT as most TVs run at 50&nbsp;Hz (PAL) or 60&nbsp;Hz (NTSC), although there are some 100&nbsp;Hz PAL TVs that are [[flicker-free]]. Typically only low-end monitors run at such low frequencies, with most computer monitors supporting at least 75&nbsp;Hz and high-end monitors capable of 100&nbsp;Hz or more to eliminate any perception of flicker.<ref>{{cite web|url=http://www.pcmech.com/article/crt-monitor-flickering/|archive-url=http://arquivo.pt/wayback/20160515111055/http://www.pcmech.com/article/crt-monitor-flickering/|url-status=dead|archive-date=15 May 2016|title=CRT Monitor Flickering?|access-date=4 October 2009}}</ref> Though the 100&nbsp;Hz PAL was often achieved using interleaved scanning, dividing the circuit and scan into two beams of 50&nbsp;Hz. Non-computer CRTs or CRT for [[sonar]] or [[radar]] may have long [[Image persistence|persistence]] phosphor and are thus flicker free. If the persistence is too long on a video display, moving images will be blurred.

===High-frequency audible noise===
50&nbsp;Hz/60&nbsp;Hz CRTs used for TV operate with horizontal scanning frequencies of 15,750 and 15,734.27&nbsp;Hz (for [[NTSC]] systems) or 15,625&nbsp;Hz (for [[PAL]] systems).<ref>{{cite book |last= Netravali |first= Arun N. |author2=Haskell, Barry G. |title= Digital pictures: representation, compression, and standards |publisher= Plenum Publishing Corporation |year= 1995 |page= 100 |url= https://books.google.com/books?id=-TXYfZrCBuQC&pg=PA100 |isbn=978-0-306-44917-8}}</ref> These frequencies are at the upper range of [[human hearing]] and are inaudible to many people; however, some people (especially children) will perceive a high-pitched tone near an operating CRT TV.<ref>{{cite web |url=http://www.pcguide.com/ts/x/comp/crt/failWhine-c.html |title=The monitor is producing a high-pitched whine |access-date=4 October 2009}}</ref> The sound is due to [[magnetostriction]] in the magnetic core and periodic movement of windings of the [[flyback transformer]]<ref>{{cite journal |last1=Rhys-Jones |first1=J. |title=Television economics |journal=Radiotronics |date=February 1951 |volume=16 |issue=2 |page=37}}</ref> but the sound can also be created by movement of the deflection coils, yoke or ferrite beads.<ref>{{cite web | title=SER FAQ: TVFAQ: High pitched whine or squeal from TV with no other symptoms | website=Sci.Electronics.Repair FAQ | url=https://www.repairfaq.org/samnew/tvfaq/tvwhine.htm | access-date=2020-12-11}}</ref>

This problem does not occur on 100/120&nbsp;Hz TVs and on non-CGA (Color Graphics Adapter) computer displays, because they use much higher horizontal scanning frequencies that produce sound which is inaudible to humans (22&nbsp;kHz to over 100&nbsp;kHz).

===Implosion===
[[File:CRT implosion.JPG|thumb|A CRT during an implosion]]
If the glass wall is damaged, atmospheric pressure can [[Implosion (mechanical process)|implode]] the vacuum tube into dangerous fragments which accelerate inward and then spray at high speed in all directions. Although modern cathode-ray tubes used in TVs and computer displays have [[epoxy]]-bonded face-plates or other measures to prevent shattering of the envelope, CRTs must be handled carefully to avoid injury.<ref name=Bali>{{cite book |last= Bali |first= S.P. |title= Colour Television: Theory and Practice |publisher= Tata McGraw–Hill |year= 1994 |page= 129 |url= https://books.google.com/books?id=53nnX4fnnNIC&pg=PA129 |isbn=978-0-07-460024-5}}</ref>

====Implosion protection====
[[File:Datapoint 1500 keyboard mounted in case.jpg|thumb|[[Datapoint]] 1500 terminal with exposed chassis, with its CRT suffering from a "cataract" due to aging PVA]]
Early CRTs had a glass plate over the screen that was bonded to it using glue,<ref name="auto63"/> creating a laminated glass screen: initially the glue was [[polyvinyl acetate]] (PVA),<ref name="auto41">{{Cite web|url=http://www.earlytelevision.org/Deksnis/doug_harland.htm|title=Illustrated Cataract Repair|website=www.earlytelevision.org}}</ref> while later versions such as the LG Flatron used a resin, perhaps a UV-curable resin.<ref>{{Cite web|url=https://patents.justia.com/patent/4841372|title=US Patent for Cathode ray tube contrast enhancement systems Patent (Patent # 4,841,372 issued June 20, 1989) - Justia Patents Search|website=patents.justia.com}}</ref><ref name="auto13"/> The PVA degrades over time creating a "cataract", a ring of degraded glue around the edges of the CRT that does not allow light from the screen to pass through.<ref name="auto41"/> Later CRTs instead use a tensioned metal rim band mounted around the perimeter that also provides mounting points for the CRT to be mounted to a housing.<ref name="braun"/> In a 19-inch CRT, the tensile stress in the rim band is 70&nbsp;kg/cm<sup>2</sup>.<ref>{{Cite web|url=https://www.freepatentsonline.com/4158419.html|title=Implosion protected CRT|accessdate=18 December 2022}}</ref>

Older CRTs were mounted to the TV set using a frame. The band is tensioned by heating it, then mounting it on the CRT; the band cools afterwards, shrinking in size and putting the glass under compression,<ref>{{Cite web|url=http://www.r-type.org/exhib/aar0024.htm|title=160AB22 @ The Valve Museum|website=www.r-type.org}}</ref><ref name="auto63"/><ref name="band">{{Cite web|url=https://patents.google.com/patent/US6757158B2/en?q=braun+tube&oq=braun+tube|title=Implosion resistant cathode ray tube with mounting lug having a compound bend|accessdate=18 December 2022}}</ref> which strengthens the glass and reduces the necessary thickness (and hence weight) of the glass. This makes the band an integral component that should never be removed from an intact CRT that still has a vacuum; attempting to remove it may cause the CRT to implode.<ref name="auto28"/>

The rim band prevents the CRT from imploding should the screen be broken. The rim band may be glued to the perimeter of the CRT using epoxy, preventing cracks from spreading beyond the screen and into the funnel.<ref>{{Cite web|url=https://patents.google.com/patent/US4930015A/en|title=Flat tension mask cathode ray tube implosion system|date=6 April 1988 |accessdate=18 December 2022 |last1=Dougherty |first1=Lawrence W. |last2=Lee |first2=Sae D. |last3=Rogers |first3=Melvin F. }}</ref><ref name="band"/>

Alternatively the compression caused by the rim band may be used to cause any cracks in the screen to propagate laterally at a high speed so that they reach the funnel and fully penetrate it before they fully penetrate the screen. This is possible because the funnel has walls that are thinner than the screen. Fully penetrating the funnel first allows air to enter the CRT from a short distance behind the screen, and prevent an implosion by ensuring the screen is fully penetrated by the cracks and breaks only when the CRT already has air.<ref name="auto63"/>

===Electric shock===
To accelerate the electrons from the cathode to the screen with enough energy<ref>{{cite web |url=https://spark.iop.org/speed-electrons |title=The speed of electrons |access-date=2021-10-18}}</ref> to achieve sufficient image brightness, a very [[high voltage]] (EHT or extra-high tension) is required,<ref>Color Television Servicing Manual, Vol-1, by M.D. Aggarwala, 1985, Television for you, Delhi, India</ref> from a few thousand volts for a small oscilloscope CRT to tens of thousands for a larger screen color TV. This is many times greater than household power supply voltage. Even after the power supply is turned off, some associated capacitors and the CRT itself may retain a charge for some time and therefore dissipate that charge suddenly through a ground such as an inattentive human grounding a capacitor discharge lead. An average monochrome CRT may use 1–1.5&nbsp;kV of anode voltage per inch.<ref>{{cite web | title=The Truth About CRTs and Shock Danger | website=Low End Mac | date=2019-10-11 | url=https://lowendmac.com/2007/the-truth-about-crts-and-shock-danger/ | access-date=2020-12-11}}</ref><ref name="auto19"/>