Editing Cathode-ray tube (section)

===Color CRTs===
[[File:CRT screen. closeup.jpg|right|thumb|150px|Magnified view of a delta-gun [[shadow mask]] color CRT]]
[[File:CRT pixel array.jpg|thumb|On the left: Magnified view of In-line phosphor triads (a slot mask) CRT. On the right: Magnified view of Delta-gun phosphor triads.]]
[[File:CRT Phosphors.jpg|right|thumb|150px|Magnified view of a [[Trinitron]] (aperture grille) color CRT. A thin horizontal support wire is visible.]]
[[File:CRT mask types en-de.svg|thumb|CRT triad and mask types]]
[[File:CRT phosphors.png|thumb|right|Spectra of constituent blue, green and red phosphors in a common CRT]]
[[File:Electron gun.jpg|thumb|280px|The in-line electron guns of a color CRT TV]]
Color CRTs use three different phosphors which emit red, green, and blue light respectively. They are packed together in stripes (as in [[aperture grille]] designs) or clusters called [[Triad (monitors)|"triads"]] (as in shadow mask CRTs).<ref name=crtmonitorwork>{{cite web |url=http://www.bit-tech.net/hardware/2006/03/20/how_crt_and_lcd_monitors_work/1 |title=How CRT and LCD monitors work |access-date=4 October 2009 |work=bit-tech.net}}</ref><ref>{{Cite web|url=http://repairfaq.cis.upenn.edu/sam/icets/tvset.htm|title=The TV Set|website=repairfaq.cis.upenn.edu|access-date=8 December 2020|archive-date=12 November 2020|archive-url=https://web.archive.org/web/20201112014823/http://repairfaq.cis.upenn.edu/sam/icets/tvset.htm|url-status=dead}}</ref>

Color CRTs have three electron guns, one for each primary color, (red, green and blue) arranged either in a straight line (in-line) or in an [[equilateral triangle|equilateral triangular]] configuration (the guns are usually constructed as a single unit).<ref name="auto6"/><ref name="auto44"/><ref>{{Cite web|url=https://patents.google.com/patent/EP0281197A1/en|title=Colour cathode ray tube|accessdate=18 December 2022}}</ref><ref>{{Cite web|url=https://patents.google.com/patent/US4766344A/en|title=In-line electron gun structure for color cathode ray tube having oblong apertures|accessdate=18 December 2022}}</ref><ref>{{Cite web|url=https://patents.google.com/patent/EP1536451A1/en|title=Cathode ray tube device with an in-line electron gun|accessdate=18 December 2022}}</ref> The triangular configuration is often called ''delta-gun'', based on its relation to the shape of the [[Greek letter]] [[Delta (letter)|delta]] (Δ). The arrangement of the phosphors is the same as that of the electron guns.<ref name="auto6"/><ref>{{Cite web|url=https://patents.google.com/patent/US20050001552A1/en|title=Cathode-ray tube}}</ref> A grille or mask absorbs the electrons that would otherwise hit the wrong phosphor.<ref name=maskgrill>{{cite web |url=http://www.pcguide.com/ref/crt/crtMask-c.html |title=The Shadow Mask and Aperture Grill |work=PC Guide |access-date=4 October 2009 |archive-url=https://web.archive.org/web/20100102114333/http://pcguide.com/ref/crt/crtMask-c.html |archive-date=2 January 2010 |url-status=dead }}</ref>

A [[shadow mask]] tube uses a metal plate with tiny holes, typically in a delta configuration, placed so that the electron beam only illuminates the correct phosphors on the face of the tube;<ref name=crtmonitorwork/> blocking all other electrons.<ref name="auto104"/> Shadow masks that use slots instead of holes are known as slot masks.<ref name="auto105"/> The holes or slots are tapered<ref>{{Cite web|url=https://patents.google.com/patent/US3973965A/en|title=Making shadow mask with slit-shaped apertures for CRT|accessdate=18 December 2022}}</ref><ref>{{cite web |url= http://www.earlytelevision.org/pdf/manufacture_of_color_pic_tubes.pdf |archive-url=https://web.archive.org/web/20150905122030/http://www.earlytelevision.org/pdf/manufacture_of_color_pic_tubes.pdf |archive-date=2015-09-05 |url-status=live|title=Manufacture of color picture tubes |website=www.earlytelevision.org |access-date=2020-12-11}}</ref> so that the electrons that strike the inside of any hole will be reflected back, if they are not absorbed (e.g. due to local charge accumulation), instead of bouncing through the hole to strike a random (wrong) spot on the screen. Another type of color CRT (Trinitron) uses an [[aperture grille]] of tensioned vertical wires to achieve the same result.<ref name=maskgrill/> The shadow mask has a single hole for each triad.<ref name="auto6"/> The shadow mask is usually {{Fraction|1|2}}&nbsp;inch behind the screen.<ref name="auto85"/>

Trinitron CRTs were different from other color CRTs in that they had a single electron gun with three cathodes, an aperture grille which lets more electrons through, increasing image brightness (since the aperture grille does not block as many electrons), and a vertically cylindrical screen, rather than a curved screen.<ref>{{Cite web|url=https://www.chicagotribune.com/news/ct-xpm-1991-10-18-9104040210-story.html|title=Several Companies Working on Flatter, Clearer TV Screens|first=Rich|last=Warren|website=chicagotribune.com|date=18 October 1991 }}</ref>

The three electron guns are in the neck (except for Trinitrons) and the red, green and blue phosphors on the screen may be separated by a black grid or matrix (called black stripe by Toshiba).<ref name="auto94"/>

The funnel is coated with aquadag on both sides while the screen has a separate aluminum coating applied in a vacuum,<ref name="auto6"/><ref name="auto38"/> deposited after the phosphor coating is applied, facing the electron gun.<ref>{{cite journal | url=https://ieeexplore.ieee.org/document/1451112 | doi=10.1109/PROC.1973.9182 | title=Cathode-ray-tube phosphors: Principles and applications | date=1973 | last1=Larach | first1=S. | last2=Hardy | first2=A.E. | journal=Proceedings of the IEEE | volume=61 | issue=7 | pages=915–926 }}</ref><ref>[https://pearl-hifi.com/06_Lit_Archive/02_PEARL_Arch/Vol_16/Sec_53/Philips_Tech_Review/PTechReview-44-1988_89-335.pdf Phosphor Screens in Cathode-Ray Tubes for Projection Television] pearl-hifi.com</ref> The aluminum coating protects the phosphor from ions, absorbs secondary electrons, providing them with a return path, preventing them from electrostatically charging the screen which would then repel electrons and reduce image brightness, reflects the light from the phosphors forwards and helps manage heat. It also serves as the anode of the CRT together with the inner aquadag coating. The inner coating is electrically connected to an electrode of the electron gun using springs, forming the final anode.<ref name="auto11"/><ref name="auto6"/> The outer aquadag coating is connected to [[Ground (electricity)|ground]], possibly using a series of springs or a harness that makes contact with the aquadag.<ref>{{Cite web|url=https://patents.google.com/patent/US3996491A/en|title=External connective means for a cathode ray tube|accessdate=18 December 2022}}</ref><ref name="auto96">{{Cite web|url=http://oldtellys.co.uk/otcolour.html|title=Colour Science|website=oldtellys.co.uk}}</ref>

====Shadow mask====
{{Main|Shadow mask}}
The shadow mask absorbs or reflects electrons that would otherwise strike the wrong phosphor dots,<ref name="auto91"/> causing color purity issues (discoloration of images); in other words, when set up correctly, the shadow mask helps ensure color purity.<ref name="auto6"/> When the electrons strike the shadow mask, they release their energy as heat and x-rays. If the electrons have too much energy due to an anode voltage that is too high for example, the shadow mask can warp due to the heat, which can also happen during the Lehr baking at ~435&nbsp;°C of the frit seal between the faceplate and the funnel of the CRT.<ref name="auto66"/><ref>{{Cite web|url=https://patents.google.com/patent/US4327307A/en|title=Shadow mask for color cathode ray tube|accessdate=18 December 2022}}</ref>

Shadow masks were replaced in TVs by slot masks in the 1970s, since slot masks let more electrons through, increasing image brightness. Shadow masks may be connected electrically to the anode of the CRT.<ref>{{Cite web|url=https://patents.google.com/patent/US3543072A/en|title=Color cathode ray tube with metallic contactor ribbon bonded on inside wall of tube between the high voltage terminal and the shadow mask frame|accessdate=18 December 2022}}</ref><ref name="auto40"/><ref>{{cite web |url=https://global.sharp/100th/pdf/gravure.pdf |archive-url=https://web.archive.org/web/20210126133217/https://global.sharp/100th/pdf/gravure.pdf |archive-date=2021-01-26 |url-status=live |title= Sharp radios over the years|website=global.sharp |access-date=2020-12-11}}</ref><ref name="auto14">{{Cite web|url=https://stweb.peelschools.org/pcsweb/pc_tut/06crtmon.htm|title=CRT Monitors|website=stweb.peelschools.org|access-date=8 December 2020|archive-date=16 April 2020|archive-url=https://web.archive.org/web/20200416205016/http://stweb.peelschools.org/pcsweb/pc_tut/06crtmon.htm|url-status=dead}}</ref> Trinitron used a single electron gun with three cathodes instead of three complete guns. CRT PC monitors usually use shadow masks, except for Sony's Trinitron, Mitsubishi's Diamondtron and NEC's [[Cromaclear]]; Trinitron and Diamondtron use aperture grilles while Cromaclear uses a slot mask. Some shadow mask CRTs have color phosphors that are smaller in diameter than the electron beams used to light them,<ref name="auto103">{{Cite web|url=https://patents.google.com/patent/US4251610A/en|title=Method of making multicolor CRT display screen with minimal phosphor contamination|accessdate=18 December 2022}}</ref> with the intention being to cover the entire phosphor, increasing image brightness.<ref name="auto55">{{Cite magazine|title=Mac's Service Shop: Zenith's 1973 Color Line<!--http://www.rfcafe.com/references/popular-electronics/macs-service-shop-zenith-1973-color-tv-popular-electronics-march-1973.htm -->|date=March 1973|magazine=Popular Electronics}}</ref> Shadow masks may be pressed into a curved shape.<ref>{{Cite web|url=https://patents.google.com/patent/US4482334A/en|title=Method for making CRT shadow masks|accessdate=18 December 2022}}</ref><ref>{{Cite web|url=https://www.freepatentsonline.com/4859901.html|title=Color CRT shadow mask with wrinkle-free corners|accessdate=18 December 2022}}</ref><ref>{{Cite web|url=https://books.google.com/books?id=jniJTaz6zDMC&q=shadow+mask+crt+tapered&pg=RA1-PA57|title = Official Gazette of the United States Patent and Trademark Office: Patents|year = 1990}}</ref>

====Screen manufacture====
Early color CRTs did not have a black matrix, which was introduced by Zenith in 1969, and Panasonic in 1970.<ref name="auto55"/><ref name="auto39">{{Cite web|url=https://patents.google.com/patent/US5081394A/en|title=Black matrix color picture tube|accessdate=18 December 2022}}</ref><ref name="auto95"/> The black matrix eliminates light leaking from one phosphor to another since the black matrix isolates the phosphor dots from one another, so part of the electron beam touches the black matrix. This is also made necessary by warping of the shadow mask.<ref name="auto94"/><ref name="auto103"/> Light bleeding may still occur due to stray electrons striking the wrong phosphor dots. At high resolutions and refresh rates, phosphors only receive a very small amount of energy, limiting image brightness.<ref name="auto97"/>

Several methods were used to create the black matrix. One method coated the screen in photoresist such as dichromate-sensitized polyvinyl alcohol photoresist which was then dried and exposed; the unexposed areas were removed and the entire screen was coated in colloidal graphite to create a carbon film, and then hydrogen peroxide was used to remove the remaining photoresist alongside the carbon that was on top of it, creating holes that in turn created the black matrix. The photoresist had to be of the correct thickness to ensure sufficient adhesion to the screen, while the exposure step had to be controlled to avoid holes that were too small or large with ragged edges caused by light diffraction, ultimately limiting the maximum resolution of large color CRTs.<ref name="auto103"/> The holes were then filled with phosphor using the method described above. Another method used phosphors suspended in an aromatic diazonium salt that adhered to the screen when exposed to light; the phosphors were applied, then exposed to cause them to adhere to the screen, repeating the process once for each color. Then carbon was applied to the remaining areas of the screen while exposing the entire screen to light to create the black matrix, and a fixing process using an aqueous polymer solution was applied to the screen to make the phosphors and black matrix resistant to water.<ref name="auto39"/> Black chromium may be used instead of carbon in the black matrix.<ref name="auto103"/> Other methods were also used.<ref>{{Cite web|url=https://patents.google.com/patent/KR100728490B1/en|title=photo-sensitive compound and photo-resist resin composition for forming black matrix of CPT containing effective amount thereof|accessdate=18 December 2022}}</ref><ref>{{Cite web|url=https://patents.google.com/patent/US5840450A/en|title=Method for forming a black matrix on a faceplate panel for a color CRT|accessdate=18 December 2022}}</ref><ref>{{Cite web|url=https://patents.google.com/patent/US6022651A/en|title=Black matrix and a phosphor screen for a color cathode-ray-tube and production thereof|accessdate=18 December 2022}}</ref><ref>{{Cite web|url=https://patents.google.com/patent/DE19655045C2/en|title=Water-soluble photocurable resin compsn. useful in black matrix punch|accessdate=18 December 2022}}</ref>

The phosphors are applied using [[photolithography]]. The inner side of the screen is coated with phosphor particles suspended in PVA photoresist slurry,<ref>{{cite journal |last1=Lakatos |first1=Andras I. |title=Introduction |journal=Journal of the Society for Information Display |date=2000 |volume=8 |issue=1 |pages=1 |doi=10.1889/1.1985254 |s2cid=57611617 }}</ref><ref>{{Cite book|url=https://books.google.com/books?id=aAQBIuPrudQC&q=crt+phosphor+photolithography&pg=PA94|title = Cathodoluminescence and Photoluminescence: Theories and Practical Applications|isbn = 9781420052732|last1 = Ozawa|first1 = Lyuji|date = 3 October 2018| publisher=CRC Press }}</ref> which is then dried using infrared light,<ref>{{Cite magazine|title=Assembly line for lower-priced TV <!--http://www.earlytelevision.org/pdf/life_12-28-59.pdf-->|date=28 December 1959|magazine=Life Magazine}}</ref> exposed, and developed. The exposure is done using a "lighthouse" that uses an ultraviolet light source with a corrector lens to allow the CRT to achieve color purity. Removable shadow masks with spring-loaded clips are used as photomasks. The process is repeated with all colors. Usually the green phosphor is the first to be applied.<ref name="auto6"/><ref>{{cite web |url=http://www.earlytelevision.org/pdf/recent_improvements_in_the_21axp22_color_kinescope.pdf |archive-url=https://web.archive.org/web/20201101112452/http://www.earlytelevision.org/pdf/recent_improvements_in_the_21axp22_color_kinescope.pdf |archive-date=2020-11-01 |url-status=live |title= Recent Improvements in the 21AXP22 Color Kinescope|date=1956 |website=www.earlytelevision.org |access-date=2020-12-11}}</ref><ref name="auto47">{{Cite web|url=https://patents.google.com/patent/US6614160B1/en|title=Fluorescent screen of color CRT and fabricating method thereof|accessdate=18 December 2022}}</ref><ref>{{Cite web|url=https://patents.google.com/patent/US5256463A/en|title=Method for manufacturing color phosphor surface|accessdate=18 December 2022}}</ref> After phosphor application, the screen is baked to eliminate any organic chemicals (such as the PVA that was used to deposit the phosphor) that may remain on the screen.<ref name="auto39"/><ref>{{Cite web |url=https://patents.google.com/patent/US3515553A/en|title=Photolithographic deposition of phosphors on faceplate of crt using spraying of photosensitive pva-phosphor suspension in plural layers |website=Google Patents |date=26 September 1951}}</ref> Alternatively, the phosphors may be applied in a vacuum chamber by evaporating them and allowing them to condense on the screen, creating a very uniform coating.<ref name="auto26"/> Early color CRTs had their phosphors deposited using silkscreen printing.<ref name="auto90"/> Phosphors may have color filters over them (facing the viewer), contain pigment of the color emitted by the phosphor,<ref>{{cite journal |last1=Ohno |first1=K. |last2=Kusunoki |first2=T. |title=ChemInform Abstract: Effect of Ultrafine Pigment Color Filters on Cathode Ray Tube Brightness, Contrast, and Color Purity. |journal=ChemInform |date=5 August 2010 |volume=27 |issue=33 |pages=no |doi=10.1002/chin.199633002 }}</ref><ref name="auto8"/> or be encapsulated in color filters to improve color purity and reproduction while reducing glare.<ref name="auto47"/><ref name="auto14"/> Such technology was sold by Toshiba under the Microfilter brand name.<ref>{{cite web | url=https://books.google.com/books?id=QVZ3k_kTQ-oC&dq=toshiba+microfilter+tube&pg=PA30 | title=PC Mag | date=26 September 1995 }}</ref> Poor exposure due to insufficient light leads to poor phosphor adhesion to the screen, which limits the maximum resolution of a CRT, as the smaller phosphor dots required for higher resolutions cannot receive as much light due to their smaller size.<ref>{{Cite web|url=https://patents.google.com/patent/US6013978A/en|title=Method for producing phosphor screens, and color cathode ray tubes incorporating same|accessdate=18 December 2022}}</ref>

After the screen is coated with phosphor and aluminum and the shadow mask installed onto it the screen is bonded to the funnel using a glass frit that may contain 65–88% of lead oxide by weight. The lead oxide is necessary for the glass frit to have a low melting temperature. Boron oxide (III) may also present to stabilize the frit, with alumina powder as filler powder to control the thermal expansion of the frit.<ref>{{Cite web|url=https://patents.google.com/patent/US6583079B1/en|title=CRT frit capable of sealing a CRT bulb at a relatively low temperature and in a short time|accessdate=18 December 2022}}</ref><ref name="auto81"/><ref name="auto50"/> The frit may be applied as a paste consisting of frit particles suspended in [[amyl acetate]] or in a [[polymer]] with an alkyl methacrylate [[monomer]] together with an organic solvent to dissolve the polymer and monomer.<ref name="auto57">{{cite web |url=https://data.epo.org/publication-server/pdf-document?pn=0889010&ki=A1&cc=EP |title=Sealing glass paste for cathode ray tubes patent applicatrion|date=1999 |website= data.epo.org|format=PDF|access-date=2020-12-11}}</ref><ref>{{Cite web|url=https://patents.google.com/patent/US5281560A/en|title=Non-lead sealing glasses}}</ref> The CRT is then baked in an oven in what is called a Lehr bake, to cure the frit, sealing the funnel and screen together. The frit contains a large quantity of lead, causing color CRTs to contain more lead than their monochrome counterparts. Monochrome CRTs on the other hand do not require frit; the funnel can be fused directly to the glass<ref name="auto104"/> by melting and joining the edges of the funnel and screen using gas flames. Frit is used in color CRTs to prevent deformation of the shadow mask and screen during the fusing process. The edges of the screen and the edges of funnel of the CRT that mate with the screen, are never melted.<ref name="auto6"/> A primer may be applied on the edges of the funnel and screen before the frit paste is applied to improve adhesion.<ref>{{Cite web|url=http://scienceon.kisti.re.kr/srch/selectPORSrchPatent.do?cn=USP1988114788471|title=Sealing for CRT components|publisher=Zenith Electronics Corporation|date=November 21, 1986|website=ScienceON}}</ref> The Lehr bake consists of several successive steps that heat and then cool the CRT gradually until it reaches a temperature of 435–475&nbsp;°C<ref name="auto57"/> (other sources may state different temperatures, such as 440&nbsp;°C)<ref>{{cite web |url=https://patentimages.storage.googleapis.com/pdfs/US5355051.pdf |archive-url=https://web.archive.org/web/20201031225810/https://patentimages.storage.googleapis.com/pdfs/US5355051.pdf |archive-date=2020-10-31 |url-status=live |title=Patent data |website=patentimages.storage.googleapis.com |access-date=2020-12-11}}</ref> After the Lehr bake, the CRT is flushed with air or nitrogen to remove contaminants, the electron gun is inserted and sealed into the neck of the CRT, and a vacuum is formed on the CRT.<ref>{{Cite web|url=https://patents.google.com/patent/US3658401A/en|title=Method of manufacture of cathode ray tubes having frit-sealed envelope assemblies|accessdate=18 December 2022}}</ref><ref name="auto75"/>

====Convergence and purity in color CRTs====
Due to limitations in the dimensional precision with which CRTs can be manufactured economically, it has not been practically possible to build color CRTs in which three electron beams could be aligned to hit phosphors of respective color in acceptable coordination, solely on the basis of the geometric configuration of the electron gun axes and gun aperture positions, shadow mask apertures, etc. The shadow mask ensures that one beam will only hit spots of certain colors of phosphors, but minute variations in physical alignment of the internal parts among individual CRTs will cause variations in the exact alignment of the beams through the shadow mask, allowing some electrons from, for example, the red beam to hit, say, blue phosphors, unless some individual compensation is made for the variance among individual tubes.

Color convergence and color purity are two aspects of this single problem. Firstly, for correct color rendering it is necessary that regardless of where the beams are deflected on the screen, all three hit the same spot (and nominally pass through the same hole or slot) on the shadow mask.{{clarify|reason=See Talk page for apparent contradiction|date=August 2019}} This is called convergence.<ref>{{cite web |url=http://www.ultimateavmag.com/howto/305picture/|archive-url=https://web.archive.org/web/20091126054428/http://www.ultimateavmag.com/howto/305picture/|archive-date=26 November 2009|title=Picture This|date=March 2005|author=Norton, Thomas J. |work= UltimateAVmag.com}}</ref> More specifically, the convergence at the center of the screen (with no deflection field applied by the yoke) is called static convergence, and the convergence over the rest of the screen area (specially at the edges and corners) is called dynamic convergence.<ref name="auto48"/> The beams may converge at the center of the screen and yet stray from each other as they are deflected toward the edges; such a CRT would be said to have good static convergence but poor dynamic convergence. Secondly, each beam must only strike the phosphors of the color it is intended to strike and no others. This is called purity. Like convergence, there is static purity and dynamic purity, with the same meanings of "static" and "dynamic" as for convergence. Convergence and purity are distinct parameters; a CRT could have good purity but poor convergence, or vice versa. Poor convergence causes color "shadows" or "ghosts" along displayed edges and contours, as if the image on the screen were [[Intaglio (printmaking)|intaglio printed]] with poor registration. Poor purity causes objects on the screen to appear off-color while their edges remain sharp. Purity and convergence problems can occur at the same time, in the same or different areas of the screen or both over the whole screen, and either uniformly or to greater or lesser degrees over different parts of the screen.

[[File:Magnet on TV.webm|thumb|A magnet used on a CRT TV. Note the distortion of the image.]]
The solution to the static convergence and purity problems is a set of color alignment ring magnets installed around the neck of the CRT.<ref>{{Cite web|url=https://www.repairfaq.org/samnew/tvfaq/tvcrtpura.htm|title=SER FAQ: TVFAQ: CRT purity adjustment|website=www.repairfaq.org}}</ref> These movable weak permanent magnets are usually mounted on the back end of the deflection yoke assembly and are set at the factory to compensate for any static purity and convergence errors that are intrinsic to the unadjusted tube. Typically there are two or three pairs of two magnets in the form of rings made of plastic impregnated with a magnetic material, with their [[magnetic fields]] parallel to the planes of the magnets, which are perpendicular to the electron gun axes. Often, one pair of rings has 2 poles, another has 4, and the remaining ring has 6 poles.<ref>{{cite web |url= http://www.arcaderepairtips.com/wp-content/uploads/2016/01/guide_setup_adjust_arcade_monitors_v1.2.0.pdf |archive-url=https://web.archive.org/web/20170612054932/http://www.arcaderepairtips.com/wp-content/uploads/2016/01/guide_setup_adjust_arcade_monitors_v1.2.0.pdf |archive-date=2017-06-12 |url-status=live|title=A Guide to Setup and Adjust the CRT of an Arcade Color Monitor |last= Karlsson|first=Ingvar |website= www.arcaderepairtips.com|access-date=2020-12-11}}</ref> Each pair of magnetic rings forms a single effective magnet whose field [[vector field|vector]] can be fully and freely adjusted (in both direction and magnitude). By rotating a pair of magnets relative to each other, their relative field alignment can be varied, adjusting the effective field strength of the pair. (As they rotate relative to each other, each magnet's field can be considered to have two opposing components at right angles, and these four components [two each for two magnets] form two pairs, one pair reinforcing each other and the other pair opposing and canceling each other. Rotating away from alignment, the magnets' mutually reinforcing field components decrease as they are traded for increasing opposed, mutually cancelling components.) By rotating a pair of magnets together, preserving the relative angle between them, the direction of their collective magnetic field can be varied. Overall, adjusting all of the convergence/purity magnets allows a finely tuned slight electron beam deflection or lateral offset to be applied, which compensates for minor static convergence and purity errors intrinsic to the uncalibrated tube. Once set, these magnets are usually glued in place, but normally they can be freed and readjusted in the field (e.g. by a TV repair shop) if necessary.

On some CRTs, additional fixed adjustable magnets are added for dynamic convergence or dynamic purity at specific points on the screen, typically near the corners or edges. Further adjustment of dynamic convergence and purity typically cannot be done passively, but requires active compensation circuits, one to correct convergence horizontally and another to correct it vertically. In this case the deflection yoke contains convergence coils, a set of two per color, wound on the same core, to which the convergence signals are applied. That means 6 convergence coils in groups of 3, with 2 coils per group, with one coil for horizontal convergence correction and another for vertical convergence correction, with each group sharing a core. The groups are separated 120° from one another. Dynamic convergence is necessary because the front of the CRT and the shadow mask are not spherical, compensating for electron beam defocusing and astigmatism. The fact that the CRT screen is not spherical<ref>{{cite web |url=https://wiki.arcadeotaku.com/images/d/dc/Nanaoms2932.pdf |archive-url=https://web.archive.org/web/20170605081955/http://wiki.arcadeotaku.com/images/d/dc/Nanaoms2932.pdf |archive-date=2017-06-05 |url-status=live |title= General notice|website=wiki.arcadeotaku.com |access-date=2020-12-11}}</ref> leads to geometry problems which may be corrected using a circuit.<ref>{{Cite web|url=https://patents.google.com/patent/US3825796A/en|title=Crt geometry correction network|accessdate=18 December 2022}}</ref> The signals used for convergence are parabolic waveforms derived from three signals coming from a vertical output circuit. The parabolic signal is fed into the convergence coils, while the other two are sawtooth signals that, when mixed with the parabolic signals, create the necessary signal for convergence. A resistor and diode are used to lock the convergence signal to the center of the screen to prevent it from being affected by the static convergence. The horizontal and vertical convergence circuits are similar. Each circuit has two resonators, one usually tuned to 15,625&nbsp;Hz and the other to 31,250&nbsp;Hz, which set the frequency of the signal sent to the convergence coils.<ref>{{Cite book|url=https://books.google.com/books?id=53nnX4fnnNIC&q=crt+convergence&pg=PA110|title=Colour Television: Theory and Practice|date=March 1994|publisher=McGraw-Hill Education |isbn=9780074600245}}</ref> Dynamic convergence may be accomplished using electrostatic quadrupole fields in the electron gun.<ref>{{Cite web|url=https://patents.google.com/patent/EP0739028A2/en|title=Method and apparatus for controlling dynamic convergence of a plurality of electron beams of a color cathode ray tube|accessdate=18 December 2022}}</ref> Dynamic convergence means that the electron beam does not travel in a perfectly straight line between the deflection coils and the screen, since the convergence coils cause it to become curved to conform to the screen.

The convergence signal may instead be a sawtooth signal with a slight sine wave appearance, the sine wave part is created using a capacitor in series with each deflection coil. In this case, the convergence signal is used to drive the deflection coils. The sine wave part of the signal causes the electron beam to move more slowly near the edges of the screen. The capacitors used to create the convergence signal are known as the s-capacitors. This type of convergence is necessary due to the high deflection angles and flat screens of many CRT computer monitors. The value of the s-capacitors must be chosen based on the scan rate of the CRT, so multi-syncing monitors must have different sets of s-capacitors, one for each refresh rate.<ref name="auto4"/>

Dynamic convergence may instead be accomplished in some CRTs using only the ring magnets, magnets glued to the CRT, and by varying the position of the deflection yoke, whose position may be maintained using set screws, a clamp and rubber wedges.<ref name="auto48"/><ref>{{Cite web|url=https://patents.google.com/patent/CA1267682A/en|title=Deflection yoke for adhesive assembly and mounting|accessdate=18 December 2022}}</ref> 90° deflection angle CRTs may use "self-convergence" without dynamic convergence, which together with the in-line triad arrangement, eliminates the need for separate convergence coils and related circuitry, reducing costs. complexity and CRT depth by 10 millimeters. Self-convergence works by means of "nonuniform" magnetic fields. Dynamic convergence is necessary in 110° deflection angle CRTs, and quadrupole windings on the deflection yoke at a certain frequency may also be used for dynamic convergence.<ref>{{cite web |url=http://sbe.org/handbook/fundamentals/Video/Video-Electron_Optics.pdf |title= Video electron optics|website=sbe.org/handbook |access-date=2020-12-11}}</ref>

Dynamic color convergence and purity are one of the main reasons why until late in their history, CRTs were long-necked (deep) and had biaxially curved faces; these geometric design characteristics are necessary for intrinsic passive dynamic color convergence and purity. Only starting around the 1990s did sophisticated active dynamic convergence compensation circuits become available that made short-necked and flat-faced CRTs workable. These active compensation circuits use the deflection yoke to finely adjust beam deflection according to the beam target location. The same techniques (and major circuit components) also make possible the adjustment of display image rotation, skew, and other complex [[raster graphics|raster]] geometry parameters through electronics under user control.<ref name="auto4"/>

Alternatively, the guns can be aligned with one another (converged) using convergence rings placed right outside the neck; with one ring per gun. The rings can have north and south poles. There can be 4 sets of rings, one to adjust RGB convergence, a second to adjust Red and Blue convergence, a third to adjust vertical raster shift, and a fourth to adjust purity. The vertical raster shift adjusts the straightness of the scan line. CRTs may also employ dynamic convergence circuits, which ensure correct convergence at the edges of the CRT. Permalloy magnets may also be used to correct the convergence at the edges. Convergence is carried out with the help of a crosshatch (grid) pattern.<ref name=":98">{{cite web |url=http://educypedia.karadimov.info/library/DX2P.pdf |archive-url=https://web.archive.org/web/20160428122828/http://educypedia.karadimov.info/library/DX2P.pdf |archive-date=2016-04-28 |url-status=live |title=1DX2P Geometry, Convergence, and Purity Adjustments |website=educypedia.karadimov.info |access-date=2020-12-11}}</ref><ref name=":99">{{Cite web|url=https://www.repairfaq.org/samnew/tvfaq/tvcrtcona.htm|title=SER FAQ: TVFAQ: CRT convergence adjustment |website=www.repairfaq.org}}</ref> Other CRTs may instead use magnets that are pushed in and out instead of rings.<ref name="auto96"/> In early color CRTs, the holes in the shadow mask became progressively smaller as they extended outwards from the center of the screen, to aid in convergence.<ref name="auto55"/>

====Magnetic shielding and degaussing====
[[File:Degauss-in-progress at Dell-Trinitron-monitor.jpg|thumb|256px|A degaussing in progress]]
[[File:Mu metal CRT shields.jpg|thumb|[[Mu metal]] magnetic shields for oscilloscope CRTs]]
If the shadow mask or aperture grille becomes magnetized, its magnetic field alters the paths of the electron beams. This causes errors of "color purity" as the electrons no longer follow only their intended paths, and some will hit some phosphors of colors other than the one intended. For example, some electrons from the red beam may hit blue or green phosphors, imposing a magenta or yellow tint to parts of the image that are supposed to be pure red. (This effect is localized to a specific area of the screen if the magnetization is localized.) Therefore, it is important that the shadow mask or aperture grille not be magnetized. The earth's magnetic field may have an effect on the color purity of the CRT.<ref name=":98" /> Because of this, some CRTs have external magnetic shields over their funnels. The magnetic shield may be made of soft iron or mild steel and contain a degaussing coil.<ref>{{Cite web|url=https://www.freepatentsonline.com/4943755.html|title=Magnetic shielding with constant-current coils for CRT|accessdate=18 December 2022}}</ref> The magnetic shield and shadow mask may be permanently magnetized by the earth's magnetic field, adversely affecting color purity when the CRT is moved. This problem is solved with a built-in degaussing coil, found in many TVs and computer monitors. Degaussing may be automatic, occurring whenever the CRT is turned on.<ref>{{Cite web|url=https://www.repairfaq.org/samnew/tvfaq/tvdegacrt.htm|title=SER FAQ: TVFAQ: Degaussing (demagnetizing) a CRT|website=www.repairfaq.org}}</ref><ref name="auto6"/> The magnetic shield may also be internal, being on the inside of the funnel of the CRT.<ref>{{Cite web|url=https://patents.google.com/patent/US6768253B1/en|title=Inner magnetic shield and cathode-ray tube|accessdate=18 December 2022}}</ref><ref>{{Cite web|url=https://patents.google.com/patent/US6768252B2/en|title=Magnetic shield structure for color cathode ray tube|accessdate=18 December 2022}}</ref><ref name="auto4"/><ref>{{Cite web|url=https://books.google.com/books?id=35EbAQAAMAAJ&q=crt+internal+magnetic+shield&pg=RA1-PA501|title = Official Gazette of the United States Patent and Trademark Office: Patents|date = July 1994}}</ref><ref>{{Cite web|url=https://patents.google.com/patent/US6911769B2/en|title=Lateral magnetic shielding for color CRT|date=20 September 2001 |last1=Antonelli |first1=Goffredo |last2=Paolis |first2=Cesare De |last3=Giannantonio |first3=Giuseppe |last4=Ginesti |first4=Paolo }}</ref><ref>{{Cite web|url=https://patents.google.com/patent/US5804912A/en|title=Magnetic shielding CRT|accessdate=18 December 2022}}</ref>

Color CRT displays in TV sets and computer monitors often have a built-in [[degaussing]] (demagnetizing) coil mounted around the perimeter of the CRT face. Upon power-up of the CRT display, the degaussing circuit produces a brief, alternating current through the coil which fades to zero over a few seconds, producing a decaying alternating magnetic field from the coil. This degaussing field is strong enough to remove shadow mask magnetization in most cases, maintaining color purity.<ref>{{cite web |url=http://www.pcguide.com/ref/crt/crtGauss-c.html|title=Magnetization and Degaussing|access-date=4 October 2009}}</ref><ref>{{Cite web|url=https://www.repairfaq.org/samnew/tvfaq/tvcrtpca.htm|title=SER FAQ: TVFAQ: CRT purity and convergence problems|website=www.repairfaq.org}}</ref> In unusual cases of strong magnetization where the internal degaussing field is not sufficient, the shadow mask may be degaussed externally with a stronger portable degausser or demagnetizer. However, an excessively strong magnetic field, whether alternating or constant, may mechanically [[deformation (engineering)|deform]] (bend) the shadow mask, causing a permanent color distortion on the display which looks very similar to a magnetization effect.

====Resolution====
[[Dot pitch]] defines the maximum resolution of the display, assuming delta-gun CRTs. In these, as the scanned resolution approaches the dot pitch resolution, [[moiré]] appears, as the detail being displayed is finer than what the shadow mask can render.<ref>{{cite web |url=http://www.displaymate.com/moire.html |title= Moiré Interference Patterns |access-date=4 October 2006 |work=DisplayMate Technologies website}}</ref> Aperture grille monitors do not suffer from vertical moiré, however, because their phosphor stripes have no vertical detail. In smaller CRTs, these strips maintain position by themselves, but larger aperture-grille CRTs require one or two crosswise (horizontal) support strips; one for smaller CRTs, and two for larger ones. The support wires block electrons, causing the wires to be visible.<ref>{{cite web |url=http://computer.howstuffworks.com/question406.htm |title=What causes the faint horizontal lines on my monitor? |access-date=4 October 2009 |work=HowStuffWorks|date=21 June 2000 }}</ref> In aperture grille CRTs, dot pitch is replaced by stripe pitch. Hitachi developed the Enhanced Dot Pitch (EDP) shadow mask, which uses oval holes instead of circular ones, with respective oval phosphor dots.<ref name="auto14"/> Moiré is reduced in shadow mask CRTs by arranging the holes in the shadow mask in a honeycomb-like pattern.<ref name="auto4"/>