Editing Cathode-ray tube (section)

===Electron gun===
{{Main|Electron gun}}
The electron gun emits the electrons that ultimately hit the phosphors on the screen of the CRT. The electron gun contains a heater, which heats a cathode, which generates electrons that, using grids, are focused and ultimately accelerated into the screen of the CRT. The acceleration occurs in conjunction with the inner aluminum or aquadag coating of the CRT. The electron gun is positioned so that it aims at the center of the screen.<ref name="auto6"/> It is inside the neck of the CRT, and it is held together and mounted to the neck using glass beads or glass support rods, which are the glass strips on the electron gun.<ref name="auto68"/><ref name="auto6"/><ref>{{Cite patent |country=US |number=4409279A |url=https://patents.google.com/patent/US4409279A/en |title=Glass support rod for use in electron-gun mount assemblies}}</ref> The electron gun is made separately and then placed inside the neck through a process called "winding", or sealing.<ref name="auto37"/><ref>{{cite web | title=CRT Manufacturing | url=https://vintagetek.org/crt-manufacturing/ | access-date=2020-12-11}}</ref><ref name="auto83">{{Cite patent |country=US |number=4561874A |url=https://patents.google.com/patent/US4561874A/en |title=Method for heat sealing a gun mount in a CRT neck}}</ref><ref name="auto75">{{Cite web|url=http://www.lgphilips-displays.com/english/download/CPT_Manu_Process.pdf|title=Manual|date=2 May 2006|archive-url=https://web.archive.org/web/20060502053742/http://www.lgphilips-displays.com/english/download/CPT_Manu_Process.pdf|archive-date=2 May 2006}}</ref><ref>{{Cite web|url=https://www.earlytelevision.org/14ap4_construction.html|title=DuMont 14AP4|website=www.earlytelevision.org}}</ref><ref name="auto65">{{Cite web|url=https://venturebeat.com/2017/03/03/what-the-death-of-the-crt-display-technology-means-for-classic-arcade-machines/|title=Donkey Kong's failing liver: What the death of the CRT display technology means for classic arcade machines|date=3 March 2017}}</ref> The electron gun has a glass wafer that is fused to the neck of the CRT. The connections to the electron gun penetrate the glass wafer.<ref name="auto83"/><ref>{{Cite patent |country=US |number=6078134A |url=https://patents.google.com/patent/US6078134A/en |title=Narrow-neck CRT having a large stem pin circle}}</ref> Once the electron gun is inside the neck, its metal parts (grids) are arced between each other using high voltage to smooth any rough edges in a process called spot knocking, to prevent the rough edges in the grids from generating secondary electrons.<ref>{{Cite patent |country=EP |number=0634771B1 |url=https://patents.google.com/patent/EP0634771B1/un |title=Method for spot-knocking an electron gun assembly of a cathode ray tube}}</ref><ref>{{Cite patent |country=US |number=4883438A |url=https://patents.google.com/patent/US4883438A/en |title=Method for spot-knocking an electron gun mount assembly of a CRT}}</ref><ref>{{Cite patent |country=US |number=4457731A |url=https://patents.google.com/patent/US4457731A/en |title=Cathode ray tube processing}}</ref>

====Construction and method of operation====
The electron gun has an indirectly heated [[hot cathode]] that is heated by a tungsten filament heating element; the heater may draw 0.5–2&nbsp;A of current depending on the CRT. The voltage applied to the heater can affect the life of the CRT.<ref>Practical Television, June 2001 issue<!--https://worldradiohistory.com/hd2/IDX-UK/Technology/Technology-All-Eras/Archive-Practical-Television-IDX/00s/Television-2001-06-OCR-Page-0034.pdf--></ref><ref>{{cite web |url=https://elektrotanya.com/files/forum/2014/03/kepcsofutes-mero_[ET].pdf |title=CRT heater voltages|date=2001 |website=elektrotanya.com|access-date=2020-12-11}}</ref> Heating the cathode energizes the electrons in it, aiding electron emission,<ref name="auto77">{{Cite patent |country=JP |number=H07245056A |url=https://patents.google.com/patent/JPH07245056A/en |title=Indirectly heated cathode body structure for cathode-ray tube}}</ref> while at the same time current is supplied to the cathode; typically anywhere from 140&nbsp;mA at 1.5&nbsp;V to 600&nbsp;mA at 6.3&nbsp;V.<ref name="auto100">{{Cite web|url=https://electronicspost.com/function-of-electron-gun-assembly-in-crt-cathode-ray-tube/|title=Function of Electron Gun Assembly in CRT (Cathode Ray Tube)|date=16 November 2015}}</ref> The cathode creates an electron cloud (emits electrons) whose electrons are extracted, accelerated and focused into an electron beam.<ref name="auto68"/> Color CRTs have three cathodes: one for red, green and blue. The heater sits inside the cathode but does not touch it; the cathode has its own separate electrical connection. The cathode is a material coated onto a piece of nickel which provides the electrical connection and structural support; the heater sits inside this piece without touching it.<ref name="auto54"/><ref>{{Cite patent |country=US |number=4305188A |url=https://patents.google.com/patent/US4305188A/en |title=Method of manufacturing cathode assembly}}</ref><ref>{{Cite web|url=https://www.circuitstoday.com/crt-cathode-ray-tube|title=CRT-Cathode Ray Tube|date=25 September 2009}}</ref><ref>{{Cite magazine|last=Blackburn |first=A. P.|url=http://www.r-type.org/articles/art-116.htm |title=The Cathode-Ray Tube|magazine=The Radio Constructor|location= United Kingdom|date= August 1955}}</ref>

There are several [[short circuit]]s that can occur in a CRT electron gun. One is a heater-to-cathode short, that causes the cathode to permanently emit electrons which may cause an image with a bright red, green or blue tint with retrace lines, depending on the cathode (s) affected. Alternatively, the cathode may short to the control grid, possibly causing similar effects, or, the control grid and screen grid (G2)<ref>{{cite web |url=http://lateblt.tripod.com/bit71.txt |title=Principles Of A CRT |website=lateblt.tripod.com|access-date=2020-12-11}}</ref> can short causing a very dark image or no image at all. The cathode may be surrounded by a shield to prevent [[sputtering]].<ref>{{Cite patent |country=FR |number=2691577A1 |url=https://patents.google.com/patent/FR2691577A1/en |title=Cathode assembly for CRT electron gun - has protective screen around cathode emitter between emitter and hole in insulator support of cylindrical grid electrode}}</ref><ref name="auto">{{cite web |url= http://www.thegleam.com/ke5fx/crt/sencrt.pdf |archive-url=https://web.archive.org/web/20110422131015/http://www.thegleam.com/ke5fx/crt/sencrt.pdf |archive-date=2011-04-22 |url-status=live|title=Sencore Blows Away CRT Failures With CR7000 |last=Fromm |first= Randy |website= www.thegleam.com|access-date=2020-12-11}}</ref>

The cathode is a layer of barium oxide which is coated on a piece of nickel for electrical and mechanical support.<ref name="auto87">{{Cite patent |country=CN |number=1400621A |url=https://patents.google.com/patent/CN1400621A/en |title=Electronic tube cathode, long-life electronic tube tube and its making process}}</ref><ref name="auto63"/> The barium oxide must be activated by heating to enable it to release electrons. Activation is necessary because barium oxide is not stable in air, so it is applied to the cathode as barium carbonate, which cannot emit electrons. Activation heats the barium carbonate to decompose it into barium oxide and carbon dioxide while forming a thin layer of metallic barium on the cathode.<ref name=Gassler2016/><ref name="auto87"/> Activation is done when forming the vacuum (described in {{Slink|Cathode-ray tube|Evacuation|nopage=y}}). After activation, the oxide can become damaged by several common gases such as water vapor, carbon dioxide, and oxygen.<ref name="ba"/> Alternatively, barium strontium calcium carbonate may be used instead of barium carbonate, yielding barium, strontium and calcium oxides after activation.<ref>{{Cite patent |country=JP |number=2004022271A |url=https://patents.google.com/patent/JP2004022271A/en |title=Cathode-ray tube}}</ref><ref name="auto68"/> During operation, the barium oxide is heated to 800–1000°C, at which point it starts shedding electrons.<ref>{{Cite web|url=https://books.google.com/books?id=GwIAAAAAMBAJ&q=CRT+corner+convergence&pg=PT90|title=Maximum PC|date=5 August 1999|publisher=Future US|via=Google Books}}</ref><ref name="auto63"/><ref name="auto77"/>

Since it is a hot cathode, it is prone to cathode poisoning, which is the formation of a positive ion layer that prevents the cathode from emitting electrons, reducing image brightness significantly or completely and causing focus and intensity to be affected by the frequency of the video signal preventing detailed images from being displayed by the CRT. The positive ions come from leftover air molecules inside the CRT or from the cathode itself<ref name="auto68"/> that react over time with the surface of the hot cathode.<ref name="auto71">{{Cite web|url=http://www.ke5fx.com/crt.html|title=CRT Restoration for the (Brave) Experimenter|website=www.ke5fx.com}}</ref><ref name="auto"/> Reducing metals such as manganese, zirconium, magnesium, aluminum or titanium may be added to the piece of nickel to lengthen the life of the cathode, as during activation, the reducing metals diffuse into the barium oxide, improving its lifespan, especially at high electron beam currents.<ref>{{Cite patent |country=KR |number=100490170B1 |url=https://patents.google.com/patent/KR100490170B1/en |title=Cathode of CRT}}</ref> In color CRTs with red, green and blue cathodes, one or more cathodes may be affected independently of the others, causing total or partial loss of one or more colors.<ref name="auto"/> CRTs can wear or burn out due to cathode poisoning. Cathode poisoning is accelerated by increased cathode current (overdriving).<ref name="auto25">{{Cite web|url=https://www.extremetech.com/electronics/53826-crt-innovations/4|title=CRT Innovations - Page 4 of 7 - ExtremeTech|website=www.extremetech.com|access-date=8 December 2020|archive-date=6 October 2020|archive-url=https://web.archive.org/web/20201006072651/http://www.extremetech.com/electronics/53826-crt-innovations/4|url-status=dead}}</ref> In color CRTs, since there are three cathodes, one for red, green and blue, a single or more poisoned cathode may cause the partial or complete loss of one or more colors, tinting the image.<ref name="auto"/> The layer may also act as a capacitor in series with the cathode, inducing thermal lag. The cathode may instead be made of scandium oxide or incorporate it as a dopant, to delay cathode poisoning, extending the life of the cathode by up to 15%.<ref name="auto26">{{Cite web|url=https://www.extremetech.com/electronics/53826-crt-innovations/5|title=CRT Innovations - Page 5 of 7 - ExtremeTech|website=www.extremetech.com|access-date=8 December 2020|archive-date=1 November 2020|archive-url=https://web.archive.org/web/20201101022250/https://www.extremetech.com/electronics/53826-crt-innovations/5|url-status=dead}}</ref><ref name="auto63"/><ref>{{Cite book|url=https://books.google.com/books?id=FvkqeL4IDMwC&q=barium+oxide&pg=PA9|title=Image Performance in CRT Displays|first=Kenneth|last=Compton|date=5 December 2003|publisher=SPIE Press|isbn=9780819441447|via=Google Books}}</ref>

The rate of emission of electrons from the cathodes is related to their surface area. A cathode with more surface area creates more electrons, in a larger electron cloud, which makes focusing the electron cloud into an electron beam more difficult.<ref name="auto25"/> Normally, only a part of the cathode emits electrons unless the CRT displays images with parts that are at full image brightness; only the parts at full brightness cause all of the cathode to emit electrons. The area of the cathode that emits electrons grows from the center outwards as brightness increases, so cathode wear may be uneven. When only the center of the cathode is worn, the CRT may light brightly those parts of images that have full image brightness but not show darker parts of images at all, in such a case the CRT displays a poor gamma characteristic.<ref name="auto"/>

A voltage negative with respect to the cathode<ref>{{Cite book|url=https://books.google.com/books?id=gw8SBQAAQBAJ&q=crt+aluminized+screen&pg=PA129|title=Instrumentation Systems: Jones' Instrument Technology|first=B. E.|last=Noltingk|date=6 February 2016|publisher=Elsevier|isbn=9781483135601|via=Google Books}}</ref> is applied to the first (control) grid (G1) to control the emission of electrons into the rest of the electron gun. G1 in practice is a [[Wehnelt cylinder]].<ref name="auto100"/><ref>{{Cite web|url=https://www.radartutorial.eu/12.scopes/sc15.en.html|title=Radartutorial|website=www.radartutorial.eu}}</ref> The brightness of the image on the screen depends on both the anode voltage and the electron beam current and in practise the latter is constant, while the former is controlled by varying the difference in voltage between the cathode and the G1 control grid. The second (screen) grid of the gun (G2) then accelerates the electrons towards the screen using several hundred DC volts. Then a third grid (G3) electrostatically focuses the electron beam before it is deflected and later accelerated by the anode voltage onto the screen.<ref>{{Cite web|url=https://www.jmargolin.com/xy/xymon.htm|title=The Secret Life of XY Monitors|website=www.jmargolin.com}}</ref> Electrostatic focusing of the electron beam may be accomplished using an [[einzel lens]] energized at up to 600 volts.<ref>{{Cite patent |country=US |number=5382883A |url=https://patents.google.com/patent/US5382883A/en |title=Multi-beam group electron gun with common lens for color CRT}}</ref><ref name=Gassler2016>{{cite book |doi=10.1007/978-3-319-14346-0_70 |chapter=Cathode Ray Tubes (CRTS) |title=Handbook of Visual Display Technology |year=2016 |last1=Gassler |first1=Gerhard |pages=1595–1607 |publisher=Springer |location=Cham |isbn=978-3-319-14345-3 }}</ref> Before electrostatic focusing, focusing the electron beam required a large, heavy and complex mechanical focusing system placed outside the electron gun.<ref name="auto53"/>

However, electrostatic focusing cannot be accomplished near the final anode of the CRT due to its high voltage in the dozens of Kilovolts, so a high voltage (≈600–8000&nbsp;V)<ref>{{Cite web|url=http://bunkerofdoom.com/crt10sp4/index.html|title=Round CRT for Video or Computer|website=bunkerofdoom.com}}</ref> electrode, together with an electrode at the final anode voltage of the CRT, may be used for focusing instead. Such an arrangement is called a bipotential lens, which also offers higher performance than an einzel lens, or, focusing may be accomplished using a magnetic focusing coil together with a high anode voltage of dozens of kilovolts. However, magnetic focusing is expensive to implement, so it is rarely used in practice.<ref name="auto54"/><ref name=Gassler2016/><ref>{{Cite web|url=http://repairfaq.cis.upenn.edu/Misc/REPAIR/F_monfaqa.html#MONFAQA_019|title=Sci.Electronics.Repair FAQ: Notes on the Troubleshooting and Repair of Computer and Video Monitors|website=repairfaq.cis.upenn.edu|access-date=8 December 2020|archive-date=31 October 2020|archive-url=https://web.archive.org/web/20201031113759/http://repairfaq.cis.upenn.edu/Misc/REPAIR/F_monfaqa.html#MONFAQA_019|url-status=dead}}</ref><ref>{{Cite book|author= Thorn-AEI Radio Valves and Tubes Limited |url=http://www.r-type.org/articles/art-004f.htm |title=Electrons in Shadow-mask Colour Tubes|location= United Kingdom|date=1968}}</ref> Some CRTs may use two grids and lenses to focus the electron beam.<ref name="auto26"/> The focus voltage is generated in the flyback using a subset of the flyback's high voltage winding in conjunction with a resistive voltage divider. The focus electrode is connected alongside the other connections that are in the neck of the CRT.<ref>{{Cite web|url=http://repairfaq.cis.upenn.edu/samnew/tvfaq/tvfocdrift.htm|title=SER FAQ: TVFAQ: Focus drift with warmup|website=repairfaq.cis.upenn.edu|access-date=8 December 2020|archive-date=30 October 2020|archive-url=https://web.archive.org/web/20201030215231/http://repairfaq.cis.upenn.edu/samnew/tvfaq/tvfocdrift.htm|url-status=dead}}</ref>

There is a voltage called cutoff voltage which is the voltage that creates black on the screen since it causes the image on the screen created by the electron beam to disappear, the voltage is applied to G1. In a color CRT with three guns, the guns have different cutoff voltages. Many CRTs share grid G1 and G2 across all three guns, increasing image brightness and simplifying adjustment since on such CRTs there is a single cutoff voltage for all three guns (since G1 is shared across all guns).<ref name="auto6"/> but placing additional stress on the video amplifier used to feed video into the electron gun's cathodes, since the cutoff voltage becomes higher. Monochrome CRTs do not suffer from this problem. In monochrome CRTs video is fed to the gun by varying the voltage on the first control grid.<ref>{{Cite web|url=https://www.radartutorial.eu/12.scopes/sc16.en.html|title=Radartutorial|website=www.radartutorial.eu}}</ref><ref name="auto53"/>

During retracing of the electron beam, the preamplifier that feeds the video amplifier is disabled and the video amplifier is biased to a voltage higher than the cutoff voltage to prevent retrace lines from showing, or G1 can have a large negative voltage applied to it to prevent electrons from getting out of the cathode.<ref name="auto68"/> This is known as blanking. (see [[Vertical blanking interval]] and [[Horizontal blanking interval]].) Incorrect biasing can lead to visible retrace lines on one or more colors, creating retrace lines that are tinted or white (for example, tinted red if the red color is affected, tinted magenta if the red and blue colors are affected, and white if all colors are affected).<ref>{{Cite web|url=https://www.repairfaq.org/samnew/tvfaq/tvrgbrlin.htm|title=SER FAQ: TVFAQ: Red, green, or blue retrace lines|website=www.repairfaq.org}}</ref><ref>{{Cite web|url=https://www.repairfaq.org/samnew/tvfaq/tvwgrlin.htm|title=SER FAQ: TVFAQ: White/gray retrace lines|website=www.repairfaq.org}}</ref><ref name="auto32">{{cite web |url=https://www.ti.com/lit/an/snla017/snla017.pdf?ts=1599041272338&ref_url=https%253A%252F%252Fwww.google.com%252F |title= AN-656 Understanding the Operation of a CRT Monitor |website=www.ti.com |format=PDF|access-date=2020-12-11}}</ref> Alternatively, the amplifier may be driven by a video processor that also introduces an OSD (On Screen Display) into the video stream that is fed into the amplifier, using a fast blanking signal.<ref>{{cite web |url= https://www.st.com/resource/en/datasheet/CD00003613.pdf |archive-url=https://web.archive.org/web/20210227224753/https://www.st.com/resource/en/datasheet/CD00003613.pdf |archive-date=2021-02-27 |url-status=live|title=Data |website=www.st.com |access-date=2020-12-11}}</ref> TV sets and computer monitors that incorporate CRTs need a DC restoration circuit to provide a video signal to the CRT with a DC component, restoring the original brightness of different parts of the image.<ref>{{cite web |url=https://www.ti.com/lit/an/snla017/snla017.pdf |title=AN-656 Understanding the Operation of a CRT Monitor |website=www.ti.com |access-date=2020-12-11}}</ref>

The electron beam may be affected by the Earth's magnetic field, causing it to normally enter the focusing lens off-center; this can be corrected using astigmation controls. Astigmation controls are both magnetic and electronic (dynamic); magnetic does most of the work while electronic is used for fine adjustments.<ref>{{Cite web|url=http://www.curtpalme.com/Focus_and_Mechanical_Aim7.shtm|title=CRT Projector Focus & Mechanical Aim Basics by Guy Kuo|website=www.curtpalme.com}}</ref> One of the ends of the electron gun has a glass disk, the edges of which are fused with the edge of the neck of the CRT, possibly using [[frit]];<ref>{{Cite patent |country=US |number=6139388A |url=https://patents.google.com/patent/US6139388A/en |title=Method of forming a frit seal between a stem and a neck of a cathode ray tube during manufacturing of a cathode ray tube}}</ref> the metal leads that connect the electron gun to the outside pass through the disk.<ref>{{Cite patent |country=US |number=6677701B2 |url=https://patents.google.com/patent/US6677701B2/en |title=Stem for cathode ray tube}}</ref>

Some electron guns have a quadrupole lens with dynamic focus to alter the shape and adjust the focus of the electron beam, varying the focus voltage depending on the position of the electron beam to maintain image sharpness across the entire screen, specially at the corners.<ref name="auto4"/><ref>{{cite journal |last1=Bae |first1=Mincheol |last2=Song |first2=Yongseok |last3=Hong |first3=Younggon |last4=Kwon |first4=Yonggeol |last5=Lee |first5=Kwangsik |title=42.1: A New Electron Gun for a 32ʺ Super Wide Deflection Angle (120°) CRT |journal=SID Symposium Digest of Technical Papers |date=2001 |volume=32 |issue=1 |pages=1112 |doi=10.1889/1.1831753 |s2cid=110552592 }}</ref><ref name="auto61">{{cite thesis |last1=Sluijterman |first1=AAS Seyno |title=Innovative use of magnetic quadrupoles in cathode-ray tubes |date=2002 |doi=10.6100/IR555490 }}</ref><ref>{{Cite patent |country=US |number=4230972A |url=https://patents.google.com/patent/US4230972A/en |title=Dynamic focus circuitry for a CRT data display terminal}}</ref><ref>{{Cite web|url=http://repairfaq.cis.upenn.edu/Misc/REPAIR/F_monfaqb.html|title=Sci.Electronics.Repair FAQ: Notes on the Troubleshooting and Repair of Computer and Video Monitors|website=repairfaq.cis.upenn.edu|access-date=8 December 2020|archive-date=13 December 2020|archive-url=https://web.archive.org/web/20201213200911/http://repairfaq.cis.upenn.edu/Misc/REPAIR/F_monfaqb.html|url-status=dead}}</ref> They may also have a bleeder resistor to derive voltages for the grids from the final anode voltage.<ref>{{Cite web|url=http://www.lgphilips-displays.com:80/english/download/hdfolderdec2002.pdf|date=26 October 2005|archive-url=https://web.archive.org/web/20051026141832/http://www.lgphilips-displays.com/english/download/hdfolderdec2002.pdf|title=Data|archive-date=26 October 2005|access-date=6 February 2021|url-status=live}}</ref><ref>{{Cite patent |country=US |number=4682962A |url=https://patents.google.com/patent/US4682962A/en |title=Method of manufacturing a cathode ray tube}}</ref><ref>{{Cite patent |country=KR |number=830000491B1 |url=https://patents.google.com/patent/KR830000491B1/en |title=Partial voltage resistor of electron gun structure}}</ref>

After the CRTs were manufactured, they were aged to allow cathode emission to stabilize.<ref>{{Cite patent |country=US |number=4832646A |url=https://patents.google.com/patent/US4832646A/en |title=Aging process for cathode ray tubes}}</ref><ref>{{Cite patent |country=JP |number=2000082402A |url=https://patents.google.com/patent/JP2000082402A/en |title=Aging device for cathode-ray tube}}</ref>

The electron guns in color CRTs are driven by a video amplifier which takes a signal per color channel and amplifies it to 40–170&nbsp;V per channel, to be fed into the electron gun's cathodes;<ref name="auto32"/> each electron gun has its own channel (one per color) and all channels may be driven by the same amplifier, which internally has three separate channels.<ref>{{cite web |url=https://www.ti.com/lit/an/snoa268/snoa268.pdf |archive-url=https://web.archive.org/web/20201030080137/https://www.ti.com/lit/an/snoa268/snoa268.pdf |archive-date=2020-10-30 |url-status=live |title= AN-861 Guide to CRT Video Design |website= www.ti.com|access-date=2020-12-11}}</ref> The amplifier's capabilities limit the resolution, refresh rate and contrast ratio of the CRT, as the amplifier needs to provide high bandwidth and voltage variations at the same time; higher resolutions and refresh rates need higher bandwidths (speed at which voltage can be varied and thus switching between black and white) and higher contrast ratios need higher voltage variations or amplitude for lower black and higher white levels. 30&nbsp;MHz of bandwidth can usually provide 720p or 1080i resolution, while 20&nbsp;MHz usually provides around 600 (horizontal, from top to bottom) lines of resolution, for example.<ref name="auto92">{{Cite web|url=https://www.audioholics.com/hdtv-formats/display-technologies-guide-lcd-plasma-dlp-lcos-d-ila-crt/display-technologies-guide-lcd-plasma-dlp-lcos-d-ila-crt-page-7|title=Cathode Ray Tube (CRT) Direct View and Rear Projection TVs|first=Clint|last=DeBoer|website=Audioholics Home Theater, HDTV, Receivers, Speakers, Blu-ray Reviews and News|date=30 August 2004 }}</ref><ref name="auto32"/> The difference in voltage between the cathode and the control grid is what modulates the electron beam, modulating its current and thus creating shades of colors which create the image line by line and this can also affect the brightness of the image.<ref name="auto"/> The phosphors used in color CRTs produce different amounts of light for a given amount of energy, so to produce white on a color CRT, all three guns must output differing amounts of energy. The gun that outputs the most energy is the red gun since the red phosphor emits the least amount of light.<ref name="auto32"/>

====Gamma====
CRTs have a pronounced [[triode]] characteristic, which results in significant [[gamma correction|gamma]] (a nonlinear relationship in an electron gun between applied video voltage and beam intensity).<ref>{{cite web |url=http://broadcastengineering.com/newsrooms/broadcasting_gamma_correction/ |title=Gamma correction |access-date=4 October 2009 |last=Robin |first=Michael |date=1 January 2005 |work=BroadcastEngineering |url-status=dead |archive-url=https://web.archive.org/web/20090531130845/http://broadcastengineering.com/newsrooms/broadcasting_gamma_correction/ |archive-date=31 May 2009}}</ref>