Editing Phosphor (section)

===Cathode-ray tubes===<!-- This section is linked from Cathode-ray tube -->
[[File:CRT phosphors.png|thumb|right|400px|Spectra of constituent blue, green and red phosphors in a common cathode-ray tube]]

[[Cathode-ray tube]]s produce signal-generated light patterns in a (typically) round or rectangular format. Bulky CRTs were used in the black-and-white television (TV) sets that became popular in the 1950s, developed into color CRTs in the late 1960s, and used in virtually all color TVs and computer monitors until the mid-2000s. In the late 20th century, advanced electronics made new wide-deflection, "short tube" CRT technology viable, making CRTs more compact, but still bulky and heavy. As the original video display technology, having no viable competition for more than 40 years and dominance for over 50 years, the CRT ceased to be the main type of video display in use only around 2010. In addition to direct-view CRTs, CRT projection tubes were the basis of all projection TVs and computer video projectors of both front and rear projection types until at least the late 1990s.

CRTs have also been widely used in scientific and engineering instrumentation, such as [[oscilloscope]]s, usually with a single phosphor color, typically green. Phosphors for such applications may have long afterglow, for increased image persistence.  A variation of the display CRT, used prior to the 1980s, was the CRT [[storage tube]], a digital memory device which (in later forms) also provided a visible display of the stored data, using a variation of the same electron-beam excited phosphor technology.

The process of producing light in CRTs by electron-beam excited phosphorescence yields much faster signal response times than even modern (2020s) [[LCD]]s can achieve, which makes [[light pen]]s and light gun games possible with CRTs, but not LCDs.  Also in contrast to most other video display types, because CRT technology draws an image by scanning an electron beam (or a formation of three beams) across a phosphor surface, a CRT has no intrinsic "native resolution" and does not require scaling to display raster images at different resolutions; the CRT can display any raster format natively, within the limits defined by the electron beam spot size and, for a color CRT, the dot pitch of the phosphor. Because of this operating principle, CRTs can produce images using either raster and vector imaging methods.  Vector displays are impossible for display technologies that have permanent discrete pixels, including all LCDs, [[Plasma display|plasma display panels]], [[Digital micromirror device|DMD]] projectors, and [[OLED]] (LED matrix, e.g. TFT OLED) panels.

The phosphors can be deposited as either [[thin film]], or as discrete particles, a powder bound to the surface. Thin films have better lifetime and better resolution, but provide less bright and less efficient image than powder ones. This is caused by multiple internal reflections in the thin film, scattering the emitted light.

'''White''' (in black-and-white): The mix of zinc cadmium sulfide and zinc sulfide silver, the {{chem2|ZnS:Ag + (Zn,Cd)S:Ag}} is the white '''P4''' phosphor used in black and white television CRTs. Mixes of yellow and blue phosphors are usual. Mixes of red, green and blue, or a single white phosphor, can also be encountered.

'''Red:''' [[Yttrium]] [[oxide]]-[[sulfide]] activated with europium is used as the red phosphor in color CRTs. The development of color TV took a long time due to the search for a red phosphor. The first red emitting rare-earth phosphor, YVO<sub>4</sub>:Eu<sup>3+</sup>, was introduced by Levine and Palilla as a primary color in television in 1964.<ref>{{cite journal| doi = 10.1063/1.1723611| title = A new, highly efficient red-emitting cathodoluminescent phosphor (YVO<sub>4</sub>:Eu) for color television| year = 1964|author = Levine, Albert K.| journal = Applied Physics Letters| volume = 5|page = 118| first2 = Frank C.| last2 = Palilla|bibcode = 1964ApPhL...5..118L| issue = 6 }}</ref> In single crystal form, it was used as an excellent polarizer and laser material.<ref>{{cite journal| doi = 10.1063/1.98500| title = Highly efficient Nd:YVO<sub>4</sub> diode-laser end-pumped laser| year = 1987|author = Fields, R. A.| journal = Applied Physics Letters| volume = 51|page = 1885| first2 = M.| first3 = C. L.| last2 = Birnbaum| last3 = Fincher|bibcode = 1987ApPhL..51.1885F| issue = 23 | doi-access = free}}</ref>

'''Yellow:''' When mixed with [[cadmium sulfide]], the resulting '''zinc cadmium sulfide''' {{chem2|(Zn,Cd)S:Ag}}, provides strong yellow light.

'''Green:''' Combination of zinc sulfide with [[copper]], the '''P31''' phosphor or {{chem2|ZnS:Cu}}, provides green light peaking at 531&nbsp;nm, with long glow.

'''Blue:''' Combination of zinc sulfide with few ppm of [[silver]], the ZnS:Ag, when excited by electrons, provides strong blue glow with maximum at 450&nbsp;nm, with short afterglow with 200 nanosecond duration. It is known as the '''P22B''' phosphor. This material, '''zinc sulfide silver''', is still one of the most efficient phosphors in cathode-ray tubes. It is used as a blue phosphor in color CRTs.

The phosphors are usually poor electrical conductors. This may lead to deposition of residual charge on the screen, effectively decreasing the energy of the impacting electrons due to electrostatic repulsion (an effect known as "sticking"). To eliminate this, a thin layer of aluminium (about 100&nbsp;nm) is deposited over the phosphors, usually by vacuum evaporation, and connected to the conductive layer inside the tube. This layer also reflects the phosphor light to the desired direction, and protects the phosphor from ion bombardment resulting from an imperfect vacuum.

To reduce the image degradation by reflection of ambient light, [[Contrast (vision)|contrast]] can be increased by several methods. In addition to black masking of unused areas of screen, the phosphor particles in color screens are coated with pigments of matching color. For example, the red phosphors are coated with [[ferric oxide]] (replacing earlier Cd(S,Se) due to cadmium toxicity), blue phosphors can be coated with marine blue ([[cobalt(II) oxide|CoO]]·''n''[[alumina|{{chem|Al|2|O|3}}]]) or [[ultramarine]] ({{chem|Na|8|Al|6|Si|6|O|24|S|2}}). Green phosphors based on ZnS:Cu do not have to be coated due to their own yellowish color.<ref name="advelectr">{{cite book|author=Peter W. Hawkes |title=Advances in electronics and electron physics |url=https://books.google.com/books?id=FE_Hvcbkpa4C&pg=PA350 |access-date=9 January 2012 |date=1 October 1990 |publisher=Academic Press |isbn=978-0-12-014679-6 |pages=350–}}</ref>

====Black-and-white television CRTs====
The black-and-white television screens require an emission color close to white. Usually, a combination of phosphors is employed.

The most common combination is {{chem2|ZnS:Ag + (Zn,Cd)S:Cu,Al}} (blue + yellow). Other ones are {{chem2|ZnS:Ag + (Zn,Cd)S:Ag}} (blue + yellow), and {{chem2|ZnS:Ag + ZnS:Cu,Al + Y2O2S:Eu(3+)}} (blue + green + red – does not contain cadmium and has poor efficiency). The color tone can be adjusted by the ratios of the components.

As the compositions contain discrete grains of different phosphors, they produce image that may not be entirely smooth. A single, white-emitting phosphor, {{chem2|(Zn,Cd)S:Ag,Au,Al}} overcomes this obstacle. Due to its low efficiency, it is used only on very small screens.

The screens are typically covered with phosphor using sedimentation coating, where particles [[suspension (chemistry)|suspended]] in a solution are let to settle on the surface.<ref name="lumdisp">Lakshmanan, p. 54.</ref>

====Reduced-palette color CRTs====
For displaying of a limited palette of colors, there are a few options.

In [[penetron|'''beam penetration tubes''']], different color phosphors are layered and separated with dielectric material. The acceleration voltage is used to determine the energy of the electrons; lower-energy ones are absorbed in the top layer of the phosphor, while some of the higher-energy ones shoot through and are absorbed in the lower layer. So either the first color or a mixture of the first and second color is shown. With a display with red outer layer and green inner layer, the manipulation of accelerating voltage can produce a continuum of colors from red through orange and yellow to green.

Another method is using a mixture of two phosphors with different characteristics. The brightness of one is linearly dependent on electron flux, while the other one's brightness saturates at higher fluxes—the phosphor does not emit any more light regardless of how many more electrons impact it. At low electron flux, both phosphors emit together; at higher fluxes, the luminous contribution of the nonsaturating phosphor prevails, changing the combined color.<ref name="lumdisp"/>

Such displays can have high resolution, due to absence of two-dimensional structuring of RGB CRT phosphors. Their color palette is, however, very limited. They were used e.g. in some older military radar displays.

====Color television CRTs====
{{missing information|section|time period of each phosphor composition|date=October 2020}}
The phosphors in color CRTs need higher contrast and resolution than the black-and-white ones. The energy density of the electron beam is about 100 times greater than in black-and-white CRTs; the electron spot is focused to about 0.2&nbsp;mm diameter instead of about 0.6&nbsp;mm diameter of the black-and-white CRTs. Effects related to electron irradiation degradation are therefore more pronounced.

Color CRTs require three different phosphors, emitting in red, green and blue, patterned on the screen. Three separate electron guns are used for color production (except for displays that use [[beam-index tube]] technology, which is rare). The red phosphor has always been a problem, being the dimmest of the three necessitating the brighter green and blue electron beam currents be adjusted down to make them equal the red phosphor's lower brightness. This made early color TVs only usable indoors as bright light made it impossible to see the dim picture, while portable black-and-white TVs viewable in outdoor sunlight were already common.

The composition of the phosphors changed over time, as better phosphors were developed and as environmental concerns led to lowering the content of cadmium and later abandoning it entirely. The {{chem2|(Zn,Cd)S:Ag,Cl}} was replaced with {{chem2|(Zn,Cd)S:Cu,Al}} with lower cadmium/zinc ratio, and then with cadmium-free {{chem2|ZnS:Cu,Al}}.

The blue phosphor stayed generally unchanged, a silver-doped zinc sulfide. The green phosphor initially used manganese-doped zinc silicate, then evolved through silver-activated cadmium-zinc sulfide, to lower-cadmium copper-aluminium activated formula, and then to cadmium-free version of the same. The red phosphor saw the most changes; it was originally manganese-activated zinc phosphate, then a silver-activated cadmium-zinc sulfide, then the europium(III) activated phosphors appeared; first in an [[yttrium vanadate]] matrix, then in [[yttrium oxide]] and currently in [[yttrium oxysulfide]]. The evolution of the phosphors was therefore (ordered by B-G-R):
* {{chem2|ZnS:Ag}}&nbsp;&ndash;&nbsp;{{chem2|Zn2SiO4:Mn}}&nbsp;&ndash;&nbsp;{{chem2|Zn3(PO4)2:Mn}}
* {{chem2|ZnS:Ag}}&nbsp;&ndash;&nbsp;{{chem2|(Zn,Cd)S:Ag}}&nbsp;&ndash;&nbsp;{{chem2|(Zn,Cd)S:Ag}}
* {{chem2|ZnS:Ag}}&nbsp;&ndash;&nbsp;{{chem2|(Zn,Cd)S:Ag}}&nbsp;&ndash;&nbsp;{{chem2|YVO4:Eu(3+)}} (1964&ndash;?)
* {{chem2|ZnS:Ag}}&nbsp;&ndash;&nbsp;{{chem2|(Zn,Cd)S:Cu,Al}}&nbsp;&ndash;&nbsp;{{chem2|Y2O2S:Eu(3+)}} or {{chem2|Y2O3:Eu(3+)}}
* {{chem2|ZnS:Ag}}&nbsp;&ndash;&nbsp;{{chem2|ZnS:Cu,Al}} or {{chem2|ZnS:Au,Cu,Al}}&nbsp;&ndash;&nbsp;{{chem2|Y2O2S:Eu(3+)}}<ref name="lumdisp"/>

====Projection televisions====
For [[projection television]]s, where the beam power density can be two orders of magnitude higher than in conventional CRTs, some different phosphors have to be used.

For blue color, {{chem2|ZnS:Ag,Cl}} is employed. However, it saturates. {{chem2|(La,Gd)OBr:Ce,Tb(3+)}} can be used as an alternative that is more linear at high energy densities.

For green, a [[terbium]]-activated {{chem2|Gd2O2Tb(3+)}}; its color purity and brightness at low excitation densities is worse than the zinc sulfide alternative, but it behaves linear at high excitation energy densities, while zinc sulfide saturates. However, it also saturates, so {{chem2|Y3Al5O12:Tb(3+)}} or {{chem2|Y2SiO5:Tb(3+)}} can be substituted. {{chem2|LaOBr:Tb(3+)}} is bright but water-sensitive, degradation-prone, and the plate-like morphology of its crystals hampers its use; these problems are solved now, so it is gaining use due to its higher linearity.

{{chem2|Y2O2S:Eu(3+)}} is used for red emission.<ref name="lumdisp"/>