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{{Short description|Type of electrode and part of cold cathode fluorescent lamp}}
{{About|light sources and indicators|cold cathode ion sources|Ion source}}
{{More citations needed|date=May 2008}}
[[File:Gase-in-Entladungsroehren.jpg|thumb|right|A set of cold cathode discharge tubes]]
A '''cold cathode'''<ref>{{US Patent|1993187}}, Cold cathode discharge tube</ref> is a [[cathode]] that is not electrically heated by a [[Electrical filament|filament]].<ref group="note">A negatively charged [[electrode]] emits [[electron]]s or is the positively charged terminal. For more, see [[field emission]].</ref> A cathode may be considered "cold" if it emits more electrons than can be supplied by [[thermionic emission]] alone. It is used in [[Gas discharge lamp|gas-discharge lamps]], such as [[neon lamp]]s, [[discharge tube]]s, and some types of [[vacuum tube]]. The other type of cathode is a [[hot cathode]], which is heated by electric current passing through a [[Electrical filament|filament]]. A cold cathode does not necessarily operate at a low temperature: it is often heated to its [[operating temperature]] by other methods, such as the current passing from the cathode into the gas.

==Cold-cathode devices==
[[Image:ZM1210-operating edit2.jpg|thumb|upright|
The stacked digit arrangement in a [[Nixie tube]] is visible here.]]

A cold-cathode vacuum tube does not rely on external heating of an electrode to provide [[thermionic emission]] of electrons. Early cold-cathode devices included the [[Geissler tube]] and [[Julius Plücker|Plucker tube]], and early [[cathode-ray tube]]s.  Study of the phenomena in these devices led to the discovery of the electron.

[[Neon lamp]]s are used both to produce light as indicators and for special-purpose illumination, and also as circuit elements displaying [[negative resistance]]. Addition of a trigger electrode to a device allowed the glow discharge to be initiated by an external control circuit; [[Bell Laboratories]] developed a "trigger tube" cold-cathode device in 1936.<ref>D. M. Neale, ''Cold Cathode Tube Circuit Design'', Francis and Taylor, 1964. pp. 1–7.</ref>

Many types of cold-cathode switching tube were developed, including various types of [[thyratron]], the [[krytron]], cold-cathode displays ([[Nixie tube]]) and others. [[Voltage regulator]] tubes rely on the relatively constant voltage of a [[glow discharge]] over a range of current and were used to stabilize power-supply voltages in tube-based instruments. A [[Dekatron]] is a cold-cathode tube with multiple electrodes that is used for counting.  Each time a pulse is applied to a control electrode, a glow discharge moves to a step electrode; by providing ten electrodes in each tube and cascading the tubes, a counter system can be developed and the count observed by the position of the glow discharges. Counter tubes were used widely before development of [[integrated circuit]] counter devices.

The [[flash tube]] is a cold-cathode device filled with [[xenon]] gas, used to produce an intense short pulse of light for photography or to act as a [[stroboscope]] to examine the motion of moving parts.

===Lamps===
Cold-cathode lamps include [[cold-cathode fluorescent lamp]]s (CCFLs) and [[neon lamp]]s. Neon lamps primarily rely on excitation of gas molecules to emit light; CCFLs use a discharge in mercury vapor to develop ultraviolet light, which in turn causes a [[fluorescence|fluorescent]] coating on the inside of the lamp to emit visible light.

Cold-cathode fluorescent lamps were used for [[backlight]]ing of [[LCD]]s, for example computer monitors and television screens.

In the lighting industry, “cold cathode” historically refers to luminous tubing larger than 20&nbsp;mm in diameter and operating on a current of 120 to 240 milliamperes. This larger-diameter tubing is often used for interior alcove and general lighting.<ref>{{cite web | title = Ifay guide info electric discharge lighting systems, cold cathode| url =http://database.ul.com/cgi-bin/XYV/template/LISEXT/1FRAME/showpage.html?name=IFAY.GuideInfo&ccnshorttitle=Electric+Discharge+Lighting+Systems,+Cold+Cathode&objid=1074096534&cfgid=1073741824&version=versionless&parent_id=1073988136&sequence=1}}</ref><ref>{{cite web|title=EGL lighting products |url=http://www.egl-lighting.com/products.html |access-date=9 February 2011 |url-status=dead |archive-url=https://web.archive.org/web/20101026042259/http://www.egl-lighting.com/products.html |archive-date=October 26, 2010 }}</ref>
The term "neon lamp" refers to tubing that is smaller than 15&nbsp;mm in diameter{{citation needed|date=September 2015}} and typically operates at approximately 40 milliamperes.  These lamps are commonly used for neon signs.

==Details==
The cathode is the negative electrode. Any gas-discharge lamp has a positive (anode) and a negative electrode. Both electrodes alternate between acting as an anode and a cathode when these devices run with [[alternating current]].

[[File:Cold Cathodes.jpg|thumb|A standard computer case fitted with blue and green cold-cathode tubes]]
[[File:Cold Cathode Fluorescent Lamp.JPG|thumb|Cold-cathode fluorescent lamp backlight]]

A ''cold cathode'' is distinguished from a [[hot cathode]] that is heated to induce [[thermionic emission]] of [[electron]]s. Discharge tubes with hot cathodes have an envelope filled with low-pressure gas and containing two electrodes. Hot cathode devices include common [[vacuum tube]]s, [[fluorescent lamp]]s, high-pressure discharge lamps and [[vacuum fluorescent display]]s.

The surface of cold cathodes can emit [[secondary electrons]] at a ratio greater than unity (breakdown).  An electron that leaves the cathode will collide with neutral gas molecules.  The collision may just excite the molecule, but sometimes it will knock an electron free to create a positive ion.  The original electron and the freed electron continue toward the anode and may create more positive ions (see [[Townsend avalanche]]). The result is for each electron that leaves the cathode, several positive ions are generated that eventually crash onto the cathode. Some crashing positive ions may generate a secondary electron. The discharge is self-sustaining when for each electron that leaves the cathode, enough positive ions hit the cathode to free, on average, another electron. External circuitry limits the discharge current.  Cold-cathode discharge lamps use higher voltages than hot-cathode ones. The resulting strong electric field near the cathode accelerates ions to a sufficient velocity to create free electrons from the cathode material.

Another mechanism to generate free electrons from a cold metallic surface is [[field electron emission]]. It is used in some [[x-ray tube]]s, the [[field-emission microscopy|field-electron microscope]] (FEM), and [[field-emission display]]s (FEDs).

Cold cathodes sometimes have a [[rare-earth]] coating to enhance electron emission. Some types contain a source of [[beta radiation]] to start [[ionization]] of the gas that fills the tube.<ref>{{US Patent|1860149}}, Discharge tube.</ref> In some tubes, [[glow discharge]] around the cathode is usually minimized; instead there is a so-called positive column, filling the tube.<ref>{{US Patent|2103033}}, Electron emissive electrode.</ref><ref>{{US Patent|1316967}}, Gaseous-conduction lamp.</ref><ref group="note">Positive column is part of a glow discharge, such as in the [[Moore lamp]].</ref> Examples are the [[neon lamp]] and [[nixie tube]]s. Nixie tubes too are cold-cathode neon displays that are in-line, but not in-plane, display devices.

Cold-cathode devices typically use a complex [[high-voltage power supply]] with some mechanism for limiting current. Although creating the initial space charge and the first arc of current through the tube may require a very high voltage, once the tube begins to heat up, the electrical resistance drops, thus increasing the [[electric current]] through the lamp. To offset this effect and maintain normal operation, the supply voltage is gradually lowered. In the case of tubes with an ionizing gas, the gas can become a very hot [[plasma (physics)|plasma]], and electrical resistance is greatly reduced. If operated from a simple power supply without current limiting, this reduction in resistance would lead to damage to the power supply and overheating of the tube electrodes.

==Applications==
{{multiple image
| total_width       = 350
| image1            = CCFL bulb.jpg
| caption1          = A photo of an illuminated compact fluorescent lamp (CFL) of the cold-cathode variety
| image2            = Cold-Cathode-CFL-illuminated.jpg
| caption2          = An illuminated cold-cathode [[Compact fluorescent lamp|CFL]]
}}
Cold cathodes are used in [[cold-cathode rectifier]]s, such as the [[crossatron]] and [[mercury-arc valve]]s, and [[cold-cathode amplifier]]s, such as in [[automatic message accounting]] and other [[pseudospark switch]]ing applications. Other examples include the [[thyratron]], [[krytron]], [[sprytron]], and [[ignitron]] tubes.

A common cold-cathode application is in [[neon sign]]s and other locations where the ambient temperature is likely to drop well below freezing, The [[Clock Tower, Palace of Westminster]] (Big Ben) uses cold-cathode lighting behind the clock faces where continual striking and failure to strike in cold weather would be undesirable. Large cold-cathode fluorescent lamps (CCFLs) have been produced in the past and are still used today when shaped, long-life linear light sources are required. {{As of|2011}}, miniature CCFLs were extensively used as [[backlight]]s for [[computer]] and television [[liquid-crystal display]]s. CCFL lifespans vary in LCD televisions depending on transient voltage surges and temperature levels in usage environments.

Due to its efficiency, CCFL technology has expanded into room lighting. Costs are similar to those of traditional [[fluorescent lighting]],{{Clarify|date=August 2011|reason=is this cost of lamps, running cost, total cost?}} but with several advantages: it has a long life, the light emitted is {{clarify span|easier on the eyes|what?|date=October 2017}}, bulbs turn on instantly to full output and are also dimmable.<ref name="Solé Lighting">[http://solelighting.com/ Solé Lighting] (commercial site advocating CCFLs).</ref>

===Effects of internal heating===
In systems using [[alternating current]] but without separate [[anode]] structures, the electrodes alternate as anodes and cathodes, and the impinging electrons can cause substantial localized heating, often to [[red heat]]. The electrode may take advantage of this heating to facilitate the thermionic emission of electrons when it is acting as a cathode. (''Instant-start'' fluorescent lamps employ this aspect; they start as cold-cathode devices, but soon localized heating of the fine [[tungsten]]-wire cathodes causes them to operate in the same mode as [[hot-cathode]] lamps.)

This aspect is problematic in the case of backlights used for [[liquid-crystal display|LCD]] TV displays. New energy-efficiency regulations being proposed in many countries will require variable backlighting; variable backlighting also improves the perceived contrast range, which is desirable for LCD TV sets. However, CCFLs are strictly limited in the degree to which they can be dimmed, both because a lower plasma current will lower the temperature of the cathode, causing erratic operation, and because running the cathode at too low a temperature drastically shortens the life of the lamps.{{citation needed|date=November 2015}} Much research is being directed to this problem, but high-end manufacturers are now turning to high-efficiency white [[LED]]s as a better solution.{{citation needed|date=December 2021}}

==References and notes==
=== Notes ===
{{Reflist|group=note}}

=== Citations ===
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[[Category:Electrodes]]
[[Category:Gas discharge lamps]]
[[Category:Types of lamp]]
[[Category:Vacuum]]
[[Category:Vacuum tubes]]