Editing Fluorescent lamp (section)

===Construction===
[[File: Germicidal Lamp 2.jpg|thumb|right|upright|Close-up of the cathodes of a [[germicidal lamp]] (an essentially similar design that uses no fluorescent phosphor, allowing the [[electrode]]s to be seen)]]

A fluorescent lamp tube is filled with a mix of [[argon]], [[xenon]], [[neon]], or [[krypton]], and mercury vapor. The pressure inside the lamp is around 0.3% of atmospheric pressure.<ref name="Alok">{{cite book
  | last = Kulshreshtha
  | first = Alok K.
  | title = Basic Electrical Engineering: Principles and Applications
  | publisher = Tata McGraw-Hill Education
  | date = 2009
  | location = India
  | page = 801
  | url = https://books.google.com/books?id=GWPfL5bxkAwC&q=pressure+in+%22fluorescent+tube%22&pg=PA801
  | isbn = 978-0-07-014100-1
  | access-date = 2020-10-17 
  | archive-date = 2023-04-20 
  | archive-url = https://web.archive.org/web/20230420064855/https://books.google.com/books?id=GWPfL5bxkAwC&q=pressure+in+%22fluorescent+tube%22&pg=PA801
  | url-status = live
  }}</ref> The partial pressure of the mercury vapor alone is about 0.8&nbsp;Pa (8&nbsp;millionths of atmospheric pressure), in a T12 40-watt lamp.{{sfn|Kane|Sell|2001|p=185}} The inner surface of the lamp is coated with a [[fluorescent]] coating made of varying blends of metallic and [[rare-earth]] [[phosphor]] salts. The lamp's electrodes are typically made of coiled [[tungsten]] and are coated with a mixture of barium, strontium and calcium oxides to improve [[thermionic emission]].

[[File:Germicidal UV discharge tube glow.jpg|thumb|left|upright|A [[germicidal lamp]] uses a low-pressure mercury-vapor glow discharge identical to that in a fluorescent lamp, but the uncoated [[fused quartz]] envelope allows ultraviolet radiation to transmit.]]

Fluorescent lamp tubes are often straight and range in length from about {{convert|100|mm|in|sp=us}} for miniature lamps, to {{convert|2.43|m|ft|sp=us}} for high-output lamps. Some lamps have a circular tube, used for table lamps or other places where a more compact light source is desired. Larger U-shaped lamps are used to provide the same amount of light in a more compact area, and are used for special architectural purposes. [[Compact fluorescent lamp]]s have several small-diameter tubes joined in a bundle of two, four, or six, or a small diameter tube coiled in a helix, to provide a high amount of light output in minimal volume.

Light-emitting phosphors are applied as a paint-like coating to the inside of the tube. The organic solvents are allowed to evaporate, then the tube is heated to nearly the melting point of glass to drive off remaining organic compounds and fuse the coating to the lamp tube. Careful control of the grain size of the suspended phosphors is necessary; large grains lead to weak coatings, and small particles lead to poor light maintenance and efficiency. Most phosphors perform best with a particle size around 10 micrometers. The coating must be thick enough to capture all the ultraviolet light produced by the mercury arc, but not so thick that the phosphor coating absorbs too much visible light. The first phosphors were synthetic versions of naturally occurring fluorescent minerals, with small amounts of metals added as activators. Later other compounds were discovered, allowing differing colors of lamps to be made.{{sfn|Van Broekhoven|2001|p=93}}

Fluorescent tubes can have an outer silicone coating applied by dipping the tube into a solution of water and silicone, and then drying the tube. This coating gives the tube a silky surface finish, and protects against moisture,  guaranteeing a predictable surface resistance on the tube when starting it.<ref>{{Cite book |last=Cayless |first=M. A. |url=https://books.google.com/books?id=x40rBgAAQBAJ&dq=fluorescent+tube+silicone+moisture&pg=PA208 |title=Lamps and Lighting |date=2012-08-21 |publisher=Routledge |isbn=978-1-135-13809-7 |language=en}}</ref>