Self-powered lighting
Encyclopedia
Tritium illumination is the use of gaseous tritium
, a radioactive isotope
of hydrogen
, to create visible light. Tritium emits electron
s through beta decay
, and when they interact with a phosphor material, fluorescent light is created, a process called radioluminescence
. As tritium illumination requires no electrical energy, it found wide use in applications such as emergency exit sign
s and illumination of wristwatches. More recently, many applications using radioactive materials have been replaced with photoluminescent materials.
Tritium lighting is made using glass tubes with a phosphor
layer in them and tritium
gas inside the tube. Such a tube is known as a "gaseous tritium light source" (GTLS), or beta light, (since the tritium undergoes beta decay
).
, releasing electrons which cause the phosphor layer to fluoresce.
During manufacture, a length of borosilicate glass
tube which has had the inside surface coated with a phosphor
-containing material is filled with the radioactive tritium. The tube is then fused with a CO2 laser
at the desired length. Borosilicate is preferred for its strength and resistance to breakage. In the tube, the tritium gives off a steady stream of electron
s due to beta decay. These particles excite the phosphor, causing it to emit a low, steady glow. Tritium is not the only material that can be used for self-powered lighting. Other beta particle
-emitting radioisotopes can also serve. Radium
was used in the past to make self-luminous
paint, but has been replaced by tritium, which is less hazardous.
Various preparations of the phosphor compound can be used to produce different colors of light. Some of the colors that have been manufactured in addition to the common phosphorus green are red, blue, yellow, purple, orange, and white.
The types of GTLS used in watches give off a small amount of light—not enough to be seen in daylight, but enough to be visible in the dark from a distance of several meters. The average such GTLS has a useful life of 10–20 years. As the tritium component of the lighting is often more expensive than the rest of the watch itself, manufacturers try to use as little as possible. Being an unstable isotope with a half-life
of about 12.32 years, tritium loses half its brightness in that period. The more tritium that is initially placed in the tube, the brighter it is to begin with, and the longer its useful life. Tritium exit signs usually come in three brightness levels guaranteed for 10, 15, or 20 year useful life expectancies. The difference between the signs is how much tritium the manufacturer installs.
These light sources are most often seen as "permanent" illumination for the hands of wristwatches intended for diving, nighttime, or tactical use. They are additionally used in glowing novelty
keychain
s and in self-illuminated exit signs. They are favored by the military for applications where a power source may not be available, such as for instrument dial
s in aircraft, compass
es, and sights for weapons.
Tritium lights are also found in some old rotary dial
telephones, though due to their age they no longer produce a useful amount of light.
Tritium lights or beta lights were formerly used in fishing lures.
of some small arms. The reticule on the SA80
's optical SUSAT
sight (Sight Unit Small Arms Trilux) contains a small amount of tritium for the same effect as an example of tritium use on a rifle sight. The electrons emitted by the radioactive decay of the tritium cause phosphor
to glow, thus providing a long lasting (several years) and non-battery-powered firearms sight which is visible in dim lighting conditions. The tritium glow is not noticeable in bright conditions such as during daylight however. As a result, some manufacturers have started to integrate fiber optic sights with tritium vials to provide bright, high-contrast firearms sights in both bright and dim conditions.
(though in quantities several thousand times larger than that in a keychain), consumer and safety devices containing tritium for use in the United States are subject to certain possession, resale, disposal, and use restrictions. Devices such as self-luminous exit signs, gauges, wrist watches, etc., which contain small amounts of tritium are under the jurisdiction of the US Nuclear Regulatory Commission
, and are subject to possession, distribution, import and export regulations found in 10 CFR
Parts, 30, 32 and 110. They are also subject to regulations for possession, use and disposal in certain states. They are readily sold and used in the US and are widely available in the UK and are regulated in England and Wales by environmental health departments of local councils. Tritium lighting is legal in most of Asia and Australia.
Advisory Group on Ionizing Radiation declared the health risks of tritium exposure to be double that previously set by the International Commission on Radiological Protection
, but encapsulated tritium lighting devices, typically taking the form of a luminous glass tube embedded in a thick block of clear plastic, prevent the user from being exposed to the tritium at all unless the device is broken apart.
Tritium presents no external radiation threat when encapsulated in non-hydrogen-permeable containers due to its low penetration depth, which is insufficient to penetrate intact human skin. The primary danger from tritium arises if it is inhaled, ingested, injected or otherwise absorbed into the body. This results in the emitted radiation being absorbed in a relatively small region of the body, again due to the low penetration depth. The biological half-life
of tritium—the time it takes for half of an ingested dose to be expelled from the body—is low, at only 12 days. Tritium excretion can be accelerated further by increasing water intake to 3-4 liters/day.
Direct, short-term exposure to small amounts of tritium is relatively harmless. If a tritium tube should break, one should leave the area and allow the gas to diffuse into the air. Tritium exists naturally in the environment, but in very small quantities.
Tritium
Tritium is a radioactive isotope of hydrogen. The nucleus of tritium contains one proton and two neutrons, whereas the nucleus of protium contains one proton and no neutrons...
, a radioactive isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
of hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
, to create visible light. Tritium emits electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s through beta decay
Beta decay
In nuclear physics, beta decay is a type of radioactive decay in which a beta particle is emitted from an atom. There are two types of beta decay: beta minus and beta plus. In the case of beta decay that produces an electron emission, it is referred to as beta minus , while in the case of a...
, and when they interact with a phosphor material, fluorescent light is created, a process called radioluminescence
Radioluminescence
Radioluminescence is the phenomenon by which luminescence is produced in a material by the bombardment of ionizing radiation such as beta particles.-Tritium:...
. As tritium illumination requires no electrical energy, it found wide use in applications such as emergency exit sign
Exit sign
An exit sign is a device in a public facility denoting the location of the emergency exit , guiding people to the closest exit in case of fire or other emergency. Most relevant codes require exit signs to be permanently lit...
s and illumination of wristwatches. More recently, many applications using radioactive materials have been replaced with photoluminescent materials.
Tritium lighting
Phosphor
A phosphor, most generally, is a substance that exhibits the phenomenon of luminescence. Somewhat confusingly, this includes both phosphorescent materials, which show a slow decay in brightness , and fluorescent materials, where the emission decay takes place over tens of nanoseconds...
layer in them and tritium
Tritium
Tritium is a radioactive isotope of hydrogen. The nucleus of tritium contains one proton and two neutrons, whereas the nucleus of protium contains one proton and no neutrons...
gas inside the tube. Such a tube is known as a "gaseous tritium light source" (GTLS), or beta light, (since the tritium undergoes beta decay
Beta decay
In nuclear physics, beta decay is a type of radioactive decay in which a beta particle is emitted from an atom. There are two types of beta decay: beta minus and beta plus. In the case of beta decay that produces an electron emission, it is referred to as beta minus , while in the case of a...
).
Physics
The tritium in a gaseous tritium light source undergoes beta decayBeta decay
In nuclear physics, beta decay is a type of radioactive decay in which a beta particle is emitted from an atom. There are two types of beta decay: beta minus and beta plus. In the case of beta decay that produces an electron emission, it is referred to as beta minus , while in the case of a...
, releasing electrons which cause the phosphor layer to fluoresce.
During manufacture, a length of borosilicate glass
Borosilicate glass
Borosilicate glass is a type of glass with the main glass-forming constituents silica and boron oxide. Borosilicate glasses are known for having very low coefficients of thermal expansion , making them resistant to thermal shock, more so than any other common glass...
tube which has had the inside surface coated with a phosphor
Phosphor
A phosphor, most generally, is a substance that exhibits the phenomenon of luminescence. Somewhat confusingly, this includes both phosphorescent materials, which show a slow decay in brightness , and fluorescent materials, where the emission decay takes place over tens of nanoseconds...
-containing material is filled with the radioactive tritium. The tube is then fused with a CO2 laser
Carbon dioxide laser
The carbon dioxide laser was one of the earliest gas lasers to be developed , and is still one of the most useful. Carbon dioxide lasers are the highest-power continuous wave lasers that are currently available...
at the desired length. Borosilicate is preferred for its strength and resistance to breakage. In the tube, the tritium gives off a steady stream of electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s due to beta decay. These particles excite the phosphor, causing it to emit a low, steady glow. Tritium is not the only material that can be used for self-powered lighting. Other beta particle
Beta particle
Beta particles are high-energy, high-speed electrons or positrons emitted by certain types of radioactive nuclei such as potassium-40. The beta particles emitted are a form of ionizing radiation also known as beta rays. The production of beta particles is termed beta decay...
-emitting radioisotopes can also serve. Radium
Radium
Radium is a chemical element with atomic number 88, represented by the symbol Ra. Radium is an almost pure-white alkaline earth metal, but it readily oxidizes on exposure to air, becoming black in color. All isotopes of radium are highly radioactive, with the most stable isotope being radium-226,...
was used in the past to make self-luminous
Radioluminescence
Radioluminescence is the phenomenon by which luminescence is produced in a material by the bombardment of ionizing radiation such as beta particles.-Tritium:...
paint, but has been replaced by tritium, which is less hazardous.
Various preparations of the phosphor compound can be used to produce different colors of light. Some of the colors that have been manufactured in addition to the common phosphorus green are red, blue, yellow, purple, orange, and white.
The types of GTLS used in watches give off a small amount of light—not enough to be seen in daylight, but enough to be visible in the dark from a distance of several meters. The average such GTLS has a useful life of 10–20 years. As the tritium component of the lighting is often more expensive than the rest of the watch itself, manufacturers try to use as little as possible. Being an unstable isotope with a half-life
Half-life
Half-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...
of about 12.32 years, tritium loses half its brightness in that period. The more tritium that is initially placed in the tube, the brighter it is to begin with, and the longer its useful life. Tritium exit signs usually come in three brightness levels guaranteed for 10, 15, or 20 year useful life expectancies. The difference between the signs is how much tritium the manufacturer installs.
Uses of tritium illumination
Novelties
A novelty item is a small manufactured adornment, especially a personal adornment.- Novelty item :This term covers a range of small manufactured goods, such as*antiques*collectables*chocolates*esoterica*executive toys*gadgets*tools and implements...
keychain
Keychain
A keychain or key chain is a small chain, usually made from metal or plastic, that connects a small item to a keyring. The length of a keychain allows an item to be used more easily than if connected directly to a keyring...
s and in self-illuminated exit signs. They are favored by the military for applications where a power source may not be available, such as for instrument dial
Dial (measurement)
A dial is generally a flat surface, circular or rectangular, with numbers or similar markings on it, used for displaying the setting or output of a timepiece, radio, clock, watch, or measuring instrument...
s in aircraft, compass
Compass
A compass is a navigational instrument that shows directions in a frame of reference that is stationary relative to the surface of the earth. The frame of reference defines the four cardinal directions – north, south, east, and west. Intermediate directions are also defined...
es, and sights for weapons.
Tritium lights are also found in some old rotary dial
Rotary dial
The rotary dial is a device mounted on or in a telephone or switchboard that is designed to send electrical pulses, known as pulse dialing, corresponding to the number dialed. The early form of the rotary dial used lugs on a finger plate instead of holes. Almon Brown Strowger filed the first patent...
telephones, though due to their age they no longer produce a useful amount of light.
Tritium lights or beta lights were formerly used in fishing lures.
Small arms sights
Tritium is used to illuminate the sightsIron sights
Iron sights are a system of shaped alignment markers used as a sighting device to assist in the aiming of a device such as a firearm, crossbow, or telescope, and exclude the use of optics as in telescopic sights or reflector sights...
of some small arms. The reticule on the SA80
SA80
The SA80 is a British family of 5.56mm small arms. It is a selective fire, gas-operated assault rifle. SA80 prototypes were trialled in 1976 and production was completed in 1994....
's optical SUSAT
SUSAT
The Sight Unit Small Arms, Trilux, or SUSAT, is a 4× telescopic sight with tritium-powered illumination, utilised at dusk or dawn. The full name of the current model is the SUSAT L9A1. The sight is not designed as a sniper sight, but is rather intended to be mounted on a variety of rifles and to...
sight (Sight Unit Small Arms Trilux) contains a small amount of tritium for the same effect as an example of tritium use on a rifle sight. The electrons emitted by the radioactive decay of the tritium cause phosphor
Phosphor
A phosphor, most generally, is a substance that exhibits the phenomenon of luminescence. Somewhat confusingly, this includes both phosphorescent materials, which show a slow decay in brightness , and fluorescent materials, where the emission decay takes place over tens of nanoseconds...
to glow, thus providing a long lasting (several years) and non-battery-powered firearms sight which is visible in dim lighting conditions. The tritium glow is not noticeable in bright conditions such as during daylight however. As a result, some manufacturers have started to integrate fiber optic sights with tritium vials to provide bright, high-contrast firearms sights in both bright and dim conditions.
Legal issues
Because tritium in particular is an integral part of thermonuclear devicesNuclear weapon design
Nuclear weapon designs are physical, chemical, and engineering arrangements that cause the physics package of a nuclear weapon to detonate. There are three basic design types...
(though in quantities several thousand times larger than that in a keychain), consumer and safety devices containing tritium for use in the United States are subject to certain possession, resale, disposal, and use restrictions. Devices such as self-luminous exit signs, gauges, wrist watches, etc., which contain small amounts of tritium are under the jurisdiction of the US Nuclear Regulatory Commission
Nuclear Regulatory Commission
The Nuclear Regulatory Commission is an independent agency of the United States government that was established by the Energy Reorganization Act of 1974 from the United States Atomic Energy Commission, and was first opened January 19, 1975...
, and are subject to possession, distribution, import and export regulations found in 10 CFR
Code of Federal Regulations
The Code of Federal Regulations is the codification of the general and permanent rules and regulations published in the Federal Register by the executive departments and agencies of the Federal Government of the United States.The CFR is published by the Office of the Federal Register, an agency...
Parts, 30, 32 and 110. They are also subject to regulations for possession, use and disposal in certain states. They are readily sold and used in the US and are widely available in the UK and are regulated in England and Wales by environmental health departments of local councils. Tritium lighting is legal in most of Asia and Australia.
Health concerns
While these devices contain a radioactive substance, it is currently believed that self-powered lighting does not pose a significant health concern. A 2007 report by the UK government's Health Protection AgencyHealth Protection Agency
The Health Protection Agency, or, in Welsh, Yr Asiantaeth Diogelu Iechyd is a statutory corporation. It is an independent UK organisation that was set up by the government in 2003 to protect the public from threats to their health from infectious diseases and environmental hazards...
Advisory Group on Ionizing Radiation declared the health risks of tritium exposure to be double that previously set by the International Commission on Radiological Protection
International Commission on Radiological Protection
The International Commission on Radiological Protection is an advisory body providing recommendations and guidance on radiation protection; It was founded in 1928 by the International Society of Radiology and was then called the ‘International X-ray and Radium Protection Committee’...
, but encapsulated tritium lighting devices, typically taking the form of a luminous glass tube embedded in a thick block of clear plastic, prevent the user from being exposed to the tritium at all unless the device is broken apart.
Tritium presents no external radiation threat when encapsulated in non-hydrogen-permeable containers due to its low penetration depth, which is insufficient to penetrate intact human skin. The primary danger from tritium arises if it is inhaled, ingested, injected or otherwise absorbed into the body. This results in the emitted radiation being absorbed in a relatively small region of the body, again due to the low penetration depth. The biological half-life
Biological half-life
The biological half-life or elimination half-life of a substance is the time it takes for a substance to lose half of its pharmacologic, physiologic, or radiologic activity, as per the MeSH definition...
of tritium—the time it takes for half of an ingested dose to be expelled from the body—is low, at only 12 days. Tritium excretion can be accelerated further by increasing water intake to 3-4 liters/day.
Direct, short-term exposure to small amounts of tritium is relatively harmless. If a tritium tube should break, one should leave the area and allow the gas to diffuse into the air. Tritium exists naturally in the environment, but in very small quantities.
External links
- Cleanup of a broken tritium sign
- Radioluminescent items
- Forum discussion on Superluminova vs. Tritium
See also
- List of light sources
- Exit signExit signAn exit sign is a device in a public facility denoting the location of the emergency exit , guiding people to the closest exit in case of fire or other emergency. Most relevant codes require exit signs to be permanently lit...
- RadioluminescenceRadioluminescenceRadioluminescence is the phenomenon by which luminescence is produced in a material by the bombardment of ionizing radiation such as beta particles.-Tritium:...