Tagging of postage stamps means that the stamps are printed on luminescent paper or with luminescent ink to facilitate automated mail processing. Both fluorescence and phosphorescence are used. The same stamp may have been printed with and without these luminescent features. The two varieties are referred to as tagged and untagged, respectively.
Letters and postcards fed into an automated mail processing plant are illuminated with ultraviolet light. The reaction of the luminescent features of the stamps on this illumination is used to position the mail items such that the stamps can be cancelled, and that the significant parts of the address such as postcodes may be read and the mail be sorted accordingly.
The luminescent features of the stamps are generally invisible or barely visible to the human eye in normal illumination. However, they can be identified under ultraviolet light similar to how it is done in the postal machinery. In general, fluorescent features can be identified with UV light of a longer wavelength than needed for phosphorescent features (see below).
The luminescent substance ("taggant") can be printed over the whole surface of the stamp, the main design, the margins only, single bands or bars or other patterns, or can be added to the paper itself.
The tagging pattern can also be varied to enable the sorting of mail according to the service class.
Upon absorption of light, fluorescent materials emit light upon of a longer wavelength (lower energy) than the absorbed radiation, but cease to do so once immediately, when the illumination is stopped. The tagging of stamps uses substances that absorb ultraviolet light of wavelengths between 300 nm and 450 nm ("Black light", UVA, long-wave UV) and emit light in the visible spectrum. Under UV illumination they usually glow a greenish or yellowish colour.
It must not be confused with the "whitening" of paper. [1] It is achieved by adding optical brighteners that usually re-emit light in the blue region of the spectrum, making the paper appear whiter by compensating a perceived deficit in reflected colours of these wavelengths.
Phosphorescent materials release the absorbed energy only slowly, so that they exhibit an "afterglow". Materials for stamp tagging absorb ultraviolet light of wavelengths between 180 nm and 300 nm (UVC, short-wave UV) and emit light of a greenish or reddish colour depending on the substances used.
Fluorescent stamps can be detected with a black light fluorescent tube. Phosphorescent stamps can be detected using a shortwave UV lamp. The effects of both processes can be recorded photographically. Lamps for both ranges of wavelengths as well as combinations of both are available. Care must be taken when using UV lamps, since their light can damage the eyes. [2]
The first tagged stamps of Canada were issued in 1962 with vertical phosphorescent bands. In 1972, fluorescent general tagging was introduced, initially as vertical bars, now normally on all four sides of the stamp. [3]
Deutsche Bundespost started issuing stamps on fluorescent Lumogen paper in 1960 in connection with trials for automated mail processing in the Darmstadt area. Fluorescent paper was generally used for stamps of Deutsche Bundespost and Deutsche Bundespost Berlin from 1961 on. [4] Deutsche Post AG continues to use this technology. Deutsche Post of the GDR did not use luminescent tagging on stamps.
Luminescent tagging has been added to postage stamps of the United Kingdom since the Wilding issues of 1959 in the shape of vertical bands. Stamps of the current Machin series have been printed with one or two such "phosphor bands"; [5] those for second-class mail bear only one such band, those for first-class mail bear two. The positions of the bands may vary: stamps from booklets may have shortened, notched, or inset bands that do not extend onto neighbouring gutters to avoid the use of the latter instead of stamps for franking. [6] Due to the presence of optical brighteners in many printing papers, phosphorescent materials were chosen for stamp tagging in the UK. [7]
The US Post Office Department started experiments with fluorescent compounds in the early 1960s. An 8¢ air mail stamp issued in 1963 was the first stamp printed for trials with new cancelling machines. [8] The 5¢ City Delivery issue of 1963 was the first commemorative issue produced with tagging. [9]
Precancelled stamps and service-inscribed stamps are not usually tagged because they need not be routed through the cancelling equipment.
Since luminescent ink or luminescent paper are only delivered to specialist printers, tagging also serves as an anti-counterfeiting measure, similar to the practice on banknotes.
When Deutsche Post of the GDR expanded automated mail processing in the 1980s, they did not use luminescent tagging, but used sideways illumination to identify the shadows of the stamp perforation in order to position mail items in cancelling and sorting machinery. Red light was used for this purpose, giving a good contrast to ordinary writing ink colours and enabling machine reading of postcodes. Some issues of Postal cards were printed entirely in orange to facilitate the latter process. However, the colours of the imprinted stamps was later changed to those of the usual definitives of the corresponding value, and simulated perforations were added around the stamp design to help locate the stamp position.
The electromagnetic spectrum is the full range of electromagnetic radiation, organized by frequency or wavelength. The spectrum is divided into separate bands, with different names for the electromagnetic waves within each band. From low to high frequency these are: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays. The electromagnetic waves in each of these bands have different characteristics, such as how they are produced, how they interact with matter, and their practical applications.
Fluorescence is one of two kinds of emission of light by a substance that has absorbed light or other electromagnetic radiation. Fluorescence involves no change in electron spin multiplicity and generally it immediately follows absorption; phosphorescence involves spin change and is delayed. Thus fluorescent materials generally cease to glow nearly immediately when the radiation source stops, while phosphorescent materials, which continue to emit light for some time after.
A light-emitting diode (LED) is a semiconductor device that emits light when current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons. The color of the light is determined by the energy required for electrons to cross the band gap of the semiconductor. White light is obtained by using multiple semiconductors or a layer of light-emitting phosphor on the semiconductor device.
Ultraviolet (UV) light is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight, and constitutes about 10% of the total electromagnetic radiation output from the Sun. It is also produced by electric arcs, Cherenkov radiation, and specialized lights, such as mercury-vapor lamps, tanning lamps, and black lights.
A phosphor is a substance that exhibits the phenomenon of luminescence; it emits light when exposed to some type of radiant energy. The term is used both for fluorescent or phosphorescent substances which glow on exposure to ultraviolet or visible light, and cathodoluminescent substances which glow when struck by an electron beam in a cathode-ray tube.
A fluorescent lamp, or fluorescent tube, is a low-pressure mercury-vapor gas-discharge lamp that uses fluorescence to produce visible light. An electric current in the gas excites mercury vapor, which produces short-wave ultraviolet light that then causes a phosphor coating on the inside of the lamp to glow. A fluorescent lamp converts electrical energy into useful light much more efficiently than an incandescent lamp. The typical luminous efficacy of fluorescent lighting systems is 50–100 lumens per watt, several times the efficacy of incandescent bulbs with comparable light output. For comparison, the luminous efficacy of an incandescent bulb may only be 16 lumens per watt.
Phosphorescence is a type of photoluminescence related to fluorescence. When exposed to light (radiation) of a shorter wavelength, a phosphorescent substance will glow, absorbing the light and reemitting it at a longer wavelength. Unlike fluorescence, a phosphorescent material does not immediately reemit the radiation it absorbs. Instead, a phosphorescent material absorbs some of the radiation energy and reemits it for a much longer time after the radiation source is removed.
A blacklight, also called a UV-A light, Wood's lamp, or ultraviolet light, is a lamp that emits long-wave (UV-A) ultraviolet light and very little visible light. One type of lamp has a violet filter material, either on the bulb or in a separate glass filter in the lamp housing, which blocks most visible light and allows through UV, so the lamp has a dim violet glow when operating. Blacklight lamps which have this filter have a lighting industry designation that includes the letters "BLB". This stands for "blacklight blue". A second type of lamp produces ultraviolet but does not have the filter material, so it produces more visible light and has a blue color when operating. These tubes are made for use in "bug zapper" insect traps, and are identified by the industry designation "BL". This stands for "blacklight".
A fluorophore is a fluorescent chemical compound that can re-emit light upon light excitation. Fluorophores typically contain several combined aromatic groups, or planar or cyclic molecules with several π bonds.
A safelight is a light source suitable for use in a photographic darkroom. It provides illumination only from parts of the visible spectrum to which the photographic material in use is nearly, or completely insensitive.
A mercury-vapor lamp is a gas-discharge lamp that uses an electric arc through vaporized mercury to produce light. The arc discharge is generally confined to a small fused quartz arc tube mounted within a larger soda lime or borosilicate glass bulb. The outer bulb may be clear or coated with a phosphor; in either case, the outer bulb provides thermal insulation, protection from the ultraviolet radiation the light produces, and a convenient mounting for the fused quartz arc tube.
A germicidal lamp is an electric light that produces ultraviolet C (UVC) light. This short-wave ultraviolet light disrupts DNA base pairing, causing formation of pyrimidine dimers, and leads to the inactivation of bacteria, viruses, and protozoans. It can also be used to produce ozone for water disinfection. They are used in ultraviolet germicidal irradiation (UVGI).
The induction lamp, electrodeless lamp, or electrodeless induction lamp is a gas-discharge lamp in which an electric or magnetic field transfers the power required to generate light from outside the lamp envelope to the gas inside. This is in contrast to a typical gas discharge lamp that uses internal electrodes connected to the power supply by conductors that pass through the lamp envelope. Eliminating the internal electrodes provides two advantages:
Yttrium aluminium garnet (YAG, Y3Al5O12) is a synthetic crystalline material of the garnet group. It is a cubic yttrium aluminium oxide phase, with other examples being YAlO3 (YAP) in a hexagonal or an orthorhombic, perovskite-like form, and the monoclinic Y4Al2O9 (YAM).
Luminous paint is paint that emits visible light through fluorescence, phosphorescence, or radioluminescence.
Strontium aluminate is an aluminate compound with the chemical formula SrAl2O4. It is a pale yellow, monoclinic crystalline powder that is odourless and non-flammable. When activated with a suitable dopant, it acts as a photoluminescent phosphor with long persistence of phosphorescence.
Ultraviolet photography is a photographic process of recording images by using radiation from the ultraviolet (UV) spectrum only. Images taken with ultraviolet radiation serve a number of scientific, medical or artistic purposes. Images may reveal deterioration of art works or structures not apparent under light. Diagnostic medical images may be used to detect certain skin disorders or as evidence of injury. Some animals, particularly insects, use ultraviolet wavelengths for vision; ultraviolet photography can help investigate the markings of plants that attract insects, while invisible to the unaided human eye. Ultraviolet photography of archaeological sites may reveal artifacts or traffic patterns not otherwise visible.
An ultraviolet (UV) marker is a pen whose marks are fluorescent but transparent; the marks can be seen only under an ultraviolet light. They are commonly used in security situations to identify belongings or to prevent the reproduction of unauthorized banknotes. UV pens can now be bought at some stationery shops to securely mark items of high value in case of theft.
Super-LumiNova is a brand name under which strontium aluminate–based non-radioactive and nontoxic photoluminescent or afterglow pigments for illuminating markings on watch dials, hands and bezels, etc. in the dark are marketed. When activated with a suitable dopant, it acts as a photoluminescent phosphor with long persistence of phosphorescence. This technology offers up to ten times higher brightness than previous zinc sulfide–based materials.
Kaede is a photoactivatable fluorescent protein naturally originated from a stony coral, Trachyphyllia geoffroyi. Its name means "maple" in Japanese. With the irradiation of ultraviolet light (350–400 nm), Kaede undergoes irreversible photoconversion from green fluorescence to red fluorescence.
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