Luminescence

• Color Tel.evision with Cathode Ray Tube (CRT)
• Fluorescent lighting
• Flat Screen Technology
• Signs and Signals
• Medical Radiography

Emissions applications have been under development since the 1960's, in conjunction with the industrial availability of Rare Earths of sufficient purity (>99,99 %): television color, fluorescent lighting, and medical X-ray photography.

The intense emissions and almost monochromatic tones obtained by diluting the Rare Earths based activators in the appropriate host networks (very often simple rare-earth compounds at 0.7 or 14 electrons ¦: yttrium, lanthanum, gadolinium et lutecium) are the primary reason for this development. They made it possible to meet the very specific criteria for use that traditional band emission phosphors could not satisfy. A great variety of emissions can be obtained, depending on the type of activator brought into play and the respective positions of the excited or fundamental energy levels.

• Color Tel.evision with Cathode Ray Tube (CRT)

• Fluorescent lighting

In the fluorescent lighting domain, rare-earth phosphors have made theoretical forecasts that daylight could be reconstituted by adding three primary emissions in defined proportions at 450, 550 and 610 mm a virtual possibility (fig 5). In a low pressure fluorescent tube, conversion ultraviolet rays emitted by excited mercury into visible light is traditionally done using an off-white band emitter, the calcium halophosphate activated by antimony and manganese. The perfection of phosphors using blue emissions from divalent europium (specific case of a narrow band emission from transitions bringing the 5d levels into play) and green emissions from trivalent terbium coactivated by cerium, respectively, and red from trivalent europium has made it possible to make trichromatic fluorescent tubes recently miniaturized in the form of compact lights for use in the home.

• Flat Screen Technology

There is a strong demand to replace the cumbersome cathode ray tubes on TV's and computers with large dimension flat screens. Different technologies using Rare Earths are emerging:

		• Field emission screens, operating at low tension (< 8000 V) use rare-earth-based phosphors: Red (Y2O3: Eu; Sr TiO3: Pr …) Green (Y3 (Al, Ga)5 O12: Tb; Y2 SiO5: Tb, SrGa2S4:Eu…) Blue (Y2 Si O5: Ce)
		• Plasma technology screens. In this case, the phosphors are excited by the discharge of a plasma placed between two electrodes. The screen is broken down into small cells or pixels, the size of which small enough to obtain high definition screens. There is a clear improvement in life span in these screens. Rare earths are present in the blue phosphors (Ba Mg Al10 O17: Eu), red (Y, Gd) BO3: Eu, which is preferable to Y2O3: Eu due to its stronger VUV absorption, and green (Y, Gd) BO3: Tb.

• Signs and Signals

There has also been a marked increase in the use of phosphors in signs and signals. Numerous safety signs (exit lights, reflective safety bands and highway markings, etc.) require the use of phosphorescents. Rare Earths are part of the mineral phosphorescent product with the longest known phosphorescence duration, namely Sr Al₂ 04: Eu Dy .

There is also a strong increase in the use of Phosphor Converters Light Emitting Diodes (PCLED) for illuminating applications: traffic signals, shop windows, advertisement boards and signals, etc. The LED is a semi-conducting material that can emit a blue light. This blue emission is used as an excitation source of phosphors such as Y₃ Al₅ O₁₂: Ce³⁺ also called YAG:Ce. The addition of blue light from the diode and the emission resulting from YAG:Ce excitation produces white light.

Diodes emitting a wide spectrum of colors are now manufactured all over the world. diodes have a longer lifetime than other lamp t so their use is particularly prevalent wherever lighting maintenance is expensive.

• Medical Radiography

A third domain favoring the use of Rare Earths luminescence is medical radiography. The radiological image is transformed into an optical image by a screen intensifier using the ability of a phosphor to transform the X-ray into visible blue or green light, to which photographic emulsions are far more sensitive than X-rays.

The use of rare-earth phosphors for this application was developed and thus replaced the calcium tungstate used in traditional systems. This results in: · the intrinsic absorption of X-rays on the range of energies brought into play · the more effective conversion into visible light (10 to 20% output rather than 6% for CaWO4) and · the spectral separation of the emission (from trivalent terbium or thulium) adapted to the maximum sensitivity of the emulsions used, three essential criteria in defining the efficiency factor of screen intensifiers.

There are three major types of rare-earth phosphors used:

• Gd₂O₂S: Tb (green), LaOBr: Tm in YTaO₄: Nb or Tm (blues),

which make it possible to appreciably improve the quality of the images obtained and at the same time decrease patient exposure time by a factor of 2 to 4.

• Contacts

If you would like more informations, please contact:
ec-electronics@eu.rhodia.com