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White Light Sources

Humans do interpret quite different visual stimuli as the colour "white". Not only broad band emissions like the ones from daylight sources produce a "white" perception, but also narrow band light sources like fluorescent tubes. These are glass tubes filled with mercury vapour and electrodes at each end. The interior of the tube is coated with a fluorescent material consisting of a semi-conducting phosphor (e.g. calcium tungstate, zinc sulfide, zinc silicate, or others) and diverse dopants (mostly heavy metals). This material absorbs most of the UV - part of the Hg emission and excites electrons from the valence band into the conduction band. Due to the dopants, a band of energies exists in the band gap of the phosphor. The position of this band depends on the type of the dopant atoms. After non-radiative decay to this band a radiative decay to the valence band follows. These transitions lie within the visible range, mostly around 600 nm (res. around 17000 cm-1) in the orange - red region. The corresponding spectral features are broader and differ from tube to tube whereas the comparatively sharp mercury emissions at around 408, 440 and 550 nm (res. 24500, 22800 and 18200 cm-1) appear in all fluorescent tube spectra. Therefore the white colour of the light produced by fluorescent tubes is essentially a combination of emission from the coating and the visible mercury lines. The luminescence spectra of two typical fluorescent tubes are given in Fig. 1. In recent years, there is a growing concern about the mercury which eventually pollutes the environment because it is a health hazard. Therefore, there is an increasing demand for light emitting devices that are operated without mercury.
White light fluorescent tubes that are operated without mercury therefore demand a phosphor that has a great light-emitting intensity and can deliver white light at high efficiency directly from UV-excitation of the phosphor in the coating.

light distribution spectra of two fluorescent tubes
  Fig. 1: Two typical spectral distributions of fluorescent tubes. The mercury lines are marked with Hg and a letter in brackets, the reddish fluorescent peaks are marked with "F".
 

White light crystals and microcrystals

Dy,Tm-; Eu,Dy,Tm-; Tb, Dy, Tm- and Eu, Tb, Dy, Tm co-doped Gadoliniumaluminiumborate (GAB) or Yttriumaluminumborate (YAB) crystals and microcrystalline powders show the desired effect. It should be emphasised that the white impression results from luminescence of the rare earth ions only and does not require the presence of mercury emission bands.White light has already been obtained with similar materials like Ce,Dy:GAB or Tm,Dy:YAB (see Fig. 2). Exchanging Ce3+ with Tm3+ increases the colour rendering of the emitted light, and a further addition of Tb3+ results in a greenish white, whereas an addition of Eu3+ leads to a phosphor with a reddish white emission. When both Tb3+ and Eu3+ are added, a polychromatic white phosphor can be obtained, whose chromaticity co-ordinates are closest to the white point whilst intensity and colour rendering index are highest (next to Eu,Dy,Tm:GAB(2,3,2)) when compared to the other phosphors presented. The CIE1931 xy chromaticity co-ordinates of the phosphors under 390nm excitation are depicted in Fig. 3, the emission spectra of the phosphors described above are given in Fig. 4, including the Russell–Saunders terms of the respective emission bands.

white light crystals
  Fig. 2: Dy,Tm:YAB(4%,5%) crystals in a transparent box. Excitation source: high pressure Hg UV lamp, 390 nm UV filter.
 
  Fig. 3: CIE1931 xy chromaticity co-ordinates of Ce,Dy:GAB(4,3) (a), Dy,Tm:GAB(3,2) (2), Eu,Dy,Tm,GAB(2,3,2) (3), Tb,Dy,Tm:GAB(2,3,2) (4) and Eu,Tb,Dy,Tm:GAB(2,2,3,2) (5) upon 390nm excitation (all marked with an ×). The asterisk marks the position of the white point.
 
  Fig. 4: Emission spectra of Ce,Dy:GAB(4,2) (a), Dy,Tm:GAB(3,2) (b), Eu,Dy,Tm:GAB(2,3,2) (c), Tb,Dy,Tm:GAB(2,3,2) (d) and
Eu,Tb,Dy,Tm:GAB(2,2,3,2) (e) upon 390nm excitation.
  A series of emission spectra with excitation from 350 to 370nm is given in Fig. 5. In order to simulate a technical application like down-conversion for UV LEDs, the excitation was relatively broad (15 nm spectral band width). As can be seen from Fig. 5, the variation of the excitation wave length results in different intensity distributions of the emittion ions. Therefore, the colour temperature of the emitted white light can be adjusted by variation of the excitation wavelength.
 
 

Fig. 5: Emission spectra of Eu,Tb,Dy,Tm:GAB(2,2,3,2), excitation from350 to 370 nm, 15nm excitation band width

 

Polycrystalline sub-micron-sized GdAl3(BO3)4 phosphors doped with rare-earth ions have been prepared by combustion synthesis with urea. Whereas Dy,Tm:GAB(3,2) exhibits a higher colour rendering index (Ra = 30) than Ce,Dy:GAB(4,3) (Ra = 3), Eu,Tb,Dy,Tm:GAB(2,2,3,2) reveals even more advantages. In addition to Dy3+ and Tm3+, this material shows the highest emission intensities of the phosphors investigated herein. A total of seven
emission bands occurs, resulting in a 40% higher total emission compared to Ce,Dy:GAB(4,3) and a higher colour rendering index (Ra = 48). The colour temperature is stable from 350 to 360nm excitation at ~3900K, and can be gradually lowered ~3300 K by increasing the excitation wavelength up to 370 nm. According to a recent work by Krames et al., such low colour temperatures close to 3200K (incandescent tungsten halogen lamp) are a common target for white LEDs in indoor illumination. These beneficial optical properties in combination with the chemical inertness, the temperature stability and the high radiation damage threshold, discloses Eu,Tb,Dy,Tm:GAB(2,2,3,2) as a promisingp hosphor material for the application with UV sources of high junction temperatures like high-power LEDs.
A very good overview and detailed descriptions on measuring colours, colour temperatures and colour differences are given in R.W.G. Hunts book "Measuring Colour", Ellis Horwood Publishers (1987), and in the technical reports obtainable at the CIE. More references on this subject and references on YAB and GAB can be found in the paper given below.

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