Led-based white-emitting illumination unit

DETAILED DESCRIPTION OF THE DRAWINGS The following text provides a more detailed description, by way of example, of the synthesis of an Eu-doped and Mn-doped chlorosilicate Ca.
sub.
8 Mg(SiO.
sub.
4).
sub.
4 Cl.
sub.
2 : (Eu.
sup.
2+,Mn.
sup.
2+).
Then, the suitability of this phosphor is documented on the basis of a number of exemplary measurements.
The phosphor powder is produced by means of a high-temperature solid-state reaction.
For this purpose, by way of example, the highly pure starting materials CaCO.
sub.
3, MgO, SiO.
sub.
2 and CaCl.
sub.
2 are mixed together in a molar ratio of 7:1:4:1.
5.
A small quantity of Eu.
sub.
2 O.
sub.
3 or MnCO.
sub.
3 is added for doping purposes, replacing the corresponding molar quantity of CaCO.
sub.
3.
This corresponds to the empirical formula Ca.
sub.
8-x-y Eu.
sub.
x Mn.
sub.
y Mg(SiO.
sub.
4).
sub.
4 Cl.
sub.
2 plus 0.
5 CaCl.
sub.
2.
After the individual components have been well mixed, the powder is heated at 1000-1200.
degree.
C.
for 1-4 h in a reducing atmosphere (H.
sub.
2 /N.
sub.
2), so that it reacts with the compound described above.
To remove excess CaCl.
sub.
2 and other water-soluble foreign phases, the powder can be washed again using fully deionized water.
The result is a phosphor powder with high quantum efficiencies (typically approximately 70%) when excited in the short-wave wavelength region around 400 nm.
FIG.
1 shows a typical excitation and emission spectrum of a europium-doped powder.
The amount of Eu.
sub.
2 O.
sub.
3 added is 0.
03 mol, i.
e.
x=0.
06.
The efficiency of excitation over a very wide wavelength region from 300 to 470 nm, primarily 360 to 400 nm, is quite clear from this figure.
The reduction in excitability at greater wavelengths is caused by the Eu.
sup.
2+ absorption band.
However, at 460 nm, quantum efficiencies which are comparable to those at 400 nm or even shorter wavelengths (down to approximately 340 nm) are still measured.
The emission spectrum has an Eu.
sup.
2+ emission band with a maximum at about 507 nm.
This emission appears green to the eye.
If desired, the emission behavior of the phosphor can be better matched to the sensitivity of the eye by co-doping with a small quantity of manganese