Luminescent screen and low pressure mercury vapor discharge lamp containing the same

The invention relates to a luminescent screen comprising a luminescent 
metaborate, provided on a support. The invention further relates to a 
low-pressure mercury vapor discharge lamp comprising such a luminescent 
screen. 
Netherlands Patent Application No. 7,607,724 (PHN No. 8464) discloses a 
luminescent screen containing a luminescent borate the host lattice of 
which is defined by the formula LnB.sub.3 O.sub.6, wherein Ln represents a 
rare earth metal. Activation of these known metaborates by Gd, Bi, Ce, Tb 
and/or Dy furnishes efficiently luminescing materials. 
The invention has for its object to provide novel luminescent metaborates 
which, when used in a luminescent screen, have greatly desired luminescent 
properties. 
A luminescent screen according to the invention comprises a luminescent 
metaborate, provided on a support, and is characterized in that the borate 
has a composition defined by the formula 
EQU (Y, La).sub.1-x-y-z Ce.sub.x Gd.sub.y Tb.sub.z (Mg, Zn).sub.1-p Mn.sub.p 
B.sub.5 O.sub.10, 
wherein, 
if y=z=p=O, it holds that 0.01.ltoreq.x.ltoreq.1.0 
if z=p=O, it holds that 
0.01.ltoreq.x.ltoreq.1-y, 
0.02.ltoreq.y.ltoreq.0.80, 
if p=0, it holds that 
0.01.ltoreq.x.ltoreq.1-y-z 
0.ltoreq.y.ltoreq.0.98 
y+z.ltoreq.0.99 
0.01.ltoreq.z.ltoreq.0.75 
if z=0, it holds that 
0.01.ltoreq.x.ltoreq.1-y 
0.ltoreq.y.ltoreq.0.99 
0.01.ltoreq.p.ltoreq.0.30, 
and if both p.noteq.0 and z.noteq.0, it holds that 
0.01.ltoreq.x.ltoreq.1-y-z 
0.ltoreq.y.ltoreq.0.98 
0.01.ltoreq.z.ltoreq.0.75 
y+z.ltoreq.0.99 
0.01.ltoreq.p.ltoreq.0.30. 
The luminescent borates of a luminescent screen according to the invention 
are crystalline materials the host lattice composition of which may be 
represented by the formula (Y, La) (Mg, Zn) B.sub.5 O.sub.10. It was found 
that these novel materials have a monoclinic crystal structure which 
corresponds to the structure of LaCoB.sub.5 O.sub.10, NdCoB.sub.5 O.sub.10 
and SmCoB.sub.5 O.sub.10. The crystal structure of the last-mentioned 
material, for example, is described in Kristallografiya, 19. 737 (1974). 
From experiments which resulted in the invention it appeared that these 
novel metaborates, which contain Y and/or La and Mg and/or Zn form very 
efficiently luminescing materials on activation by one or more of the 
activator elements Ce, Gd, Tb and Mn. They can be excellently excited by 
ultraviolet radiation, and particularly by short-wave ultraviolet 
radiation. 
They are, consequently, very suitable for practical use in the luminescent 
screen of a low-pressure mercury vapor discharge lamp. It further appeared 
that the borates according to the invention can also be excited in a 
different manner, for example by electrons. The spectral distribution of 
the radiation emitted by the borates depends on the activator or 
activators used. The concentration of a certain activator is chosen within 
a certain range, since, below a certain minimum value of the concentration 
of that activator too low an absorption of the excitation energy and/or 
too low a quantum efficiency occurs, and as above a certain maximum value 
radiation fluxes are obtained which are too low owing to concentration 
quenching. It was found that the choice of the component cations for the 
host lattice, Y or La or both and Mg or Zn or both, have little influence 
on the luminescent properties of the luminescent borate. 
When the borate defined by the above-mentioned general formula are 
activated by Ce alone (that is to say y=z=p=0) materials are obtained 
which have an efficient narrow band emission of short wavelength. The 
emission maximum is located at 300-305 nm and the half-value width of the 
emission band is 35 nm. In these materials which are particularly suitable 
for use in the luminescent screen of low-pressure mercury vapor discharge 
lamps for photo-chemical purposes, for example for generating erythema, 
the Ce-content x is chosen to be at least 0.01. The maximum value of x is 
1.0, as concentration quenching does not occur in these materials to any 
significant extent. 
The borates activated by Ce and Gd (wherein z=p=0) are efficiently 
luminescing materials, which emit the characteristic line emission of Gd 
with an emission maximum at 313 nm. In these materials the excitation 
energy is transferred from the Ce-ions to the Gd-ions. The 
Gd-concentration y may then be chosen between wide limits (from 0.02 to 
0.80). The minimum Ce-concentration x is here also 0.01. As there is no 
significant concentration quenching effect, the Ce-content may be 
increased until all the available Y- and/or La-positions are occupied 
(x.ltoreq.1-y). The borates with Gd-emission are used in the luminescent 
screen of low-pressure mercury vapor discharge lamps for photo-chemical 
purposes, irradiation purposes in particular, for example the 
photo-therapy of skin diseases such as psoriasis. 
The borates according to the invention activated by Ce and Tb, (p=0) are 
very efficiently, green-luminescing materials (characteristic 
Tb-emission). In these materials the excitation energy is absorbed by the 
Ce and transferred to the Tb. Also here the Ce content x is at least 0.01. 
The Tb-content z should be at least 0.01 and not more than 0.75. These 
borates may also contain Gd (y from 0 to 0.98), the energy transfer from 
the Ce to the Tb proceeding partly via Gd-ions. The sum of Gd- and 
Tb-contents y+z must not be more than 0.99, as at least 0.01 mole Ce must 
be present. The upper limit of the Ce-content is determined by the number 
of available positions in the lattice (x.ltoreq.1-y-z). The green 
luminescing, Tb-activated borates are used in the luminescent screen of 
special purpose low-pressure mercury vapor discharge lamps (for example 
for electro-photography) or, in combination with other luminescent 
materials, in low-pressure mercury vapor discharge lamps used for general 
lighting purposes. 
The Mn-activated borates (z=0) according to the invention luminesce in a 
band (half-value width approximately 80 nm) having a maximum in the red 
portion of the spectrum at approximately 622 nm. They are therefore very 
suitable for use as the red component or as the colour-correcting 
component in the luminescent screen of low-pressure mercury vapor 
discharge lamps used for general lighting purposes. Also in these 
materials the excitation energy is transferred from the Ce (Ce-content 
from 0.01 to 1-y) to the Mn. These materials may also contain Gd (y from 0 
to 0.99). The Mn-content p should be chosen in the range from 0.01 to 
0.30. 
The borates, which contain both Mn and Tb (consequently p.noteq.0 and 
z.noteq.0) show the red Mn-emission as well as the green Tb-emission. 
These materials are suitable for use in the screen of low-pressure mercury 
vapor discharge lamps used for general lighting purposes. In these 
materials the Ce is again present in a concentration of at least 0.01 and 
not more than 1-y-z. The materials may contain Gd (y from 0 to 0.98). The 
range for the Tb- and Mn-content is equal to the content of the materials 
having only Tb- emission or only Mn-emission (0.01.ltoreq.z.ltoreq.0.75 
and 0.01.ltoreq.p.ltoreq.0.30). 
It was found that the Ce-emission is most efficient in the borates (wherein 
y=z=p=0) which have a Ce-content x from 0.01 to 0.50. A luminescent screen 
containing these materials is therefore preferred, if Ce-emission is 
desired. 
According to the invention, another embodiment of a luminescent screen is 
characterized in that 
z=p=0 
0.01.ltoreq.x.ltoreq.0.50 
0.05.ltoreq.y.ltoreq.0.75 
x+y.ltoreq.1. 
It was found that the most efficient Gd-emission is obtained with 
Gd-contents from 0.05 to 0.75 and Ce-contents from 0.01 to 0.50. 
The most efficient green Tb-emission is found in the borates defined by the 
above-mentioned general formula and conditions, which do not contain Mn 
(p=0) and wherein all the available Y- and La-positions are occupied by 
Ce, Tb and, possibly, Gd (x+y+z=1) and wherein up to 20 mole % of the B be 
replaced by Al and/or Ga. From experiments it was found that such a slight 
replacement in the borates activated by Ce and Tb may result in higher 
luminous fluxes. 
The highest luminous fluxes are obtained with the above-mentioned 
green-luminescing borates (p=0, x+y+z=1) when the Gd-concentration y is 
chosen in the range from 0.50 to 0.90, and 0.5 to 8 mole % of B is 
replaced by Al and/or Ga. It appeared, that the energy transfer from Ce to 
Tb proceeds very well via Gd-ions. The use of Gd in the above-mentioned 
range renders it possible to choose a lower Tb-content. Screens comprising 
these luminescing borates are therefore preferred, if green emission is 
desired. 
Yet another advantageous embodiment of a luminescent screen according to 
the invention contains a borate having a composition defined by the 
above-mentioned general formula, and is characterized in that z=0, x+y=1, 
and wherein up to 20 mole % of the B may be replaced by Al and/or Ga. It 
was also found for the borates which were activated by Ce and Mn and which 
have the red Mn-emission that the most efficient materials are obtained 
when the Y and La have been wholly replaced by Ce and, possibly by Gd. It 
appears again that the transfer of excitation energy proceeds very well 
via Gd-ions. Therefore, preference is given to screens of this type, for 
which it holds that 0.50.ltoreq.y.ltoreq.0.99, and wherein 0.5 to 8 mole % 
of B has been replaced by Al and/or Ga. The use of Gd renders it possible 
to choose a lower Mn-content, causing concentration quenching of the 
Mn-emission to become less likely. Also for these borates, activated by Ce 
and Mn, it holds that a small replacement of the B by Al and/or Ga may 
result in higher luminous fluxes. 
A preferred embodiment of luminescent screen by means of which white light 
can be generated in combination with a low-pressure mercury vapor 
discharge comprises a borate, activated by Ce, Tb and Mn, and is 
characterized according to the invention in that the borate has a 
composition defined by the above-mentioned general formula and that 
p.noteq.0 and z.noteq.0, that x+y+z=1, and that up to 20 mole % of the B 
may be replaced by Al and/or Ga. It was found that also in these borates 
it is advantageous when Y and La are wholly replaced by Ce, Tb and, 
possibly, Gd. Preference is again given to the borates which contain Gd, 
wherein 0.50.ltoreq.y.ltoreq.0.90, and wherein from 0.5 to 0.8 mole % of B 
is replaced by Al and/or Ga. 
Some embodiments of the invention will now be further described and 
explained in greater detail with reference to the accompanying drawings.

In FIG. 1 reference numeral 1 denotes the glass wall of a low-pressure 
mercury vapor discharge lamp according to the invention. Electrodes 2 and 
3 in which the discharge takes place during operation of the lamp are 
disposed, one each, at the ends of the lamp. The lamp contains a rare gas 
mixture, which serves as the igniting gas, and a small quantity of 
mercury. The wall 1 is constructed as a luminescent screen and is coated 
on its inside with a luminescent layer 4, which contains a luminescent 
metaborate according to the invention. The luminescent layer 4 may be 
applied to the wall 1 in a conventional manner, for example by means of a 
suspension containing the luminescent material. 
EXAMPLE 1 
A mixture was made of 
1.76 g La.sub.2 O.sub.3 
0.21 g CeO.sub.2 
1.29 g MgCO.sub.3 Mg(OH).sub.2.3H.sub.2 O 
4.01 g H.sub.3 BO.sub.3 
For each mole of metaborate to be formed this mixture comprises an excess 
of 0.1 mole Mg and 0.4 mole B. As generally known, the use of a slight 
excess of one or more of the constituent components may have an 
advantageous influence on the formation of the luminescent material. If so 
desired, the excess of B may be removed from the final product by washing 
with water. The above-indicated mixture was heated for 1 hour at 
400.degree. C. in a weakly reducing atmosphere. After cooling and 
pulverizing, the product thus obtained was subjected three times to a heat 
treatment, each time for 1 hour at 1045.degree. C. in a weakly reducing 
atmosphere. After cooling and pulverizing, a luminescent metaborate having 
a composition defined by the formula La.sub.0.90 Ce.sub.0.10 MgB.sub.5 
O.sub.10 was obtained. X-ray diffraction analysis showed that this 
material (and also all the examples of materials according to the 
invention given hereafter) has a monoclinic crystal structure similar to 
the structure of, for example LaCoB.sub.5 O.sub.10. On excitation by 
shortwave ultraviolet radiation (predominantly 254 nm-radiation), the 
borate appears to have a quantum efficiency of 72%. The emission spectrum 
of this material consists of a band (half-value width approximately 40 nm) 
with a maximum at approximately 302 nm. FIG. 2 shows this spectrum. In 
this Figure the wavelength .lambda. is plotted in nm on the horizontal 
axis and the emitted radiant energy E is plotted in arbitrary units on the 
vertical axis. 
EXAMPLES 2 to 6 
In a similar manner to that described in Example 1, a number of 
Ce-activated borates were prepared. The following Table I shows the 
formulae of these materials, together with the results of the measurements 
of quantum efficiency q (in %) and absorption A (in %) of the exciting 
radiation. 
TABLE I 
______________________________________ 
Example formula q A 
______________________________________ 
1 La.sub.0.90 Ce.sub.0.10 MgB.sub.5 O.sub.10 
72 84 
2 La.sub.0.95 Ce.sub.0.05 MgB.sub.5 O.sub.10 
67 82 
3 La.sub.0.85 Ce.sub.0.15 MgB.sub.5 O.sub.10 
65 88 
4 CeMgB.sub.5 O.sub.10 
15 98 
5 La.sub.0.90 Ce.sub.0.10 ZnB.sub.5 O.sub.10 
52 86 
6 Y.sub.0.90 Ce.sub.0.10 MgB.sub.5 O.sub.10 
70 86 
______________________________________ 
EXAMPLE 7 
A mixture was made of 
0.98 g Gd.sub.2 O.sub.3 
0.68 g Y.sub.2 O.sub.3 
0.10 g CeO.sub.2 
0.51 g MgO (excess of 0.05 mole per mole of borate to be formed) 
4.45 g H.sub.3 BO.sub.3 (excess of 1.0 mole per mole of borate to be 
formed). 
This mixture was fired three times, each time for 1 hour, at 1035.degree. 
C. in a weakly reducing atmosphere. The product was cooled and pulverized 
after each firing operation. The luminescent borate obtained had a 
composition defined by the formula Gd.sub.0.45 Y.sub.0.50 Ce.sub.0.05 
MgB.sub.5 O.sub.10. On excitation by 254 nm-radiation, the borate emits 
the characteristic Gd-radiation (some very narrow lines at approximately 
312 nm). FIG. 3 shows the spectral energy distribution of the emission of 
this borate. The peak height of the maximum emission is 59% of the peak 
height of the known, Bi-activated gadolinium lanthanum borate having a 
composition defined by the formula Gd.sub.0.5 La.sub.0.487 Bi.sub.0.013 
B.sub.3 O.sub.6. 
EXAMPLES 8 to 13 
In a similar manner to that described in Example 7, a number of borates, 
activated by Ce and Gd, were prepared. The formulae of these materials and 
the results of measurements of peak height P (in a % figure with respect 
to the above-mentioned standard) and absorption A (in %) of the exciting 
radiation are summarized in Table II. 
TABLE II 
______________________________________ 
Example formula P A 
______________________________________ 
7 Gd.sub.0.45 Y.sub.0.50 Ce.sub.0.05 MgB.sub.5 O.sub.10 
59 85 
8 Gd.sub.0.25 Y.sub.0.70 Ce.sub.0.05 MgB.sub.5 O.sub.10 
53 83 
9 Gd.sub.0.75 Y.sub.0.20 Ce.sub.0.05 MgB.sub.5 O.sub.10 
47 83 
10 La.sub.0.4 Gd.sub.0.5 Ce.sub.0.1 ZnB.sub.5 O.sub.10 
45 90 
11 La.sub.0.55 Gd.sub.0.35 Ce.sub.0.1 MgB.sub.5 O.sub.10 
36 89 
12 La.sub.0.35 Gd.sub.0.55 Ce.sub.0.1 MgB.sub.5 O.sub.10 
57 90 
13 La.sub.0.25 Gd.sub.0.65 Ce.sub.0.1 MgB.sub.5 O.sub.10 
56 91 
______________________________________ 
EXAMPLE 14 
A mixture was made of 
2.72 g Gd.sub.2 O.sub.3 
0.86 g CeO.sub.2 
0.93 g Tb.sub.4 O.sub.7 
1.06 g MgO (an excess of 0.05 mole per mole of borate to be formed) 
8.11 g H.sub.3 BO.sub.3 (an excess of 0.25 mole per mole of borate to be 
formed). 
This mixture was heated for 1 hour at 1035.degree. C. in a weakly reducing 
atmosphere. After cooling and pulverizing, the product obtained was once 
again subjected for 1 hour to the same temperature treatment. The 
luminescent borate obtained had a composition defined by the formula 
Gd.sub.0.6 Ce.sub.0.2 Tb.sub.0.2 MgB.sub.5 O.sub.10, and, on excitation by 
short-wave ultraviolet radiation (254 nm) it has a quantum efficiency of 
the terbium emission of 76%. The emission spectrum consists of the 
characteristic Tb-radiation and is shown in FIG. 4. 
EXAMPLES 15 to 26 
A number of borates activated by Ce and Tb were prepared in the same manner 
as described for Example 14. The formulae of the materials obtained and 
the measured quantum efficiency q of the Tb-emission and the absorption A 
(in %) are shown in Table III. 
TABLE III 
______________________________________ 
Example formula q A 
______________________________________ 
14 Gd.sub.0.6 Ce.sub.0.2 Tb.sub.0.2 MgB.sub.5 O.sub.10 
76 94 
15 Gd.sub.0.7 Ce.sub.0.15 Tb.sub.0.15 MgB.sub.5 O.sub.10 
75 94 
16 Gd.sub.0.4 Y.sub.0.3 Ce.sub.0.15 Tb.sub.0.15 MgB.sub.5 O.sub.10 
70 94 
17 Gd.sub.0.4 La.sub.0.3 Ce.sub.0.15 Tb.sub.0.15 MgB.sub.5 O.sub.10 
69 94 
18 Ce.sub.0.95 Tb.sub.0.05 MgB.sub.5 O.sub.10 
44 98 
19 Ce.sub.0.6 Tb.sub.0.4 MgB.sub.5 O.sub.10 
73 98 
20 Ce.sub.0.3 Tb.sub.0.7 MgB.sub.5 O.sub.10 
73 97 
21 Gd.sub.0.94 Ce.sub.0.05 Tb.sub.0.01 MgB.sub.5 O.sub.10 
60 85 
22 Gd.sub.0.75 Ce.sub.0.05 Tb.sub.0.2 MgB.sub.5 O.sub.10 
80 86 
23 Gd.sub.0.65 Ce.sub.0.05 Tb.sub.0.3 MgB.sub.5 O.sub.10 
80 85 
24 Gd.sub.0.94 Ce.sub.0.05 Tb.sub.0.01 ZnB.sub.5 O.sub.10 
59 89 
25 Gd.sub.0.70 Ce.sub.0.05 Tb.sub.0.25 ZnB.sub.5 O.sub.10 
76 90 
26 Gd.sub.0.66 Ce.sub.0.14 Tb.sub.0.20 MgB.sub.5 O.sub.10 
77 92 
______________________________________ 
EXAMPLES 27 to 32 
A number of borates according to the invention were prepared wherein a 
small quantity of the B had been replaced by Al or Ga in order to examine 
the influence of this substitution on the luminous flux of these 
materials. A mixture was made of 
20.66 g CeO.sub.2 
65.25 g Gd.sub.2 O.sub.3 
22.43 g Tb.sub.4 O.sub.7 
58.16 g MgCO.sub.3 Mg(OH).sub.2.3H.sub.2 O 
181.78 g H.sub.3 BO.sub.3 
3.06 g Al.sub.2 O.sub.3. 
This mixture, which contains the constituent components in stoichiometrical 
quantities, was heated twice, each time for 1 hour at 1035.degree. C. in a 
weakly reducing atmosphere. The product obtained was washed for half an 
hour in water (at room temperature) and thereafter was filtered and dried 
(at approximately 120.degree. C.). The borate obtained had a composition 
defined by the formula Ce.sub.0.2 Gd.sub.0.6 Tb.sub.0.2 MgB.sub.4.9 
Al.sub.0.1 O.sub.10 and shows the characteristic Tb-radiation. 
In the same manner as described in Example 27, and additional number of Al- 
or Ga-containing borates were prepared (Examples 28 to 31). The material 
of Example 32 was produced for the purpose of comparison. This material 
does not contain Al or Ga and, in all other respects, is fully equal to 
the material of Example 27. The formulae and measurements of the quantum 
efficiency q of the terbium emission and absorption A (254 nm-excitation) 
for these materials are summarized in Table IV. 
TABLE IV 
______________________________________ 
Example formula q A 
______________________________________ 
27 Ce.sub.0.2 Gd.sub.0.6 Tb.sub.0.2 MgB.sub.4.9 Al.sub.0.1 O.sub.10 
80 95.9 
28 Ce.sub.0.2 Gd.sub.0.6 Tb.sub.0.2 MgB.sub.4.8 Al.sub.0.2 O.sub.10 
80 95.5 
29 Ce.sub.0.2 Gd.sub.0.6 Tb.sub.0.2 MgB.sub.4.6 Al.sub.0.4 O.sub.10 
77 94.5 
30 Ce.sub.0.2 Gd.sub.0.6 Tb.sub.0.2 MgB.sub.4.2 Al.sub.0.8 O.sub.10 
63 91.3 
31 Ce.sub.0.2 Gd.sub.0.6 Tb.sub.0.2 MgB.sub.4.9 Ga.sub.0.1 O.sub.10 
79 96.5 
32 Ce.sub.0.2 Gd.sub.0.6 Tb.sub.0.2 MgB.sub.5 O.sub.10 
76 93.5 
______________________________________ 
EXAMPLES 33 to 38 
A luminescent borate, activated by Ce and Mn, defined by the formula 
Gd.sub.0.95 Ce.sub.0.05 Zn.sub.0.9 Mn.sub.0.1 B.sub.5 O.sub.10 was 
obtained by heating a mixture of 
4.30 g Gd.sub.2 O.sub.3 
0.22 g CeO.sub.2 
1.93 g ZnO (an excess of 0.05 mole per mole of borate) 
0.32 g MnCO.sub.3 
8.11 g H.sub.3 BO.sub.3 an excess of 0.25 mole per mole of borate), 
twice each time for 1 hour at 940.degree. C. in a weakly reducing 
atmosphere. On 254 nm-excitation this borate (Example 33) shows the red 
Mn-emission band with a maximum at approximately 635 nm and a half-value 
width of approximately 75 nm). FIG. 5 shows the emission spectrum. Some 
further borates with Mn-emission were obtained in a similar manner to that 
described above. The formulae of these materials and the measured quantum 
efficiency q (in %) of the manganese emission and absorption A (in %) at 
254 mn-excitation are shown in Table V. 
TABLE V 
______________________________________ 
Example formula q A 
______________________________________ 
33 Gd.sub.0.95 Ce.sub.0.05 Zn.sub.0.9 Mn.sub.0.1 B.sub.5 O.sub.10 
80 86 
34 La.sub.0.95 Ce.sub.0.05 Zn.sub.0.9 Mn.sub.0.1 B.sub.4 O.sub.10 
60 90 
35 Y.sub.0.95 Ce.sub.0.05 Zn.sub.0.9 Mn.sub.0.1 B.sub.5 O.sub.10 
60 89 
36 CeMg.sub.0.98 Mn.sub.0.02 B.sub.5 O.sub.10 
48 98 
37 CeMg.sub.0.89 Mn.sub.0.11 B.sub.5 O.sub.10 
63 98 
38 CeMg.sub.0.83 Mn.sub.0.17 B.sub.5 O.sub.10 
61 98 
______________________________________ 
EXAMPLES 39, 40 and 41 
A borate, activated by Ce, Tb and Mn, defined by the formula Gd.sub.0.6 
Ce.sub.0.2 Tb.sub.0.2 Mg.sub.0.97 Mn.sub.0.03 B.sub.5 O.sub.10 was 
obtained starting from a mixture of 
20.66 g CeO.sub.2 
65.25 g Gd.sub.2 O.sub.3 
22.43 g Tb.sub.4 O.sub.7 
23.48 g MgO 
2.30 g MnCO.sub.3 
193.40 g H.sub.3 BO.sub.3 (an excess of 0.21 mole per mole of borate). 
This mixture was heated twice, each time for 1 hour, at 1035.degree. C. in 
a weakly reducing atmosphere. After cooling, the product obtained was 
washed in water, filtered and dried. On excitation by 254 nm-radiation 
this borate shows both the characteristic Tb-radiation and also the red 
Mn-emission. Another two borates of this type were prepared in a similar 
manner. The formulae and measuring results of these materials are shown in 
Table VI. The values of the quantum efficiency q in this Table relate to 
the total efficiency of the Tb- and the Mn-emission. 
TABLE VI 
______________________________________ 
Example formula q A 
______________________________________ 
39 Gd.sub.0.6 Ce.sub.0.2 Tb.sub.0.2 Mg.sub.0.97 Mn.sub.0.03 B.sub.5 
O.sub.10 70 91 
40 Ce.sub.0.8 Tb.sub.0.2 Mg.sub.0.96 Mn.sub.0.04 B.sub.5 O.sub.10 
64 98 
41 Ce.sub.0.7 Tb.sub.0.3 Mg.sub.0.97 Mn.sub.0.03 B.sub.5 O.sub.10 
74 98 
______________________________________ 
A low-pressure mercury vapor discharge lamp of the type shown in FIG. 1, 
which is of the 40 W-type, was provided with a luminescent borate, 
activated by Ce and Tb, in accordance with the above Example 14. This lamp 
had an initial luminous flux of 109.3 lm/W, which was still substantially 
the same after the lamp had been in operation for 100 hours (109.1 lm/W). 
Such a lamp, provided with a Ce, Tb and Mn-activated borate in accordance 
with the above Example 39 had an initial luminous flux of 55.6 lm/W. After 
the lamp had been in operation for 100 hours the luminous flux was still 
54.7 1m/W. The colour point of the radiation emitted by this lamp was 
x=0.438 and y=0.418.