Illumination device with at least one LED as light source

Details Illumination device with at least one LED as light source Illumination device with at least one LED as light source

2002-09-12

2004-07-06

Patel, Ashok (Department: 2879)

Electric lamp and discharge devices

With luminescent solid or liquid material

Solid-state type

C313S503000, C313S502000, C313S501000, C257S099000, C362S800000

Reexamination Certificate

active

06759804

ABSTRACT:

TECHNICAL FIELD
The invention is based on an illumination device with at least one LED as light source in accordance with the preamble of claim
1
. It deals in particular with an LED which emits in the visible or white region and is based on an LED which emits primarily UV/blue light.
BACKGROUND ART
An illumination device with at least one LED as light source which emits, for example, white light is currently produced predominantly by combining a Ga(In)N LED which emits in the blue region at approximately 460 nm and a yellow-emitting YAG:Ce
3+
phosphor (U.S. Pat. Ser. No. 5,998,925 and WO 98/12757).
A more complex concept with improved color rendering is a three-color mixture. In this case, the fundamental colors red-green-blue (RGB) are mixed to produce white. In this context, it is either possible to use a blue LED for partial conversion of two phosphors which emit red and green (WO 00/33390) or to use a UV-emitting LED which excites three phosphors, which respectively emit in the red, green and blue (cf. WO 97/48138). Examples are line emitters, such as YOB:Ce,Tb (green) and YOS:Eu (red). In this case, however, a relatively short-wave emission (UV region<370 nm) is required in order to be able to achieve high quantum efficiencies. This requires the use of sapphire substrates for the UV LED, which are very expensive. On the other hand, if a UV LED based on the less expensive SiC subtrates is used, one has to be satisfied with an emission in the range from 380 to 420 nm, which makes the use of line emitters in the green and red difficult to impossible. In the case of blue phosphors, this leads to absorption problems.
Furthermore, a specific problem in this context is the additional absorption loss of blue radiation on account of the broad-banded nature of the absorption of the red- and green-emitting phosphors. All this together leads to considerable limitations when setting the luminous color and light efficiency.
U.S. Pat. No. 5,535,230 has disclosed the use of ordinary “calcium halophosphate” for semiconductor components or fluorescent lamps. This term is understood as meaning the commercially available Ca
5
(PO
4
)
3
Cl:(Sb,Mn), as discussed in detail, for example, in U.S. Pat. No. 5,838,101. A further halophosphate discussed in that document is (Sr,Ba,Ca)
5
(PO
4
)
3
Cl:Eu. In this context, these phosphors are used for excitation by the far UV (254 nm).
Finally, U.S. Pat. No. 3,513,103 proposes the use of (Sr,Ca)
9
(PO
4
)
6
Cl
2
:(Eu,Mn) in fluorescent lamps for conversion of the UV radiation.
DISCLOSURE OF THE INVENTION
It is an object of the present invention to provide an illumination device with at least one LED as light source, the LED emitting primary radiation in the range from 380 to 420 nm, this radiation being partially or completely converted into longer-wave radiation by phosphors which are exposed to the primary radiation from the LED, which is distinguished by high stability. A further object is to provide an illumination device which emits white light and in particular has a high color rendering. A further object is to provide a high-efficiency LED which absorbs well in the range from 380 to 420 nm and is easy to produce.
This object is achieved by the following features: the conversion is achieved at least with the aid of a phosphor which emits over a broad band and originates from the class of the (Eu,Mn)-coactivated halophosphates, the halophosphate corresponding to the formula M
5
(PO
4
)
3
(Cl,F):(Eu
2+
,Mn
2+
), where M=Sr, Ca, Ba individually or in combination. Particularly advantageous configurations are given in the dependent claims.
High-efficiency phosphors which emit red light and are suitable for LEDs which emit primarily in the near UV are rare. The term near UV is in this context intended to cover wavelengths between approximately 380 and 420 nm. By way of example, the line emitter Ln
2
O
2
S:Eu
3+
(Ln=La, Y, Gd, Lu) is suitable for use with UV LEDs with primary emission below 380 nm. Hitherto, only Eu-doped nitrides or sulfides have been serious candidates for wavelengths above 380 nm. However, nitrides, which are sensitive to air, are difficult to produce. Sulfides have two drawbacks: they have an absorption gap between 380 and 410 nm and they are not particularly stable, i.e. are temperature-sensitive. Organic phosphors are too sensitive for LED applications. Virtually all known oxides have a low absorption at wavelengths greater than 380 nm.
According to the invention, the phosphor used for the LED-based illumination device is a halophosphate of type (Sr,Ba,Ca)
5
(PO
4
)
3
(Cl, F):(Eu,Mn). The same halophosphate, but activated only with Eu, is well known as a blue-emitting phosphor for fluorescent lamps and is commercially available. It has very good absorption at 254 and 365 nm. However, its absorption in the near UV (380 to 420 nm) is unsatisfactory. Quite unexpectedly, it has been found that adding the coactivator Mn hugely increases the absorption in the desired region without the efficiency suffering significantly.
In particular, it has been found that adding a melting agent in a high concentration (SrCl
2
) and the use of a relatively high level of Eu are favorable. Furthermore, a high level of purity and a small grain size of the raw materials should be ensured. Then, depending on the concentration of the activators Eu and Mn, a yellow to red emission results. The Eu content based on the divalent cation is preferably 0.5 to 2 mol %, and the Mn content, based on the divalent cation, is 3 to 8 mol %. The cation used is primarily Sr, if appropriate with a small amount of added Ba and/or Ca. The Mn:Eu ratio should be approximately 4 to 8.
Overall, therefore, it is possible to achieve highly efficient conversion of primary radiation in the range from 380 to 420 nm, since this excitation range is close to the emission of the phosphor.
In addition to producing a colored light source by excitation by means of UV radiation or blue primary emission from an LED (380 to 420 nm), it is advantageous in particular to generate white light with the aid of this phosphor. In the case of a UV-emitting LED as primary light source, this is achieved using at least two phosphors. In this way, it is possible to achieve good color rendering of over Ra=75. The point is that the phosphor according to the invention itself simultaneously provides two emission peaks in the blue and the yellow-red spectral regions, and therefore achieves something which normally requires two different phosphors.
This phosphor can be used together with other phosphors to achieve a white-emitting LED. It is particularly preferable for it to be used together with chlorosilicate phosphors as are already known per se. In this context, see for example WO 01/95400, corresponding to U.S. Ser. No. 10/048,963 and WO 01/93342, corresponding to U.S. Ser. No. 10/031,578, to which reference is expressly made. Therefore, the white point can be hit almost exactly given a suitable mixture.
A white mixture can also be produced on the basis of a UV-emitting LED by means of this (Eu,Mn)-doped halophosphate together with a blue-green phosphor, such as for example BaMgAl
10
O
17
:Eu
2+
(BAM). If necessary, the color rendering can be improved still further by adding a green phosphor (for example Eu-doped thiogallates or chlorosilicates or Sr aluminate).


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patent: 5535230 (1996-07-01), Abe
patent: 5838101 (1998-11-01), Pappalardo
patent: 5998925 (1999-12-01), Shimizu et al.
patent: 6351069 (2002-02-01), Lowery et al.
patent: 6488392 (2002-12-01), Lu
patent: 6504179 (2003-01-01), Ellens et al.
patent: 6670748 (2003-12-01), Ellens et al.
patent: 201 08 013 U 1 (2001-09-01), None
patent: 100 26 435 (2002-04-01), None
patent: WO 97/48138 (1997-12-01), None
patent: WO 98/12757 (1998-03-01), None
patent: WO 00/33390 (2000-06-01), None

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