Abstract:
The invention relates to a light emitting diode device comprising a light emitting layer ( 103 ) and a filter layer ( 105 ) arranged on a surface of the light emitting layer ( 103 ), the filter layer ( 105 ) being adopted to receive light from the light emitting layer, to pass light components within a predetermined angular range and not to pass light components outside the predetermined angular range.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to light emitting diodes. 
       BACKGROUND OF THE INVENTION 
       [0002]    Light emitting diodes (LED) comprise a light emitting layer arranged on a substrate being formed by e.g. a LED chip. The light emitting layer, e.g. a line emitting phosphor layer, emits light of e.g. a certain color associated with a certain wavelength thus generating e.g. a red or green light. However, the light emitting layer may also reflect light impinging on its surface which widens an angular range of the emitted light. 
         [0003]    Light emitting devices are described in the U.S. Pat. No. 5,813,753, the U.S. Pat. No. 5,813,752, the EP 170320 and the EP 275601. Further approaches are described in the US 2005/0243570 A1, the EP 0922305 B1, the WO 2006/031352 A2, the US 2003/0169385 A1, the U.S. Pat. No. 4,882,617, the U.S. Pat. No. 5,813,752 and the U.S. Pat. No. 5,813,753. 
       SUMMARY OF THE INVENTION 
       [0004]    It is the object of the invention to provide a light emitting diode device with improved light emitting characteristics. 
         [0005]    This object is achieved by the features of the independent claims. 
         [0006]    The invention is based on the finding that light emitting characteristics of a light emitting diode may be improved when arranging a filter layer on a surface of a light emitting layer. In order to improve the angular radiation characteristic, the filter layer passes only light components which are within a predetermined angular range e.g. with respect to a normal of the filter layer. The light components which are outside the predetermined angular range are not passed by the filter layer. These components may be reflected by the filter layer towards e.g. the light emitting layer. 
         [0007]    The invention relates to a light emitting diode device comprising a light emitting layer and a filter layer arranged on a surface of the light emitting layer, the filter layer being adopted to receive light from the light emitting layer, to pass light components within a predetermined angular range and not to pass light components outside the predetermined angular range. 
         [0008]    According to an embodiment, the predetermined angular range is a range of ±5° with respect to a normal of the filter layer or a range of ±10° with respect to the normal of the filter layer or a range of ±15° with respect to the normal of the filter layer or a range of ±20° with respect to a normal of the filter layer or a range of ±25° with respect to a normal of the filter layer. Preferably, the angular range is between 5° to 15°. 
         [0009]    The reduced angular range can be used in projection systems, street lights, car lights or indoor car lights e.g. to adjust the desired angular light distribution, with one or more LEDs and flashes. In general, projection systems benefit from a small angular range, preferably 5° to 15°, and lighting systems profit most from a somewhat larger angular range, preferably larger than 15°. For zoom flashes, it would be advantageous to have flexible angular ranges by using a number of different interference filters. 
         [0010]    According to an embodiment, the filter layer is arranged to suppress light components or to reflect light components towards the light emitting layer if the light components are outside the predetermined angular range. 
         [0011]    According to an embodiment the filter layer is an interference filter. 
         [0012]    According to an embodiment, the light emitting layer comprises phosphor, in particular a green line emitting Ln 2 O 3 :Er or Ln 2 O 3 :Ho (Ln=Sc, Y, Gd, Lu) or a red line emitting K 2 M(IV)F 6 :Mn, M denoting a four valent metal ion. 
         [0013]    According to an embodiment, the filter layer is configured to suppress or to reflect or to absorb certain spectral light components to adjust a spectral characteristic of light emittable by the light emitting diode, in particular to adjust a spectral characteristic of a color point. 
         [0014]    According to an embodiment, the filter layer comprises a plurality of sub-layers wherein reflecting indices of subsequent sub-layers are different. 
         [0015]    According to an embodiment, the filter layer comprises a plurality of sub-layers having a thickness in a range between 0.2λ and 0.3λ, wherein λ denotes a desired emission wavelength. 
         [0016]    According to an embodiment, the filter layer is further adapted to form a lens. 
         [0017]    The invention further relates to a display device comprising the inventive light emitting diode device. 
         [0018]    The invention further relates to light emitting diode flash device comprising the inventive light emitting diode device. 
         [0019]    The invention further relates to a method for manufacturing a light emitting diode device with manufacturing a light emitting layer; and arranging a filter layer on a surface of the light emitting layer, the filter layer being adopted to receive light from the light emitting layer, to pass light components within a predetermined angular range and not to pass light components outside the predetermined angular range. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    Further embodiments of the invention will be described with reference to the following Figures, in which: 
           [0021]      FIG. 1  shows a light emitting diode; 
           [0022]      FIG. 2  shows a light emitting diode; and 
           [0023]      FIG. 3  shows optical spectra for excitation, emission and reflection of Y 2 O 3 :Er. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0024]      FIG. 1  shows a light emitting diode device comprising a substrate layer  101 , a light emitting layer  103  arranged on a surface of the substrate layer  101  and a filter layer  105  arranged on a surface of the light emitting layer  103 . 
         [0025]    The substrate layer  101 , for example a LED chip, may comprise further layers, e.g. a contact layer, a band-gap confining layer etc., which are provided to excite the light emitting layer  103 . The light emitting layer  103  may be a phosphor layer or may comprise LUMIRAMIC plates. 
         [0026]    As depicted in  FIG. 1 , the filter layer forming e.g. an interference filter (IF) may be used in connection with a light emitting device or in LED based systems. The interference layer  105  may further contribute to increasing a brightness and a contrast in LED based display systems, may enhance a color purity for e.g. phosphor converted LEDs, may relax LED binning issues associated with white light emitting phosphor converted LEDs or may provide a zoom function in e.g. LED flashes. 
         [0027]    According to an embodiment, the light emitting layer  103  may comprise phosphor powder layers and/or LUMIRAMIC plates. Furthermore, the interference filter may also have a curved shape, e.g. a convex or a concave shape to process a lens function as depicted in  FIG. 2 . 
         [0028]    The LED device shown in  FIG. 2  has a substrate  201 , e.g. a LED chip, having e.g. the characteristics of the substrate  101 , a light emitting layer  203  having e.g. the characteristics of the light emitting layer  103  and a filter layer  205  having e.g. the characteristics of the filter layer  105  of  FIG. 1 . 
         [0029]    Furthermore, a surface of the light emitting layer may have e.g. a convex or a concave shape wherein the filter layer  205  may have a surface following the shape of the surface of the light emitting layer  203 . A top surface of the interference layer  205  may also be curved, e.g. convex or concave, to form a lens. 
         [0030]    For example, the light emitting layer  103  and  203  may comprise green and/or red line emitting phosphor, by way of example, wherein, when using line emitters, the light output gain is increased. Furthermore, the use of line emitting phosphor reduces chromatic aberration which may be induced by optical components in the optical system. In order to provide a green line emitting phosphor, e.g. for application in phosphor conversed LEDs, e.g. Ln 2 O 3 :Er or Ln 2 O 3 :Ho (Ln=Sc, Y, Gd, Lu) may be used. In order to provide red line emitting phosphor, e.g. K 2 M(IV)F 6 :Mn, denoting a four-violent metal ion, may be used. Blue primaries may, however, also be employed in order to obtain more light yield in e.g. a forward direction. As far as broad-band emitters are concerned, the optical gain may be reduced but may however, still be significant. In such cases, the filter layer  105  or  205  forming e.g. interference filters may contribute to obtaining more saturated colors, for example to enlarging a color gamut. As the case may be, the interference filter according to the invention may enhance the contrast when e.g. used in display systems. 
         [0031]    Referring again to the filter layers  105  and  205  disclosed in  FIGS. 1 and 2 , a light reflected back onto the reflective light emitting layer  103  or  203  or the substrate  101 ,  201  may be scattered so that a component of the light may pass through the respective filter layer  105  or  205 . In this way, an angle at which the light leaves the LED device may be reduced which enables smaller optics and a higher light output in a desired direction. 
         [0032]    The filter layer forming e.g. an interference filter may introduce multiple reflections of light components arriving at the interference filter within or outside a certain angular range whereas the interference filter may be transparent with respect to a fraction of the light generated by the substrate forming e.g. a LED and/or by the light emitting layer on a top of the substrate  101  when arriving at the interference filter within an angular range differing from the above outlined range. 
         [0033]    Furthermore, the filter layer may be employed to correct small deviations of e.g. a central wavelength of e.g. a blue LED in e.g. white light emitting LED alarms. By suppressing a part of the emission generated by the light emitting layer also corrections of the color point of the LED light are possible. Moreover, the inventive approach may also be used to realize a zoom flash wherein the interference filter may be inserted in the optical pathway. 
         [0034]    Furthermore, the filter layer may be used in combination with LUMIRAMIC plates which also may comprise a phosphor powder layer. 
         [0035]    The filter layers shown in  FIG. 1  or  2  may consist of a number of layers wherein succeeding layers may have different reflection indices. For example, a first layer may have a first reflecting index, a second layer following the first layer may have a second reflecting index which has a lower or higher reflecting index and so forth, so that, alternately, the reflecting indices are reduced or increased. Furthermore, the layers may be formed such that they do not absorb light. Furthermore, the filter layers may comprise or may consist of SiO 2  and/or TiO 2 . However, other materials can also be used. The number of layers may vary and may amount to 20 in case of e.g. line emitters or may amount to an increased number, e.g. 40, in the case of broad-band emitters. The number of layers may also be dependent upon a difference in excitation wavelength of a light generated by the LED chip  101  or  201  and/or the emission wavelength of the light generated by the light emitting layer  103  or  203 . The optical thickness of the filter layers, nd, n denoting the refractive index, d denoting a physical index, may be between 0.2λ and 0.3λ, λ denoting a desired central wavelength of the emitted light. 
         [0036]    The filter layers may further be manufactured independently by e.g. sputtering or upon a basis of a gas phase on e.g. a piece of glass or plastic or silicon which may be applied to the powder layer or to the LUMIRAMIC plate. However, the filter layer may also be directly applied to LUMIRAMIC plates. Preferably, the filter layers  105  or  205  directly contact to a luminescent structure formed e.g. by the light emitting layers  103  and  203 . 
         [0037]      FIG. 3  shows optical spectra resulting when employing e.g. Y 2 O 3 :Er in a filter layer in the case of λ Peak =564 nm and Y 2 O 3 :0.8%Er. In particular,  FIG. 3  shows a curve of a reflection spectrum  301 , an emission spectrum  303  and an excitation spectrum  305  with respects to a relative intensity over wavelength in nanometers.