WO2009016585A2

WO2009016585A2 - Color conversion device

Info

Publication number: WO2009016585A2
Application number: PCT/IB2008/053034
Authority: WO
Inventors: Lingli Wang; Martijn H. R. Lankhorst; Martinus P. J. Peeters
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2007-08-02
Filing date: 2008-07-29
Publication date: 2009-02-05
Also published as: WO2009016585A3; TW200926453A

Abstract

The present invention relates to a color conversion device (204) for a light output device (102) emitting light within a first wavelength range, comprising a first wavelength converting element (106) adapted to receive light emitted by the light output device (102) and to output light at a second wavelength range, and a second wavelength converting element (108) arranged to receive the light outputted from the first wavelength converting element (108) and to output light at a third wavelength range, wherein one of the first or the second wavelength converting element (106, 108) is arranged to partly cover the other one of the first or the second wavelength converting element (106, 108), thereby allowing for the color conversion device (204) to output light within the second and third wavelength ranges. By only partly cover either the first wavelength converting element (106) with the second wavelength converting element, or only partly cover the second wavelength converting element (108) with the first wavelength converting element (106), it is possible to achieve a higher color rendering index (CRI) as not all of the light emitted by the light output device (102) is filtered by both the first and the second wavelength converting element (106, 108).

Description

COLOR CONVERSION DEVICE

FIELD OF THE INVENTION

The present invention relates to a color conversion device and a light emitting arrangement comprising such a color conversion device.

DESCRIPTION OF THE RELATED ART

At present, light emitting diodes (LEDs) have become the most efficient sources of colored light in almost the entire visible spectral range. For providing white light emission needed for general lighting applications, it is possible to for example use a combination of a green LED, a red LED, and a blue LED that are mixed and thereby provides white light. Conventionally, gallium nitride GaN material is used to provide the green and blue colors, and the gallium phosphide GaP material system to provide the red color. A drawback with such a combination of LEDs is however that multiple light sources are needed, thus resulting in a complicated fabrication process and an expensive end product.

Another method for providing white light emission needed for general lighting applications includes the conversion of light of a first (peak) wavelength into light having a longer (peak) wavelength using a process known as luminescence/fluorescence. The fluorescent process involves absorbing the light having the first wavelength by a wavelength- converting material such as a phosphor, exciting the luminescent centers of the phosphor material, which emit the light having the longer wavelength. This process can for example be used for covering a blue LED chip by an overlying yellow/orange phosphor layer, whereby unconverted blue light and converted yellow/orange light mix to white light.

A recent development in the area of such so called phosphor converted LEDs comprises using a ceramic layer as the overlying phosphor layer, as disclosed in the document US2005/0569582. In US2005/0569582, a light emitting layer is combined with a ceramic layer which is disposed in the path of the light emitted by the light emitting layer. The ceramic layer is composed of or includes a wavelength-converting material such as a phosphor. The ceramic layer may be more robust and less sensitive to temperature than other prior art phosphor layers, which typically comprise a translucent resin, silicone gel, or similar including wavelength-converting material. US2005/0569582 further discloses an embodiment where an additional ceramic layer is placed on top of the first ceramic layer, i.e. two ceramic layers are stacked over the light emitting layer. The two ceramic layers can comprise different phosphors, for example provided as a yellow layer and a red layer. The arrangement of the different phosphors in the two ceramic layers or the two ceramic layers themselves may be chosen to control the interaction between the multiple phosphors in the LED module, so that a certain color point can be provided.

However, a drawback with the stacked structure disclosed in US2005/0569582 is that a ceramic layer combination having specific properties (such as certain phosphor concentrations and/or a certain thickness of the layers) must be produced for each desired overall color point. Furthermore, by stacking the red layer on top of the yellow layer, less yellow light is provided, thus resulting in light emitting device having a low color rendering index (CRI), opposite to what is desired in for example general purpose lighting.

OBJECT OF THE INVENTION

There is therefore a need for an improved light emitting arrangement usable in a general lighting environment, and more specifically that handles the prior art problems with LED based general purpose lighting having a low color rendering index.

SUMMARY OF THE INVENTION

According to an aspect of the invention, the above object is met by a color conversion device for a light output device emitting light within a first wavelength range, comprising a first wavelength converting element adapted to receive light emitted by the light output device and to output light at a second wavelength range, and a second wavelength converting element arranged to receive the light outputted from the first wavelength converting element and to output light at a third wavelength range, wherein one of the first or the second wavelength converting element is arranged to partly cover the other one of the first or the second wavelength converting element, thereby allowing for the color conversion device to output light within the second and third wavelength ranges. It is according to the present invention possible to arrange at least two wavelength converting elements, a first and a second element, in the vicinity of a light output device. The light output device emits light within a first wavelength range, light which is converted by the first wavelength converting element to light within a second wavelength range. The light outputted by the first wavelength converting element is in turn again converted by the second wavelength converting element to a third wavelength range, together with parts of light within the first wavelength range that passes the first wavelength converting element without being converted to the second wavelength range. By according to the invention only partly cover either the first wavelength converting element with the second wavelength converting element, or only partly cover the second wavelength converting element with the first wavelength converting element, it is possible to achieve a higher color rendering index (CRI) as not all of the light emitted by the light emitted by the first wavelength converting element is filtered by the second wavelength converting element. Furthermore, as the shape and position of the one wavelength converting element partly covering the other wavelength converting element can be adjusted, it is possible to also, possible dynamically, adjust the white color point of the resulting light outputted by the color conversion device according to the present invention, thus not having to produced different wavelength converting elements for each desired overall color point. Also, as the second wavelength converting element does not fully cover the first wavelength converting element (possible asymmetric placement of the second wavelength converting element), it is possible to adjust the color point of the color conversion device by small displacements of the second wavelength converting element.

Preferably, the first wavelength converting element is arranged to fully cover the light output device, and thereafter only partly cover the first wavelength converting element with the second wavelength converting element. However, it is also possible to partly cover the light output device with the second wavelength converting element, and thereafter cover the second wavelength converting element and the complete light output device with the first wavelength converting element. It might however in this case be necessary to fully cover at least the edges of the light output device with second wavelength converting element, thus eliminating the risk of light leakage from the light output device which could lead to a bad color uniformity. To achieve a flat surface of the second wavelength converting element, the uncovered portions of second wavelength converting element may be filled with a translucent resin or the like.

In a preferred embodiment of the present invention, at least one of the first or the second wavelength converting element is optically translucent (or transparent) to light within the first wavelength range, thereby allowing for the color conversion device to output light within the first, second and third wavelength ranges. In this case, not only light outputted by the first and the second wavelength converting elements are mixed and thus outputted, but instead, also light of the first wavelength range is allowed to be mixed with light within the second and the third wavelength ranges, resulting in an overall improvement of the color rendering index for light outputted by the color conversion device. The term "optically translucent" is understood to mean that light is allowed to pass through the wavelength converting element, being only to some extent scattered by the wavelength converting element.

Preferably, at least one of the first or the second wavelength converting element are selected from a group comprising a phosphor foil and a ceramic plate, such as in the form of a lumiramic plate. In the case of a ceramic plate, particles of phosphor compounds, or phosphor precursor compound are compressed and sintered at high temperatures so that they become ceramic, for example by pressing and sintering methods well known in the art. Also, it is possible to use a polymer dispersed fluorescent element, where phosphor particles are dispersed in a polymer. The polymer should typically be essentially optically clear. For example, the polymer may be selected from the group comprising epoxy and silicone resins. The polymer dispersed fluorescent elements may be manufactured by introducing phosphor particles into a solution of polymer precursors and then causing the solution to polymerize. In an embodiment, the first wavelength converting element is a yellow lumiramic plate, and the second wavelength converting element is selected to be at least one of a red phosphor coating, a red phosphor foil, or a red lumiramic plate. The thickness of the yellow lumiramic plate is selected to be between approximately 80 - 250 μm. The thickness of the second wavelength converting element, i.e. the red layer depends on the type of layer used. When using a red phosphor coating, the typical thickness is up to approximately 50 μm (dense powder stacking with a packing density of approximately 50 % ), or can be much thicker by diluting the phosphor particles in a matrix material. In any case, the wavelength converting element partly covering the other element is however most of the times selected to be thinner than the other wavelength converting element.

It is possible to use different types of light outputting devices, but preferably, the light output device is a light emitting diode (LED) emitting blue light. In this case, the fully covering wavelength converting element is preferably yellow, and the partly covering wavelength converting element is red. However, it is also possible to use a green wavelength converting element instead of the yellow wavelength converting element. Also, it is according to the present invention possible to use other types of LEDs emitting light within other wavelength ranges, such as for example in the UV range. The color conversion device according to the present invention is preferably used as a component in a light emitting arrangement also comprising a light emitting diode (LED). As mentioned above, such a light emitting arrangement can for example be used in conventional general purpose lighting, thus having an improved color rendering index. Furthermore, the light emitting arrangement according to the present invention can also be used in a display device further comprising a display unit.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing currently preferred embodiments of the invention, in which:

Figure 1 illustrates a block diagram of a light emitting arrangement comprising a color conversion device according to prior art;

Figure 2a - 2c illustrates block diagrams of light emitting arrangements comprising color conversion devices according to different preferred embodiments the present invention; and

Figure 3 illustrates diagram curves of the intensity distribution vs. wavelength for a prior art color conversion device and a conversion device according to the present invention.

DETAILED DESCRIPTION OF CURRENTLY PREFERRED EMBODIMENTS

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which currently preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, theses embodiments are provided for thoroughness and completeness, and fully convey the scope of the invention to the skilled addressee. Like reference characters refer to like elements throughout.

Referring now to the drawings and to figure 1 in particular, there is depicted a side view and a top view of a prior art light emitting arrangement 100. The light emitting arrangement 100 comprises a light emitting element 102, for example in the form of a light emitting die, and a conversion device 104 arranged on top of the light emitting element 102. The color conversion device 104 comprises a yellow wavelength converting element 106 and a red wavelength converting element 108, where the yellow wavelength converting element 106 is arranged directly on top of the light emitting element 102, such that the yellow wavelength converting element 106 is sandwiched between the light emitting element 102 and the red wavelength converting element 108.

In the illustrated prior art embodiment in figure 1 , the yellow wavelength converting element 106 is a yellow lumiramic plate, and the red wavelength converting element 108 is a red lumiramic plate. It is however also possible to instead arrange a red phosphor foil on top of the yellow lumiramic plate 104. Furthermore, the light emitting element 102 is adapted to emit blue light, for example in the wavelength range of 430 to 480 nm. The thickness of the yellow lumiramic plate 106 is approximately around 80 - 250 μm, and the thickness of the red wavelength converting element 108 is approximately up to 50 μm.

As mentioned above, the lumiramic layers 106, 108 are composed of or includes a wavelength-converting material such as a phosphor, in this case yellow and red, providing an element being more robust and less sensitive to temperature than for example phosphor foils. Furthermore, the yellow lumiramic plate 106 is arranged to fully cover, and to some extent protrude outside of the light emitting die 102, thus minimizing the possibility for blue light "leaking" outside and around the color conversion device 104. The same counts for the red lumiramic plate 108 which fully cover the yellow lumiramic plate 106. This is further illustrated by the top view of the prior art light emitting arrangement 100, where the dashed line illustrates the fully covered light emitting die 102.

During use of the prior art light emitting arrangement 100, blue light emitted by the light emitting die 102 will impinge on the color conversion device 104. The yellow lumiramic plate 106 will then convert some of the blue light to yellow, or at least yellowish, light, and will also allow some of the blue light to pass through the yellow lumiramic plate 106. However, the blue light passing through the yellow lumiramic plate 106 will to some extend be scattered by the yellow lumiramic plate 106. Yellow light emitted by the yellow lumiramic plate 106, and the to some extent scattered blue light, will then impinge on the red lumiramic plate 108, which will convert the light and emit red, or at least reddish, light. The red lumiramic plate 108 will also allow some of the blue and the yellow light to pass through the red lumiramic plate 108, thereby allowing for the blue, the yellow, and the red light to mix, which thus will be perceived as white light.

Even though the light emitted by the prior art light emitting arrangement 100 is perceived as white, or at least whitish, the color rendering index of the emitted light is some what low due to the absorption of the wavelength of yellow/green light by the red phosphor as the blue light has to pass through both the yellow lumiramic plate 106 and the red lumiramic plate 108. This is especially noticeable in the yellow wavelength region, i.e. from approximately 550 to 590 nm.

Turning now to figure 2a - 2c, illustrating three different embodiments of the present invention, in which the color conversion device 204, 204', 204" has been adjusted such that the resulting light emitted by the light emitting arrangement 200, 200', 200" has a higher color rendering index. As in figure 1, the color conversion device 204, 204', 204", is arranged on top of the light emitting element 102, the color conversion device 204, 204', 204" comprising a yellow lumiramic plate 106 extending over, and at least partly protruding outside of, the light emitting element 102. However, according to the present invention, the red lumiramic plate 108 does not fully cover the yellow lumiramic plate 106, thus allowing for more yellow light to be emitted by the light emitting arrangement 200, 200', 200", resulting in a higher color rendering index.

In figure 2a, the red wavelength converting element 108 has been divided into four different rectangular segments each having an area that is less that a quarter of the total area of the yellow wavelength converting element 106, the four segments of the red wavelength converting element 108 together having a total area approximating 40% of the area of the yellow wavelength converting element 106. The four different segments have been arranged such that a corner of each of the segments coincides with a different corner of the yellow wavelength converting element 106, thereby leaving an uncovered area between the different segments in the shape of a Swiss cross. As the four segments of the red wavelength converting element 108 does not fully cover the total area of the yellow wavelength converting element 106, not all of the blue light passing through the yellow wavelength converting element 106, and the yellow light emitted by the yellow wavelength converting element 106, will have to pass through the red wavelength converting element 108, thus resulting in a larger component of both blue, but especially yellow, light in the resulting light emitted by the light emitting arrangement 200. As the resulting light emitted by the light emitting arrangement 200 comprises a larger yellow component, there will be less of a local minima in the resulting intensity distribution for the light emitting arrangement 200, thus resulting in a higher color rendering index.

The size of the area of the different segments of the red wavelength converting element 108 are in the present embodiment illustrated as less than a quarter of the total area of the yellow wavelength converting element 106. However, it is of course possible to arrange the areas of the different segments to be as smaller. It is also possible, and within the scope of the present invention, to use more, or less, than four segments, such as for example eight segments.

The placement of the red wavelength converting element 108 does furthermore not necessary have to be at the boarder of the yellow wavelength converting element 106. An example of this is illustrated in figure 2b, where the four different segments of the red wavelength converting element 108 have been replaced with only one segment of the red wavelength converting element 108. The size of the red wavelength converting element 108 in figure 2b is selected to be approximately 50% of the total size of the yellow wavelength converting element 106. Furthermore, instead of placing a corner of the red wavelength converting element 108 to coincide with a corner of the yellow wavelength converting element 106, the red wavelength converting element 108 is rotated 45 degrees, such that the corners of the red wavelength converting element 108 touches the boarder of the yellow wavelength converting element 106. As in figure 2a, the red wavelength converting element 108 in figure 2b does not fully cover the yellow wavelength converting element 106, result in a larger component of blue and yellow light in the resulting light emitted by the light emitting arrangement 200'.

An alternative embodiment of the present invention is illustrated in figure 2c. The red wavelength converting element 108 is in the present embodiment shaped as a rectangle arranged on top of and at the center of the yellow wavelength converting element 106, having an area that is approximately 40% of the total area of the yellow wavelength converting element 106. It is of course possible to use different sizes of the red wavelength converting element 108, as the size of the red wavelength converting element 106, in relation to the yellow wavelength converting element 106, controls the white point of the mixed light emitted by the light emitting arrangement 200". It is of course possible, and within the scope of the present invention, to use a different shape than the rectangular shapes used for the red wavelength converting element 108 in figure 2a - 2c. It is for example possible to deposit a circular red wavelength converting element 108 on top of the yellow wavelength converting element 106. Furthermore, as in figure 1, the red and the yellow wavelength converting elements 106, 108 in figure 2a - 2c can be formed using for example lumiramic plates, or can be replaced with a local coating of red phosphor, a red phosphor foil, and/or a polymer dispersed fluorescent element. Also, it should be noted that the yellow wavelength converting element 106 in figure 2a - 2c is arranged, as in figure 1, to extend and protrude outside of the light emitting die 102, as this minimizes the possibility of blue light leaking from the light emitting die 102. This is further illustrated by the dashed line in the top views (figure 2a - 2c) depicting the light emitting element 102. Furthermore, as the red wavelength converting element 108 does not fully cover the total area of the yellow wavelength converting element 106, it is possible to have a somewhat increased thickness of the red wavelength converting element 108, e.g. approximately up to 50 μm.

In figure 3 there is illustrated two different diagram curves, 302, 304, of the intensity distribution vs. wavelength for a prior art color conversion device, for example the color conversion device 104 illustrated in figure 1, and a conversion device according to an embodiment of the present invention, for example the color conversion device 204 illustrated in figure 2a.

From viewing the diagram curve 302 illustrating the intensity distribution vs. wavelength for the prior art color conversion device 104 it can be seen that there is a suppression of yellow light present in the total light emitted by the light emitting arrangement 100 comprising the color conversion device 104. This suppression of yellow light results in a low color rendering index of the total light emitted by the light emitting arrangement 100, which as mentioned above is a property that is especially undesirable in relation to a light source for general purpose lighting.

In comparison, the diagram curve 304 illustrating the intensity distribution vs. wavelength for the color conversion device 104 according to an embodiment of the present invention shows a larger component of light in the yellow/orange wavelength region, i.e. between 480 - 580 nm. This addition of light in the yellow/orange wavelength region allows for a higher color rendering index and a lower color temperature, and is due to the fact that not all light emitted by the yellow wavelength converting element 106 has to pass through the red wavelength converting element 108. The skilled addressee realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. For example, by adjusting the size, shape or position of the red wavelength converting element/locally coating of red phosphor/red phosphor foil, the color-point of the resulting (mixed) light can be directed from red to cyan. Furthermore, it is possible to use a different color than blue for the light emitting element, for example in the UV wavelength range. It is also possible to use different types of light emitting elements, or light output devices, instead of the light emitting element illustrated above, for example selected from a group comprising a light emitting diode (LED), an organic light emitting diode (OLED), or a polymeric light emitting diode (PLED). In conclusion, it is according to the present invention possible to provide a new color conversion device that allows for the possibility to emit light having a higher color rendering index when receiving light in the blue wavelength region, as in comparison to a prior art color conversion device. The color conversion device according to the present invention comprises a yellow wavelength converting element and a red wavelength converting element, where the red wavelength converting element only partly covers the yellow wavelength converting element.

Claims

CLAIMS:

1. A color conversion device for a light output device emitting light within a first wavelength range, comprising: a first wavelength converting element adapted to receive light emitted by the light output device and to output light at a second wavelength range; and - a second wavelength converting element arranged to receive the light outputted from the first wavelength converting element and to output light at a third wavelength range, c h a r a c t e r i z e d i n that one of the first or the second wavelength converting element is arranged to partly cover the other one of the first or the second wavelength converting element, thereby allowing for the color conversion device to output light within the second and third wavelength ranges.

2. Color conversion device according to claim 1, wherein at least one of the first or the second wavelength converting element is optically translucent to light within the first wavelength range, thereby allowing for the color conversion device to output light within the first, second and third wavelength ranges.

3. Color conversion device according to any of claims 1 or 2, wherein at least one of the first or the second wavelength converting element are selected from a group comprising a phosphor foil and a ceramic plate.

4. Color conversion device according to any one of the preceding claims, wherein the light output device is a light emitting diode (LED) emitting diode emitting blue light.

5. A light emitting arrangement, comprising: a light emitting diode (LED); and a color conversion device according to any one of the preceding claims, the color conversion device arranged to receive light emitted by the LED.

6. A display device comprising a display unit and a light emitting arrangement according to claim 5.