Abstract:
A lamp cover containing phosphor for providing white light emission is disclosed. The lamp cover is comprised of a light-partial-reflective cap structure providing the outer surface of the lamp cover, wherein the light-partial-reflective cap structure is composed of a plurality of light transparent layers and a plurality of vacuum layers that are alternatively stacked from outside to inside, a supporting transparent cap structure providing the inner surface of the lamp cover, and a phosphor mixed structure mechanically supported by the outer surface of the supporting transparent cap structure, wherein the outer surface of the phosphor mixed structure is adjacent to the most inner vacuum layer of the light-partial-reflective cap structure. Once the lamp cover is combined with a phosphor exciting light source, the light-partial-reflective cap structure partially prevents phosphor exciting light from escaping from the lamp cover by using Fresnel reflection.

Description:
BACKGROUND 
       [0001]    1. Field 
         [0002]    Example embodiments of inventive concepts relate to a lamp cover for light emitting device. 
         [0003]    2. Description of the Related Art 
         [0004]    Combining the uses of blue LEDs and blue excited phosphor is a popular technology to produce white light emission. In such a technology, the blue light hits the phosphor and is partially converted into a longer wavelength, such as a yellow one. The excited light portion mixed with the residual blue light produces a pseudo white light. 
         [0005]    A common method to implement such a white light emitting device is to mix phosphor with a liquid resin binder and dispense the phosphor mixed resin binder around the LED chip. The main disadvantage of this method is that it is hard to get a uniform color distribution at different angles since the light path length at different angles cannot be easily controlled to be the same. Meantime, the displacement of LED dies during die bonding process make the uniform color control more difficult. 
         [0006]    To get a more uniform color distribution, a method to use a pre-made phosphor structure with the same thickness at all position is proposed in U.S. Pat. No. 7,582,914. In such a method, the pre-made phosphor structure is placed far away from the LED chip. In many cases, it is separated from the blue LED package and attached onto the PCB, where the blue LED package is located. Since the phosphor structure is far away from the LED chip and the leadframe house cavity where the LED chip is located, the scattering effect of the phosphor particles is much less than that in the phosphor-near-chip case so that blue light can easily escape without being absorbed to emit fluorescent light. As a result, phosphor usage and weight percentage should be greatly increased to make the phosphor structure for achieving the same correlated color temperature of light emission in comparison to the phosphor-near-chip case. The bigger the distance between the phosphor structure and the LED chip is, the more phosphor usage is needed, which dramatically increases the cost of lumen per watt for white LED lighting system since the current commercial phosphor prices are very expensive. 
         [0007]    As for general lighting, a pre-made phosphor structure with a big dimension or large area is highly desired since it not only provides a uniform light distribution, but also reduces the LED flickering issue, which has been complained by customers for long time. 
       SUMMARY OF INVENTION 
       [0008]    A lamp cover for a light emitting device is provided, the lamp cover comprising: an outer surface of the lamp cover; a supporting transparent cap structure providing an inner surface of the lamp cover; a phosphor mixed structure supported by the supporting transparent cap structure; and a light-partial-reflective cap structure disposed between the outer surface and the phosphor mixed structure, wherein the light-partial-reflective cap structure comprises at least one light transparent material layer and at least one space layer that are alternatively stacked. 
         [0009]    At least one interface between the at least one transparent material layer and the at least one space layer may serve as a light-partial-reflective surface by using Fresnel reflection. 
         [0010]    The at least one interface may be coated with an anti-reflection layer for fluorescent light emitting from the phosphor mixed structure to prevent the fluorescent light from being reflected back into the phosphor mixed structure. 
         [0011]    An outer surface of the phosphor mixed structure may be adjacent to the most inner space layer of the light-partial-reflective cap structure. 
         [0012]    The most inner space layer of the light-partial-reflective cap structure may be a vacuum in order to protect the phosphor mixed structure by preventing it from being oxidized, and the other space layers may be air-filled layers when the light-partial-reflective cap structure comprises a plurality of the space layers. 
         [0013]    The number of the pair of transparent material layer and space layer may be only one. 
         [0014]    A light emitting device is provided, the light emitting device comprising: a substrate; a LED package mounted on the substrate; and a lamp cover disposed on the substrate to surround the LED package, wherein the lamp cover comprises: an outer surface of the lamp cover; a supporting transparent cap structure providing an inner surface of the lamp cover; a phosphor mixed structure supported by the supporting transparent cap structure; and a light-partial-reflective cap structure disposed between the outer surface and the phosphor mixed structure, wherein the light-partial-reflective cap structure comprises at least one light transparent material layer and at least one space layer that are alternatively stacked. 
         [0015]    The LED package may emit a blue light ray or an UV light ray. 
         [0016]    For example, the substrate may be a PCB. 
         [0017]    The lamp cover can be combined with UV/Blue LEDs to generate white light emission. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    Example embodiments of inventive concepts will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings.  FIGS. 1-2B  represent non-limiting, example embodiments of inventive concepts as described herein. 
           [0019]      FIG. 1  is a schematic cross-sectional view of the lamp cover according to example embodiments of inventive concepts. 
           [0020]      FIG. 2A  is a schematic cross-sectional view of a light emitting device utilizing the lamp cover of  FIG. 1 . 
           [0021]      FIG. 2B  is a magnified view of  FIG. 2A  for a clearer illustration. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    Example embodiments of inventive concepts will now be described more fully with reference to the accompanying drawings, in which example embodiments of inventive concepts are shown. The invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, example embodiments of inventive concepts are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the sizes of components may be exaggerated for clarity. 
         [0023]    It will be understood that when an element or layer is referred to as being “on”, “connected to”, or “coupled to” another element or layer, it can be directly on, connected to, or coupled to the other element or layer or intervening elements or layers that may be present. In contrast, when an element is referred to as being “directly on”, “directly connected to”, or “directly coupled to” another element or layer, there are no intervening elements or layers present. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
         [0024]    It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, components, regions, layers, and/or sections, these elements, components, regions, layers, and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer, and/or section from another element, component, region, layer, and/or section. Thus, a first element, component, region, layer, or section discussed below could be termed a second element, component, region, layer, or section without departing from the teachings of example embodiments of inventive concepts. 
         [0025]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments of inventive concepts. As used herein, the singular forms “a,” “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises”, “comprising”, “includes” and/or “including,” if used herein, specify the presence of stated features, integers, steps, operations, elements and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components and/or groups thereof. 
         [0026]    Spatially relative terms, such as “beneath”, “below”, “lower”, “above”, “upper”, and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the exemplary term “below” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. 
         [0027]    Example embodiments of inventive concepts described herein will refer to plan views and/or cross-sectional views by way of ideal schematic views. Accordingly, the views may be modified depending on manufacturing technologies and/or tolerances. Therefore, example embodiments of inventive concepts are not limited to those shown in the views, but include modifications in configuration formed on the basis of manufacturing processes. Therefore, regions exemplified in figures have schematic properties and shapes of regions shown in figures exemplify specific shapes or regions of elements, and do not limit example embodiments of inventive concepts. 
         [0028]    Reference will now be made in detail to example embodiments of inventive concepts, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout and the size of each element may be exaggerated for clarity and convenience of explanation. 
         [0029]      FIG. 1  is a schematic cross-sectional view of a lamp cover  20  for a light emitting device according to example embodiments of inventive concepts. As shown in  FIG. 1 , the lamp cover  20  may include an outer surface  1  of the lamp cover  20 , a supporting transparent cap structure  21  providing an inner surface  2  of the lamp cover  20 , a phosphor mixed structure  22  mechanically supported by an outer surface  3  of the supporting transparent cap structure  21 , and a light-partial-reflective cap structure  10  disposed between the outer surface  1  and the phosphor mixed structure  22 . 
         [0030]    The light-partial-reflective cap structure  10  may include two transparent material layers  11  and  13 , and two space layers  12  and  14 , which are alternatively stacked from outside to inside by the order of number, that is ( 11 - 12 - 13 - 14 ). The light-partial-reflective cap structure  10  may also include a first interface  5  between a first space layer  14  and a first transparent material layer  13 , a second interface  6  between the first transparent material layer  13  and a second space layer  12 , and a third interface  7  between the second space layer  12  and a second transparent material layer  11 . 
         [0031]    The phosphor mixed structure  22  is adjacent to a first space layer  14  by a surface  4  of the phosphor mixed structure  22 . The phosphor mixed structure  22  can be a transparent silicone resin mixed with phosphor. When excited by UV or blue light, the phosphor emits fluorescent light. If the wavelength of exciting light is UV, the fluorescent light can have blue, green, and red or blue and yellow colors to be mixed for white light. If the wavelength of exciting light is blue, the fluorescent light can have yellow color or green and red colors. When mixed with the residual blue light, the fluorescent light can be viewed as a white light. It is preferable that the thickness of the phosphor mixed structure ( 22 ) is the same at all positions to achieve a uniform color distribution of white light at all viewing angles. 
         [0032]    Although  FIG. 1  shows two pairs of transparent material layers and space layers, it can be one or any integer number larger than two. The first, most inner space layer  14  may be a vacuum so that the first space layer  14  protects the phosphor mixed structure  22  by preventing it from being exposed and oxidized in the air. However, except the first space layer  14 , the other space layer  12  can be an air-filled layer. The first and second transparent material layers  13  and  11  may made of, for example, a PPA (polyphthalamide) plastic. 
         [0033]      FIG. 2A  is a schematic cross-sectional view of a light emitting device  70  utilizing the lamp cover  20  of  FIG. 1 . Referring to  FIG. 2A , the light emitting device  70  may comprise a substrate  31 , a LED package  30  mounted on the substrate  31 , and the lamp cover  20  which is disposed on the substrate  31  to surround the LED package  30 . The substrate  31  may be a printed circuit board (PCB). The light emitting device  70  may be a white light emitting device. 
         [0034]      FIGS. 2A and 2B  show the mechanism how the lamp cover  20  works with the LED package  30 . Although the LED package  30  may emit a blue light ray or emit an UV light ray, for explanatory purposes, it will be assumed hereafter that the LED package  30  emits the blue light ray. First, the blue light ray  41  is emitted from the blue LED package ( 30 ). Then, the blue light ray  41  hits the phosphor mixed structure  22  and is at least partially absorbed to emit fluorescent light with a longer wavelength, such as represented by rays  60 . The residual blue light ray  42  will keep going into the light-partial-reflective cap structure  10 . At the first interface  5  between the first transparent material layer  13  and the first space layer  14 , the blue light ray is split into a reflected ray  43  and a transmitted ray  44  by Fresnel reflection and refraction laws, respectively. The residual transmitted ray  44  will keep going and hit the second interface  6  between the first transparent material layer  13  and the second space layer  12 , again it will be split into a reflected ray  45  and a transmitted ray  46 . The phenomenon is the same for the transmitted blue light ray  46  when it hits the third interface  7  to get a reflected ray  47  and a transmitted ray ( 48 ), and the transmitted ray  48  to get a reflected ray  49  and a transmitted ray  50  at the outer surface  1 . 
         [0035]    The last transmitted ray  50  is the residual blue light to be mixed with the transmitted fluorescent light  43  to achieve white light perception, while the reflected blue light rays  43 ,  45 ,  47  and  49  will hit back into the phosphor mixed structure  22  to be reused. Therefore, less phosphor amount may be used to achieve the same correlated color temperature (CCT) than the related art. The phosphor reduction ratio strongly depends on the number of pairs of the transparent material layers and space layers used for the light-partial-reflection cap structure  10 . According to an example experiment by using the same amount of phosphor, the method using one pair of the transparent material layers and space layers achieves a CCT 5700K compared to a CCT˜6500K without using any pair of the transparent material layers and space layers. The CCT can be dropped up to about 800K. Professional designers know that the data may vary a lot as different transparent materials, such as in refractive indexes, are used for the light-partial-reflective cap structure  10 . 
         [0036]    It is noted that the fluorescent light rays  60  can also be reflected back into the phosphor mixed structure  22  when they are transmitted through the light-partial-reflective cap structure  10 . However, the absorption coefficient of the phosphor mixed structure  22  to the reflected fluorescent light is much less than that to the reflected blue light, because the wavelength of the fluorescent light is longer than that of the blue right. Therefore, the light loss of the reflected fluorescent light is negligible if the number of pairs of the transparent material layers and space layers used for the light-partial-reflection cap structure  10  is not large. When it is required to consider the light loss caused by the Fresnel reflection of the fluorescent light inside the light-partial-reflection cap structure  10 , for example, an anti-reflection layer for fluorescent light can be coated at each interface between each pair of the transparent material layers and space layers adjacent to each other in order to prevent the fluorescent light from being reflected back into the phosphor mixed structure  22 , while each interface still produces Fresnel reflection for blue light.