WO2012156860A1

WO2012156860A1 - Light emitting device for edge-lit or direct-lit lcd monitor/display

Info

Publication number: WO2012156860A1
Application number: PCT/IB2012/052254
Authority: WO
Inventors: Marcus Jozef Henricus Kessels
Original assignee: Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 2011-05-18
Filing date: 2012-05-07
Publication date: 2012-11-22
Anticipated expiration: 2013-11-18

Abstract

A light emitting device is provided comprising a plurality of LEDs arranged in clusters, in which each LED has an operating parameter or characteristic X_i and each cluster has a corresponding operating parameter or characteristic Y_j, wherein Y_j is the combined effect of all X_i of the LEDs in a cluster and wherein the clusters and LEDs are arranged such that - the distance between two LEDs within a cluster is smaller than the distance between two adjacent clusters. Further, the variation in Y_j between clusters is smaller than the variation in X_i between the LEDs of the device, and a parameter "A" relates to the variation in X_i among the plurality of LEDs and a parameter "B" relates to the variation in Y_j among the clusters, and the ratio B/A is below 0.55*M^(-1.5)wherein the parameter "M" is the number of LEDs in each cluster. An edge-lit or direct-lit LCD monitor comprising the light emitting device according to the invention and a method of distributing a plurality of LEDs in an array of clusters are also provided.

Description

Light emitting device for edge-lit or direct-lit LCD monitor/display

BACKGROUND OF THE INVENTION

In Light emitting diode (LED) lighting fixtures, edge-lit or direct-lit TV's and monitors, multiple LEDs are used to create the required amount of light. To ensure that the lighting fixtures or LED displays have a uniform color and brightness, binning and kitting of LEDs may be applied. Binning is a process in which the characteristics of a plurality of LEDs are measured and LEDs of the same color, brightness or any parameter, or LEDs falling within a specific interval for a specific parameter, are classified as belonging to the same "bin". LEDs from matching bins are combined when used in a fixture or a TV. Kitting is a process for obtaining a homogeneous color distribution in which LEDs of different color bins are more or less homogenously distributed over an array. This means for example that a LED with a high flux may be combined with a led with a low flux to ensure that the average meets a specification of interest. The same can be done for color. For example, LEDs with a high v' (a coordinate in color space CIE 1976) may be combined with an LED with low v', and again the mix could meet a desired specification. An example of such a binning and kitting approach is described in US 7 201 493.

Due to the improved efficiency of LEDs, the number of LEDs in displays or lighting fixtures is reducing since the light output per LED is increasing. However, such applications usually require a predefined lumen flux and usually have a standard or constant lighting fixture size or display size, which means that the number or density of LEDs necessary for such applications decreases. As a consequence, the pitch of the array increases, which may result in color in-homogeneities over the display or lighting fixture since light from neighboring LEDs may not be sufficiently mixed. An observer may then see the individual LED colors when viewing the display or lighting fixture. SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improvement of the above techniques and prior art.

The above object is provided according to a first aspect of the invention by a light emitting device comprising a plurality of LEDs arranged in clusters. In these clusters, each LED has an operating parameter or characteristic X; and each cluster has a corresponding operating parameter or characteristic Y_j. Y_j is the combined effect of all Xi of the LEDs in a cluster and the clusters and LEDs are arranged such that the distance between two LEDs within a cluster is smaller than the distance between two adjacent clusters, the variation in Y_j between clusters is smaller than the variation in Xi between the LEDs of the device, and a ratio B/A is below about 0.55*M^^{1 -5} wherein the parameter "A" relates to the variation in Xi among the plurality of LEDs, the parameter "B" relates to the variation in Y_j among the clusters and the parameter "M" is the number of LEDs in each cluster.

The term "variation" refers to any measured parameter that relates to the spread or variation between Xi or Y_j. The variation could thus for example be, or be related to, the variance, the standard deviation or root mean square deviation.

In an embodiment of the first aspect, the variation is the standard deviation of the Xi and Y_j, respectively.

If the device comprises N LEDs and N' clusters, then l≤i≤N and 1< j≤N' . If there is M LEDs in each cluster, then N -N/M and l≤i≤N and 1< j≤N/M.

The "operating parameter or characteristic" refers to any electrical and/or optical operating parameter or characteristic of a LED or a cluster of LEDs, such as color coordinate, flux, forward voltage, etc.

The first aspect of the invention is thus based on the general idea of arranging LEDs in clusters instead of distributing them evenly on an array when used in for example a fixture, LCD-screen or TV. The cluster of LEDs may then act as a single light source with improved properties, such as improved color, flux etc compared to a single LED. According to the first aspect of the invention, the parameter or data of the individual LEDs have been utilized for forming the light emitting device, i.e. traditional binning methods are not required for distributing the LEDs of the device. Thus, tracking the individual measured

characteristics of LEDs instead of the conventional binning methods allows arranging the LEDs in clusters according to the first aspect of the invention such that the homogeneity of parameters over the array of clusters is increased.

In other words, the first aspect of the invention is based on the idea that LEDs having complementary properties, such as complementary colors or brightness values, are clustered. Clusters having equal color and brightness are spaced at a larger distance from each other and the LEDs are put at a smaller distance from each other within a cluster compared to the distance between clusters. As an example, the distance between clusters could be at least two times larger than the distance between LEDs in a cluster, such as at least three times larger, such as at least four times larger, such as at least five times larger, such as at least ten times larger, such as at least twenty times larger, than the distance between LEDs in a cluster.

The properties of a cluster is the combined, or mixed, effect of the properties of the individual LEDs of that cluster. According to the first aspect of the invention, the

LEDs are further arranged such that the variation in an operating parameter or characteristic between clusters is smaller than the variation in the corresponding operating parameter or characteristic between all LEDs of the device.

The inventor has found that a ratio of B/A below about 0.55*Μ^^1-5^ results in a light emitting device or array with improved homogeneity in for example color and brightness. The parameter "A" could for example be the variation in X; or be proportional to the variation in X;. In analogy, the parameter "B" could be the variation in Y_j or be proportional to the variation in Y.

Consequently, the inventor has found that it is advantageous that the ratio B/A depends on the LED count "M" in a cluster.

In an embodiment of the first aspect of the invention, the ratio B/A is below about 0.50*M^{(~1 5)}.

In a further embodiment of the first aspect of the invention, the ratio B/A is below about 0.45*M^{(~1 5)}.

In a further embodiment of the first aspect of the invention, the ratio B/A is below about 0.35*M^{(~1 5)}.

In a further embodiment of the first aspect of the invention, the ratio B/A is below about 0.33*M^{(~1 5)}.

The first aspect of the invention is advantageous since it provides for displays with a very low LED count and with improved uniformity. Thus, the homogeneity in e.g. color or brightness over the device or array is increased. In other words, the first aspect of the invention provides a light emitting device having less spread in specific parameter values compared to a light emitting device in which LEDs are distributed according to conventional bin and kitting methods.

In an embodiment, the clusters may be arranged in positions forming an array.

An array refers to a systematic arrangement, usually in rows or columns. The clusters may for example form a one-dimensional array, i.e. a row, or a two-dimensional array, e.g. several rows and columns. In an embodiment the array may be a regular or an irregular array. In an embodiment of the first aspect of the invention, the operating parameter or characteristic X; may be selected from the forward voltage V_f, the flux Φ, a color parameter and the brightness of a LED.

The flux Φ refers to the luminous flux of the LED and is often used as an objective measure of the useful power emitted by a light source.

Y_j, as described above, is the proper sum or mixed effect of the characteristic X; of the LEDs in a cluster. Y_j is thus the operating parameter or characteristic of the cluster that corresponds to the operating parameter or characteristic X; of a single LED. As an example, if X is the flux Φ, Y is the arithmetic sum of the individual fluxes of the LEDs in a cluster. Further, if X is the forward voltage V_f, Y is the sum of the individual forward voltages of the LEDs in a cluster.

As an example, the color parameter may be the color coordinate of a color point.

A "color point" refers to a point in a color spectral chart, for example represented by a set of x and y coordinates in a CIE 1931 spectral chart and/or by u', v' coordinates from the 1976 CIE diagram. A "color coordinate" thus refers to a coordinate in a color chart, such as x, y, u', v'. A further example of a color point is a point in a RGB color chart. A color coordinate may thus also be a R, G or B value in a RGB color chart.

The "tristimulus values" of a color refers to the amounts of three primary colors in a three-component additive color model needed such that the mixed color of the tristimulus values matches the color or a color coordinate of a color chart. The color parameter may also be at least one tristimulus value of the color of a LED.

If X; is a color point of a LED, then Y_j is the color point which results from converting the color points of all LEDs in a cluster to individual tristimulus values, summing the individual tristimulus values accordingly and then converting the summed tristimulus values back to a color point. In other words, if a cluster comprises a first LED with color point Xi and a second LED with color point X₂, these color points are converted into tristimulus values (xn, x₁₂, x₁₃) and (x₂₁, x₂₂, x₂₃). The tristimulus values are summed to form a set of summed tristimulus values (xn+x₂₁, xi₂+x₂₂, xi₃+x₂₃) and this set is converted into a color point to get the Y value for the cluster. Methods for converting color points to tristimulus values, and vice versa, are known in the art and therefore not further described herein. If X is a coordinate, e.g. u', v' coordinates from the 1976 CIE diagram, Y is calculated as described above for the case when X is a color point. In certain applications, the parameter "A" is better described as a weighted average in a variation or as a maximum variation. As an example, the LEDs of the device may have "n" different colors and a parameter "A_n" may be calculated for each color, wherein "A_n" relates to the variation in the X; that are associated with a color. The parameter "A" may then be selected as the average of all "A_n" or the maximum of each "A_n".

As an example, the LEDs of the device could be a mix of red (R), green (G) and blue (B) LEDs.

Since "the distance" of the red (R), green (G) and blue (B) colors in the color chart is by definition far from the mixed color point, a useful definition of the parameter "A" is found by calculating a variation for each of R, G and B separately.

Thus, the parameter "Ai" is calculated for the red (R) LEDs, the parameter "A₂" is calculated for the green (G) LEDs and the parameter "A₃" is calculated for the blue (B) LEDs. The parameter "A" could then be defined as the average of parameters "Ai", "A₂" and "A₃" or the maximum of parameters "Ai", "A₂" and "A3". The definition of the parameter "B" thus depends on how the parameter "A" is calculated, such that the parameter "B" relates to the corresponding variation between clusters. In the example above, if the maximum variation in R, G and B between LEDs is in R and the parameter "A" is selected as the variation in R, then the parameter "B" is selected as the similar variation in R between clusters. It will be appreciated that the parameter "B" may be calculated based on how the parameter "A" is calculated.

Consequently, in an embodiment of the first aspect, the device comprises LEDs of red (R), green (G) and blue (B) colors.

In another embodiment of the first aspect, the device comprises LEDs of green (G) and purple (P) colors. It may be advantageous to calculate the parameter "A" when the device comprises LEDs of green (G) and purple/pink (P) colors with the same approach as described above when LEDs of red (R), green (G) and blue (B) colors are used.

As a further embodiment, the device may comprise a total of N LEDs with M LEDs in each cluster, and the parameter "A" and the parameter "B" may be defined as

In the context of the present disclosure, X is the mean of all Xi, i.e.

X - iSW X, and Ϋ is the mean of aU Y, i.e. T = J L£T TJ . Thus, the pa_rameter -_A ^» reflects the variation in Xi between N number of LEDs of the light emitting device and the parameter "B" reflects the variation in Y_j between all clusters of the light emitting device.

As an example, the light emitting device may comprise a total of N LEDs with

2 LEDs in each cluster, and the parameter "A" and the parameter "B" are defined as

Consequently, if the ratio of B/A above is below about 0.55*M^{( 1'5)}„ such as below about 0.45*M^(_1'5)„ such as below about 0.33*M^(_1'5)„ the homogeneity of the device or array increases.

According to an embodiment, there is provided an edge-lit or direct-lit LCD monitor comprising the light emitting device according to any one of the embodiments described above. The present embodiment is advantageous in that it provides monitors having an increased homogeneity in terms of e.g. color and brightness.

According to an embodiment, there is provided edge-lit lighting fixtures comprising the light emitting device according to any one of the embodiments described above. The present embodiment is advantageous in that it provides lighting fixtures having an increased homogeneity in terms of e.g. color and brightness.

According to a second aspect of the invention, there is provided a method of distributing a plurality of LEDs in an array of clusters. The method comprises arranging the LEDs with "M" LEDs in each cluster such that the distance between the LEDs within a cluster is smaller than the distance between two adjacent clusters and the variation of an operating parameter or characteristic Y_j between clusters is smaller than the variation of an operating parameter or characteristic Xi between all LEDs in the array, wherein Y_j is the combined effect of all Xi of the LEDs in a cluster, and wherein a parameter "A" relates to the variation in Xi among the plurality of LEDs and a parameter "B" relates to the variation in Y_j among the clusters and the ratio B/A is below about 0.55*Μ^^1-5\ The terms and definitions used in the second aspect of the invention are the same as discussed in relation to the other aspects of the invention above. In analogy with the first aspect above, a method according to the second aspect provides for a more uniform display at a very low LED count and consequently, the homogeneity in e.g. color over the device or array is increased.

In an embodiment of the second aspect of the invention, the ratio B/A is below about 0.50*M^{(~1 5)}.

In a further embodiment of the second aspect of the invention, the ratio B/A is below about 0.45*M^{(~1 5)}.

In a further embodiment of the second aspect of the invention, the ratio B/A is below about 0.35*M^{(~1 5)}.

In a further embodiment of the second aspect of the invention, the ratio B/A is below about 0.33*M^{(~1 5)}.

In an embodiment of the second aspect, the method does not comprise binning of LEDs before distributing the plurality of LEDs in an array of clusters.

As described in relation to the first aspect above, tracking the individual measured characteristics of LEDs and using the individual data for each LED does not require conventional binning methods for distributing the LEDs over an array of clusters. This allows arranging the LEDs in clusters such that the homogeneity of parameters over the array of clusters is increased.

It will be appreciated that the features of the embodiments described with reference to the first aspect of the invention are also combinable with the method as defined in accordance with the second aspect of the invention.

Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description. Those skilled in the art realize that different features of the present invention can be combined to create embodiments other than those explicitly described in the following.

BRIEF DESCRIPTION OF THE DRAWINGS

This 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.

Fig. la shows a conventional lightbar assembly (prior art) in which LEDs are distributed in a one-dimensional array.

Fig. lb shows a lightbar assembly according to an embodiment of the present invention. DETAILED DESCRIPTION

Fig. la shows a conventional lightbar assembly as known in the prior art. LEDs, in this case 54 LEDs defined by operating parameterers X1-X54 (e.g. the fluxes Φι- Φ54), are distributed at different array positions. The distance or spacing between the different LEDs is constant (Di). Due to improved efficiency of each single LED, the number of LEDs is relatively low, or at least has decreased as compared to devices using less efficient LEDs in terms of light output, which in turn means that the distance Di is relatively large. As a result, the light from the LEDs are not sufficiently mixed, i.e. there is not enough overlap of the light beams from the LEDs in the lightbar assembly. Consequently, individual colors from the different LEDs become visible to the user or viewer of the lightbar assembly, e.g. if it is mounted in a fixture or a display.

Fig. lb shows a lightbar assembly according to an exemplifying embodiment of the present invention. The lightbar assembly comprises 54 LEDs, i.e. the same number of LEDs as the lightbar assembly of Fig. la, which are defined by operating parameters X1-X54. The 54 LEDs are, according to the present embodiment, distributed in 27 clusters with two LEDs in each cluster.

The clusters and LEDs are distributed such that the distance between the LEDs in each cluster (d) is smaller than the distance between two adjacent clusters (D₂). In other words, the distance between two LEDs of a cluster is smaller than the distance from that cluster to any other cluster of the device (lightbar).

Each cluster of LEDs acts a single light source defined by operating parameters Y1-Y27. As an example, the operating parameter Xi may be the flux Φ; of each LED, which means that Yi is the sum of the fluxes from the first two LEDs, i.e. Φι + Φ₂, and Y₂ is the sum of the fluxes from the next two LEDs, i.e. Φ3 + Φ4, etc.

Further, the LEDs provided with complementary colors and/or brightness levels and the LEDs are distributed in the clusters such that the variation in Y_j between clusters is smaller than the variation in Xi between the LEDs of the device. In the specific case shown in Fig. lb, in which the characteristic or operating parameter X_n is the flux Φ_η, the variation in the flux between the LEDs of the lightbar assembly is calculated as

A= Φ,

and the variation in the flux from each cluster of the assembly is calculated as

B in which Yi = Φι + Φ₂, Y₂ = Φ₃ + Φ₄ etc. and Υ is the mean of all

Y_j, i.e. Y = m^.

According to an embodiment of the invention, the LEDs and clusters are distributed such that the ratio of B/A is below about preferably below 0.45*M^^~

^{I S} and more preferably below The parameter "M" is the number of LEDs in each cluster and is in this case 2. Such a ratio of B/A leads to a more homogeneous properties, e.g. a more homogeneous flux, over the entire area or extension of the lightbar assembly compared to the prior art lightbar assembly of Fig. la.

For example, the lightbar assembly could be a two-dimensional array of columns and rows of clusters of LEDs. In analogy with the example described above, such a two-dimensional array having LEDs distributed according to the present invention, would have a more homogenous flux compared to prior art two-dimensional arrays in which LEDs are distributed according to conventional binning and kitting methods.

Even though the invention has been described with reference to specific exemplifying embodiments thereof, many different alterations, modifications and the like will become apparent for those skilled in the art. The described embodiments are therefore not intended to limit the scope of the invention, as defined by the appended claims. For instance, the skilled person appreciates that it is not necessary that each cluster needs to have the same number of LEDs M. Instead, in an embodiment the clusters may have a different number of LEDs, such that cluster j may comprise M_j LEDs. As an example, due to symmetry differences the clusters near the ends of the lightbar or near the sides of an areal fixture or display may have a different number of LEDs than the clusters in the middle of the lightbar or fixture. The skilled person will appreciate that the formulas for parameter 'B' will have to be adjusted accordingly.

Claims

CLAIMS:

1. A light emitting device comprising a plurality of LEDs arranged in clusters, in which each LED has an operating parameter or characteristic Xi and each cluster has a corresponding operating parameter or characteristic Y_j, wherein Y_j is the combined effect of all Xi of the LEDs in a cluster and wherein the clusters and LEDs are arranged such that - the distance between two LEDs within a cluster is smaller than the distance between two adjacent clusters,

the variation in Y_j between clusters is smaller than the variation in Xi between the LEDs of the device, and

a ratio B/A is below about 0.55*M^^{1 -5} wherein the parameter "A" relates to the variation in Xi among the plurality of LEDs, the parameter "B" relates to the variation in Y_j among the clusters and the parameter "M" is the number of LEDs in each cluster.

2. A light emitting device according to claim 1 , wherein the ratio B/A is below about 0.45*M^{(~1 5)}.

3. A light emitting device according to claim 1 or 2, wherein the ratio B/A is below about 0.33*M^(~L5).

4. A light emitting device according to any previous claim, wherein the clusters are arranged in positions forming an array.

5. A light emitting device according to any previous claim, wherein the operating parameter or characteristic Xi is at least one of the forward voltage V_f, the flux Φ, a color parameter and the brightness of a LED.

6. A light emitting device according to claim 5, wherein the color parameter is a color coordinate of a color point.

7. A light emitting device according to claim 6, wherein the LEDs of the device have "n" different colors and a parameter "A_n" is calculated for each color, wherein "A_n" relates to the variation in the X; that are associated with a color and wherein the parameter "A" is selected as the average of all "A_n" or the maximum of each "A_n".

A light emitting device according to claim 7, wherein the device comprises of red (R), green (G) and blue (B) colors, or LEDs of green (G) and purple (P) colors.

9. A light emitting device according to any previous claim, wherein the device comprises a total of N LEDs with M LEDs in each cluster, and the parameter "A" and the parameter "B" are defined as

10. A light emitting device according to claim 9, wherein the device comprises total of N LEDs with 2 LEDs in each cluster, and the parameter "A" and the parameter "B' are defined as

1 1. An edge-lit or direct-lit LCD monitor comprising a light emitting device according to any previous claim.

12. A method of distributing a plurality of LEDs in an array of clusters, the method comprising arranging the LEDs with "M" LEDs in each cluster such that the distance between the LEDs within a cluster is smaller than the distance between two adjacent clusters and the variation of an operating parameter or characteristic Y_j between clusters is smaller than the variation of an operating parameter or characteristic X; between all LEDs in the array, wherein Y_j is the combined effect of all Xi of the LEDs in a cluster, and wherein a parameter "A" relates to the variation in X; among the plurality of LEDs, a parameter "B" relates to the variation in Y_j among the clusters and the ratio B/A is below about 0.55 *M^^"

13. A method according to claim 12, wherein the ration B/A is below about 0.45*M^{(~1 5)}.

A method according to claim 13, wherein the ration B/A is below about

15. A method according to any one of claims 12-14, wherein the method does not comprise binning of LEDs before distributing the plurality of LEDs in an array of clusters.