WO2019149649A1

WO2019149649A1 - A lighting device having multiple lighting units including different colors

Info

Publication number: WO2019149649A1
Application number: PCT/EP2019/051962
Authority: WO
Inventors: Chen MA
Original assignee: Signify Holding B.V.
Priority date: 2018-02-01
Filing date: 2019-01-28
Publication date: 2019-08-08

Abstract

A lighting device comprises a set of lighting units including different colors arranged around a circle having a center (60). Each lighting unit comprises a light source (40) and a lens (20) over the light source (40). Each lens (20) receives light from the respective light source (40) and provides an output light distribution, which is skewed towards the center of the circle (60). This arrangement gives the light distributions from the lighting units a large degree of overlap which improves mixing of light and hence reduces color over position (COP) problems.

Description

A LIGHTING DEVICE HAVING MULTIPLE LIGHTING UNITS INCLUDING

DIFFERENT COLORS

FIELD OF THE INVENTION

This invention relates to lighting devices in which mixing of the light output from different lighting units is desired.

BACKGROUND OF THE INVENTION

Good color mixing and illuminance uniformity are important for lighting devices such as panel and ceiling lighting. A light mixing chamber is one option for providing color mixing but it is bulky and heavy. A known preferred way to provide good mixing of light from an array of light sources is to provide each light source with an associated lens.

There is generally a trade-off between color mixing and uniformity. For example, for a direct lit panel luminaire with a particular number of LEDs, the smaller the pitch size of the multiple color LEDs the better the color mixing, but the harder to reach the required illuminance uniformity on the exit window (i.e. the output surface of the lighting device which is visible in use). Thus, a color module with a small pitch may enable very good color uniformity, but extra modules are needed to reach a desired illuminance uniformity at the exit window.

Figure 1 shows an example of a known lens which may be applied over an individual light source such as a chip on board LED, or over a sub-array of light sources. It is for example used in LED backlights.

The lens comprises a lens body 10 with an outer surface 12 and a cavity 14 formed in the base of the lens. The lens is designed to extend the output beam from the LED 16 to a large angle, which enables light mixing to take place within a short distance. However, when this kind of lens is applied to a color lighting module, the edge of the module will show color differences, which result in so-called color over position (COP) problems, namely visible differences in color at the light exit window of the lighting device.

When using different color light sources in one system, the light rays travel from the light source to the system exit window, to positions which depend on the emission position of the light. Therefore, the color of the emitted light is position-dependent. If the outputs from the different light sources do not fully overlap on the exit window, the edge will show color differences.

A reduced pitch size and an increased number of lighting modules within a luminaire is a way to overcome this edge color leakage problem, but this increases the cost.

US2014/0071674 discloses a light emitting apparatus with such a reduced pitch. Its lens includes a first curving surface and a second curving surface opposite to the first curving surface. An optical axis of the second curving surface is close to the central area with respect to an optical axis of the first curving surface.

US2012/0199852 discloses an LED component includes a monolithic substrate, an array of LED chips disposed on a surface of the substrate, and an array of optical lenses, each optical lens overlying at least one of the LED chips and having a lens base attached to the substrate, where at least one of the LED chips is positioned to provide a peak emission shifted from a perpendicular centerline of the respective lens base. The lens is designed to shape the emitted light beam as well as to provide environmental and/or mechanical protection for the LED chip(s) thus is a primary lens instead of a secondary lens.

There is therefore a need for a design of lighting device in which color uniformity can be achieved from light sources which can be arranged at a relative large pitch, so that a desired illuminance level and uniformity can be achieved with a relatively small number of light sources.

SUMMARY OF THE INVENTION

The invention is defined by the claims.

According to examples in accordance with an aspect of the invention, there is provided a lighting device comprising a set of lighting units arranged around a circle having a center,

wherein each lighting unit comprises a light source and a lens over the light source, wherein the lens of each lighting unit comprises a lens body comprising a base, a cavity formed into the lens body from the base and an outer lens surface over the cavity,

wherein each lens is adapted to receive light from the respective light source and to provide an output light distribution, wherein a principle axis of the output light distribution is offset from a direction perpendicular to a plane of the circle, towards the center of the circle, wherein at least two of the lighting units have different color light output. The lens function is to shape, i.e. skew, the output light distribution so that it is generally tilted towards the center of the circle around which the lighting units are arranged. This means that the light distributions have a large degree of overlap. This improves mixing of light, in particular color mixing, and hence reduces color over position (COP) problems, by which differences in color are visible at different locations of the light exit window of the lighting device.

For each lens:

the cavity may terminate at an opening which is coplanar with the base, the opening having a center of area;

the center of area of the opening is formed offset from a center of area of the base, and

the light source is offset from the center of area of the opening towards the center of area of the base.

The light source is located in an off-center position at the bottom opening of the cavity, and the cavity itself is also offset from a center of the base. The effect of this is to skew the output light distribution inwardly towards the circle center as explained above. In particular, the lighting units are arranged with the light sources radially further out than centers of area of the base.

Note that the center of area of the base is to be understood as derived from only the outer shape of the base, i.e. it ignores that there is an opening forming part of the cavity. For each lighting unit, the light source may be at a first distance, a, from an edge of the lens in a direction away from the center of area of the base and at a second distance, b, from an edge of the lens in a direction towards the center of area of the base, and wherein the ratio a/b is in the range 0.53 to 0.75.

This range of ratios defines the degree of asymmetry of the lens, for providing the particular light output distribution desired for improving mixing when multiple lenses are combined.

The multiple lenses may be arranged with a relatively large pitch while still avoiding COP problems. The freedom to increase the pitch enables exit window light uniformity (i.e. the appearance of a lighting unit when looking directly at it) to be achieved more easily. Further, a larger pitch allows more spreading thermal load in the set of lighting units, thus the lighting device can achieve a much better thermal management than that in prior art, e.g., US 2014/0071674. The principle axis may be considered to be the light output direction in which the light intensity is highest, and it is for example the redirected ray which was emitted in a normal direction (i.e. perpendicularly) from the light source.

Note that the different color light outputs may be different color temperatures of the same general color (e.g. different white color temperatures) and/or different colors.

An outer profile of the output light distribution as projected to a light exit window is preferably skewed from a direction perpendicular to a plane of the circle towards the center of the circle.

The whole light distribution is thus skewed to one side so that overlap between these distributions from different lenses may be maximized.

Each light source may comprise a single LED or an array of LEDs.

A single LED may be more accurately positioned at a desired location for the optimum function of the lens, whereas an array of LEDs enables greater light output.

Each lighting unit for example generates a circular output light distribution at a light exit window which is parallel to the circle at a predetermined distance from the circle, wherein the output light distributions from each lighting unit cover between 80% and 100% of the light exit window.

The light exit window for example comprises a diffuser plate. By illuminating as much of the exit window for example by each lighting unit, the color uniformity is improved.

The device may comprise three lighting units each having different color light outputs.

The improved mixing of the light output means that color mixing is improved. In one example, the colors comprise 6500k cool white, 2200k warm white and a lime color. Of course, any combination of color and/or color temperatures may be used, such as RGB lighting units.

Each lens may have a plane of mirror symmetry which is perpendicular to a plane of the base and extends between the center of area of the opening and the center of area of the base.

The lens body is symmetric about one plane of symmetry, and this gives some symmetry to the light output. Note that "the plane extends between A and B" is intended to signify that "the vector between A and B is within the plane" and should thus be understood accordingly. The light distribution from each lighting unit is preferably symmetric, in a first plane perpendicular to the base and perpendicular to a line between the center of area of the opening and the center of area of the base, about an axis of mirror symmetry. This follows from the symmetry of the lens body shape.

The light output is thus symmetric in a plane which may be considered to be a side-to- side cross section (through the center of area of the base and perpendicular to the base).

The light distribution from each lighting unit may be asymmetric in a second plane perpendicular to the base and parallel to a line between the center of area of the opening and the center of area of the base.

The light output is thus asymmetric in a plane which may be considered to be an end- to-end cross section (through the center of area of the base and perpendicular to the base).

The outer lens surface of each lens is for example a dome, and the dome has a negative draft angle at the interface to the base.

The lens body may be formed from silicone.

This enables a small negative drift angle to be tolerated even in an injection molding process.

Of course other materials may be used such as plastics (poly(methyl methacrylate) (PMMA), polycarbonate (PC), polystyrene (PS)) or glass and other manufacturing techniques than molding may be used, such as additive manufacturing techniques or reverse molding techniques.

The lighting device may be a light bulb, and a luminaire may then comprise the light bulb and a housing for the light bulb. The lighting device may instead itself be a luminaire.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of the invention will now be described in detail with reference to the accompanying drawings, in which:

Figure 1 shows an example of a known lens which may be applied over an individual light source;

Figure 2 shows a lens design which forms part of each lighting unit in a device of the invention;

Figure 3 shows a view of the lens through a cross sectional plane;

Figure 4 shows a view of the lens through a plane perpendicular to Figure 3; Figure 5 shows the light output distribution for a lens of the type shown in Figure 1 ;

Figure 6 shows the light output distribution for a lens of the type shown in Figure 2 for a side-to-side plane;

Figure 7 shows the light output distribution for a lens of the type shown in Figure 2 for an end-to-end plane;

Figure 8 shows one lighting unit and is used to show shape of the light output distribution as projected onto a light exit window;

Figure 9 explains the light output distribution more clearly; and

Figure 10 shows an arrangement of three lighting units in a circle to form a device in accordance with the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The invention will be described with reference to the Figures.

It should be understood that the detailed description and specific examples, while indicating exemplary embodiments of the apparatus, systems and methods, are intended for purposes of illustration only and are not intended to limit the scope of the invention. These and other features, aspects, and advantages of the apparatus, systems and methods of the present invention will become better understood from the following description, appended claims, and accompanying drawings. It should be understood that the Figures are merely schematic and are not drawn to scale. It should also be understood that the same reference numerals are used throughout the Figures to indicate the same or similar parts.

The invention provides a lighting device which comprises a set of lighting units including different colors arranged around a circle having a center. Each lighting unit comprises a light source and a lens over the light source. Each lens receives light from the respective light source and provides an output light distribution, which is skewed towards the center of the circle. This arrangement gives the light distributions from the lighting units a large degree of overlap which improves mixing of light and hence reduces color over position (COP) problems.

Before describing the full lighting device of multiple lighting units, a lens design which forms part of each lighting unit will first be explained with reference to Figure 2.

The lens 20 is for providing over a light source to shape and direct the light output form the light source. The lens is a divergent lens, e.g., thin in middle and thick at periphery. The lens comprises a lens body 22 comprising a base 24, a cavity 26 formed into the lens body 22 from the base 24 and an outer lens surface 32 over the cavity 26. The lens body is for example formed from a plastics material such as PMMA, PC or PS or it may be formed from silicone or from glass.

The cavity 26 terminates at an opening 28 which is coplanar with the base 24 and the opening 28 has a center of area 30 which is offset from a center of area 34 of the base. The lens has a light source location 36 at which the light source is to be located. The light source location 36 is offset from the center of area 30 of the opening 26 towards the center of area 34 of the base 24.

The light source is thus located in an off-center position at the bottom opening of the cavity, and the cavity itself is also offset from a center of the base. The effect of this is to skew the output light distribution inwardly (and particular towards a center of a circle of the lighting units as shown further below).

Note that the center of area 34 of the base is to be understood as derived from only the outer shape of the base, i.e. it ignores that there is an opening 28 forming part of the cavity.

The lens has a plane 38 of mirror symmetry which is perpendicular to a plane of the base 24 and extends between the center of area 30 of the opening and the center of area 34 of the base. This may be considered to be side-to-side symmetry. When arranged in a circle, this side-to-side symmetry direction is a tangential direction.

Figure 3 shows a view of the lens through the plane 38 as a cross section and additionally shows the light source 40 at the light source location. This plane 38 is perpendicular to the base and parallel to a line between the center of area 30 of the opening and the center of area 34 of the base.

In this plane, the skewing of the light output distribution can be seen. In particular, ray 44 represents the principle light output axis from the light source 40, namely the direction of peak light intensity (for a Gaussian light output) or else an axis of rotational symmetry of the light output (since, for example, a light distribution may be rotationally symmetric but with a dip at the central axis).

The bending of the ray 44 can be seen at both refractive index interfaces, namely from the cavity 28 to the lens body 22 and from the lens body 22 to the ambient surroundings. The end result is that the exit beam is deflected by an angle a from the normal. This double bending results from the off-center position of the light source in the cavity and the off-center position of the cavity in the overall lens body. The cavity shape may define a positive lens portion towards the edge (the left part in Figure 3) and a freeform lens towards the middle (the right part in Figure 3).

The light output is thus asymmetric in a plane which may be considered to be an end- to-end cross section (through the center of area of the base and perpendicular to the base). When arranged in a circle, this end-to-end asymmetry direction is a radial direction.

The degree of asymmetry may be defined. The light source may be at a first distance, a, from an edge of the lens in a direction away from the center of area of the base and at a second distance, b, from an edge of the lens in a direction towards the center of area of the base. The ratio a/b is for example in the range 0.53 to 0.75.

Further, the lens height c may fall in the range 0.45(a+b) to 0.85(a+b).

Figure 3 also shows that the outer lens surface may have a negative draft angle at the interface 46 to the base. The lens body may be formed from silicone so that the negative draft angle can be tolerated in an injection molding method.

Figure 4 shows a view of the lens perpendicular to the plane of Figure 3. In this plane, the lens shape and correspondingly the light output distribution is symmetric about an axis of mirror symmetry 42, which corresponds to the principle light output axis from the light source 40.

The light distribution is thus symmetric, in a plane perpendicular to the base and perpendicular to a line between the center of area 30 of the opening and the center of area 34 of the base.

Figure 5 shows the light output distribution for a lens of the type shown in Figure 1. The light output distribution is rotationally symmetric so that the plot of Figure 5 represents two orthogonal planes (L=0 and L=90) and all other angles are the same. Angle zero corresponds to the direction of principle optical axis.

Figure 6 shows the light output distribution for a lens of the type shown in Figure 2. The light output distribution is shown for the side-to-side plane of Figure 4 (defined as L=0). The symmetry can be clearly seen.

Figure 7 shows the light output distribution for a lens of the type shown in Figure 2, for the end-to-end plane of Figure 5 (defined as L=90). The asymmetry can be clearly seen.

In the symmetrical distribution, the width is around 150 degrees whereas in the asymmetrical distribution the width is around 70 degrees.

Figure 8 shows one lighting unit 50 (lens 20 and light source 40) and is used to show the shape (i.e. outer profile) 52 of the light output distribution as projected onto a light exit window. The light distribution has a generally circular shape as shown, but when that circular shape is projected down to the plane of the lighting unit, the lighting unit 50 is offset from the center. Thus, the overall light distribution is skewed to one side (not only the principle axis as shown in Figure 3) as compared to a normal light output direction.

Figure 9 is used to explain this more clearly. It shows two lighting units 50a, 50b equally spaced from a central axis 51 , which axis is perpendicular to a base plane. The angular extent of their output rays is shown. This angular extent is shown in a plane which contains the central axis 51 and also contains the axis of symmetry 42. The asymmetric skewing of the light output towards the central axis 51 can clearly be seen.

The output rays overlap at the light exit window 53.

To avoid any confusion, by "outer profile" in this context is meant the shape of the light output beam (i.e. the edge where the light intensity drops below a threshold intensity) on the exit window 53. The exit window is a plane parallel to the plane common to the lighting units but spaced by a distance D. The exit window is perpendicular to the axis of symmetry 42 and hence perpendicular to the central axis 51.

The distance D is designed such that the desired overlap of the light output is achieved at the exit window. The distance D thus depends on the way the light output is skewed by each lens, for example the exit angle of the ray 44 shown in Figure 3. In other words, for a given value of D, the corresponding required value for a for each lens can be obtained. In an actual lighting device, there will be a constraint for the parameter D, such as the thickness of an optical chamber, or a height of the luminaire. For a more closely spaced exit window location such as shown as 53', an increased angle a would be needed to provide the same degree of overlap.

The invention relates to lighting devices in which a light exit window (where the overlap is desired) is part of the device structure. Thus, the lighting device is a bulb or luminaire having a light exit window. For example, the distance D is less than the diameter of the light exit window, and comparable with and preferably smaller than the spacing between lighting units. For example, the distance D is preferably less than 1/3 and more preferably less than 1/5 of the pitch between lighting units.

The distance D is typically less than lOOmm, for example in the range 20mm to 80mm, and a typical value of D is around 40mm. Thus, the invention enables an ultra-thin luminaire to be provided, with acceptable light mixing with fewer LED chips than existing designs. This overlap provides color mixing and it avoids color over position problems at the exit window 53. If the exit window is offset from the optimized position, for example as shown by exit window 53’, severe COP issues will arise at the periphery of the window.

Figure 10 shows an arrangement of three lighting units 50a, 50b, 50c in a circle for which the center is shown as 60. The lighting units are arranged so that the skewing of light as explained above is to bend the light towards the center of the circle. In particular, a principle axis 44 of the output light distribution for each lighting unit is offset from a direction perpendicular to a plane of the circle, towards the center 60 of the circle.

The multiple lighting units may be arranged with a relatively large pitch while still avoiding COP problems. The freedom to increase the pitch enables the exit window light uniformity (i.e. the appearance of a lighting unit when looking directly at it) to be achieved more easily. The different color light outputs may be different color temperatures of the same general color (e.g. different white color temperatures) and/or different colors.

Each light source 40 may comprise a single LED or an array of LEDs. A single LED may be more accurately positioned at a desired location for optimum function of the lens, whereas an array of LEDs enables greater light output.

As shown in Figure 10, the output light distribution (on the exit window) from each lighting unit covers a large area of the exit window, such as between 80% and 100% of the light exit window. This ensures a large area of overlap. Regions 54a, 54b, 54c where only one light source illuminates the exit window are kept to a minimum, for example less than 15% of the area of the light exit window, preferably less than 10%.

The light exit window for example comprises a diffuser plate.

The example of Figure 10 has three lighting units each having different color light output. In one example, the colors comprise 6500k cool white, 2200k warm white and a lime color. Of course, any combination of colors and/or color temperatures may be used, such as RGB lighting units. There may be more than 3 lighting units. Preferably, they are all evenly angularly space in a circle. There may be between 3 and 10 such lighting units.

Experiments have been conducted to demonstrate the performance of the lens.

One standard lens of the type shown in Figure 1 was manufactures and one of the type explained above. The lenses were formed from polycarbonate (PC).

Weighted averages and standard deviations were calculated to evaluate the color differences on the exit window. A smaller color difference means better color uniformity. A color difference of less than 0.005 generally means that the color difference cannot be detected by human eyes. The weighted average is the average difference between the color (uV) being considered and a reference color, weighted by flux (Y). The weighted average is calculated as follows:

The standard deviation is the square root of the variance, calculated as follows:

The table below shows a comparison between the two lens types.

The efficiency is comparable. The central beam intensity was found to be slightly for the modified lens of Figure 2. The weighted average and standard deviation are used to evaluate the color deviation.

For the standard lens, one module has a weighted average over 0.021, whereas with the modified lens the value became lowered to 0.014. This is a significant improvement and achieved with the same pitch (60mm). Thus, the color uniformity can be improved without requiring a corresponding reduction in pitch. The luminance uniformity is also much better.

In this way, the number of LED chips can be reduced to achieve a require luminance uniformity.

By way of example, one module may comprise 4 chips and 3 lenses, with one 2200k chip and associated lens, one 6500k chip and associated lens, and two lime chips with a shared lens.

The invention may be applied to different lumen packages and different sizes.

The example above shows one circle of lighting units. There may be multiple circles of lighting units. Different lens designs may be provided for lighting units on different circle radii. Thus, the design may be extended in size in an unlimited way. Hence, the width of the luminaire can be also unlimited, for example even to cover the whole surface of a ceiling or wall.

However, for a typical application such as a discrete lamp or luminaire, the circle on which the lighting units are arranged for example has a diameter of less than l50mm, for example less than lOOmm, such as in the range 50mm to 60mm.

Each lens is typically designed for a single LED chip. However, for LED chips with a small light emitting surface, there may be multiple LEDs associated with each lens. The smaller the light emitting surface, the more LED chips each lens may cover. For existing LED chip designs, it may typically be appropriate to provide two or three chips for each lens. However, as chip scale packages and micro LEDs evolve, it may be appropriate to have an increased number of LEDs per lens.

The pitch between the lighting units (i.e. between the lenses) is related to the distance D to the exist window. For a general luminaire and a reasonable size of the lens, a pitch in the range 30mm to 150mm is appropriate.

The lighting device shows above may be formed as a light bulb or as a luminaire. A luminaire may have one set of lighting units or multiple sets of lighting units.

Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a” or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. A lighting device comprising a set of lighting units arranged around a circle having a center (60),

wherein each lighting unit comprises a light source (40) and a lens (20) over the light source (40), wherein the lens of each lighting unit comprises a lens body (22) comprising a base (24), a cavity (26) formed into the lens body (22) from the base (24) and an outer lens surface (32) over the cavity (26),

wherein each lens (20) is adapted to receive light from the respective light source (40) and to provide an output light distribution, wherein a principle axis (44) of the output light distribution is offset from a direction perpendicular to a plane of the circle, towards the center (60) of the circle,

wherein at least two of the lighting units have different color light output;

wherein for each lens (20):

the cavity (26) terminates at an opening (28) which is coplanar with the base (24), the opening (28) having a center of area (30);

the center of area (30) of the opening is formed offset from a center of area (34) of the base, and

the light source is offset from the center of area (30) of the opening (26) towards the center of area (34) of the base (24);

wherein, for each lighting unit, the light source is at a first distance, a, from an edge of the lens in a direction away from the center of area (34) of the base and at a second distance, b, from an edge of the lens in a direction towards the center of area (34) of the base, and wherein the ratio a/b is in the range 0.53 to 0.75.

2. A lighting device as claimed in claim 1, wherein an outer profile of the output light distribution of each lighting unit as projected to a light exit window is skewed from a direction perpendicular to a plane of the circle towards the center (60) of the circle,

3. A lighting device as claimed in claim 1 or 2, wherein each light source (40) comprises a single LED or an array of LEDs.

4. A lighting device as claimed in any preceding claim, wherein each lighting unit generates a circular output light distribution (52; 52a, 52b, 52c) at a light exit window (53) which is parallel to the circle at a predetermined distance (D) from the circle, wherein the output light distributions from each lighting unit cover between 80% and 100% of the light exit window.

5. A lighting device as claimed in any preceding claim, comprising three lighting units (50a, 50b, 50c), each having different color light output.

6. A lighting device as claimed in any preceding claim, wherein each lens has a plane (38) of mirror symmetry which is perpendicular to a plane of the base and extends between the center of area (30) of the opening and the center of area (34) of the base.

7. A lighting device as claimed in any preceding claim, wherein the light distribution from each lighting unit is symmetric, in a first plane perpendicular to the base and

perpendicular to a line between the center of area (30) of the opening and the center of area (34) of the base, about an axis (42) of mirror symmetry.

8. A lighting device as claimed in any preceding claim, wherein the light distribution from each lighting unit is asymmetric in a second plane perpendicular to the base and parallel to a line between the center of area (30) of the opening and the center of area (34) of the base.

9. A lighting device as claimed in any preceding claim, wherein the outer lens surface (32) of each lens is a dome, and wherein the dome has a negative draft angle at the interface (46) to the base.

10. A lighting device as claimed in any preceding claim wherein the lens body (22) is formed from silicone.

11. A lighting device as claimed in any preceding claim wherein the lighting device is a light bulb.

12. A luminaire comprising a light bulb as claim 11 and a housing for the light bulb.

13. A lighting device as claimed in any one of claims 1 to 10, wherein the lighting device is a luminaire, and comprises multiple sets of lighting units.