WO2015173025A1

WO2015173025A1 - Micro-optical designs addressing color over angle

Info

Publication number: WO2015173025A1
Application number: PCT/EP2015/059437
Authority: WO
Inventors: Hendrikus Hubertus Petrus Gommans; Marcellinus Petrus Carolus Michael Krijn
Original assignee: Koninklijke Philips N.V.
Priority date: 2014-05-13
Filing date: 2015-04-30
Publication date: 2015-11-19

Abstract

The invention provides a lighting unit (10) comprising a light source (100) and a light transmissive first window (200), wherein the light source (100) is configured to provide polychromatic light source light (101) with a spatially inhomogeneous color distribution to an upstream face (210) of the light transmissive first window (200), wherein the light transmissive first window (200) comprises a plurality of optical elements (230) configured to provide a beam of light (11) downstream of a downstream face (220) of the light transmissive first window (200), and wherein the shape of the optical elements (230) varies over the light transmissive first window (200) to provide said beam of light (11) with a spatially homogenous color distribution at a predetermined distance (d) downstream from the downstream face (220) of the light transmissive first window (200).

Description

Micro-optical designs addressing color over angle

FIELD OF THE INVENTION

The invention relates to a lighting unit comprising a light source and a light transmissive window, as well as to such light transmissive window per se. BACKGROUND OF THE INVENTION

The use of diffusers is known in the art. US2013277690, for instance, describes a low profile lighting module. According to US2013277690 devices can produce a uniform light intensity output profile, limiting the perceived appearance of individual point sources, from direct lighting modules comprising several light emitting diodes.

US2013277690 describes that individual lighting device components can contribute to this uniform profile, including: primary optics, secondary optics, and contoured housing elements. These components can interact with and control emitted light, thus adjusting its pattern.

These components can alter the direction of emitted light, providing a more uniform light intensity over a wider range of viewing angle. For instance, the diffuser comprises embedded isotropic disks that correlate with the position of the LED package.

US20130242568A1 discloses an illumination device comprising a light source and a lens sheet including a plurality of prisms with different shapes and disposed on and concentrically around the optical axis. SUMMARY OF THE INVENTION

Beam shaping is essential in many lighting applications. One category of beam shaping optical elements includes classical elements such as reflectors and collimators as used in most luminaries. Traditionally, elements belonging to this category are bulky whereas those in the second category are difficult to design and expensive to manufacture.

Using solid state light sources, or other light sources that may have an inhomogeneous light distribution, may lead to light generated by a lighting unit that has a spatially inhomogeneous color distribution, such as an inhomogeneous color temperature distribution e.g. at the exit window (near field) and/or at a surface on which the light of the lighting unit is shed (far field). The use of diffusers is possible, but may lead to efficiency reduction and/or lead to loss in directionality (i.e. collimation properties) of the beam. When using a diffuser, bulky beam shaping elements are necessary to (re)shape the beam.

Hence, it is an aspect of the invention to provide an alternative lighting unit, which preferably further at least partly obviates one or more of above-described drawbacks. Especially, it is an aspect of the invention to provide an alternative lighting unit which provides lighting unit light that which when shed on a surface remote (such as near field or far field) from the lighting unit may be substantially homogenous in color. Even more especially, it is an aspect of the invention to provide an alternative lighting unit with provides lighting unit light that may have an exit window that may be perceived substantially homogenous in color when a spectator would look into the direction of the source of the light. Alternatively or additionally, it is an aspect of the invention to provide an alternative lighting unit with which the beam can be shaped with a relative large freedom, while on the other hand having a relative thin construction (of the essential elements of light source(s) and optics).

Micro-optical structures and solid state light sources appear to provide a good combination that can be used for such alternative lighting unit. It surprisingly appears that it is possible to recombine the beamlets (i.e. beams generated by a micro optical structure or element) with different colors (or color compositions) into a single beam with a more homogenous color distribution with such micro-optical structures. It further surprisingly appears that when a specific multi-layer system of windows with micro-optical structures is chosen, beams can be produced that can be selected to be narrow or broad, and that can be directed, and which may substantially not suffer from a color over angle problem.

Hence, in a first aspect the invention provides a lighting unit comprising a light source and a light transmissive first window (also indicated as "first window" or

"window", or "transmissive window"), which comprises, or especially is a foil, wherein the light source is configured to provide polychromatic light source light with a (spatially) inhomogeneous color distribution to an upstream face of the light transmissive first window (such as by way of example blue and yellow) with a (spatially) inhomogeneous color distribution (i.e. e.g. more yellow at one spot and more blue at another spot), wherein the light transmissive first window comprises a plurality of optical elements, especially configured to provide a (collimated) beam of light downstream of a downstream face of the light transmissive first window, and wherein in a specific embodiment the shape of the optical elements varies over the light transmissive first window to especially provide said beam of light with a (spatially) homogenous color distribution at a predetermined distance (d) downstream from the downstream face of the light transmissive first window, wherein the optical elements have one or more of a refractive functionality and a total internal reflection functionality to the light source light, wherein the optical elements comprise a first facet fl and a second facet f2 mutually angled at a top angle B, wherein the first facet fl is oriented at a tilt angle a with a normal to the light transmissive window, wherein the optical elements are arranged within an off-axis distance (d2) in the range of 0-d2max, with the off-axis distance (d2) being the distance relative to an optical axis of the light source measured along the light transmissive first window, and wherein the top angle and/or tilt angle have a fluctuating value along the off-axis distance d2.

Fluctuating of the value of the tilt angle and/or top angle in this respect means that said value shows at least two sequences of a local minimum and local maximum. Due to the fluctuating value of a and/or β of the optical elements, some blurring and mixing of light source light occurs, resulting in a beam of somewhat less imaging quality but with an increased color uniformity. The blurring and/or broadening of the beam of light can be controlled by the degree of fluctuation of the tilt angle a, the top angle β or by both a and B. To limit the degree of blurring and/or broadening of the beam of light, the fluctuation of a and/or β is within a range of ± 15% of a local average value.

In an embodiment of the lighting unit only the top angle or tilt angle of the TIR optical elements have a fluctuating value along the distance (d2) to further limit the degree of blurring and to maintain imaging in the beam of light to a desired extent. Typically the TIR optical elements deflect light rays issued at relatively large angles with the optical axis of the light source, for example angles larger than 40°, which is also the angle region where the color over angle is most prominent visible. Hence, then color uniformity is significantly improved while the blurring and increase in the beam of light is relatively small.

In some Fresnel lenses there is a continuous, gradual increase/decrease in tilt angle and/or top angle with increasing d2, thus relatively narrow respectively relatively broad beams of light can easily be generated. Hence, it is convenient, if of the optical elements an average value of the tilt angle and/or top angle has increased from off-axis distance d2 = 0.5*d2max to d2 = d2max.

To ensure mixing/blurring by a minimum amount of optical elements for generating directional light with a spatially homogenous color distribution at a predetermined distance from the light transmissive first window, the lighting unit is characterized in that the top angle and/or tilt angle have at least three local maxima in their respective fluctuating value. As indicated above, such lighting device may provide directional light with a spatially homogenous color distribution at a predetermined distance from the light transmissive first window. This predetermined distance may be selected based on the dimensions of the optical elements (herein also called micro-optical structures or micro- optical elements). This distance can be the distance to a second window (such as a (second) foil), which can have the function of an exit window, which distance can then especially be selected from the range of 1-100 mm, such as 5-50 mm ("near field"), or such distance can be the distance selected from the range of e.g. 0.2-50 m ("far field"), especially 0.5-20 m, such as 1-10 m. Hence, the predetermined distance may especially be selected from the range of 1- 100 mm, such as 5-50 mm, in the case wherein downstream of the first window, a second window is arranged (near field). The predetermined distance may especially be selected from the range 0.2-50 m, especially 0.5-20 m, especially in the case spot projection (far field).

Upon application from the lighting unit, at the predetermined distance, a more homogeneous beam of light will be perceived (then without such specific light transmissive first window). In state of the art systems, the color inhomogeneity may be solved with a diffuser, or is simply ignored. However, users in general dislike such color inhomogeneity. The appearance of objects illuminated with light that has a spatially inhomogeneous color distribution may also not be desired and can under circumstance even be dangerous, for instance when observing safety instructions or icons. Further, the lighting unit of the invention may not only solve the above problem(s) with respect to color inhomogeneity, it can also be relatively thin, which is also desired by users and adds to a versatile application.

The useful features of the lighting device are especially obtained by the variation of the optical elements over the light transmissive window. By tuning one or more of length, width, height, angle of the optical elements, i.e. the micro-optical structures, one can locally direct the beam (or beamlets) to get a better distribution of the different colors (or color temperatures). Further, one may also tweak the facets of the optical structures, for instance tilt angles may vary over the window. These ways to tune the beam(lets) are embodiments of the feature that the shape of the optical elements varies over the light transmissive window. In this way one may direct the beamlets to better overlap. For instance, the beamlets escaping from the first window at positions closer to the light source may be narrowed and/or the beamlets escaping from the first window at positions more remote from the light source may be broadened and/or redirected. Hence, the invention also provides the use of the light source and the transmissive window as defined herein, to provide a beam of polychromatic light with a spatially homogenous color distribution in the near field or far field while maintaining degree of collimation of the beam of light. Hence, beams which are homogenous in color and intensity may be generated with the lighting unit as described herein.

The optical structures may e.g. be obtainable by laser ablation or by 3D printing (of transparent material; see also below), etc.. Hence, the optical elements may comprise two or more facets, with at least two facets having a mutual angle (γ). Further, the optical elements have a height and a width. The shape of the optical elements varies over the light transmissive first window, as indicated above, especially in one or more of (i) the mutual angle (γ), a (ii) height-width ratio, and (iii) a shape of a facet. The optical structures may be arranged in a regular array or an irregular array or a combination thereof. In a specific embodiment, the distribution of the (micro) optical elements and the variation in shape over the first window is symmetric.

The optical elements are arranged within an off-axis distance (d2) in the range of 0-d2_max, with said distance being the distance relative to an optical axis of the light source measured along the light transmissive first window. Especially, a first domain (of the e.g. the upstream face) with optical elements is configured within a distance from the optical axis of d2/dl=l, and a second domain is configured within an off-axis distance (d2) of the optical axis selected from the range of d2/dl= 1 to d2 _max. Of course, within the distance d2/dl=l there may be a plurality of domains and/or at an off-axis distance d2 larger than d2/dl=l there may be a plurality of domains. In general, there will be a plurality of optical elements, such as in the order of at least 100, or at least 200, which may mutually vary in one or more of the above aspects. However, one or more domains, each comprising one or more optical elements, within the range larger than d2/dl=l may compensate for the color inhomogeneity. The reference d2_max may indicate the edge of the light transmissive first window. Hence, the optical elements are arranged within an off-axis distance (d2) in the range of 0-d2_max.

Especially, the light transmissive first window has a dimension, such as length/width or diameter such that d2 is at least 2*dl, especially at least 5*dl .

For instance, the optical elements arranged within the distance d2/dl=l (i.e. arranged at an off-axis distance d2, wherein d2/dl<l), i.e. within about a cone having an opening angle of 90° (2*45°), the optical elements, which may especially be configured to use refraction to configure the light, may provide light having more of a first component of light such, as blue light, and the optical elements beyond that range (i.e. especially with d2 being in the range of dl-d2_max, even more especially with d2 being in the range of 1.2dl- d2_max), which may especially be configured to use refraction and/or total internal reflection, may provide light having less of said first component of light, i.e. may e.g. be more yellow. At least part of these optical elements may be configured to correct for angle differences and may provide at the predetermined distance the spatially homogeneous color distribution.

In yet a further embodiment, for instance a base angle of one or more facets of the optical elements is varied (over the transmissive window). This may be done by varying the mutual angle and/or other dimensions of the facets (over the transmissive window). As indicated above, the optical elements are arranged within a distance in the range of 0-d2_max, with d2 being the distance relative to an optical axis of the light source measured along the light transmissive first window. Especially, the optical elements comprise first facets which receive direct radiation from the light source, wherein at least at values of d2/dl larger than about 1 , an angle a between the first facets and a normal to the light transmissive first window increases with increasing off-axis distance (d2), with a subset of the first facets having smaller angles a than a first facet at a shorter distance (but still within the range of d2/dl>l, such as d2/dl> 1.2). This subset may include a plurality of facets. The angle a is thus not constant over the transmissive window or may not gradually increases with increasing distance from the optical axis, but may gradually increase with increasing distance from the optical axis with one or more, especially a plurality, of discontinuities in this decrease. By providing one or more facets a smaller angle than a facet closer to the optical axis, the beams are (in an enhanced way) redirected in the direction of the optical axis.

Would there be no, or a gradual increase in this angle, then the above indicated, and in the drawings displayed, spatial inhomogeneous color distribution would be obtained. However, by tweaking one or more, especially a plurality of the facets, the rays are redirected and can be redirected such that the spatially homogenous color distribution at the predetermined distance may be obtained. Hence, by changing the angle of the facet with the normal (or alternatively defined, by changing the base angle), the rays can be directed such, that the rays overlap well at the predetermined distance, and the spatially homogenous color distribution at the predetermined distance may be obtained.

The phrase "the shape of the optical elements varies over the light transmissive window" may those especially apply to those optical elements in the second domain(s) (see also below), i.e. especially this condition may apply for those optical elements with d2 being selected from the range of dl-d2_max, especially 1.2dl-d2_max. Hence, for instance the second domain may then include one or more optical elements with a first angle al, i.e. the angle between the first facets and a normal to the light transmissive first window, arranged at a first distance xl from optical axis, one or more optical elements with a second angle α2, arranged at a second distance x2 from optical axis, and one or more optical elements with a third angle a3, arranged at a third distance x3 from optical axis, with x3>x2>xl, and with α1<α2>α3 (with e.g. a3>al). Therefore, the second domain may also be considered to include a plurality of (second) domains. Good correction may especially be obtained at distances from the optical axis of at least 1.2dl . The angles a will in general be in the range of 0-60 °, especially 0-45 °, such as 0-30°, such as at least 10°.

The first facets (in this second domain) are especially those facets that may after direct receipt of the light of the light source may refract in the direction of the second facets (of the same optical elements) which then (after total internal reflection) redirect the beam out of the transmissive window at the downstream face. The first facets are thus especially directed to the light source and the second facets are at another side of the optical element, and will especially not directly receive the light source radiation. The first facets arranged with a distance (d2) being in the range of 0-1.2d 1, especially within 0-dl may especially be configured as Fresnel lens.

For instance, assuming a solid state LED providing blue light with a yellow phosphor in a dome on the die of the LED, the white light along an optical axis may have a high color temperature whereas the white light more remote from the optical axis may have a lower color temperature. This may differ hundreds of degrees, i.e. Δχ and/or Ay may differ more than 0.02 (i.e. the differences between the color coordinates x and y at the CIE diagram (CIE 1931 color space chromaticity diagram) between the light of the beam closer to the optical axis and more remote from the optical axis).

At the predetermined distance, the colors of the polychromatic light may be well distributed. This feature may also be used to apply a second window. This second window can be used to (further) shape the beam. Hence, the invention provides in a further embodiment a lighting device wherein at the predetermined distance (d) a light transmissive second window ("second window") is arranged, comprising a plurality of optical elements configured to provide a beam of light downstream of a downstream face of the light transmissive second window.

Especially, the first window comprises a foil. Foils can be very thin and can e.g. easily be stretched between walls of a light chamber. Also the second window, if applicable, can be a foil. In an embodiment, the term "window" refers to a self-supporting (transmissive) element. The window especially comprises material that is transmissive for visible light. Hence, the window is light transmissive. This applies to the first window and also the optional further windows. The term "first window" or "light transmissive first window" may optionally also refer to a plurality of first windows. For instance, two of such windows may be provided to further improve the homogeneous distribution of the colors. The invention thus especially allows solving the problem of color over angle differences by using the micro-optical elements.

Further, especially the light transmissive first window is configured to provide said beam of light having an opening angle of 120° or less. This may be different when also a second window is available. Then, the opening angle (of the beam of light to the first window) can also be larger or smaller, dependent upon the dimension of the lighting device. Hence, when a second window is available, the light transmissive second window is especially configured to provide said beam of light having an opening angle of 120° or less. The final beam, emanating from the lighting device thus especially has an opening angle of 120° or less. Hence, there may be no substantial glare. The opening angle may also be smaller, like 90°, or less. The opening angle is especially defined with respect to the full width half maximum (FWHM).

The (spatial) (in)homogeneity (of color the light) may be determined in different ways.

As indicated above, due to the better distribution of the different types of light the light at the predetermined distance, such as at the second foil, or in the absence of the second foil at a non-zero distance from the first foil at a predetermined distance, especially selected from the range of 0.2-20 m (see above) has a substantial more homogeneous distribution of the light than the light emanating from the light source. For instance, there may be points at the upstream side of the first foil that differ relative much in color points, i.e. the CIE x and/or y values may differ much. However, at the predetermined distance, this difference will be substantially smaller. Hence, in a further embodiment, the upstream face a has a first cross-sectional area, wherein the beam of light at the predetermine distance (d) has a second cross-sectional area, wherein a color difference (or color point difference) between two points is defined as ΙΔχΙ + lAyl, with x and y being CIE coordinates, and wherein the largest color difference in the second cross-sectional area is 90% or less of the largest color difference in the first cross-sectional area. For instance, assuming xl ;yl=0,32;0,3 and x2,y2=0,35;0,28 at the upstream face, then Δχ=-0,03 and Ay=0,02. Hence, ΙΔχΙ + lAyl is 0,05. Due to a better homogenization of the light, without substantial loss in intensity and/or directionality (as may be the case with diffusers or dichroic filters, etc.), with the present invention this inhomogeneity may be reduced with at least 10%, such as at least 25%, even more a reduction with 50%. For instance, at the predetermined distance the following color coordinates may be found xl ;yl=0,33;0,29 and x2,y2=0,34;0,29, with ΙΔχΙ + lAyl then being 0,01. Hence, the largest differences in color points may substantially be reduced.

Alternatively or additionally, the color (point) difference is calculated as sqrt (Delta(u')^A2 +Delta(v')^A2). Due to a better homogenization of the light, without substantial loss in intensity and/or directionality (as may be the case with diffusers or dichroic filters, etc.), with the present invention this inhomogeneity, calculated on the basis of sqrt

(Delta(u')^A2 +Delta(v')^A2), may be reduced with at least 10%, such as at least 25%, even more reduced with 50%.

Especially, a ratio of the color difference at the upstream face (of the light transmissive first window) and at the predetermined distance (d), defined as (Au'v')_u/(Au'v')d, is larger than 1, such as larger than 1.2, like larger than 1.5, like at least 1,67. The color inhomogeneity at the upstream face may e.g. be in the range of 50.10^" - 150.10 , whereas the color inhomogeneity at the predetermine distance may be less than 30.10^" , such as less than 10.10^"3.

Therefore, the invention provides a beam of light downstream from the transmissive first window that has at a predetermined distance, especially selected from the range of 1 mm - 50 m, a spatially more homogeneous color distribution than at the upstream face of said window.

Hence, the indication herein that the light source light includes a spatially inhomogeneous color distribution and the beam of light has a spatially homogenous light distribution especially indicates that upstream from the first light transmissive window the color distribution of light source light is less homogenously distributed than the color distribution of the beam of light at the predetermined distance downstream from this first transmissive window.

Returning to the first window (the light transmissive first window), the window may distribute the light source light in different ways to obtain a homogeneous distribution of the polychromatic light and thereby reduce color differences. For instance, this can be done by a better distribution of all light over a specific (virtual) area and/or by redistribution light over domains within this (virtual) area (at the predetermined distance). This virtual area can be the upstream area of the upstream face of the second window or this can be a virtual plane more remote from the lighting unit (especially in the absence of this second window). In the latter case, this may especially be the area formed by the spot of light provided by the beam of light at the predetermined distance (at a plane perpendicular to an optical axis relative to the lighting device). Hence, the invention also provides said lighting unit according, having a virtual plane at the predetermined distance (d) downstream from the downstream face, wherein a first domain with a plurality of the optical elements closest to the light source distributes light source light over 90% or more of the area of virtual plane and wherein one or more second domains further away from the light source also distribute light source light over 90% or more of the area of virtual plane. In a specific embodiment, the first window may comprise two or more second domains further away from the light source also distribute light source light over 90% or more of the area of virtual plane, and wherein the individual domains each distribute light source light over less than 90% of the area of virtual plane. In case of a single light source, the upstream face of the first window may e.g. show a circular distribution of the light with at a core and at the outer ring color points that may differ most. The core may be closest to the light source. Such distribution may e.g. be addressed by two domains, one around the core, and the other the remaining. However, also more than two domains may be used. The value of 90% or more is chosen to allow some imperfections at e.g. the edges. However, it is also possible to provide a very homogeneous distribution of the polychromatic light over the entire virtual plane (i.e. 100%). Note that herein the term "very homogeneous distribution" or "very homogeneous distribution of the polychromatic light" and similar terms and phrases especially indicate an even color distribution, i.e. an observer does not perceive substantial differences in color point (at the predetermined distance from the first window).

Further, especially the light transmissive first window comprises two or more domains each domain including one or more of said optical elements, especially a plurality of such optical elements, with mutually different shapes of the optical elements between the domains, wherein the two or more domains are configured to provide overlapping spots of light at the predetermined distance (with the beam of light at the predetermined distance (thus) being composed of said spots (of light), wherein each spot has an area overlap with another spot of at least 90%.

Especially, the number of domains per light source is selected from the range of 2-50, such as 2-20, especially 2-4, though much more may be used. Assuming a solid state light source with a wavelength converter, 2-4, such as two domains may suffice, with especially one domain closest to the light source, and the other domain(s) surrounding this domain. However, of course more than 4 may be used. Further there may be a gradual variation, with a plurality of domains (of e.g. more than 4). The light transmissive first window is especially arranged at a non-zero distance from the light source (or at least at a non-zero distance from the light emitting surface of the light source (such as a LED die). The distance may especially be in the range 0.1-100 mm, such as especially 0.50 mm, even more especially in the range of 0.1-25 mm, such as 0.5-15 mm, especially in the range of 0.5-10 mm, like 1-8 mm.

The lighting unit may comprise arrangement of a plurality of light sources. This arrangement is especially a 2D arrangement, and especially the arrangement is regular, such as a cubic arrangement or a hexagonal arrangement. However, the arrangement may also be irregular. In case the light sources are arranged regular, the shortest distance between adjacent light sources (measured from a central point from the light sources) can also be indicated as pitch. In an embodiment, there may be (two) different pitches in (two) different directions. As the arrangement may be regular or irregular, or a combination thereof, and as the pitches may be constant all over the arrangement or may be different in (two) different directions, herein also a mean shortest distance (measured from a central point from the light sources) is defined. This mean shortest distance may in general be in the range of 0.5-100 mm, such as at least 1 mm, like especially in the range of 5-50 mm.

The lighting unit may include a plurality of light sources, such as e.g. at least 4, like at least 16, such as at least 25, like at least 49, or even more at least 100 light sources. Note however that substantially larger numbers are also possible.

In a specific embodiment, the light source comprises a solid state light source

(such as a LED or laser diode).

In a further specific embodiment, the plurality of light sources comprises two or more subsets, which may be independently controllable. The light source itself may in a specific embodiment include one light source, such as a solid state light source, like a LED or laser diode. In such embodiment, the light source may essentially consist of the light source. However, in yet another embodiment, the light source comprises two or more light sources, which may optionally also be independently controllable.

In a further specific embodiment, the light source comprises a solid state light source, especially configured to provide blue solid state light source light, and a wavelength converter configured to convert part of the solid state light source light, especially thus the blue light into wavelength converter light having larger wavelength (such as green, yellow, orange and/or red), whereby the light source light comprises said solid state light source light and said wavelength converter light. Especially, the lighting unit is configured to provide a beam of white light (downstream of the first window). As indicated above, the light source may provide light source light with an uneven distribution of the colors constituting this light source light. Hence, in an embodiment the (solid state) light source is configured to provide light source light to a first domain closest to the light source having a larger ratio of (solid state) light source light to wavelength converter light than to a second domain further away from the light source. For instance, a solid state light source with a yellow phosphor may provide at the first window upstream face light with bluer core and a more yellows shell surrounding this blue core.

The invention may also be used with those light sources that have two or more spatially distinct different positions from which light with different colors escape, such as RGB LEDs based on a combination of three solid state LEDs in a single mount.

As indicated above, "the light source is configured to provide polychromatic light source light with a spatially inhomogeneous color distribution to an upstream face of the light transmissive first window". This especially implies that the light source provides light that is not monochromatic, but that includes emission at different wavelengths, such is the case with blue LEDs, that have a bandwidth of several nanometers or more, and as is e.g. the case with the yellow cerium doped garnet phosphor, that has an emission bandwidth of several tens of nanometers. However, it especially indicates that the light source light includes light consisting of two or more colors selected from the group consisting of blue, blue green, green, yellow, orange, and red. The phrase "with a spatially inhomogeneous color distribution to an upstream face" especially indicates that the light source provides (during use) light that will have a certain color distribution over the upstream face of the first transmissive window. As indicated above, this may be the case in a number of state of the art light sources. The term "spatially" especially indicates that at different places on a plane of face, etc. different colors can be found, such as blue in a center and more yellow more eccentric. The phrase "configured to provide a beam of light downstream of the downstream face" and similar phrases especially indicate that the light of the light source being transmitted through the light transmissive first window is shaped into a beam (or a plurality of beamlets forming said beam) by the optical elements.

As indicated above, the lighting unit comprises at least one window, although more than one window may be available. Here, the invention is mainly illustrated with respect to one or two windows.

In case of two or more windows, the windows are especially arranged parallel to each other and parallel to the arrangement of the plurality of light sources. In general, there will be non-zero distances between the light sources and the first window (see also below), and between the first window and the second window (if any).

The total thickness of the windows(s) (or foils) may be in the range of 0.2-20 mm, especially 0.2-5 mm, including the optical elements. The window(s) may have cross- sectional areas in the range of 4 mm 2 - 50 m 2, although even larger may be possible. Also tiles of windows, arranged adjacent to each other, may be applied. The windows are transmissive, i.e. at least part of the light, especially at least part of the visible light illuminating one side of the window, i.e. especially the upstream side, passes through the window, and emanates from the window at the downstream side. This results eventually in the lighting unit light. Especially, the windows comprise, even more especially substantially consist of, a polymeric material, especially one or more materials selected from the group consisting of PE (polyethylene), PP (polypropylene), PEN (polyethylene naphthalate), PC (polycarbonate), polymethylacrylate (PMA), polymethylmethacrylate (PMMA) (Plexiglas or Perspex), cellulose acetate butyrate (CAB), silicone, polyvinylchloride (PVC), polyethylene terephthalate (PET), (PETG) (glycol modified polyethylene terephthalate), PDMS

(polydimethylsiloxane), and COC (cyclo olefin copolymer). However, other (co)polymers may also be possible. Hence, also the window regions of the respective windows are transmissive for at least part of the light of the light source(s).

The optical structures may include optical structures that are configured to couple light out after total internal reflection (TIR) (and then refraction). Alternatively or additionally, optical structures may include optical structures that are configured to (directly) couple light out after refraction. Hence, the beam shaping properties of the beam shaping element may especially be provided by optical structures that impose total internal reflection to the light source light, and provide lighting device light after outcoupling via refraction of the light source light after internal reflection. Alternatively or additionally, the beam shaping properties of the beam shaping element may especially be provided by optical structures that impose refraction to the light source light without previous reflection within the optical structure, and (thus) provide lighting device light after outcoupling via (only) refraction of the light source light. The former structures are herein also indicated as TIR structures, wherein the latter are herein also indicated as refractive structures. Hence, TIR optical structures may also be indicated as TIR+re fraction optical structures. As indicated below, an optical structure may also provide both effects, dependent upon the base angles of the facets of the optical structures. The optical structures, as indicated above, may have different facets. Hence, a single optical structure may in embodiments also provide via one facet outcoupling via (first) TIR and via another facet outcoupling via (direct) refraction. Especially, the optical structures provide at least the function of outcoupling via total internal reflection (especially at larger distances from the optical axis of the light source, such as at a distance at least equaling the distance from the transmissive window to the light source). As in such embodiments the facets may be relatively steep, though still a large beam opening angle range of the lighting device beam can be chosen. For instance in the case of beam shaping element made of polycarbonate, facets having base angles in the range of about 50°-80°, such as in the range of 50°-70°, can provide (via TIR) beams having opening angles in the range of >2*0° up to 2*80°. Especially, the base angles are selected from the range of 10°-80°, such as 10°- 70°. This will also further be discussed below. Especially, the opening angle (of the thus obtained beam) is equal to or less than 2*65° in view of glare reduction, especially in offices, even more especially equal to or less than 2*60°. Within the opening angle, at least 90%, even more at least 95% of all intensity of the light may be found.

Especially, the optical elements have one or more of a refractive functionality and total internal reflection functionality to the light source light. Of course, both types of functionalities may be available. Further, as indicated above, elements may have both functionalities. For instance, a face may provide refraction only and another face shows refraction as subsequent effect on reflection at another face. In yet a further specific embodiment the optical elements especially have prismatic shapes having one or more dimensions especially in the range of 0.01-5 mm.

The first window is arranged downstream of the light sources, and the optional second window is arranged downstream of the first window; the first window thus being arranged upstream of the second window. The terms "upstream" and "downstream" relate to an arrangement of items or features relative to the propagation of the light from a light generating means (here the especially the light source(s)), wherein relative to a first position within a beam of light from the light generating means, a second position in the beam of light closer to the light generating means is "upstream", and a third position within the beam of light further away from the light generating means is "downstream".

Each window comprises a plurality of optical elements. These optical elements may especially comprise one or more of prismatic elements, lenses, total internal reflection (TIR) elements, refractive elements, facetted elements. Optionally, a subset of elements may be translucent or scattering (see also below). In general, at least a subset, or all of the optical elements are transparent. The optical elements may be embedded in the window, and may especially be part of a window side (or face), such as especially a downstream side or an upstream side, or both the downstream and upstream side. Herein, the optical elements are especially further described in relation to optical elements having a Fresnel or refractive function and optical elements having a total internal reflection function. Each optical element may comprise one or more facets

The optical elements (including facets) may be arranged at an upstream side or a downstream side or both the upstream side and downstream side of the window (first and/or second window, etc.). Especially, TIR elements are especially available at an upstream side of the window (first and/or second window), whereas the refractive elements, such as Fresnel lenses, may be arranged at the upstream and/or downstream side of the window (first and/or second window).

One or more of the dimensions of the facets (of these elements), especially of the TIR elements, like height, width, length, etc., may in embodiments be equal to or below 5 mm, especially in the range of 0.01-5, such as below 2 mm, like below 1.5 mm, especially in the range of 0.01-1 mm. The diameters of the refractive Fresnel lenses may in embodiments be in the range of 0.02-50 mm, such as 0.5-40 mm, like 1-30 mm, though less than 30 mm may thus (also) be possible, like equal to or smaller than 5 mm, such as 0.1-5 mm. The height of these facets will also in embodiments be below 5 mm, such as below 2 mm, like below 1.5 mm, especially in the range of 0.01-1 mm. Here the term "facet", especially in TIR embodiments, may refer to a (substantially) flat (small) faces, whereas the term "facet", especially in Fresnel embodiments, may refer to curved faces. Thus curvature may especially be in the plane of the window, but also perpendicular to the plane of the window ("lens"). The Fresnel lenses are not necessarily round, they may also have distorted round shapes or other shapes.

It further appears advantageous to control the distance to the first window and the mutual distance between light sources. Hence, in a specific embodiment the light sources have a mean shortest distance (p) and wherein the light sources have a shortest distance (dl) to the first window, wherein dl/p<0.3. In a regular arrangement such as a hexagonal or cubic arrangement, the mean shortest distance is the pitch. Hence, in an embodiment the lighting unit comprises a plurality of light sources, wherein the light sources have a mean shortest distance (p) and wherein the light sources have a shortest distance (dl) to the first window, wherein dl/p<0.3. The prismatic shapes or elements may essentially comprise two (substantially flat) facets arranged under an angle with each other and especially arranged under angle (>0° and < 90° relative to a plane through the window).

The lighting device may be part of or may be applied in e.g. office lighting systems, household application systems, shop lighting systems, home lighting systems, accent lighting systems, spot lighting systems, theater lighting systems, fiber-optics application systems, projection systems, self-lit display systems, pixelated display systems, segmented display systems, warning sign systems, medical lighting application systems, indicator sign systems, decorative lighting systems, portable systems, automotive applications, green house lighting systems, horticulture lighting, or LCD backlighting.

The term white light herein, is known to the person skilled in the art. It especially relates to light having a correlated color temperature (CCT) between about 2000 and 20000 K, especially 2700-20000 K, for general lighting especially in the range of about 2700 K and 6500 K, and for backlighting purposes especially in the range of about 7000 K and 20000 K, and especially within about 15 SDCM (standard deviation of color matching) from the BBL (black body locus), especially within about 10 SDCM from the BBL, even more especially within about 5 SDCM from the BBL.

The terms "violet light" or "violet emission" especially relates to light having a wavelength in the range of about 380-440 nm. The terms "blue light" or "blue emission" especially relates to light having a wavelength in the range of about 440-490 nm (including some violet and cyan hues). The terms "green light" or "green emission", including blue- green, especially relate to light having a wavelength in the range of about 490-560 nm. The terms "yellow light" or "yellow emission" especially relate to light having a wavelength in the range of about 540-570 nm. The terms "orange light" or "orange emission" especially relate to light having a wavelength in the range of about 570-600. The terms "red light" or "red emission" especially relate to light having a wavelength in the range of about 600-750 nm. The term "pink light" or "pink emission" refers to light having a blue and a red component. The terms "visible", "visible light" or "visible emission" refer to light having a wavelength in the range of about 380-750 nm.

The term "substantially" herein, such as in "substantially all light" or in

"substantially consists", will be understood by the person skilled in the art. The term

"substantially" may also include embodiments with "entirely", "completely", "all", etc. Hence, in embodiments the adjective substantially may also be removed. Where applicable, the term "substantially" may also relate to 90% or higher, such as 95% or higher, especially 99% or higher, even more especially 99.5% or higher, including 100%. The term "comprise" includes also embodiments wherein the term "comprises" means "consists of. The term "and/or" especially relates to one or more of the items mentioned before and after "and/or". For instance, a phrase "item 1 and/or item 2" and similar phrases may relate to one or more of item 1 and item 2. The term "comprising" may in an embodiment refer to "consisting of but may in another embodiment also refer to "containing at least the defined species and optionally one or more other species".

Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

The devices herein are amongst others described during operation. As will be clear to the person skilled in the art, the invention is not limited to methods of operation or devices in operation.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "to comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. 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.

The invention further applies to a device comprising one or more of the characterizing features described in the description and/or shown in the attached drawings. The invention further pertains to a method or process comprising one or more of the characterizing features described in the description and/or shown in the attached drawings. The various aspects discussed in this patent can be combined in order to provide additional advantages. Furthermore, some of the features can form the basis for one or more divisional applications.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

Figs.l a-li schematically depict some basic aspects of the invention;

Figs. 2a-2c schematically depict some aspects of the invention;

Figs. 3a-3c schematically depict some aspects of the invention (and variations thereon);

Figs. 4a-4f schematically depict some rays of light that can be generated, such as with the devices schematically depicted in figures 2a-2c, 3b and 3c, respectively; and

Figs. 5a-5c schematically depict some examples of a course of fluctuation in tilt angle and/or top angle in relation to the distance d2.

The drawings are not necessarily on scale.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The invention especially addresses color over angle issues that are currently hampering acceptance of beam collimating designs for mid-power LEDs. Herein, uniform illumination is simultaneously solved with color consistency for both the beam spot (far field) as on the exit window of the luminaire (near field).

A relevant feature of the invention is the optical function assignment for micro facets that is position dependent. In summary, the color-over-angle has been modified into color-over-position using a first foil and then treat it, for instance, as if the emitting surface area (at the first foil) consists of 2 circular sources with two different color temperatures (inner circle 4000 K; outer ring 2300 K) that need to be projected on top of each other.

Especially two cases can be distinguished:

Creating a color-consistent beam spot (far field);

Creating a color consistent screen visible to the user.

The size of the facets on the exit window is chosen below the resolution of a human eye, when viewed from a typical viewing distance (e.g. one meter) That is, the user cannot identify the individual micro-facets on the exit window under regular operation. Fig. 1 a schematically depicts an embodiment of a lighting unit 10 comprising a light source 100 and a light transmissive first window 200. The light source 100 is especially configured to provide polychromatic light source light 101 (such as blue + yellow, or blue + yellow and red, or blue + green + red) with a spatially inhomogeneous color distribution to an upstream face 210 of the light transmissive first window 200 (see also fig. Id). Individual rays of light are indicated with references lr. The light transmissive first window 200 or foil comprises a plurality of optical elements 230 configured to provide a beam of light 1 1 downstream of a downstream face 220 of the light transmissive first window 200. The shape of the optical elements 230 varies over the light transmissive first window 200 (see e.g. also fig. lh) to provide said beam of light 1 1 with a (more) homogenous color distribution at a predetermined distance d downstream from the downstream face 220 (see also fig. le) of the light transmissive first window 200. The opening angle of the beam 1 1 is indicated with θ (Θ/2 has been depicted). Reference O indicates the optical axis. References Al and A2 indicate cross-sectional areas of the upstream face 210 and a virtual plane at a distance d, respectively. The distance between the light source 100 and the first window is indicated with reference dl .

As indicated above, the optical elements 230 may be arranged at an upstream side or a downstream side or both the upstream side and downstream side of the window (first and/or second window, etc.). In fig. la and other figures, by way of example the optical elements are only displayed at a downstream side or downstream face. The invention is however not limited to such embodiments.

Fig. lb schematically depicts an embodiment of the lighting unit 10 wherein further at the predetermined distance d a light transmissive second window 1200 is arranged, comprising a plurality of optical elements 1230. This transmissive second window 1200 is configured to provide a beam of light 101 1 downstream of a downstream face 1220 of the light transmissive second window 1200. Here, the optical elements do not necessarily vary over the window, though e.g. for beam shaping this may nevertheless be the case. This embodiment may especially be used to create at the near field, i.e. at the second window 1200, a spatially homogenous light distribution, making the second window 1200 appear to have a spatially homogenous color (such as white) (when the lighting unit is in the on state). Of course, in the far field, here virtual plane 17 may appear to have substantially the same good color homogeneity. The second window 1200 has an upstream face 1210 directed to the first window 200 and a downstream face 1220, from which the beam of light, here indicated with referncelOl 1 emanates to the surrounding external from the lighting unit 10. The optical element 230, and optionally also the optical elements 1230, shape the light source light in the beam of light, respectively.

Fig. l c schematically depict two type of light sources 100, here solid state light sources 1 10. The solid state light source has a light exit face or die 1 13 from which light 1 1 1 may escape. This light may partially be converted by a wavelength converter 120, having an upstream face 121 directed to the solid state light source, and a downstream face 122. From the downstream face 122 converter light or wavelength converter light 125 may escape, in general having a wavelength that is longer than of the excitation light or light source light 1 1 1. The wavelength converter light 125, optional in combination with the solid state light source light 1 1 1 downstream of the wavelength converter 120, is indicated as the light source light 101. This light source light may thus be composed of different colors, which may lead to the color over angle problem. The wavelength converter 120 may be arranged on the die or at a non-zero distance (right).

Fig. I d shows that the light of the light source on the upstream face 201 may e.g. provide a circular color distribution with other values for CIE x and or y in the core, indicated here as a first domain 251 and around the core, here indicated as second domain 252. The CIE coordinates at point PI may substantially differ from those at point P2. This is schematically depicted in the right drawing, wherein on the x as the spatial position over the upstream face 201 is indicated, and on the y-axis a CIE coordinate, such as x or y (purely a schematic drawing). The first domain 251 is thus closer to the light source 100 than the second domain 252; the latter is more remote (from the optical axis thus also from the light source 100).

At distance the predetermined distance or in case of a two-foil system (as depicted in Fig. lb) the color distribution can again be evaluated. P71 and P72 will now have color points that differ less in x and/or y value than points PI and P2. For instance, assuming PI : xl ;yl =0,32;0,3 and P2: x2,y2=0,35;0,28 at the upstream face, then ΙΔχΙ + lAyl is 0,05. For instance, at the predetermined distance the following color coordinates may be found P71 : xl ;yl=0,33;0,29 and P72L x2,y2=0,34;0,29, with ΙΔχΙ + lAyl then being 0,01. Hence, a the largest differences in color points may substantially be reduced.

Fig. 1 f schematically depicts an embodiment wherein the lighting unit comprises a plurality of light sources 100. Then, especially the light sources 100 have a mean shortest distance p and the light sources 100 have a shortest distance dl to the first window 210, with dl/p<0.3. Fig. l g on the left side shows bad overlapping spots SI, S2 S3 or colors CI, C2, C3. First spot SI with color CI is much smaller than spot S3 with color C3. This implies that at the there is a spatially inhomogeneous distribution of the colors ranging from relatively more color C 1 in the center to relatively more color C3 at the edge of the spot. In contrast, the present invention allows a good overlap of the spots by varying the dimensions of the optical elements that provide the spots. The differences between the spot areas can be less than 10% at the predetermined distance. Hence, each spot has an area overlap with another spot of at least 90%. Assume two spots having areas at a distance d of 95 and 100 on top of each other, then the overlap of the latter with the former is 100% and of the former with the latter is 95%. Note that other shapes the circles may also be possible. Further, the spots are not necessarily co-centric. In the left figure in lg, the overlap between SI and S3 may be 100% (the entire surface area of SI overlaps with S3), whereas the overlap between S3 and SI may be only 10%) (assuming a 10 times smaller spot SI).

Reference dl indicates the distance between the light source 100 and the first window 200. The distance dl may especially be in the range of 0.1-25 mm, such as 1-8 mm.

Fig. lh schematically depicts some options to vary the optical elements 230, here very schematically depicted as prismatic structures, for instance with slightly changes facets 231 , like including convex (or concave) facets at some of the facets of the prismatic structures. This may broaden the beam escaping from these optical elements (see e.g. Figs. 2b and 2c). Reference γ indicates the top angle of the optical structures, reference h indicates the height and reference w indicates the width. Additionally or alternatively, one or more of these parameters may vary over the window 200 to provide the desired optical properties. Hence, the shape of the optical elements 230 varies over the light transmissive first window 200 in one or more of (i) the mutual angle (γ), a (ii) height-width ratio, and (iii) a shape of a facet. In this graph, especially the shape of the facets are varied from flat to curved.

By way of example, only three optical elements 230 have been indicated in this drawing, at each side of the optical axis O of the light source 100. Reference d2 indicates the off-axis distance along the transmissive window, calculated from the optical axis. The value of d2 at the edge is d2_max, which may e.g. be in the range of 0.2-50 mm, especially 0.5- 10 mm. Hence, reference d2_max indicates the edge of the transmissive window 200.

The optical elements 230 may include different facets, which are by way of example indicated as first facet fl and second facet f2. In fig. lh, the first facets fl may (re)direct the rays, indicated with reference(s) lr, via (direct) refraction, whereas the second facets may (re)direct the rays after total internal reflection (TIR) and refraction. Up to a value of about d2/dl=l, the first facets f 1 may be configured to redirect the light source light via direct refraction, i.e. a single refraction. At a value larger than about d2/dl = l, i.e. at angles of the rays lr with the light transmissive first window 200 smaller than about 45°, the first facets or refractive facets, may refract in the direction of the second facets f2, and after TIR (and refraction) the rays are redirected. Reference d2 indicates the distance along the light transmissive first window 200 calculated from the optical axis 0. Hence, d2=0 right above the light source 100 in this schematic embodiment. Especially, refractive facets may be arranged at one or more of the upstream face 210 and the downstream face 220, whereas TIR faces may only be arranged on the upstream face of the transmissive window 200.

Fig. li schematically depicts a further embodiment of the light transmissive first window 200, here with by way of example the optical elements 230 arranged at the upstream face 210 instead of the downstream face 220. Figs, lh and lh are only examples of possible embodiments. Alternative embodiments, such as shown below, or combinations thereof, etc., may also be possible to obtain the desired mixing.

Fig. 2a shows the problem definition: as the beamlets exiting the LED source under oblique angles (by way of example 5°) have a narrow beam spread compared to the normal incident beamlets (by way of example 20°) the beam spot created by collimation demonstrates color break-up. This is commonly observed in commercial applications.

Reference BW indicates the beam width; reference CI indicates a first color, such as blue, and reference C2 indicates a second color, such as yellow. The color temperatures are indicated, with at an outer domain (more remote from the light source) lower color temperatures than a more inner domain (closer to the light source).

Fig. 2a very schematically depict the optical structures as little squares. Of course, these optical elements may especially include facets at an angle with a base plane, such as prismatic structures.

Fig. 2b schematically shows an embodiment: beamlets are broadened as a function of position while the beamlet orientation remains parallel as in top. In the far field, a spectator will see a spot at the predetermined distance d a spot with a (more) homogeneous color distribution. Fig. 2b schematically shows two domains, a first domain 251 and two second domains 252 (which may surround the first domain 251). Note that there may be much more domains and that there may be a gradual variation. This also applies to the other embodiments described herein. In this way, a plurality of different colors (or color temperatures) may be addressed in an even more even way. Fig. 2b, but also other figures may e.g. relate to a square, rectangular or circular first window 200 (and optional second window (1200)). The length and or width (of the upstream face 210) may individually be selected, and may be indicated as d2_max (though the value for length and width may differ), or the diameter may be indicated as d2_max (see also the radius R in figs. 4, thereby assuming that figs. 2a-2e refer to systems with circular windows, centered around the optical axis).

Fig. 2c shows a further embodiment. The beamlets from the light source are collected onto a facetted foil. All beamlets are redirected such that the wall is uniformly illuminated by the individual parts of the foil. In the cross-section shown in Figure 2c at the exiting side of the foil we now have created two light sources that can illuminate the wall uniform in both intensity and color. In this case the beam spot can be broader than the original beamlet width. Hence, in this embodiment: beamlets are redirected as a function of position creating uniform illumination on a surface, such as a wall.

Fig. 3a shows a similar problem as depicted in Fig. 2a, but now with two windows. The second window receives the inhomogeneous light, leading to an exit window that shows a color distribution when the lighting device 10 is in operation. Hence, as the assignment of beamlets is non-crossing the color consistency on the second foil is not uniform.

Fig. 3b shows another solution. The second foil is illuminated with each of the three light sources from the first foil uniformly. In this manner we have broadened the beamlets exiting the second foil up to maximum 45°, that however is color uniform. For most (office) down lighters we need to avoid any light beyond 65° (glare angle), with respect to a normal to the second foil, which is easily reached with this solution. Here, a uniform illumination (intensity and color) in the exit beam and in the exit screen with a beam divergence of e.g. ~ 45° is obtained. Other angles are of course also possible.

Fig. 3 c shows a further option, analogous to fig. 2c. We illuminate the second foil with two of the three light sources from the first foil uniformly in intensity. In this manner we have broadened the exiting beamlets of the second foil up to a maximum of 26° that is color uniform. Here, a uniform illumination (intensity and color) in the exit beam and in the exit screen with a beam divergence of e.g. ~ 26° is obtained. Other angles are of course also possible.

Figs. 4a-4e substantially correspond to the schematic embodiments depicted in figs. 2a-2c and 3b-3c, respectively. Only part of the light transmissive first window 200, here the right part, has been depicted. Further, the rays, indicated with lr, are depicted. The light source is, for the sake of understanding, assumed to be a point source. On the x-axis the off- axis distance d2 is depicted, here by way of example the radius R. On the y-axis, the distance from the light source 100 to the window 200 is indicated. In fact, the y-axis is the optical axis. Further, by way of example the optical elements 230 are arranged at the upstream face 220. The way in which the rays lr are drawn is schematic. This implies that in some instances refraction is shown at positions where no optical elements 230 are depicted. The rays are only shown to indicate how the light transmissive first window 200 functions. In general, there will be much more optical elements 230 than schematically depicted and it is not possible to draw all these elements. Hence only a few are depicted.

As can be seen, the top angle β increases from a position of about d2/dl= 1 to d2 _max. The first facets fl may receive direct radiation from the light source 100. At least at values of d2/dl larger than about 1, an angle a between the first facets fl and a normal to the light transmissive first window 200 increases with increasing distance d2. This allows generation of a collimated beam, as shown in fig. 1 a.

In the invention however, this collimation is herein not desired, as the rays should overlap, at least at the predetermined distance (see above). Hence, a gradual increase of the angle a with increasing off-axis distance d2, starting at about d2/dl= 1 up to d2 _max, such as d2/dl is at least 1.2, is in one or more domains with a smaller angle a than at a shorter distance. This is shown in fig. 4b, wherein different a's have been depicted, wherein a increases with increasing off-axis distance d2, but locally some a's are smaller than expected based on the increase in a with increasing off-axis distance d2.

Fig. 4b schematically depicts this for the embodiment of fig. 2b in which both the tilt angle a and top angle β fluctuates in relation to the off-axis distance d2; fig. 4c schematically depicts this for the embodiment of fig. 2c. Both embodiments show how the spots of light can be mixed well (see the schematic drawings 2b and 2c). In fig. 4b the optical structures are chosen such that the beamlets are broadened, whereas in fig. 4c the beamlet direction is (also) changed. The variation in a is here also available, but less visible. In fig. 4b, a subset of the first facets having smaller angles a than (a) first facet at (a) shorter distance(s) is (are) schematically depicted. As is shown, this subset may include a plurality of facets. The angle a 2^nd and 4^th from left, are smaller than the angle alpha 1^st and 3^rd from left,

respectively. Hence there may be a plurality of optical elements having first facets having angles a being smaller than first facets arranged closer to the optical axis. Hence, though the angles a for those facets having an off-axis distance d2 in the range of dl -d2max, especially at least 1.2dl, may increase with increasing distance, a subset within the range have a smaller angle than expected on the base of this increase. Similar observations made for the tilt angle a can be made with respect to the top angle B.

Figs. 4d and 4e relate to figs. 3b and 3c, respectively. In both embodiments, the first domain(s) close(r) to the optical axis O of the light source 100 (i.e. at d2<dl), is relatively larger than those of the schematic embodiments of figs. 2b/4b and 2c/4c. Figs. 4d and 4e schematically show different ways of mixing. In fig. 4d (see also fig. 3b), the optical elements in the second domain 252 do also redistribute the light over the entire beam to provide at the predetermined distance (see fig. 3b) the homogeneous spot. In an enlarged detail of the optical elements (230) it is schematically shown that the tilt angle l of the optical elements is constant and independent of the distance d2, while the top angle B of the optical elements fluctuates (and on average increases) in relation to the distance d2, with Bl < B2 < B3 < B4. In fig. 4e, the optical elements in the second domain 252 do also redistribute the light over part of the beam that is also addressed by the first domains 251, to provide at the predetermined distance (see fig. 3c) the homogeneous spot. These effects (also in other embodiments) may be executed symmetrical at both sides of the optical axis (or Centro symmetric all around the optical axis O). Figs. 3b show that the facets in the second domain are especially such, that the angle a increases with increasing off-axis distance d2, with a subset of the facets having an angle a smaller than one or more facets in the second domain but at a shorter off-axis distance d2 (but still with d2 especially being at least dl, such as at least 1.2dl).

The differences in top angles and/or angle a between the first facets fl and a normal to the light transmissive first window 200 can be very small, as can also be seen in figs. 4d and 4e. However a slight deviation from the general trend of increasing angle a with increasing off-axis distance d2 may already correct the rays in such a way that the desired good overlap and thus color homogeneity is obtained.

Figure 4f it is schematically shown for an embodiment that the top angle Bl of the TIR optical elements is constant and independent of the off-axis distance d2, while the tilt angle a of the TIR optical elements fluctuates (and on average increases) along the distance d2, with al < a2 < a3 < a4 < a5. Figs. 5a-5c schematically depict some examples of a course of fluctuation in the tilt angle a and/or top angle B in relation to the off-axis distance d2.Fig. 5a in this respect shows a course in the fluctuating, gradual, smooth increase in top angle B via a curve 310, while the tilt angle a is constant over the off-axis distance d2 as is shown in curve 320. It is also shown that the curve 310 of the top angle B comprises at least three local maxima in the fluctuating value of B. In Fig. 5b, it is schematically shown that top angle B remains constant over the off-axis distance d2, see curve 310, and that the tilt angle a shows a course of a fluctuating, gradual, smooth increase along the off-axis distance d2, see curve 320. Figure 5c shows a smooth fluctuating curve 310 for tilt angle a which on average does not gradually increases, and an abruptly fluctuating curve 320 for top angle B with an increase in average top angle B from d2 is about 0.5*d2max to d2 is about d2max.

Claims

CLAIMS:

1. A lighting unit (10) comprising a light source (100) and a light transmissive first window (200), wherein the light source (100) is configured to provide polychromatic light source light (101) with a spatially inhomogeneous color distribution to an upstream face (210) of the light transmissive first window (200), wherein the light transmissive first window (200) comprises a plurality of optical elements (230) configured to provide a beam of light (1 1) downstream of a downstream face (220) of the light transmissive first window (200), and wherein the shape of the optical elements (230) varies over the light transmissive first window (200) to provide said beam of light (1 1) with a spatially homogenous color distribution at a predetermined distance (d) downstream from the downstream face (220) of the light transmissive first window (200),

wherein the optical elements (230) have one or more of a refractive functionality and a total internal reflection functionality to the light source light (101), wherein the optical elements (230) comprise a first facet fl and a second facet f2 mutually angled at a top angle B, wherein the first facet f 1 is oriented at a tilt angle a with a normal to the light transmissive first window, wherein the optical elements (230) are arranged within an off-axis distance (d2) in the range of 0-d2max, with off-axis distance (d2) being the distance relative to an optical axis (O) of the light source (100) measured along the light transmissive first window (200), and wherein the top angle and/or tilt angle have a fluctuating value along the off-axis distance (d2).

2. The lighting unit according to claim 1, wherein only the top angle or tilt angle of the TIR optical elements have a fluctuating value along the off-axis distance (d2).

3. The lighting unit according to claim 1 or 2, wherein the fluctuation of a and/or B is within a range of ± 15% of a local average value.

4. The lighting unit according to claim 1, 2 or 3, wherein an average value of the tilt angle and/or top angle has increased from off-axis distance d2 = 0.5*d2max to d2 = d2max.

5. The lighting unit according to claim 1, 2, 3, or 4, wherein the top angle and/or tilt angle have at least three local maxima in their respective fluctuating value.

6. The lighting unit (10) according to claim 1, 2, 3, 4 or 5, wherein the light transmissive first window (200) is arranged at distance (dl) from the light source (100) selected from the range of 0.1-25 mm, wherein the light transmissive first window (200) comprises a foil, and wherein the predetermined distance (d) is selected from the range 0.2- 50 m.

7. The lighting unit (10) according to any one of the preceding claims, wherein the ratio of the color difference at the upstream face (210) and at the predetermined distance (d), defined as (Au'v')_u/(Au'v')_d is larger than 1.2.

8. The lighting unit (10) according to any one of the preceding claims, wherein at the predetermined distance (d) a light transmissive second window (1200) is arranged, comprising a plurality of optical elements (1230) configured to provide a beam of light (101 1) downstream of a downstream face (1220) of the light transmissive second window (1200), wherein the predetermined distance (d) is selected from the range of 1 - 100 mm.

9. The lighting unit (10) according to any one of the preceding claims, wherein two bordering facets of adjacent optical elements (230) are angled at a mutual angle (γ), wherein the optical elements (230) have a height (h) and a width (w), and wherein the shape of the optical elements (230) varies over the light transmissive first window (200) in one or more of (i) the mutual angle (γ), a (ii) height-width ratio, and (iii) a shape of a facet.

10. The lighting unit (10) according to any one of the preceding claims, wherein the light transmissive first window (200) is arranged at a distance (dl) from the light source (100), wherein the optical elements (230) are arranged within an off-axis distance (d2) in the range of 0-d2_max, with off-axis distance (d2) being the distance relative to an optical axis (O) of the light source (100) measured along the light transmissive first window (200), wherein a first domain (251) with optical elements (230) is configured within an off-axis distance (d2) from the optical axis (O) of d2/dl=l, and wherein a second domain (251) is configured within a distance (d2) of the optical axis (O) selected from the range of d2/dl= 1 to d2 _max.

1 1. The lighting unit (10) according to any one of the preceding claims, wherein the optical elements (230) are arranged within a distance (d2) in the range of 0-d2_max, with d2 being the off-axis distance relative to an optical axis (O) of the light source (100) measured along the light transmissive first window (200), wherein the optical elements (230) comprise first facets (fl) which receive direct radiation from the light source (100), wherein at least at values of d2/dl larger than about 1, an angle a between the first facets fl and a normal to the light transmissive first window 200 increases with increasing off-axis distance (d2), with a subset of the first facets having smaller angles a than a first facet at a shorter off-axis distance (d2).

12. The lighting unit (10) according to any one of the preceding claims, comprising two or more domains (251,252, ...), each domain including one or more of said optical elements (230), with mutually different shapes of the optical elements (230) between the 251 ,252, ...), wherein the two or more domains (251 ,252, ...) are configured to provide overlapping spots (S1,S2, ...) of light (11) at the predetermined distance (d), with the beam of light (1 1) at the predetermined distance (d) being composed of said spots (S1,S2, ...), wherein each spot has an area overlap with another spot of at least 90%.

13. The lighting unit (10) according to any one of the preceding claims, wherein the each having a virtual plane (7) at the predetermined distance (d) downstream from the downstream face (220), wherein a first domain (251) with a plurality of the optical elements (230) closest to the light source (101) distributes light source light (101) over 90% or more of the area of virtual plane (7) and wherein one or more second domains (252) further away from the light source (101) also distribute light source light (101) over 90% or more of the area of virtual plane (7).

14. The lighting unit (10) according to any one of the preceding claims, wherein the source unit (100) comprises a solid state light source (1 10) configured to provide blue solid state light source light (1 1 1) and a wavelength converter (120) configured to convert part of the blue light (1 1 1) into wavelength converter light (125) having a larger wavelength, whereby the light source light (101) comprises said solid state light source light (1 1 1) and said wavelength converter light (125).

15. Use of the light source (100) and the transmissive window (200) as defined in any one of the preceding claims, to provide a beam of polychromatic light (1 1,101 1) with a spatially homogenous color distribution in the near field or far field while maintaining degree of collimation of the beam of light (1 1).