FIELD OF THE INVENTION

The present invention relates to lighting devices having light sources and an optical element arranged to transmit light emitted by the light sources, and to manufacturing of such lighting devices.

BACKGROUND OF THE INVENTION

The development of new and more energy efficient illumination devices is one of the important technical challenges which society faces. Technologies based on light emitting diodes (LEDs) are normally more energy efficient than traditional lighting solutions. Currently, the reduced cost, improved performance and small size of LEDs facilitates integration of LEDs as a retrofit in lighting fixtures commonly used today.

Optical elements may be used in lighting devices for shaping, redirecting, or in any other manner influence, the light output of the lighting device. For example, such an optical element may be used to transmit light towards a wavelength converting element comprised in the lighting device. Wavelength converting elements are normally used in order to adjust the color of light output by the lighting device. The wavelength converting element comprises wavelength converting material, such as phosphor, which absorbs photons in a particular wavelength spectrum and emits photons in another wavelength spectrum, thereby changing the color of the light first emitted by a light source. Light transformed into a longer wavelength will result in light having a warmer appearance. For example, phosphor may be used to convert a portion of blue light emitted by an LED into yellow light, thereby providing a white light output.

It may be desirable to influence the light output by the optical element e.g. for influencing the optical load on the wavelength converting element and/or the homogeneity of the light distribution of the lighting device (such as the spectral and spatial distribution of the light output). The lifetime of the wavelength converting element may be affected by the optical load on the wavelength converting element produced by the light source. The optical load affects the temperature in the wavelength converting element and thereby affects the lifetime of the wavelength converting element. SUMMARY OF THE INVENTION

It would be desirable to influence the light output by an optical element of a lighting device. In particular, it would be advantageous to adapt the optical element with regard to an actual light output from the light sources of a lighting device.

To better address these concerns, a lighting device and a method of manufacturing a lighting device having the features defined in the independent claims are provided. Preferable embodiments are defined in the dependent claims.

According to a first aspect, a lighting device is provided. The lighting device comprises a plurality of light sources arranged in an array, and an optical element arranged to transmit light emitted by the light sources. The optical element is adapted (which also may be referred to as customized in the present specification) with respect to a deviation of at least one actual light emission property of each light source from a default value of the light emission property and with respect to a system model, which represents a sensitivity of at least one required system property of the lighting device for variations in the at least one light emission property of the light sources, so as to at least partly compensate for the deviation.

According to a second aspect, a method of manufacturing a lighting device is provided. The method comprises providing a plurality of light sources in an array, measuring at least one light emission property of each light source, calculating a deviation of the measured light emission properties of the light sources from a default value of the light emission property, and adapting at least one optical property of an optical element adapted to transmit light emitted by the light sources. The adaptation of the optical property is made with respect to the calculated deviation and with respect to a system model of the lighting device, so as to at least partly compensate for the deviation. The system model represents a sensitivity of at least one required system property of the lighting device for variations in the at least one light emission property of the light sources.

The present aspects are based on the idea of customizing the optical element to the actual light sources arranged in the array with respect to the system model of the lighting device. The light emission properties may slightly vary between different light sources and/or with respect to default values, e.g. as a result of manufacturing tolerances. With the present aspects, deviations in the light emission properties of the light sources resulting from such variations are at least partly compensated by the optical element. The optical element controls light emitted by the light sources and thereby influences the light distribution of the lighting device. By customizing the optical element, it is possible to obtain a light distribution adapted to the actual light emission of the light sources used in a particular device and with regard to aspects of the required system properties as indicated by the system model. For example, it may be possible to adjust the optical element to (at least partly) compensate for irregularities in the light distribution output by the light sources so as to obtain a more homogenous light distribution from the optical element, and thereby also from the lighting device.

In the context of the present specification, an "array" should be understood as any arrangement of a plurality of light sources, such as according to any desired pattern.

According to an embodiment, the lighting device may further comprise a wavelength converting element, wherein the optical element may be arranged to transmit light emitted by the light sources towards the wavelength converting element. A more homogenous light distribution transmitted by the optical element results in a more even optical load on the wavelength converting element. Peaks in the optical load on the wavelength converting element result in peaks in the temperature distribution in the wavelength converting element. As high temperature reduces the life time of the wavelength converting material (such as phosphor) in the wavelength converting element, the lifetime of the wavelength converting element may be reduced by such peaks. Hence, the present embodiment is advantageous in that the lifetime of the wavelength converting element is extended. Further, a more even optical load on the wavelength converting element provides a more uniform light output from the lighting device.

According to embodiments, the at least one required system property may be at least one of: a desired light intensity distribution output by the optical element, a required temperature distribution in the wavelength converting element, a desired aging rate of the wavelength converting material (such as of the phosphor) in the wavelength converting element, and an optical load on the wavelength converting element. Different required system properties may (or may not) have different degrees of sensitivities for variations in the light emission properties of the light sources. The required system properties may be predefined. An optical load on the wavelength converting element may partly determine the temperature distribution in the wavelength converting element, and also the light output of the lighting device. Thus, customizing the optical element with regard to the optical load on the wavelength converting element enables a longer lifetime and a desired light output.

According to another embodiment, the at least one light emission property may be at least one of: peak wavelength, dominant wavelength, full width half maximum, radiometric power, photometric power (which also may be referred to as luminous flux), color point, strength, intensity and spectrally resolved intensity. Optionally, the distribution of power across the wavelength spectrum may be measured or calculated from measurements. Variations in any one of these properties may potentially result in a variation in the light output by the lighting device and, in case a wavelength converting element is used, the optical load on the wavelength converting element, which may influence the temperature and aging of the wavelength converting element. With the present embodiment, the optical element is customized so as to compensate for a potential deviation in at least one of these properties. Measuring the strength of each light source may include measuring the power (radiant flux) of each light source, which e.g. may be measured in Watt. The intensity of the light source, i.e. the light produced per angle or area unit, may e.g. be measured in candela or lux.

According to an embodiment, the system model of the lighting device may be based on a theoretical simulation, e.g. of one or more components of the lighting device. Hence, the system model may not necessarily be obtained by direct measurements on the system of the lighting device. The theoretical simulation may be made prior to manufacturing of the lighting device, which may facilitate the customization procedure. The theoretical simulation may comprise ray tracing. Ray tracing is a technique used to simulate and trace the rays of light theoretically emitted by a light source by using simulation software. Data from such a simulation may be used to customize the optical element.

According to an embodiment, the system model may be based on measurements of the spectral properties of the wavelength converting element. The measurements of the spectral properties of the wavelength converting element may not necessarily be made more than once for a particular type of lighting device (i.e. not necessarily each time a new lighting device is manufactured). As the system model is based on such measurements, it may be more accurately adapted to the wavelength converting element.

For example, the system model may be based on measurements of a particular setup of light sources and a wavelength converting element, which may be referred to as a heuristic model. The light sources may be variably (and individually) controlled with respect to the light emission property, and a resulting variation in the required system property (e.g. optical load on, or temperature in the wavelength converting element) may be detected. The system model may then be based on the detected variations.

The at least one optical property of the optical element may be customized (so as to at least partly compensate for the deviation) by customizing one or more optical features of the optical element. Optical features may be any features of the optical element adapted to act upon (such as shape, diffuse, reflect and/or redirect) light transmitted by the optical element. In the following, several embodiments of how the optical element may be customized will be described.

According to an embodiment, the optical element may comprise a white dot pattern, and the optical property of the optical element may be customized by the white dot pattern being customized. Hence, customizing the at least one optical property of the optical element may comprise forming a white dot pattern on the optical element. By forming a white dot pattern on the optical element, it is possible to use shadowing effects to create a more even light distribution output by the optical element and, in case a wavelength converting element is used, a more even optical load on the wavelength converting element. An advantageous shadowing effect may be referred to as the half shadowing effect and may occur when the light sources are relatively extended (i.e. not point-like) sources of light, and the pitch of the white dot pattern is relatively small compared to a projected size of one light source on the wavelength converting element (or any other optics arranged to transmit light output by the optical element) when projected through a pinhole located at a position of a white dot. Shadows and light projections on the wavelength converting element from different points of the light source resulting from the dot pattern (i.e. shadows results from the dots and light projections results from the areas of the optical element between the dots) may average out each other, whereby the wavelength converting element is exposed to a more even optical load (or wear) and, thus, age more evenly. Thus, a white dot pattern of the optical element may be customized for creating the half shadowing effect, which may limit (or even out) the optical load on desired areas of the wavelength converting element.

Customizing the optical properties of the optical element may comprise adjusting the white dot pattern with respect to at least one of the parameters: dot size, dot pitch, the distance between a dot and the wavelength converting element, and the density of the dot pattern (i.e. dots per area unit).

According to an embodiment, the optical element may comprise scattering structures within the bulk of the optical element, and the optical property of the optical element may be customized by the scattering structures being customized. Hence, customizing the at least one optical property of the optical element may comprise forming scattering structures within the bulk of the optical element. Scattering structures within the bulk of the optical element enables scattering of light transmitted through the optical element and may thereby influence light output by the optical element and, in case a wavelength converting element is used, the optical load on the wavelength converting element, such as making it more uniform. Further, the scattering structures may be adapted so as to provide shadowing effects and, preferably, the half shadowing effect.

According to an embodiment, the optical element may comprise embossed prismatic structures, and the optical property of the optical element may be customized by the embossed prismatic structures being customized. Hence, customizing the at least one optical property of the optical element may comprise forming embossed prismatic structures in the optical element. The prismatic structures may reflect light for certain angles of incidence and thereby influence light output by the optical element and, in case a wavelength converting element is used, the optical load on the wavelength converting element. The prismatic structures may be adapted (e.g. with respect to prismatic structure size and pitch, and the distance between the prismatic structure and the wavelength converting element) so as to produce a shadowing effect and, preferably, the half shadowing effect.

In an embodiment, the method may comprise locally flattening out prismatic structures embossed in the optical element so as to create the desired optical properties of the optical element. Hence, prefabricated prismatic structures may be adjusted based on the calculated deviation and the system model, whereby configuration of the optical element is enabled in a late stage of the manufacturing procedure. Locally flattening out the embossed prismatic structures may e.g. be made by means of laser.

According to an embodiment, the optical element may be an optical foil, whereby the size of the lighting device may be reduced (e.g., the lighting device can be made thinner).

According to an embodiment, the lighting device may be a tube-type, light emitting diode based lighting device (also referred to as a TLED).

Further features of, and advantages with, the present aspects will become apparent when studying the appended claims and the following description. It is noted that embodiments of the invention relate to all possible combinations of features recited in the claims. Further, it will be appreciated that the various embodiments described for the lighting device are all combinable with the method as defined in accordance with the second aspect of the present invention. Those skilled in the art realize that different features of the present invention can be combined to create embodiments other than those described in the following.

BRIEF DESCRIPTION OF THE DRAWINGS These and other aspects will now be described in more detail, with reference to the appended drawings showing embodiments.

Fig. 1 is a perspective view of a lighting device according to an embodiment.

Fig. 2 is a flowchart illustrating a method for manufacturing a lighting device according to an embodiment.

All the figures are schematic, not necessarily to scale, and generally only show parts which are necessary in order to elucidate the invention, wherein other parts may be omitted or merely suggested. DETAILED DESCRIPTION

A lighting device according to an embodiment will be described with reference to Fig. 1. The lighting device 100 comprises light sources 102 arranged in an array, an optical element 104 and, optionally, a wavelength converting element 106. The optical element 104 may be arranged between the light sources 102 and the wavelength converting element 106. The light emitted from the light sources 102 propagates towards an inner surface 108 of the optical element 104 and is transmitted through the bulk 110 of the optical element 104 to the wavelength converting element 106. The wavelength converting element 106 may comprise wavelength converting material, such as phosphor or quantum dots, for converting a wavelength of light transmitted by the wavelength converting element 106. The optical element 104 and the wavelength converting element 106 may be separate components or integrated in a single component of the lighting device 100. In the present example, the lighting device 100 is a tube-type lamp. However, the lighting device may alternatively be of any other type comprising several light sources. The light sources 102 may be light emitting diodes (LEDs) and the optical element 104 may optionally be an optical foil. An optical foil may e.g. be a relatively thin light transmissive sheet.

The optical properties of the optical element 104 may be customized based on one or more actual light emission properties, such as a strength and/or at least one property of a wavelength spectrum, of each individual light source 102 with respect to a default

(predetermined) value of each light emission property, and on a system model, so as to compensate for eventual deviations in the actual light emission properties of the light sources 102 from default values. The system model represents (e.g. defines) the sensitivity of at least one required system property of the lighting device 100 for variations in the light emission property of the light sources 102. The optical element 104 may preferably be customized such that the wavelength converting element 106 receives a substantially uniform optical load, whereby the wavelength converting element 106 reaches substantially the same maximum temperature at any point illuminated by the light sources 102 via the optical element 104. Further, a substantially uniform optical load on the wavelength converting element 106 may result in a substantially uniform luminance from the lighting device 100.

For example, a white dot pattern on the inner surface 108 of the optical element 104 may be customized based on the one or more actual light emission properties of each individual light source 102 with respect to a default value of that light emission properties, and on the system model. The white dot pattern may be customized regarding dot size, dot pitch, and/or dot density so as to obtain a shadowing effect where the dots cast shadows on portions of the wavelength converting element 106. Preferably, the white dot pattern may be adapted so as to produce the half shadowing effect for evening out the optical load on the wavelength converting element 106. The dot pattern may e.g. be adjusted so as to limit the optical load on certain portions the wavelength converting element 106 to compensate e.g. for a higher strength of a particular light source 102, which otherwise would have caused an increased optical load on that portion of the wavelength converting element 106. Furthermore, the distance from the white dot pattern on the inner surface 108 to the wavelength converting element 106 may be adjusted by customizing the thickness of the optical element 104 and/or by any adjustment of the distance between the dots and the wavelength converting element 106. The white dot pattern may e.g. be produced by a programmable printer.

Alternatively (or as a complement), the optical element 104 may be customized by having scattering structures within the bulk 110 of the optical element 104, and/or embossed prismatic structures in the optical element 104 being adapted based on the actual light emission properties of the light sources 102 and the system model. Any optical property, which may influence the light output by the optical element and, optionally, the optical load on the wavelength converting element 106, may be customized based on the actual light sources 102 and the system model.

A method of manufacturing the lighting device 100 (as illustrated in Figure 1) according to an embodiment will be described with reference to Fig.2.

The method 200 comprises providing 201, such as arranging, a plurality of light sources 102 in an array, e.g. by mounting the light sources 102 to a circuit board.

Further, at least one light emission property of each light source 102 is measured 202. The measuring 202 may be made either prior to or after arrangement 201 of the light sources 102 in the array. The at least one light emission property may be a property of a wavelength spectrum of each light source 102, such as a peak wavelength, full width half maximum, radiometric power, photometric power and/or color point of the wavelength spectrum of the light source 102. Alternatively, or additionally, the at least one light emission property may be the strength (power) of each light source 102, the light intensity and/or the spectrally resolved intensity of the light sources 102. Subsequently, a deviation of the measured light emission properties of the light sources 102 from corresponding default values of the light emission properties is calculated 203. The default values may be predetermined based on the light source type to be used.

Further, the optical properties of the optical element 104 are customized 204 (such as by adjusting an optical feature of the optical element, e.g. a white dot pattern, scattering or prismatic structures of the optical element 104, as previously described) based on the calculated deviation and on a system model of the lighting device 100, so as to at least partly compensate for the calculated deviation. Customizing the optical properties may comprise originally forming optical structures (or features) or adjusting prefabricated optical structures of the optical element 104. For example, prefabricated embossed prismatic structures may be locally flattened out as desired, e.g. by means of a computer controlled laser.

The system model indicates a sensitivity of at least one required system property of the lighting device 100 for variations in the at least one light emission property of the light sources 102. The system model may be used to evaluate to which extent the optical properties of the optical element 104 need to be modified (e.g. compared to a default configuration of the optical element 104) to obtain a desired system property with the actual light sources in the array (which may possibly have actual light emission properties deviating from the default values). System properties being more sensitive for certain variations in the light from the light sources 102 may require more extensive adjustments of the optical element 104 for a certain degree of variation of the light from the light sources 102.

The required system property may be expressed as a desired light intensity distribution output by the optical element 104, a required temperature distribution in the wavelength converting element 106, a desired aging rate of the phosphor in the wavelength converting element 106, and/or a desired optical load on the wavelength converting element 106.

Further, the system model of the lighting device 100 may be based on a theoretical simulation, e.g. based on ray tracing. Alternatively, or as a complement, the system model may be based on measurements of the spectral properties of the wavelength converting element 106. For example, the system model may be obtained by simulating ray tracing/measuring the spectral properties of the wavelength converting element for different predetermined strengths and properties of a wavelength spectrum in order to estimate the sensitivity of the required system property.

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. For example, the light sources may be any kind of light sources such as lasers, flash lamps, Xenon lamps or even X-ray sources.

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 measured cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.