Light source carrier for illuminating a TOF camera

Abstract

Light source carrier (15) for a lighting module with several light sources (10) arranged on the light source carrier (15), wherein the light sources (10) are covered with a potting compound (12), and a phase shift of an emitted light for each light source can be individually adjusted via a thickness of the potting compound (12), wherein the thickness of the potting compound (12) is adjusted in such a way that phase differences of the emitted light of the light sources (10) are compensated.

Description

[0001] The invention is based on a light source carrier for a lighting device according to claim 1.

[0002] The object of the invention is to improve the performance of a ToF system.

[0003] The problem is solved by the light source carrier according to the invention.

[0004] Advantageously, a light source carrier for a lighting module is provided, comprising a plurality of light sources arranged on the light source carrier, wherein the light sources are covered with a potting compound, wherein a phase shift of an emitted light for each light source can be individually adjusted via a thickness of the potting compound, wherein the thickness of the potting compound is in each case adjusted such that phase differences of the emitted light of the light sources are compensated.

[0005] This approach allows phase differences, which occur, for example, due to varying installation positions of the light sources, to be precisely compensated so that the emitted light has a uniform phase and thus improves the performance of the TOF system.

[0006] The invention is explained in more detail by the figures.

[0007] They show schematically Fig. 1 a cross-section of a VCSEL / light source chip with contact wires to the SMD housing interior, Fig. 2 an advantageous light source arrangement, Fig. 3 a layout for the arrangement according to Fig. 2, Fig. 4 a multilayer printed circuit board with a built-in VCSEL and a potting compound according to the invention, Fig. 5 a radiation profile of a planar and a curved circuit board, Fig. 6 a structure with several planar light source-equipped circuit boards.

[0008] The idea of ​​production is as follows: A) The lasers / VCSELs are mounted as chips on a possibly flexible carrier material. B) The lasers are mounted as chips recessed onto a carrier material. C) The lasers are embedded as chips and mounted on a carrier material protected by transparent encapsulation material. D) The lasers are mounted as chips on a substrate at a distance that allows eye safety even without a diffuser. The spacing takes into account the bending radius and the specific properties of the VCSEL arrays used. E) The lasers are mounted as chips on a carrier material, with the electrical leads to single or multiple light sources being of equal length, so that signal delays are calculated to be less than 25 ps. F) Remaining runtime differences that may be expected due to fluctuations in the components can be measured during the final inspection and, if necessary, compensated by adjusting the thickness of a potting compound. G) The lasers are mounted as chips on a carrier material, whereby (directly or indirectly) by convection the heat distribution 0.1 to 1 W / cm 2 can be achieved.

[0009] LEDs, edge-emitting lasers and, more recently, mainly VCSELs are used as light sources for ToF.

[0010] The principle that all of these light sources have in common is that the heat generated must be dissipated so that the light source does not overheat.

[0011] Eye safety requirements must also be met when using these light sources. This is achieved either by limiting the driver current and / or by using optical components such as diffusers. For example, the laser source size is virtually enlarged using diffusers.

[0012] In the case of VCSELs, several watts of light output are used to illuminate the space to be scanned. This high light output is achieved by large arrays with a large number of individual emitters located on a small chip area.

[0013] For applications requiring high levels of light, several of these modules may be used. This can quickly lead to problems with delay times in the electrical control system.

[0014] Fig. Figure 1 shows a cross-section of a VCSEL chip with contact wires to the SMD package interior. The light emitted by the VCSEL is spread by the diffuser. The diffuser is very critical for eye safety and is typically held in place by a clamp.

[0015] To ensure eye safety, VCSELs with lower power are also used.

[0016] Another known prior art approach involves detaching thin VCSEL elements from the GaAs substrate using a special stamping process and then transferring them onto a third-party substrate. The transfer process is parallelized, and the VCSELs are placed on the third-party substrate with high alignment accuracy. The tolerance is typically in the micrometer range, allowing subsequent conductor patterning and, if necessary, microlens processing on the third-party substrate.

[0017] There are several disadvantages associated with the SMD-based solution.

[0018] The use of AIN SMD component packages dominates the overall price of the module. The package cost is very high, as the heat from the SMD package must be further distributed on the outside so that, for example, convection can dissipate the heat into the room / exhaust air.

[0019] A similar problem also exists in the electrical control (driver), since the fast power transistors that can be used have relatively high losses and their waste heat must be distributed by a small transistor chip.

[0020] Since the light source chips require increasingly higher electrical currents with increasing output power, while simultaneously requiring short switching times for modulating the light output (e.g., turn-on and turn-off times of less than 2 ns), this can only be achieved to a limited extent with power electronics. The drivers must be operated with high power dissipation and, at the same time, positioned close to the light source, which also generates a lot of waste heat. This leads to further local thermal stress, which must be distributed accordingly.

[0021] Some solutions currently being presented use an integrated driver, which is even placed within the SMD housing.

[0022] The driver electronics used, which are usually based on GaN, are again a significant cost factor.

[0023] The use of single chips with high optical output powers means that the resulting light is not only emitted from a small area but can also be refocused onto a small area. This poses significant problems when it comes to ensuring eye safety from light radiation and requires various protective measures. These include the driver current, or rather the temporal profile of the driver current pulse, as well as other optical components that scatter the generated light.

[0024] To overcome the disadvantages of state-of-the-art solutions, a scalable, eye-safe illumination source is proposed that can be realized using available standard technologies.

[0025] Fig. Figure 2 schematically shows a solution consisting of a light source, in this case a VCSEL, a driver (e.g., a transistor IC), and electrical lines (e.g., a trace on a PCB). The proposed layout allows for the most even distribution of light output across the PCB / substrate surface, optimizing eye safety. Furthermore, the heat output is evenly distributed across the PCB / substrate surface and can be easily dissipated.

[0026] Furthermore, the conductor tracks to the VCSELs are of equal length, so that in particular in each unit cell EZ the length of the conductor tracks to each emitter is the same.

[0027] The idea also includes the use of symmetry elements to achieve these goals. However, it would also be conceivable to adjust track lengths, etc., by deliberately extending individual tracks or creating detours. Furthermore, the use of symmetry elements achieves a high level of uniformity.

[0028] The symmetry elements consist of so-called unit cells (EZ), each of which builds upon the other. The basic type is designated "EZ0" and contains only one light source element (here, a VCSEL) and one driver transistor in this design.

[0029] The selection of a suitable unit cell with an ideal VCSEL chip enables a cost-effective and practical implementation.

[0030] Using the design of the basic type "EZ0", the next higher-level composite single cell "EZ1" can be designed. For this purpose, the individual cells are combined, as in crystal physics, using symmetry elements such as reflection, rotation, or translation. This essentially corresponds to the approach of Bravais lattices in quasi-2-dimensional space, which in the simplest case corresponds to a planar surface, but can also be a curved, arched, or regularly structured plane. This surface structuring can be used to shape the radiation characteristics, as it has a focusing effect as a concave mirror, or a defocusing effect with the opposite curvature, for example to illuminate a surface homogeneously and over a large area.

[0031] In the example above, only the conductor tracks for the modulation signal, which are primarily relevant for signal propagation times, were represented in "EZ0." The connection point of this electrical line to the next higher unit cell, "EZ1," is located at a corner point of the EZ0 surface. The connection point from "EZ1" to the next higher unit cell, "EZ2," was also placed at a similar corner point, and so on. Numerically, this corresponds to a type of recursion similar to the patterns in Hilbert curves. Since the driver current increases as the light sources are interconnected, above a certain current level, the need for an additional driver stage arises, as indicated in EZ2.

[0032] Depending on the specific application's conditions, the optical power density can be adjusted by selecting the spacing or size of the unit cells. In addition to eye safety, these conditions can also include temperature parameters or power dissipation, optical output power values, and radiation characteristics. Thermal power dissipation density can be verified using appropriate test boards, etc., or suitable simulation.

[0033] Fig. Figure 3 shows a schematic example layout for EZ2. In addition to the diamond-shaped area representing EZ2, additional components for power and signal supply were integrated.

[0034] By using chip-on-board (CoB) technology, chips (lasers, transistors) can be mounted very densely on the circuit board or similar substrate at low cost. To prevent damage to bond wires, for example, a potting compound is applied over the components and their connecting wires. Alternatively, recessed chips are possible in certain layers of the PCB board or corresponding substrate. This can also be combined with bending the substrate if necessary.

[0035] In Fig. Figure 4 shows an example of a multilayer, preferably flexible, printed circuit board with encapsulated components. The printed circuit board is constructed in several layers, with an upper layer, middle layers S1, S2, and a lower layer. In the example shown, the light source is installed in a recess in the area between layers S1 and S2, which is filled with a transparent encapsulating compound for protection.

[0036] In principle, the light source can also be arranged on a surface of the circuit board and covered there with a potting compound.

[0037] For production reasons, despite identical line lengths, small delay differences may occur due to component fluctuations in the phase control of the individual emitters.

[0038] Since the solution according to the invention consists of switchable individual emitters, it is possible, for example, to carry out a functional check after the assembly of such a circuit board and to also eliminate the phase differences of the individual emitters.

[0039] Typical potting compounds, for example, have a refractive index of approximately n=1.4 and naturally delay the light propagation time. According to the invention, the phase position of the individual VCSELs can be automatically measured / evaluated during production by measuring the phase values ​​of the light sources / VCSELs. Individual quantities for dispensing the potting compounds can then be calculated, so that the phase differences in the emitted light between the light sources can be compensated for by applying a specific thickness of the potting compound.

[0040] It is particularly advantageous to use the casting compound to partially focus the light in the form of a lens effect.

[0041] A VCSEL typically has a beam angle of 20°. The curved emitter according to the invention enables the realization of an illumination module with significantly higher divergence. Some areas of the scene can be illuminated by multiple VCSELs in an overlapping manner.

[0042] For some applications, it may be advantageous if each individual VCSEL chip emits light with reduced divergence, for example, to selectively switch off specific areas. For this purpose, the encapsulant can be applied in a slightly curved, lens-like manner.

[0043] In order to be able to bend the circuit board, either a flexible carrier material is used, or a less flexible carrier material is deliberately weakened in certain places to enable bending at these points.

[0044] The encapsulation compound used is a transparent protective coating, which should have low optical losses and good mechanical properties. The chips and, if applicable, ICs are encapsulated as in Fig. 4, placed in a lower layer, thus providing mechanical protection. The VCSEL emits light upwards and is connected to layer S1 via bond wires. Since PCB solutions with integrated waveguides and steel splitters or tilted mirror surfaces are also available, concepts with edge-emitting lasers, for example, can also be implemented.

[0045] With a flexible support, the radiation profile can preferably be adjusted via the bend.

[0046] Fig. Figure 5 shows the radiation profile of a planar and a curved circuit board. The bending radius does not necessarily have to be constant, but could also resemble a parabolic or elliptical profile, for example, to homogeneously illuminate image areas near the edges (large beam angles). A "negative" bending radius is also conceivable, which acts as a kind of concave mirror to partially focus the light cones of the individual emitters.

[0047] In the designed example, for example, a minimum spacing of approximately 2 mm between the VCSELs is required to ensure eye safety. Designs can also be used that achieve a significantly greater spacing between the individual emitters for thermal reasons.

[0048] In a further embodiment of the invention, the encapsulation is applied specifically to achieve a lens effect. Furthermore, the shape of the structure can be achieved by bending the carrier substrate.

[0049] Fig. Figure 6 shows a structure according to the invention with a flat carrier plate 20, which can be arranged, for example, on a heat sink. The supply electronics with capacitors and other components can be arranged on the carrier plate 20, for example. The VCSEL carriers 15, which are oriented in different spatial directions, are connected to the supply voltage via supply pins 18. The supply pins 18 are preferably designed such that they can also be used to dissipate heat from the VCSEL or the VCSEL carrier 18.

[0050] This arrangement also allows the VCSEL carriers 15, and thus the VCSELs themselves, to be arranged differently in space, thus achieving a desired light distribution. Furthermore, the VCSEL carriers 15 can also be designed to be flexible, as previously described, allowing for further degrees of freedom in beam shaping due to the possible bending radii.

[0051] With such an arrangement, and in particular with the aid of flexible VCSEL carriers 15, the VCSELs can, if necessary, be arranged on the carrier substrate at a distance greater than the distance permissible for eye safety. Bending the carrier substrate ensures that the light is visible in the far field in such a way that a permissible distance for eye safety is maintained.

[0052] By distributing the VCSELs over a larger area, self-heating can be advantageously reduced and heat can be dissipated more effectively.

[0053] As already described, the light distribution and phase can be adjusted in the desired manner by targeted application of the casting compound.

[0054] Preferably, the VCSEL carrier 15 together with the supply lines 18 can be manufactured as a common carrier component, so that the flat carrier substrate can be equipped with prefabricated carrier components during production.

[0055] The procedure according to the invention was illustrated using VCSELs, but in principle the structure according to the invention can also be realized with other light sources, in particular with LEDs or edge-emitting laser diodes. List of reference symbols 10 Light source, VCSEL 12 filling 15 VCSEL carriers 18 support pins 20 flat carrier plate

Claims

[1] Light source carrier (15) for a lighting module with several light sources (10) arranged on the light source carrier (15), wherein the light sources (10) are covered with a potting compound (12), and a phase shift of an emitted light can be individually adjusted for each light source via a thickness of the potting compound (12), wherein the thickness of the potting compound (12) is adjusted in such a way that phase differences of the emitted light of the light sources (10) are compensated. [2] Light source carrier (15) according to claim 1, wherein the light sources (10) are arranged in recesses of the light source carrier (15). [3] Light source support (15) according to one of the preceding claims, wherein the light source support (15) is bent to provide a desired light distribution. [4] Light source support (15) according to claim 3, wherein the light sources (10) are arranged at a distance on the light source support (15) which is greater than a distance permissible for eye safety, wherein the light source support (15) is bent such that a permissible distance for eye safety is maintained in the far field. [5] Light source carrier (15) according to claim 2 or according to claim 2 in combination with claim 3 or 4, in which the potting compound (12) is introduced into the recesses of the light source carrier (15) in such a way that the light emitted by the light sources (10) is beam-shaped via the potting compound (12). [6] Light source carrier (15) according to one of the preceding claims, in which phase differences of the light emitted by the light sources (10) are compensated by a targeted adjustment of a refractive index of the potting compound (12). [7] Lighting module with a light source carrier (15) according to one of the preceding claims.

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