Light shaping optics and light guide structure for a time-of-flight sensor

Abstract

Light shaping optics (15) for a light source (12), with a light entry surface for the light source (12) on a bottom side and a light exit surface on a top side of the light shaping optics (15), wherein a reference light output (17) for coupling out a reference light is arranged on the underside or side, characterized by that at least one inner surface (16) of the light shaping optics (15) is designed such that the light reaches the reference light output (17) via at least one total internal reflection.

Description

[0001] The invention relates to a light shaping optic and a light guide structure for a time-of-flight sensor according to the preamble of the independent claims.

[0002] The time-of-flight sensor is particularly relevant for time-of-flight or 3D-TOF camera systems that derive time-of-flight information from the phase shift of emitted and received radiation. PMD cameras with photomixing detectors (PMDs), such as those described in patent applications EP 1 777 747 B1, US 6 587 186 B2, and DE 197 04 496 C2, are especially suitable as time-of-flight or 3D-TOF cameras. These cameras are available, for example, from ifm electronic GmbH or PMD-Technologies GmbH as the O3D frame grabber and the CamCube, respectively. The PMD camera allows for a flexible arrangement of the light source and the detector, which can be housed in a single unit or mounted separately. Naturally, the terms "camera" and "camera system" also include cameras and devices with at least one receiving pixel, such as the O1D distance measuring device from ifm electronic.

[0003] DE 20 2012 102 729 U1 discloses an optoelectronic sensor for detecting objects in a monitoring area, in which an optical element is arranged in the beam path of a light source, wherein a part of the transmitted light is reflected via light reflection on an outside of the optical element.

[0004] US 7 586 077 B2 reveals a TOF sensor with reference pixels, in which light is directed to the reference pixels via a light guide.

[0005] DE 10 2004 047 498 A1 further discloses a light guide with a structured surface and a method for its manufacture.

[0006] The object of the invention is to improve the reliability of the distance measurements of a time-of-flight camera or a time-of-flight sensor.

[0007] The problem is advantageously solved by the light shaping optics and light guide structure according to the invention as defined in the preamble of the independent claims.

[0008] Advantageously, a light-shaping optic is provided for a light source, which has a light entry surface for the light source on a bottom side and a light exit surface on a top side, wherein a reference light output for coupling out a reference light is arranged on the bottom side or side.

[0009] This approach has the advantage that the reference light can be decoupled in a defined manner via the reference light output.

[0010] It is particularly useful if the light shaping optics and the beam guidance within the light shaping optics are designed in such a way that 0.01 - 3%, in particular 0.1 - 3% of the light emitted by the light source is available as reference light at the reference light output.

[0011] This approach allows the amount of extracted reference light to be defined and specified simply by designing the light-shaping optics and arranging the reference light output.

[0012] For the beam guidance of the reference light to be extracted within the light shaping optics, it is advantageous if at least one inner surface is designed in such a way that the light reaches the reference light output via at least one total internal reflection.

[0013] This approach has the advantage that light can be deflected very efficiently via total internal reflection.

[0014] A particularly advantageous optical fiber structure is provided with light entry surfaces for coupling to the reference light outputs of at least two light shaping optics and forwarding the reference light coupled in via the light entry surfaces to a light exit area of ​​the optical fiber structure, in which the optical fiber structure is designed such that the light paths from each light entry surface to the light exit surface are of the same length and have the same light losses.

[0015] This approach is particularly useful for bringing modulated reference light from different light sources to the light-emitting surface of the light guide structure in phase synchronization. To ensure that the light contribution of each light source at the light-emitting surface is equal, it is essential to ensure that the light losses along the light path are also equal.

[0016] This approach has the advantage that the time-of-flight pixels illuminated by the coupling element can be used as reference time-of-flight pixels, with the particular advantage that the reference time-of-flight pixels are illuminated with different light intensities, so that a reference time-of-flight pixel can always be found, regardless of the illumination intensity or available integration time, which can provide a usable reference signal.

[0017] Preferably, the coupling area for generating an intensity gradient over the longitudinal extent is designed as solid material or as a cavity with light-scattering particles and / or structures.

[0018] Furthermore, the coupling element is designed in such a way that no extraneous light can penetrate into the coupling element and no coupled-in light can escape outside the intended exit areas.

[0019] Preferably, the coupling element is designed to be opaque in the areas where no coupled light is guided.

[0020] The invention will now be explained in more detail using exemplary embodiments and with reference to the drawings.

[0021] They show schematically: Fig. 1. The basic principle of a time-of-flight camera based on the PMD principle, Fig. 2 a modulated integration of the time-shifted generated charge carriers, Fig. 3 a cross-section of a PMD pixel, Fig. 4. a dependence of the amplitude and the distance error on the amount of incident light, Fig. 5 Light time-of-flight sensor with one reference pixel, Fig. 6 a top view of a time-of-flight sensor with a reference pixel array, Fig. 7 an overview of the arrangement according to the invention, Fig. 8 an arrangement according to the invention with a coupling area in solid material, Fig. 9 an arrangement according to the invention with a coupling area designed as a cavity, Fig. 10 a light shaping optic according to the invention, Fig. 11 a light guide structure according to the invention, Fig. 12 a first perspective view of the light guide structure, Fig. 13 a second perspective view of the light guide structure.

[0022] In the following description of preferred embodiments, identical reference numerals denote identical or comparable components.

[0023] Fig. Figure 1 shows a measurement situation for an optical distance measurement with a time-of-flight camera, as is known, for example, from DE 197 04 496 C2.

[0024] The time-of-flight camera system 1 comprises a transmitter unit or illumination module 10 with an illumination light source 12 and associated beam-shaping optics 15, and a receiver unit or TOF camera 20 with receiving optics 25 and a time-of-flight sensor 22. The time-of-flight sensor 22 has at least one pixel, but preferably a pixel array, and is in particular configured as a PMD sensor. The receiving optics 25 typically consist of several optical elements to improve the imaging properties. The beam-shaping optics 15 of the transmitter unit 10 are preferably configured as a reflector. However, diffractive elements or combinations of reflective and diffractive elements can also be used.

[0025] The measurement principle of this arrangement is essentially based on the fact that the travel time of the emitted and received light can be determined from the phase shift of the emitted and received light. For this purpose, the light source 12 and the time-of-flight sensor 22 are simultaneously modulated with a specific modulation frequency or signal with a first phase position a via a modulator 30. Corresponding to the modulation frequency, the light source 12 emits an amplitude-modulated signal with phase a. In the illustrated case, this signal, or electromagnetic radiation, is reflected by an object 40 and, due to the distance traveled, arrives at the time-of-flight sensor 22 with a second phase shift b.In the light time-of-flight sensor 22, the signal of the first phase position a of the modulator 30 is mixed with the received signal, which has the time-of-flight-related second phase position b, whereby the phase shift or the object distance d is determined from the resulting signal.

[0026] To determine the second phase position b more precisely, and thus the object distance d, it may be possible to change the phase position a with which the light time-of-flight sensor 22 is operated in order to modify pre-tuned phase shifts Δφ. Simultaneously, it may also be possible to selectively shift the phase with which the illumination is driven.

[0027] The principle of phase measurement is schematically illustrated in Fig. Figure 2 shows the time course of the modulation signal used to control the lighting 12 and the light-time-of-flight sensor 22, here without phase shift. The light b reflected from object 40 reaches the sensor according to its light-time-of-flight t. L The light-time-of-flight sensor 22 receives the photonically generated charges q during the first half of the modulation period at a first integration node Ga and during the second half of the period at a second integration node Gb. The charges are typically collected and integrated over several modulation periods. The phase shift, and thus the distance to the object, can be determined from the ratio of the charges qa, qb collected at the first and second gates Ga, Gb.

[0028] As already known from DE 197 04 496 C2, the phase shift of the light reflected by the object, and thus the distance, can be determined, for example, by a so-called IQ (in-phase quadrature) method. To determine the distance, preferably two measurements are carried out with phase positions of the modulation signal shifted by 90°, i.e., for example, φ mod + φ0 and φ mod + φ 90 , whereby the phase shift of the reflected light can be determined from the charge difference Δq(0°), Δq(90°) determined in these phase positions via the known arctan relation. φ=arctanΔq(90°)Δq(0°)

[0029] To improve accuracy, further measurements can be carried out with phase shifts of, for example, 180°. φ=arctanΔq(90°)−Δq(270°)Δq(0°)−Δq(180°)

[0030] Of course, measurements with more than four phases and their multiples, and a correspondingly adapted evaluation, are also conceivable.

[0031] Fig. Figure 3 shows a cross-section through a pixel of a photomixing detector such as that known from DE 197 04 496 C2. The modulation photogates Gam, G0, Gbm form the light-sensitive area of ​​a PMD pixel. Depending on the voltage applied to the modulation gates Gam, G0, Gbm, the photonically generated charges q are directed either to one or the other accumulation gate or integration node Ga, Gb.

[0032] Fig. 3b shows a potential profile in which the charges q flow towards the first integration account Ga, while the potential according to Fig. 3c allows the charge q to flow towards the second integration node Gb. The potentials are set according to the applied modulation signals. Depending on the application, the modulation frequencies are preferably in the range of 1 to 100 MHz. With a modulation frequency of, for example, 1 MHz, the period is one microsecond, so the modulation potential changes accordingly every 500 nanoseconds.

[0033] In Fig. Figure 3a further shows a readout unit 400, which may optionally already be part of a PMD time-of-flight sensor designed as a CMOS. The integration nodes Ga, Gb, configured as capacitors or diodes, integrate the photonically generated charges over a multitude of modulation periods. In a known manner, the voltage then applied to the gates Ga, Gb can, for example, be tapped off via the readout unit 400 with a high impedance. The integration times are preferably selected such that the time-of-flight sensor or the integration nodes and / or the light-sensitive areas do not reach saturation for the expected amount of light.

[0034] Fig. Figure 4 schematically illustrates the dependence of an electrical parameter of the time-of-flight sensor or an integration node on the amount of light. The amount of light is determined in a known manner from the luminous flux and the irradiation time. Charge carriers are generated proportionally to the amount of light in the photosensitive area of ​​the modulation gates Gam, G0, Gbm and distributed to the integration nodes Ga, Gb in phase correlation according to the modulation signal. These charges can either be tapped off at the integration nodes Ga, Gb as a voltage signal or amplitude with high impedance, or, if applicable, measured as a current during discharge of the integration nodes. These electrical parameters thus correspond to the phase-correlated luminous flux or the corresponding amount of light.

[0035] The potential dynamic range of a time-of-flight pixel typically spans several orders of magnitude. The size of the dynamic range depends primarily on the area of ​​the pixel's photosensitive layer and the capacity of the integration nodes. The integration time for the time-of-flight sensor, or for a single pixel, is preferably set such that the sensor does not saturate in the given application.

[0036] However, with decreasing light intensity, or analogously with decreasing integration time, the voltage swing at the integration nodes Ga and Gb decreases more and more, causing, among other things, an increasing uncertainty in distance determination due to the decreasing signal-to-noise ratio, as shown by the dashed curve of the standard deviation in Fig. Figure 4 illustrates this. The lower limit of the integration time working range should therefore be chosen such that an expected distance error remains within a permissible tolerance or standard deviation, while the upper limit should preferably be below saturation.

[0037] Fig. Figure 5 shows a time-of-flight sensor 22 with several time-of-flight pixels 24 and reference time-of-flight pixels 26. The reference time-of-flight pixels 26 are illuminated by a reference light via a light channel 260. The reference light can, for example, originate from a reference light source or be directed directly from the illumination light source 12, preferably via a light guide or the light channel 260, onto the reference time-of-flight pixels 26. Preferably, the reference time-of-flight pixels 26 are identical in structure and function to the time-of-flight pixels 24 of the rest of the sensor 22 and are preferably controlled identically. In the illustrated case, the reference time-of-flight pixels 26 are spatially separated from the other time-of-flight pixels 24 by covering two rows of time-of-flight pixels with an opaque mask 28.

[0038] Such a masking 28 has several advantages. Firstly, the spatial separation prevents crosstalk of the reference light supplied via the light guide 260 onto the active time-of-flight pixels 24; secondly, dark measurements can also be carried out as a further reference via the masked pixels 25.

[0039] Of course, arrangements are also conceivable in which masked pixels 25 are omitted and the reference light time-of-flight pixels 26 are spatially offset from the pixel array of light time-of-flight pixels 24.

[0040] It is also conceivable, in addition to or as an alternative to the considerations mentioned above, to discard the measurement results of the light time-of-flight pixel 24, which are affected by the light input at the reference light time-of-flight pixel 26.

[0041] In Fig. 6 is a top view of a light time-of-flight sensor 22 according to Fig. Figure 5 shows that next to the array of time-of-flight pixels 24, a row with several reference time-of-flight pixels 26 is spatially separated. A portion of the light emitted by the illumination light source 12 is directed onto the reference pixels 260 via a light guide or light channel 260. Depending on the application and requirements, several rows of reference time-of-flight pixels 26 may be provided.

[0042] Decoupling the optical signals from the illumination light source 12 allows a reference for distance measurement to be provided via the reference time-of-flight pixels 26. Based on signals from the reference time-of-flight pixels 26, reference values ​​can be determined, which can then be used to compensate for system-related effects that influence the distance measurement. In particular, effects during the conversion of electrical to optical signals can be taken into account and compensated for, such as a changing response behavior of the electro-optical transducers due to temperature and aging effects. Advantageously, the reference time-of-flight pixels 26 are preferably operated with the same modulation signals and integration times as the other time-of-flight pixels 24.

[0043] Furthermore, to avoid saturation of the reference light time-of-flight pixels 26, it may be possible to influence the light coupling in or out of the light guide or light channel 260 in such a way that the reference light time-of-flight pixels 26 operate in an optimal range.

[0044] With the coupling element according to the invention, it is now provided to subject the reference light time-of-flight pixels 26 to different light intensities, so that, for example, regardless of the integration times used on the sensor 22, at least one reference light time-of-flight pixel 26 operates in a preferred working range.

[0045] In Fig. Figure 7 shows an exemplary arrangement with a coupling element 200 according to the invention. Several components 510 and a time-of-flight sensor 22 with a time-of-flight pixel area 24 and a reference time-of-flight pixel area 26 are arranged on a component carrier 500. A cover glass 310 is also provided for the mechanical protection of the time-of-flight sensor 22, which, in the installed state, is mounted on a frame 300 above the sensor 22. The frame has a recess on one side for receiving the coupling element 200.

[0046] In Fig. 8 is the arrangement according to Fig. Figure 7 shows the assembled state in cross-section along line XX'. The sensor 22 is located within the frame 300 and is protected by the cover glass 310 mounted above it. The coupling element 200 is arranged between the frame 200 and the cover glass 310 within the recess of the frame 300. The coupling element 300 has a groove corresponding to the recess of the frame 300, with the recess and groove being aligned such that the coupling element 200 is laterally fixed. Vertical fixation is achieved by the contact pressure of the resting cover glass 310.

[0047] Frame 300 and cover glass 310 are preferably bonded together, but clamp connections are also conceivable. The frame, in turn, is connected to the component carrier 500 (not shown here), for example by gluing, screwing, clamping, soldering, etc.

[0048] The output coupling element 200 has an input coupling area 210, which, in the illustrated example, guides the light channel 260 to a lateral edge of an output coupling area 220. The output coupling element 200 is transparent, at least in the output coupling area 220, and is designed such that the light introduced via the light channel 260 can penetrate the output coupling area 220 and be directed to the reference time-of-flight pixels 26 via an exit surface. The arrangement is dimensioned such that the exit surface of the output coupling area 220 rests over the entire area of ​​the reference time-of-flight pixels 26.

[0049] In principle, it is also conceivable that the light channel already leads into the coupling area 210 and the light is guided via the coupling area 210 to the coupling area 220.

[0050] Preferably, the coupling element 200 or at least the coupling area 220 is dimensioned in height such that pressure can be exerted on the coupling element 200 via the cover glass, resulting in a preferred contact pressure of the exit surface on the reference light time-of-flight pixel 26.

[0051] Preferably, the output element 200 is made of an elastic material, for example silicone or another elastic and transparent material, at least in the output area. Due to its elasticity, the output surface conforms particularly closely to the surface of the sensor 22 or the reference time-of-flight pixels 26. The arrangement is preferably designed such that no air inclusions are present between the output surface and the reference time-of-flight pixels 26 and / or any air inclusions are forced out.

[0052] The areas of the coupling element that are not intended to conduct light are preferably designed to be opaque and / or light-absorbing. It is also possible that, additionally or alternatively, the surfaces of the coupling element 200 from which no light is to escape are coated with an opaque material.

[0053] To illuminate the reference time-of-flight pixels 26 with different light intensities, the extraction area 220 is designed such that the light intensity decreases along its longitudinal extent. For example, the extraction area can be filled with light-scattering and / or light-absorbing particles. To achieve or enhance an intensity gradient, the particle fill level, quantity, and / or size can be varied. Coloring with other materials is also conceivable.

[0054] In the exemplary embodiment according to Fig. 9. The extraction area is not designed as a solid material, but as a cavity. The inner surfaces of the cavity are preferably coated with a light-absorbing material. Alternatively or additionally, the inner surfaces can also have a light-absorbing structure. This approach also achieves a gradient in light intensity along the longitudinal extent of the extraction area 220.

[0055] Furthermore, a fiber optic structure is proposed to direct light from the source precisely onto a surface on the imager or time-of-flight sensor, which serves as a reference area. This fiber optic structure must meet a number of requirements: 1. Couple multiple light sources 2. Front-mounted optics with a rearward-facing exit surface, since light must be emitted forwards. 3. Combining light from different sources a. same light paths b. equal amounts of light 4. Feed to the reference channel

[0056] Time-of-flight (TOF) cameras rely on very stable conditions to achieve high absolute accuracy. Thermal and long-term drifts over their lifespan can cause unavoidable delays in the signal path, particularly in the illumination. These changes then lead to a shift in the measured distance. Calibration at the time of delivery cannot counteract these unpredictable long-term drifts.

[0057] To get a sense of the magnitude, the following calculation can be made. If the phase shift between chip modulation and light modulation is 100 ps, ​​this results in an error of 15 mm. To reduce such errors, a reference measurement using the methods according to the invention is proposed.

[0058] Since more than one light source is usually required to provide the necessary and simultaneously eye-safe amount of light, light from each light source must be directed onto the reference pixels. Using light from only one source is not possible because the light sources can exhibit different drifts and may also emit slightly different pulse shapes.

[0059] The signal propagation time of the light in the optical fiber should also be as uniform as possible in order to avoid pulse expansion due to different light paths.

[0060] If one light source directed more light onto the reference channel than another, it would dominate the reference signal. As long as the light sources behave identically, this is not a problem. However, if the light sources exhibit different aging effects, this would lead to an incorrect correction.

[0061] The light collected and guided in this way must then be mixed and directed to the reference pixels. This mixing ensures that all pixels receive the same amount of light from each light source.

[0062] Additionally, the optics must be designed so that light falling on the illumination from the outside is not coupled into the reference channel. If this were the case, a mirror on the outside of the camera could severely distort the reference signal.

[0063] To solve the problem, components with different properties are required. These are, in detail: Light-shaping optics with a rearward or side-facing reference light output. The requirements are that approximately 0.01–3%, preferably 0.1–3%, of the light should be directed to the output. A particularly advantageous way to achieve this is to incorporate surfaces in the light-shaping optics that direct light to the output via total internal reflection. It is especially advantageous to keep the divergence angle at the output less than 10°. This has the advantage of simplifying further light guidance. The étendue should preferably be maintained.

[0064] The advantage of using total internal reflection lies in the fact that there is no angle at which light can be coupled into the light-shaping optics from the outside, and that the coupled light propagates in the same direction as the totally reflected light. Therefore, no light can directly enter the reference optical fiber from the outside.

[0065] The design of the optical fiber structure is characterized by its ability to guide the reference light from the light-shaping optics to the reference channel inlet. All light paths should be of equal length. This means that light from light sources located close to the reference channel must be routed via indirect paths, while light from light sources located farther from the reference channel should take the direct path. The losses of the individual strands should be as similar as possible. To achieve this, surface roughness can be intentionally introduced into the optical fiber to induce losses in particularly efficient strands. Alternatively, the cross-section can be modified or apertures can be introduced. Roughness can be created, in particular, by making cuts, especially saw cuts, using mechanical sawing and / or laser cutting. Of course, other methods for adjusting the surface roughness are also conceivable.

[0066] In Fig. Figure 10 schematically shows a light extraction from the light-shaping optics 15 according to the invention. The light-shaping optics 15 are arranged above a light source 12 and concentrate the light rays from the light source 12 to illuminate the detection area of ​​the time-of-flight camera 20. A portion of the light rays is reflected at an inner surface 16 and guided to a reference light output 17, and from there into a light guide structure 250. The reference light output 17 and the light guide structure 250, or the light entry area 251 of this structure, do not necessarily have to be mechanically connected. However, socket-plug solutions are also conceivable.

[0067] Fig. Figure 11 shows a top view of the optical fiber structure 250 according to the invention for four light sources 12 with corresponding light shaping optics 15. As already described, the optical fiber structure 250 is designed such that the light paths from the light entry surfaces 251 to the light exit area 255 are the same for all light entry surfaces 251 or light sources 12.

[0068] Fig. Figure 12 shows a perspective view of the device according to Fig. 11. Downstream of the light exit area 255 of the light guide structure 250 is the light channel 260 or reference channel 260, which directs the light supplied via the light guide structure to the output coupling element 200. The light guide structure 250 and the light channel 260 can optionally be integrally connected, but mechanical separation is also possible. The light exit area 255 can be tapered, in particular pointed.

[0069] Fig. 13 shows the arrangement according to Fig. 12 from another perspective. Reference symbol list 10 transmitting units 12 Lighting light source 15 Light shaping optics 16 interior surface 17 Reference light output 20 receiver unit, TOF camera 22 Light time-of-flight sensor 24 light-time-time pixels 25 masked pixels 26 reference light time-of-flight pixels 25 Receiving optics 28 Masking 30 Modulator 40 objects 80-phase control 85 multiplexers 200 decoupling element 210 coupling area 220 coupling area 250 fiber optic structure 260 light channel 300 frames 310 Cover glass 400 reading units 500 component carriers 510 component Gam, G0, Gbm modulation photogate Ga, Gb Integration node q charges qa, qb Charges at the integration node Ga, Gb

Claims

[1] Light shaping optics (15) for a light source (12), with a light entry surface for the light source (12) on a bottom side and a light exit surface on a top side of the light shaping optics (15), wherein a reference light output (17) for coupling out a reference light is arranged on the underside or side, characterized by , that at least one inner surface (16) of the light shaping optics (15) is designed such that the light reaches the reference light output (17) via at least one total internal reflection. [2] Light guide structure (250) with light entry surfaces (251) for coupling to the reference light outputs (17) of at least two light shaping optics (15) according to claim 1 and forwarding the reference light coupled in via the light entry surfaces (251) to a light exit area (255) of the light guide structure (15), wherein the light guide structure (250) is designed such that the light paths from each light entry surface (251) to the light exit surface (255) are of the same length and have the same light losses. [3] Light guide structure (250) according to claim 2, wherein the light losses are selectively adjusted via a roughness of the surfaces of the light guide structure (250). [4] Light guide structure (250) according to claim 3, wherein the roughness of the surface is produced by sawing or laser cutting. [5] Arrangement comprising a light guide structure (250) and a light shaping optic (15) according to one of the preceding claims as well as a light time-of-flight sensor (22) and a coupling element (200); in which an output coupling element (200) is arranged at the light exit surface (255) for forwarding the reference light to at least one reference pixel (26) of a light time-of-flight sensor (22).

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