# Patent

## Title
Device and method for measuring distance values and distance images

## Abstract

A distance camera includes at least one photo element, a trigger generator activating the photo element during a temporal integration gate, a light source illuminating an object with light pulses having a predetermined temporal intensity profile with a duration Tp, and an intensity sensor determining the intensity Ip of the light pulses arriving on the photo element. The integration gate has a predetermined delay to the light pulse emission start point in order to capture the light pulses back reflected from the object. The photo element outputs a signal value U at an integration end point in time T1e and in accordance with an intensity Ip and a duration of the light pulse arriving on the photo element during its activation.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a Continuation of International Application No. PCT/EP2013/072819, filed on Oct. 31, 2013, which claims benefit under 35 U.S.C. § 119(e) to U.S. Provisional Application No. 61/722,465, filed Nov. 5, 2012, and which claims priority under 35 U.S.C. § 119(a) to German Patent Application No. 10 2012 110 538.3, also filed on Nov. 5, 2012. The disclosures of all three related applications are considered part of and are incorporated by reference into the disclosure of the present application in their respective entireties.


FIELD AND BACKGROUND OF THE INVENTION
The invention relates to a device and a method for determining distance values and distance images.
Distance values can be measured between a measuring device and an object without a physical contact between the device and the object by optical methods. In these methods, the object is illuminated by the device and the light back reflected from the object is then captured by a light detector of the device.
Distance values can for example be determined by periodically modulating the light intensity which is emitted from the device and by measuring the phase difference between the emitted light and the back reflected light arriving on the detector. However, due to the periodicity of the light intensity this method results in an ambiguous distance measurement. Unambiguous distance values can be determined by measuring the time of flight between the emission of light and the arrival of the back reflected light on the detector.
For a time of flight measurement with a precision of the distance value in the order of centimeters fast electronics acting and responding in the picosecond range are required. The distance value measured by the device can therefore depend on manufacturing tolerances of the detector. If an image sensor with multiple picture elements is used for the detector, different picture elements can measure different distance values due to the manufacturing tolerances of the picture elements.
SUMMARY
It is an object of the invention to provide a device and a method for measuring distance values with a high precision.
A distance camera according to the invention for determining a distance value Rabs between an object and the distance camera within a predetermined distance range comprises at least one photo element, a trigger generator for activating the photo element during a temporal integration gate, a light source for illuminating the object with light pulses having a predetermined temporal intensity profile with a duration Tp, and an intensity sensor for determining the intensity Ip of the light pulses arriving on the photo element, wherein the integration gate has a predetermined delay to the light pulse emission start point in time in order to capture the light pulses back reflected from the object such that either To or To+Tp is between an integration start point in time T1b of the integration gate and an integration end point in time T1e of the integration gate, with To being the first point in time when the light pulse arrives on the photo element, wherein the photo element is adapted to output a signal value U at the integration end point in time T1e with the signal value U depending on the intensity Ip and the duration of the light pulse arriving on the photo element during its activation, wherein the camera comprises a memory unit for storing predetermined parameters of an explicit correction value function Δ=f(Ip) for the photo element, and an evaluation unit for determining a raw distance value Rraw from the signal value U and the intensity Ip and adding the raw distance value Rraw and the correction value Δ(Ip) for obtaining the distance value Rabs.
The method according to the invention for determining the distance value Rabs between the object and the distance camera comprises the steps: a) arranging a target object within the predetermined distance range with a predetermined distance Rabs,cal to the distance camera; b) obtaining a plurality of sampling points Rraw,cal, Ip,cal by:—illuminating the target object with the light source with variable and determined intensities Ip,cal of the light pulses arriving on the photo element, and

    
    
        determining the respective raw distance values Rraw,cal from the respective signal values Ucal and the corresponding intensities Ip,cal; c) calculating the parameters of the explicit correction value function Δ=Rraw,cal Rabs,cal=f(Ip,cal) being an interpolation of the sampling points Rraw,cal, and storing the parameters for the photo element in the memory unit; d) arranging the object within the predetermined distance range; e) illuminating the object with the light source with the light pulse, and determining the raw distance value Rraw from the signal value U and the corresponding intensity Ip determined using the intensity sensor; f) calculating the correction value Δ(Ip) for the raw distance value Rraw in dependence on the intensity Ip using the parameters stored in the memory unit; g) calculating the distance value by Rabs=Rraw,cal Δ(Ip).
    
    


With the distance camera according to the invention and the method according to the invention the distance values Rabs can advantageously be determined with a high precision. Distance images can be taken by either scanning the single photo element over the object and determining the single distance value Rabs with the respective light pulse or by providing the camera with multiple photo elements and by determining the multiple distance values Rabs with the single light pulse. In case the multiple photo elements are provided smooth distance images can advantageously be taken even if there are variations in individual photo elements present due to manufacturing tolerances since a respective correction value function Δ is provided for each photo element. Since the interpolation of the sampling points leads to a parameterisation of the correction value function Δ, this function is fully described by its parameters. Therefore, only these parameters need to be stored in the memory unit and the number of accesses to the memory unit is low compared to, for example, storing a high number of sampling points necessary for obtaining a distance measurement with a similar high precision. Due to the fact that the number of accesses to the memory unit according to the invention is low, the correction of the distance values Rraw is fast so that even the high precision in a centimeter range can be obtained with a high repetition rate.
Furthermore, all distance values can be corrected by using the parameterised correction value function Δ and it is not necessary to store correction values Δ(Ip) for a large number of every conceivable intensitiy Ip.
It is preferred that the memory unit is a non volatile memory. Therefore, a dynamic access to the parameters during the correction of the raw distance values Rraw is advantageously provided. Also, it is advantageously assured that an update of the correction value function Δ can optionally only be provided by the manufacturer of the distance camera by a delivery of the memory unit.
The photo element comprises preferably a condenser and a photo diode accessible from the outside such that the light pulse back reflected from the object is captured by the photo diode and is electrically connected to the condenser such that, when the light pulse back reflected from the object is captured by the photo diode, the condenser is discharged. The signal value U being output at the integration end point in time T1e is therefore preferably the voltage of the condenser at the integration end point in time T1e. The voltage is a measure for the energy of the light pulse arriving at the photo diode within the integration gate. It is preferred that the trigger generator comprises a first electrical switch for activating the photo element and a second electrical switch for deactivating the photo element and outputting the signal value U. The first and/or the second electrical switches are preferably transistors, in particular of identical type. By correcting the raw distance value Rraw with the correction value Δ(Ip) it is advantageously achieved that the specific inertia of each individual photo element, in particular the inertia of the photo diode, the condenser and both switches, is corrected.
It is preferred that the distance camera comprises a plurality of photo elements and for every photo element the respective memory unit, wherein the number of the photo elements and the memory units is at least 3*105, in particular at least 106. Since each correction value function Δ is parameterized and its parameters are stored in the respective memory unit, it is advantageously achieved that the distance values Rabs for this high number of photo elements can be determined at a high repetition rate such as 50 Hz.
It is preferred that the predetermined temporal intensity profile of the light pulse is substantially rectangular, trapezoidal, saw tooth shaped, triangular, a Lorentz profile, a Voigt profile or a Gaussian. Further, the temporal intensity profile of the light pulse is preferably predetermined by a measurement. The measurement can for example be carried out by the use of a photo multiplier or a fast photo diode. In case the predetermined temporal intensity profile of the light pulse is not rectangular, the intensity Ip is defined, for example, as maximum intensity of the temporal intensity profile. Other definitions like half maximum of the intensity of the temporal intensity profile are also conceivable. With the predetermined temporal intensity profile, and the determined signal value U and intensity Ip, it is possible to determine the distance Rraw.
Multiple distance values Rabs are preferably determined by repeating steps d) to g). Therefore only a single correction value function Δ has to be determined for the determination of the multiple distance values Rabs.
It is preferred that the explicit correction value function Δ is a monotonic function, in particular a polygonal line or a polynomial or a spline. The number of parameters is preferably four. The number of sampling points is preferably four. It has been found that the high precision of the determination of the distance value Rabs can advantageously be achieved with this low number of parameters and/or sampling points which results in a high feasible repetition rate.
It is preferred that the intensity sensor is the photo element and the intensity Ip,cal and/or the intensity Ip are determined by measuring the signal value Ural and/or U, respectively, the photo element within a second integration gate with an integration start point in time T2b earlier than or equal to To and an integration end point in time T2e later than or equal to To+Tp. Therefore, no extra intensity measuring apparatus needs to be provided for the intensity determination.



BRIEF DESCRIPTION OF THE DRAWINGS
In the following the invention is explained on the basis of schematic drawings.
FIG. 1 shows a diagram with an exemplary correction value function Δ,
FIG. 2 shows an array with multiple photo elements,
FIG. 3 shows a schematic electrical diagram of a photo element,
FIG. 4 shows a diagram of a condenser voltage in dependence on time, and
FIGS. 5 to 7 show temporal profile diagrams with light pulses and different integration gates.



DETAILED DESCRIPTION
As it can be seen in FIGS. 1 to 4 a distance camera according to the invention for determining a distance value Rabs between an object and the distance camera within a predetermined distance range comprises a light source for illuminating the object with light pulses having a substantially rectangular temporal intensity profile with a duration Tp. Conceivable are also other temporal profiles. The light source is preferably a light emitting diode or a laser preferably emitting the light pulses with the duration Tp in the nanosecond range. The predetermined distance range is the range in which the distance camera can determine distance values, for example from 0.5 m to 10 m. The distance camera further comprises at least one photo element 9 and a trigger generator for activating the photo element 9 during a temporal integration gate 30, 31 and preferably for controlling the emission of the light pulses with the light source. The trigger generator comprises a first electrical switch 15 for activating the photo element 9 and a second electrical switch 16 for deactivating the photo element 9.
FIG. 3 shows that the photo element 9 comprises a condenser 13 and a photo diode 14 accessible from the outside such that the light pulses back reflected from the object are captured by the photo diode 14. The photo diode 14 is electrically connected to the condenser 13 in a parallel circuit such that, when the light pulses back reflected from the object are captured by the photo diode 14, the condenser 13 is discharged. FIG. 4 shows a diagram where the corresponding condenser voltage 18 is plotted versus time 19. The condenser voltage 18 is held at a voltage VDD 22 by a power supply 11 and a corresponding electrical grounding 12 as long as the first electrical switch 15 is closed. At the integration start point in time T1b 20 of the integration gate 30, 31 the trigger generator opens its first electrical switch 15 and after a time delay 23 the condenser voltage 18 begins to drop which results in a voltage decay 24. The time delay 23 can vary between different photo elements 9 due to manufacturing tolerances and can depend on the intensity Ip of the light pulse arriving on the photo element 9. At the integration end point in time T1e 21 of the integration gate 30, 31 the trigger generator closes its second electrical switch 16, whereby a signal value U 25 being the condenser voltage 18 at T1e 21 is output by action of a voltage buffer 17 of the photo element 9. The signal value U 25 is a measure of the energy of the light arriving on the photo element 9 during its activation, wherein the signal value U 25 is approximately inversely proportional to the energy. After closing and opening the first 15 and second 16 electrical switch, respectively, the photo element 9 is reset and another measurement can be taken.
The distance camera further comprises an intensity sensor for determining the intensity Ip of the light pulses arriving on the photo element 9 and an evaluation unit for determining a raw distance value Rraw from the signal value U 25 and the intensity Ip 6. The evaluation unit is further adapted to add a correction value Δ(Ip) 7 taken from a correction value function Δ=f(Ip) 5 to the raw distance value Rraw so as to obtain a distance value Rabs. Each photo element 9 comprises a memory unit, wherein the parameters 10 of the correction value function Δ 5 for each photo element 9 are stored in the corresponding memory unit. Each memory unit is corresponding to its photo element 9, wherein each memory unit can be located on a chip of the photo element 9 or externally to the chip. All memory units form a memory of the distance camera. FIG. 2 shows an array 8 of the photo elements 9, wherein the number of the photo elements 9 and the memory units is at least 3*105, in particular at least 106.
FIG. 1 shows an exemplary correction value function Δ 5 in a diagram, in which raw distance values Rraw,cal 1 are plotted versus intensity Ip,cal 2. The correction value function Δ 5 is based on four sampling points 4 Rraw,cal, Ip,cal, wherein the four sampling points 4 Rraw,cal are obtained under known laboratory conditions. For obtaining the sampling points 4 a target object is arranged within the predetermined distance range with a predetermined distance Rabs,ca1 3 to the distance camera. It has been found that by choosing the predetermined distance Rabs,cal to be 2 m and the predetermined distance range to extend from 0.5 m to 10 m results in a particularly high precision for the distance values Rabs.
The target object is then illuminated by the light source with variable and predetermined intensities Ip,cal 2 of the light pulses arriving on the photo element 9, and the respective raw distance values Rraw,cal 1 are determined from the respective signal values Ucal and the corresponding intensities Ip,cal 2. The intensities Ip,cal 2 are preferably chosen such that most of the dynamic range of the photo element 9 is covered. The intensity Ip,cal 2 can, for example, be varied by inserting grey filters or an optical attenuator comprising a polarisation filter and a lambda half plate in the path of the light pulse. Each pair of a raw distance value Rraw,cal 1 and an intensity Ip,cal 2 forms the respective sampling point 4.
In order to predetermine the intensities Ip,cal 2 of the light pulses arriving on the photo element 9 it is conceivable to adjust the emission intensity of the light source by taking into consideration the reflectance of the target object and the transmission of atmosphere under the laboratory conditions. The emission intensity of the light source is adjusted such that the intensities Ip,cal 2 of the light pulses arriving on the photo element 9 correspond to the predetermined intensities Ip,cal 2.
The parameters 10 of the explicit correction value function Δ=Rraw,cal Rabs,cal=f(Ip,cal) 5 are an interpolation of the sampling points 4 Rraw,cal, Ip,cal and are calculated and stored for the photo element 9 in its corresponding memory unit. The interpolation can also comprise sections that are extrapolated from the sampling points 4 Rraw,cal, Ip,cal. In FIG. 1 the correction value function Δ 5 is a third order polynomial in the form of Δ=A1+A2Ip,cal+Δ3Ip,cal2+A4Ip,cal3 with the parameters A1, A2, A3, A4 being the parameters of this correction value function Δ 5 for the first photo element 9 in FIG. 2. The parameters A1, A2, A3, A4 for the first photo element 9 are stored in its respective memory unit. The parameters B1, B2, B3, B4 for the second photo element 9 are stored in its memory unit, whereas the parameters X1, X2, X3, X4 are stored in the memory unit of the last photo element 9. However, also polynomials with other orders or other functions, such as exponentials or a polygonal line with the individual lines connecting adjacent sampling points 4 can be employed.
For determining the distance value Rabs the object is arranged within the predetermined distance range, illuminated by the light source with the light pulse, and the raw distance value Rraw is determined from the signal value U 25 and the corresponding intensity Ip 6 determined using the intensity sensor. The correction value Δ(Ip) 7 for the raw distance value Rraw is then determined in dependence on the intensity Ip 6 using the parameters 10 stored in the respective memory unit and the distance value is afterwards calculated by Rabs=Rraw,cal−Δ(Ip) for each photo element 9.
FIGS. 5 to 7 show three trigger schemes, wherein the intensity 26 of the emitted light pulse 28 and of the light pulse 29 arriving on the photo element 9 is plotted versus time 27. Also shown are different integration gates 30, 31 with a predetermined delay to the light pulse emission start point in time. The temporal profiles of the light pulses and integration gates 30, 31 are rectangular. Conceivable are also other temporal profiles for the light pulses. For all conceivable temporal intensity profile shapes, the emission of the light pulse 28 starts at time zero and ends at Tp. The light pulse 29 is then back reflected from the object and arrives on the photo element 9 with the intensity Ip, which is lower than the intensity of the emitted light pulse 28. The light pulse 29 arrives on the photo element 9 from To to To+Tp. In FIG. 5 the integration gate 30 is chosen such that To+Tp is between the integration start point in time T1b 20 and the integration end point in time T1e, whereas To is outside of the integration gate. In contrast thereto, in FIG. 6 To is between the integration start point in time T1b 20 and the integration end point in time T1e, whereas To+Tp is outside of the integration gate. The grey marked areas in the diagrams correspond to the signal value U being a measure for the energy of the light pulse 29 arriving on the photo element 9 within the integration gates 30, 31.
The intensity Ip,cal and/or the intensity Ip are determined by measuring the signal value Ucal and/or U, respectively, using the photo element within a second integration gate 31, as shown in FIG. 7, with an integration start point in time T2b earlier than or equal to To and an integration end point in time T2e later than or equal to To+Tp. With the signal values Ucal and/or U, which correspond to the total energy of the light pulse 29 arriving on the photo element 9 and the known duration Tp of the light pulse 29, it is possible to calculate the intensity Ip.
The intensity Ip and/or Ip,cal can be determined simultaneously with a single light pulse by using two photo elements 9, the first photo element with the first integration gate 30 and the second photo element with the second integration gate 31. The first integration gate 30 is chosen such that either To or To+Tp is between the first integration start point in time T1b of the first integration gate 30 and the first integration end point in time T1e of the integration gate 30. The second integration gate 31 is chosen such that the second integration start point in time T2b is earlier than or equal to To and the second integration end point in time T2e is later than or equal to To+Tp. The first and the second photo elements can for example be arranged adjacent to each other or the light pulse 29 can be split with a beam splitter and each split light pulse can be directed to the first and second photo elements, respectively. The intensity Ip and/or Ip,cal can also be determined consecutively with a single photo element 9 and two light pulses 29.
With the integration gate 30 according to FIG. 5, To can be calculated by To=E/Ip+T1b−Tp and with the integration gate 30 according to FIG. 6 by To=T1e−E/Ip with E being the energy of light pulse 29 arriving on the photo element 9 and being preferably reciprocal to the signal value U 25. Rraw can then be determined via Rraw=0.5*c*To, with c being the speed of light. It should be noted that it is possible to either correct Rraw or To with Δ(Ip). It should be noted also that a change of the light source of the camera or a change in T1b and T1e required by a change in the predetermined distance range usually requires a new calibration function Δ.
LIST OF REFERENCE SIGNS


    1 raw distance values Rraw,cal 
    2 intensity Ip,cal 
    3 predetermined distance Rabs,cal 
    4 sampling point
    5 correction value function Δ
    6 intensity Ip 
    7 correction value Δ(Ip)
    8 array
    9 photo element with memory unit
    10 parameters
    11 power supply
    12 electrical grounding
    13 condenser
    14 photo diode
    15 first electrical switch
    16 second electrical switch
    17 voltage buffer
    18 condenser voltage
    19 time
    20 integration start point in time T1b 
    21 integration end point in time T1e 
    22 voltage VDD 
    23 time delay
    24 voltage decay
    25 signal value U
    26 intensity
    27 time
    28 emitted light pulse
    29 light pulse arriving on the photo element
    30 integration gate
    31 second integration gate

## Claims

1. A distance camera for determining a distance value (Rabs) between an object and the distance camera within a predetermined distance range, comprising:
at least one photo element, a trigger generator for activating the photo element during a temporal integration gate, a light source for illuminating the object with light pulses having a predetermined temporal intensity profile with a duration (Tp), and an intensity sensor for determining an intensity (Ip) of the light pulses arriving on the photo element, wherein the integration gate has a predetermined delay to the light pulse emission start point in time in order to capture the light pulses back reflected from the object such that either a first point in time when the light pulse arrives on the photo element (To) or To+Tp is between an integration start point in time (T1b) of the integration gate and an integration end point in time (T1e) of the integration gate, wherein the photo element is adapted to output a signal value (U) at the integration end point in time T1e with the signal value U depending on the intensity Ip and the duration of the light pulse arriving on the photo element during the time the photo element is activated, wherein the camera further comprises:
a memory unit for storing predetermined parameters of an explicit light-pulse-intensity-dependent correction value function Δ=f(Ip) for the photo element, and an evaluation unit for determining a raw distance value (Rraw) from the signal value U and the light-pulse-intensity Ip and adding the raw distance value Rraw and a correction value Δ(Ip) taken the from light-pulse-intensity-dependent correction value function Δ=f(Ip) for obtaining the distance value Rabs.

2. The distance camera according to claim 1, wherein the memory unit is a non volatile memory.

3. The distance camera according to claim 1, wherein the photo element comprises a condenser and a photo diode accessible from outside the camera such that the light pulse back reflected from the object is captured by the photo diode and electrically connected to the condenser such that, when the light pulse back reflected from the object is captured by the photo diode, the condenser is discharged.

4. The distance camera according to claim 1, wherein the trigger generator comprises a first electrical switch for activating the photo element and a second electrical switch for deactivating the photo element and outputting the signal value U.

5. The distance camera according to claim 1, further comprising a plurality of additional photo elements and a plurality of additional memory units associated respectively with the additional photo elements, wherein a number of the photo elements and the memory units is at least 3*105.

6. The distance camera according to claim 5, wherein the number of the photo elements and the memory units is at least 106.

7. The distance camera according to claim 1, wherein the predetermined temporal intensity profile of the light pulse is rectangular, trapezoidal, saw tooth shaped, triangular, a Voigt profile or a Gaussian.

8. The distance camera according to claim 1, wherein the temporal intensity profile of the light pulse is predetermined by a measurement.

9. A method for determining a distance value (Rabs) between an object and a distance camera according to claim 1, comprising:
a) arranging a target object within the predetermined distance range with a predetermined distance (Rabs,cal) to the distance camera;
b) obtaining a plurality of sampling points (Rraw,cal, Ip,cal) by:
illuminating the target object with variable and determined intensities (Ip,cal) of the light pulses arriving on the photo element, and
determining respective raw distance values Rraw,cal from respective signal values (Ucal) and the variable and determined intensities Ip,cal;

c) calculating the parameters of the explicit light-pulse-intensity-dependent correction value function Δ=Rraw,cal−Rabs,cal=f(Ip,cal) as an interpolation of the sampling points Rraw,cal, Ip,cal, and storing the parameters for the photo element in the memory unit;
d) arranging the object within the predetermined distance range;
e) illuminating the object with the light pulses, and determining the raw distance value Rraw from the signal value U and the corresponding intensity Ip determined by the intensity sensor;
f) calculating the correction value Δ(Ip) for the raw distance value Rraw in dependence on the intensity Ip with the parameters stored in the memory unit; and
g) calculating the distance value Rabs in accordance with Rabs=Rraw−Δ(Ip).

10. The method according to claim 9, further comprising repeating the limitations (d) through (g) for determining multiple distance values Rabs.

11. The method according to claim 9, wherein the explicit correction value function Δ is a monotonic function.

12. The method according to claim 11, wherein the monotonic function is a polygonal line or a polynomial or a spline.

13. The method according to claim 9, wherein the number of parameters is four.

14. The method according to claim 9, wherein the number of sampling points is four.

15. The method according to claim 9, wherein the intensity sensor is the photo element and the determined intensities Ip,cal and/or the intensity Ip are determined by measuring the signal value Ucal and/or U, respectively, with the photo element within a second integration gate with a further integration start point in time (T2b) earlier than or equal to To and a second integration end point in time (T2e) later than or equal to To+Tp.