System and method for detecting a light spot

Abstract

System and method for detecting a light spot. Method for detecting a light spot (T) formed by a laser pulse delivered by a laser source (10) onto a target surface (S), the target surface being located in an environment delivering a substantially static luminous flux, this method comprising: - observation of the surface by at least one pixel sensor (30) operating in photovoltaic mode, - synchronization of the end of the exposure, within a delay, with the laser pulse, leading to a sensor response that is substantially linear to the pulse and logarithmic with respect to the substantially static luminous flux. Figure for the abstract: Fig. 1

Description

Title of the invention: System and method for detecting a light spot Technical field

[0001] The present invention relates to the detection of a light spot within an environment generating a static light flux relatively large compared to that due to said spot. Prior art

[0002] Many applications require the ability to visualize, using an optical sensor, a light spot produced by a laser pulse impacting a surface.

[0003] CMOS optical sensors of the CTIA (Capacitive Trans-Inductance Amplifier) ​​type in particular comprise a reverse-biased photodiode used in photoconductive mode.

[0004] The applicant company has developed optical sensors whose pixels include a forward-polarized photodiode used in photovoltaic mode.

[0005] Examples of such sensors are described in applications WO2002059974A1, WO2009027449A1 and WO2010103464A1.

[0006] The advantage of these sensors is that they can intrinsically present a logarithmic response and do not saturate in the face of a strong static light flux coming from the observed scene.

[0007] In the dark or in a dark environment it is easy to visualize the spot produced by a short duration laser pulse on a scene; however, in a relatively bright environment, for example during the day, it is the opposite. Indeed, the photon flux corresponding to the laser return on the image is relatively low compared to that coming from the entire scene.

[0008] As a result, most solutions aimed at detecting a laser spot integrate a function for detecting the laser pulse within the pixels, but this has the particular disadvantage of producing a binary result and of not providing a faithful image of the light spot in its environment. Statement of the invention

[0009] There is therefore a need to benefit from a technical solution making it possible to visualize more faithfully and directly a light spot corresponding to a laser pulse within a scene, even when the static light flux coming from the scene is relatively high, such as in broad daylight. Summary of the invention

[0010] The invention meets this need by proposing a method for detecting a stain light formed by a laser pulse on a target surface, delivered by a laser source, the target surface being located in an environment delivering a substantially static light flux, this method comprising

[0011] - observation of the surface by at least one pixel sensor operating in photo mode voltaic,

[0012] - synchronization of the end of the exposure, with a delay, with the laser pulse.

[0013] This synchronization can lead to a substantially linear response of the sensor to the pulse and logarithmic with respect to the substantially static luminous flux.

[0014] Thanks to the invention, the signal / noise ratio of the laser spot compared to the light background is improved, which allows better visualization of the spot even in a light environment.

[0015] By "substantially static" it is meant that the light intensity does not vary or varies little (for example less than + / - 10%) over a duration texp which is large compared to the duration d of the laser pulse, with typically texp / d>10, better 102, even better 103. The substantially static light flux corresponding for example to the illumination of the target surface by daylight or any continuous source.

[0016] By "substantially linear response" is meant a signal response proportional to the input flow with a maximum deviation from linearity of less than 20%.

[0017] By "within a delay" it is meant that the end of the exposure is temporally shifted from the end of the pulse by a predefined magnitude, preferably less than or equal to 1 ms, ideally 100ps.

[0018] The end of the exposure corresponds to the beginning of the process of reading the pixels to produce an image.

[0019] In an exemplary implementation of the invention, the method comprises detecting the pulse using a photodetector receiving the light from the laser. An exemplary embodiment of such a photodetector is a mono-diode filtered on the wavelength of the laser. It is thus not necessary to establish a transmission of information between the laser source and the circuit. This photodetector may or may not share optical components with the sensor, for example lenses or groups of lenses making it possible to focus the image of the scene on the sensor. Sharp focusing on the photodetector is not necessary; it is sufficient that the observed field is substantially the same.

[0020] In an alternative implementation of the method, it comprises the transmission by the laser of a synchronization signal to the sensor. This signal is for example transmitted to the sensor by the laser source, or by a common master synchronizer, for example by a wired or wireless link. This can make it possible to dispense with the aforementioned photodetector. This signal can mark the start of the pulse or its end; in in the case where it marks the beginning of the pulse, knowing the duration of the pulse makes it possible to determine the moment when the end of the exposure must take place to arrive after the end of the pulse.

[0021] The duration of the laser pulse is for example less than or equal to 100ns, better between 10 and 30ns.

[0022] The end of the exposure may take place with a delay less than or equal to 2 milliseconds after the end of the pulse, better still 1 microsecond. This delay may be induced solely by the reaction time of the reading electronics.

[0023] The exposure time in the absence of a laser pulse can be between 1 and 30 ms. This time can be chosen depending, for example, on the aperture of the optics, or if the observed scene is highly reflective. One of the advantages of the invention is that the time has relatively little influence on the performance.

[0024] Preferably, the pulses are repeated, in particular at a frequency greater than or equal to 10 Hz and preferably less than or equal to 20 Hz. The images can then be acquired with the sensor at a frequency sufficient to acquire at least one image between two laser pulses, for example at a frequency greater than or equal to 50 Hz. This image without the spot can be subtracted from that with the spot so as to generate an image of the spot alone, or quite simply to see the spot flashing in order to observe it better.

[0025] The method can find various applications, among which, in a non-exhaustive manner, we can cite the alignment between a transmitter and a receiver in free field, the alignment of a transmitter with any target, the analysis of the energy distribution or of a wavefront of laser radiation, or the positioning of a laser beam on a surface on which a weld must be carried out with the laser. It is also possible to use the image of the light spot on a scene, provided with the method according to the invention to carry out guidance, for example of a tool or an object, such as a package, towards an objective designated by this spot, for example with a view to dropping a package by a drone.

[0026] The invention also relates, according to another of its aspects, to a system for detecting a light spot formed by a laser pulse from a laser source on a scene delivering a substantially static light flux, in particular for implementing the aforementioned method, this system comprising:

[0027] - at least one pixel sensor operating in photovoltaic mode,

[0028] - a circuit for synchronizing the end of the sensor exposure, with a delay, with the laser pulse, leading to a sensor response that is substantially linear to the pulse and logarithmic to the static flow of the scene.

[0029] The system may comprise a photodetector capable of receiving the light from the laser, the synchronization circuit triggering, within said delay, the end of the exposure in the event detection by the photodetector of the laser pulse. Alternatively, the system includes an input allowing the reception by the synchronization circuit of a synchronization signal providing information on the emission of the pulse by the illuminator.

[0030] The synchronization circuit can be arranged such that the end of the exposure takes place with a delay less than or equal to 2 milliseconds after the end of the pulse, better still 1 microsecond.

[0031] The exposure time in the absence of a laser pulse can be between 1 and 30 ms.

[0032] The invention also relates to an assembly comprising the laser source and the detection system according to the invention. The illuminator may in particular be configured to emit the pulses repeatedly, in particular at a frequency greater than or equal to 10 Hz and preferably less than or equal to 20 Hz.

[0033] The sensor may be arranged to acquire images at a frequency higher than that with which the pulses are emitted. This makes it possible to acquire images without the spot linked to the pulse, which can serve as reference images to be subtracted from the image of the scene with the spot, and allow the spot to be viewed alone, or to display a flashing of the spot to better view it. Brief description of the drawings

[0034] The invention may be better understood by reading the detailed description which follows, non-limiting examples of its implementation, and by examining the attached drawing, in which:

[0035] [Fig.l] schematically represents an example of an assembly according to the invention,

[0036] [Fig.2] is a view similar to [Fig.l] of a variant of the assembly,

[0037] [Fig.3] schematically represents an example of pixel architecture of a sensor according to the invention,

[0038] [Fig.4] illustrates, by analogy with the filling of a tank, the behavior of a pixel with logarithmic response to an incident light flux,

[0039] [Fig 5] illustrates the pixel response as a function of exposure time,

[0040] [Fig.6] is a view similar to [Fig.4] illustrating the effect of a light pulse,

[0041] [Fig.7] represents the response of the pixel to the light pulse,

[0042] [Fig 8] illustrates the reduction in exposure time after detection of the end of the pulse,

[0043] [Fig.9] represents the decay of the pixel signal after the end of the pulse,

[0044] [Fig. 10] illustrates the signal seen by the pixels in the presence and absence of laser illumination, and

[0045] [Fig. 11] represents the image of the spot after subtraction of the signal corresponding to the static flow of the scene. Detailed description

[0046] [Fig. 1] illustrates an example of an assembly 1 according to the invention, comprising a laser source 10 and a detection system 20 making it possible to image a scene S on which the spot T projected by the laser source 10 is present.

[0047] The detection system 20 comprises an optical sensor 30 which comprises pixels with a logarithmic response, for example such as that shown schematically and partially in [Fig.3]. Laser source

[0048] The laser source 10 can emit in any wavelength range compatible with its visualization by the detection system, for example in the visible or non-visible range, in particular in the infrared, in particular in the SWIR band ranging from 900 nm to 2500 nm.

[0049] The source emits laser pulses whose duration is preferably less than or equal to 30 ns, better between 1 and 30 ns, in particular between 10 and 20 ns.

[0050] The source may comprise any type of laser, for example solid-state or gas.

[0051] In the example of [Fig.l], the laser source transmits a synchronization signal to the detection system 20 via a link 40, which is for example wired.

[0052] This signal makes it possible to inform the detection system 20 of the moment when the pulse begins to be emitted or finishes being emitted.

[0053] In the variant of [Fig. 2], the connection 40 does not exist, and the detection system 20 comprises additional means 50 making it possible to detect the light spot of the laser pulse independently of the optical sensor 30. These means 50 comprise, for example, a photodetector such as a photodiode, sensitive to the light from the laser and preferably provided with a bandpass filter centered on the wavelength of the laser and rejecting the other wavelengths. This photodetector sends its output signal to an input 22 of a synchronization circuit 21 of the detection system 20, which controls the instant of reading of the pixels of the sensor 30.

[0054] Referring to [Fig. 3], it can be seen that each pixel of the sensor 30 operates in photovoltaic mode, with a forward-biased photodiode junction 31, a short-circuit transistor 32 which is turned on before each measurement, and a read amplifier 33. The sensor also comprises pixel selection transistors 34 and others 35 and 36 making it possible to carry out, with a differential amplifier 37, a differential reading in the absence of a signal (i.e. the pixel is completely in the dark) and in the presence of the signal. The storage capacities are not shown in this figure, the architecture of a pixel of this type being known, in particular from the applications mentioned in the introduction to the application.

[0055] This pixel architecture has the advantage of not saturating in front of a strong flow in compressing the "static" flow coming from the scene. On the other hand, this structure has the advantage of responding linearly to an impulse response, before seeing the response gradually return to the compressed response of the static flow.

[0056] According to the invention, the instant of return of the laser is known, either by the fact that the detection system is synchronized with the laser source as in [Fig. 1], or because an additional detector 50 is used as in [Fig. 2].

[0057] The advantage of the logarithmic sensor is then exploited so that the end of the exposure period coincides, with a predefined delay, for example of the order of Ips, with the end of the pulse on the image.

[0058] Thus, with a longer exposure time, for example of the order of 5 to 20 ms, the signal from the scene is compressed, but the signal from the laser is not, which makes it possible to have a signal-to-noise ratio of the laser task compared to the background which is relatively high, as will be explained below.

[0059] The accumulation of charges related to incident light within a sensor pixel is in some ways analogous to filling a reservoir with water, as illustrated in [Fig.4].

[0060] To have the analogy with a pixel whose logarithmic response is based on the use of a photodiode in photovoltaic mode, that is to say that the accumulation of charges evolves in a logarithmic manner past a given threshold, we can consider that the reservoir has a lateral overflow evacuation opening, creating a leak at a fairly low height, corresponding to said threshold.

[0061] The tap corresponds in [Fig.4] to the photon flux, which is considered to be constant for a given exposure time. The water level reaches a certain height h in the tank, which depends on the flow rate of water flowing from the tap. This level no longer changes after a certain time, as illustrated in [Fig 5], due to the presence of the overflow.

[0062] The effect of the laser pulse can be thought of as pouring a bucket of water into the tank, as illustrated in [Fig.6]. This pouring causes the level in the tank to rise momentarily, before the water flows out the side opening to return to the level before the bucket was poured, as illustrated in [Fig.7].

[0063] The higher the light environment, the greater the photon flux, and by analogy the lower the quantity provided by the bucket compared to that provided by the tap during the measurement period (corresponding by analogy to the exposure time).

[0064] With a conventional CTIA type pixel sensor, the tank does not have an overflow and the total quantity of water is measured at the end of the measurement period. If the tap has a high flow rate, the volume of the bucket remains small compared to the quantity total amount of accumulated water. In other words, the charges related to the laser pulse are small compared to the total amount of charges accumulated. In addition, it is not possible to wait too long for the pulse to arrive, because the tank continues to fill, in the absence of a side drain opening.

[0065] With a logarithmic sensor whose pixel operates in photovoltaic cell mode, the voltage is measured at the end of the exposure period, i.e. by analogy the level in the tank, not the total quantity of water accumulated.

[0066] In a pixel of this type, the charges are emptied at the start of the exposure, by activating the short-circuit transistor, then at the end of the exposure, the voltage is measured, i.e. by analogy the reservoir is empty at the start of the exposure, and at the end of the exposure, the height in the reservoir is measured.

[0067] Pixels with a logarithmic response can be exposed to the scene to be observed well in advance, since they are not affected by saturation. Thus, in a logarithmic pixel, the time at which the exposure ends makes no difference in the presence of a constant flow of photons, but in the case of a momentary input, the level increases before returning to the equilibrium level. This momentary growth is used to carry out the measurement. We see in [Fig.7] that if the exposure continues too long after the end of the pulse, the pixel has time to return to its equilibrium level.

[0068] In accordance with the invention, the pixel is read, i.e. the exposure is ended, sufficiently early after the end of the pulse so that the pixel has not had time to return to its equilibrium state, as illustrated in [Fig 8].

[0069] In this case, the voltage read includes the increase linked to the pulse, and the signal-to-noise ratio is improved.

[0070] As illustrated in [Fig.9], for a typical example of a pixel with a logarithmic response, the decay is significant from 100ps.

[0071] The pixel reading will therefore preferably be carried out with a delay of less than 100ps after the end of the pulse. The start of the exposure period can take place well before the arrival of the laser pulse, without fear of missing it. Regardless of the start of the exposure time, the pixel is not saturated. This is not possible with a standard CTIA type pixel, which is completely saturated if the exposure time becomes too long.

[0072] Another interesting aspect of the behavior of the logarithmic response pixel is that the response to a pulse is not symmetrical, i.e. the increase is very rapid while the decay is more slowly. This means that the pixel will generally have a complete increase in its output signal almost instantaneously upon arrival of the pulse, and will have a sufficiently slow decay to allow measurement.

[0073] The invention makes it possible to visualize the spot linked to the laser pulse and the scene having received the pulse on the same image, and the image of the spot is not a binary representation of the latter but an image comprising the information on the light intensity in each pixel of the spot, linked in particular to the distribution of the light energy within the laser beam.

[0074] The invention can also make it possible to avoid the problems of efficiency of resetting photodiodes encountered with CTIA type sensors in certain situations.

[0075] The decay rate in [Fig.9] can be adjusted by varying the bias of the photodiode, which allows the decay rate to be optimized and the response to be improved. The decay rate can be adjusted according to the required pulse response time.

[0076] In a variant, the acquisition of images of the scene without the spot and of images with it is carried out, by illuminating the scene at a frequency lower than that at which the acquisition of the images takes place.

[0077] For example, the frequency of the laser pulses is between 10 and 20 Hz, and the acquisition is carried out at a frequency greater than or equal to 50 Hz.

[0078] By comparing images taken with the stain and those taken without the stain, the perception of the latter can be improved. The stain can also be made to "flash" to make it easier for an operator to visualize. Examples

[0079] [Fig. 10] shows an example of the response of a sensor according to the invention to a scene lit with a laser illuminator in broad daylight.

[0080] [Fig. 11] illustrates the subtraction from the global signal of the signal corresponding to the constant luminous flux of the scene; we see the spot image, and in particular the spatial distribution of the luminous intensity of the spot, with a stronger intensity in the center of it.

Claims

Claims

1. Method for detecting a light spot (T) formed by a laser pulse delivered by a laser source (10) on a target surface (S), the target surface being located in an environment delivering a substantially static light flux, this method comprising: - observation of the surface by at least one pixel sensor (30) operating in photovoltaic mode, - synchronization of the end of the exposure, to within a delay, with the laser pulse, leading to a response of the sensor which is substantially linear to the pulse and logarithmic with respect to the substantially static light flux.

2. A method according to claim 1, comprising detecting the pulse using a photodetector (50) receiving light from the laser (10).

3. A method according to claim 1, comprising transmitting a synchronization signal to the sensor, in particular a signal from the laser source.

4. Method according to any one of the preceding claims, the duration of the laser pulse being less than or equal to 30ns, better between 10 and 30ns.

5. Method according to any one of the preceding claims, the end of the exposure taking place with a delay less than or equal to 2 milliseconds after the end of the pulse, better still 1 microsecond.

6. Method according to any one of the preceding claims, the exposure time in the absence of a laser pulse being between 1 and 30 ms.

7. Method according to any one of the preceding claims, the pulse being repeated, in particular at a frequency greater than or equal to 10 Hz and preferably less than or equal to 20 Hz.

8. Method according to the preceding claim, in which the acquisition of images is carried out with the sensor at a frequency sufficient to acquire at least one image between two laser pulses.

9. System for detecting a light spot formed by a laser pulse from a laser source (10) on a scene delivering a substantially static light flux, comprising - at least one pixel sensor (30) operating in photovoltaic mode, - a synchronization circuit (21) of the end of the exposure of the sensor, to within a delay, with the laser pulse, leading to a response of the sensor substantially linear to the pulse and logarithmic with respect to the static flow of the scene.

10. System according to claim 9, comprising a photodetector (50) capable of receiving the light from the laser, the synchronization circuit (21) triggering, within said delay, the end of the exposure in the event of detection by the photodetector (50) of the laser pulse.

11. System according to claim 9, comprising an input (22) allowing the reception by the synchronization circuit of a synchronization signal providing information on the emission of the pulse by the source.

12. System according to any one of claims 9 to 11, the synchronization circuit being arranged such that the end of the exposure takes place with a delay less than or equal to 2 milliseconds after the end of the pulse, better than 1 microsecond.

13. System according to any one of claims 9 to 12, the exposure time in the absence of a laser pulse being between 1 and 30 ms.

14. Assembly (1) comprising a laser source (10) and the detection system (20) according to any one of claims 9 to 13.

15. Assembly according to claim 14, the laser source (10) being configured to emit a pulse repeatedly, in particular at a frequency greater than or equal to 10 Hz and preferably less than or equal to 20 Hz.

16. Assembly according to the preceding claim, the sensor (30) being arranged to carry out the acquisition of images at a frequency higher than that with which the pulses are emitted.

17. Assembly according to any one of claims 14 to 16, the duration of the laser pulse being less than or equal to 100ns, better between 10 and 30ns.

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