Multispectral imager with spectral-focusing optical group and associated acquisition method

Abstract

The invention relates to a multispectral imager (1) comprising a sensor (3) and an optical system (5) that comprises a spectral-focusing optical group (9) and an axial movement system (13) for axially moving the spectral-focusing optical group (9), wherein the optical system (5) is configured to refract electromagnetic radiation (R) incident on the sensor (3) and to implement spectral defocusing of the refracted electromagnetic radiation (R), wherein the spectral defocusing is greater than 100 micrometres for a difference of 100 nanometres in wavelength, and wherein the axial movement system (13) makes the spectral-focusing optical group (9) movable between a focusing position on the sensor (3) for electromagnetic radiation (R) with a wavelength of 500 nm, and a focusing position for electromagnetic radiation (R) with a wavelength of 1800 nm.

Description

[0001] DESCRIPTION

[0002] TITLE: Multispectral imager with spectral focusing optical group and associated acquisition method

[0003] Technical field

[0004] The present invention relates to optical systems for multispectral imaging.

[0005] A particularly interesting application of the invention concerns the use of a multispectral imager for the detection of a laser spot of unknown wavelength in a scene observed by said imager.

[0006] Previous techniques

[0007] In certain applications, particularly those carried on binoculars or gyro-stabilized observation balls, it is necessary to be able to visualize a scene and detect a light spot resulting from the emission of a laser beam or the reflection of said laser beam on a surface located in the scene.

[0008] In a very bright scene such as a sunny scene, all the details of the scene appear, often with contrast, and it is difficult to distinguish the bright spot in an image of said scene.

[0009] Indeed, lasers typically have wavelengths between 700nm and 1 pm, and merge with the wavelengths of the visible range present in the observed scene.

[0010] Lasers operate in this type of situation in continuous or quasi-continuous emission, the integration time of conventional imager sensors not allowing pulsed emissions to be reliably detected.

[0011] Quasi-continuous emission means a pulsed laser emission with a periodicity of at least one pulse every 10 ps, ​​and whose pulses are, for example, 100 nanoseconds wide.

[0012] Solutions for detecting a laser spot exist and work, for example, using filters. However, these solutions require prior knowledge of the laser emission wavelength in order to be able to center the filters around this wavelength.

[0013] A laser spot of unknown wavelength cannot therefore be detected, and even less so almost instantly.

[0014] A technology based on a Fabry-Perot interferometer makes it possible to scan several wavelengths by varying the interval between the mirrors of said interferometer, but the wavelength thus filtered depends on the angle of inclination of the mirrors relative to the direction of the light, so it is impossible to image the entire scene.

[0015] Statement of the invention

[0016] The present invention therefore aims to overcome the aforementioned drawbacks and to provide an imager capable of observing an entire scene and highlighting a laser spot there, even for a laser of unknown wavelength.

[0017] The present invention relates to a multispectral imager comprising a sensor and an optical system comprising on an optical axis a spectral focusing optical group, as well as a system for axial displacement of said spectral focusing optical group, the optical system being configured to refract electromagnetic radiation incident on the sensor and to implement spectral defocusing along the optical axis of the refracted electromagnetic radiation, the spectral defocusing being greater than 100 micrometers along the optical axis for a difference of 100 nanometers in wavelength and for a wavelength between 500 nm and 1800 nm, preferably 1000 nm, the axial displacement system making the spectral focusing optical group movable between a focusing position on the sensor of electromagnetic radiation with a wavelength between 500 nm and 700 nm,and a focusing position on the sensor of electromagnetic radiation with a wavelength between 1000 nm and 1800 nm. Thus, the multispectral imager has exacerbated chromatism properties and makes it possible, by axial displacement of the spectral focusing optical group, to obtain a set of images of the scene at different observation wavelengths. If a laser spot is present in the scene, at least one image of this set of images highlights said laser spot. The imager obtained is also compact and can make it possible to detect the laser spot without a priori knowledge of the wavelength of the laser.,

[0018] Furthermore, the present invention goes against a conventional optical design, which generally seeks to minimize the chromatism of an optical system in the spectral band of interest so as to increase the optical quality of the images. Here, the search for significant defocusing, and therefore strong chromatism, is essential.

[0019] Advantageously, the spectral focusing optical group is made of dispersive glass with an Abbe number of less than 45, preferably less than 30, even more preferably less than 20.

[0020] In one embodiment, the axial displacement system is configured to axially move the spectral focusing optical group with a minimum increment of 5 micrometers.

[0021] In a particular embodiment, the spectral focusing optical group comprises a lens, or several lenses, or a plate with planar and parallel faces, at least one of the faces comprising a metasurface.

[0022] Advantageously, the axial displacement system of the spectral focusing optical group comprises a cam displacement system, and / or a stepping motor, and / or a continuous motor, and / or a piezoelectric motor.

[0023] The invention also relates to a method for acquiring images of the same scene by the imager as defined above, the method comprising the following steps, implemented repeatedly for different positions of the spectral focusing optical group corresponding to particular focused wavelengths: - Positioning of the spectral focusing optical group in an axial position corresponding to a particular focused wavelength on the sensor; and

[0024] - Acquisition of an image by the sensor.

[0025] In a particular embodiment, two images acquired at different focused wavelengths and included in a range of 50 nm are called adjacent images, the method comprising a step of obtaining a processed image for each image acquired at a particular focused wavelength, the processed image being obtained by subtraction, preferably pixel by pixel, of the acquired image with an adjacent image or with an average of a group of adjacent images.

[0026] Advantageously, the subtraction is performed by subtracting the gray intensities of each pixel.

[0027] Advantageously, a sufficient number of images is acquired during the method so as to obtain an image acquired for wavelengths focused at least every 20 nm, preferably at least every 10 nm, for a wavelength range between 500 nm and 1800 nm, preferably 1000 nm.

[0028] In a particular embodiment, the method further comprises a step of processing the acquired images by applying an algorithm for stretching the histogram of each acquired image.

[0029] Brief description of the drawings

[0030] Other aims, characteristics and advantages of the invention will appear on reading the following description, given solely by way of non-limiting example, and made with reference to the appended drawings in which:

[0031] [Fig 1] is a schematic view of a spectral imager according to the invention in a first position in which electromagnetic radiation of wavelength 700 nm is focused on the sensor;

[0032] [Fig 2] is a schematic view of a spectral imager according to the invention in a second position in which electromagnetic radiation of wavelength 800 nm is focused onto the sensor; and

[0033] [Fig 3] is a schematic view of the steps of the image acquisition method according to the invention.

[0034] Detailed description of at least one embodiment

[0035] Figures 1 and 2 schematically show a multispectral imager 1 according to the invention.

[0036] The multispectral imager 1 is, for example, embedded in binoculars or a gyro-stabilized observation ball.

[0037] The multispectral imager 1 comprises a sensor 3 and an optical system 5 comprising, aligned in this order on the same optical axis X-X', a front optical block 7, a spectral focusing optical group 9, and a focusing optical block 11, as well as a system 13 for axial displacement of said spectral focusing optical group 9.

[0038] Sensor 3 is for example an FPA (“Focal Plane Array”) sensor, for example a Silicon or InGaAs sensor.

[0039] The optical system 5 is configured to refract electromagnetic radiation R incident on the sensor 3 via the front optical unit 7, the spectral focusing optical group 9 and the focusing optical unit 11.

[0040] For example, optical system 5 has a focal length of 40 mm and an aperture number of 2.

[0041] In particular, the front optical block 7 is configured to refract electromagnetic radiation R incident on the spectral focusing optical group 9.

[0042] The front optical unit 7 comprises a lens, or a set of several lenses, depending on the optical design envisaged.

[0043] Optionally, the front optical unit 7 has a high chromatism. For example, the front optical unit 7 is made of dispersive glass with an Abbe number of less than 45, preferably less than 30, even more preferably less than 20. Equivalently, the focusing optical unit 11 is configured to refract electromagnetic radiation R incident on the sensor 3.

[0044] The focusing optical unit 1 1 comprises a lens, or a set of several lenses.

[0045] Optionally, the optical focusing block 11 has a high chromatism. For example, the optical focusing block 11 is made of dispersive glass with an Abbe number of less than 45, preferably less than 30, even more preferably less than 20.

[0046] The spectral focusing optical group 9 is configured to refract electromagnetic radiation R incident on the focusing optical block 11.

[0047] In various particular embodiments, the spectral focusing optical group 9 comprises one or more lenses.

[0048] In a particular embodiment as shown, the optical spectral focusing group 9 comprises a plate with flat and parallel faces 15, at least one of the faces 15 comprising a metasurface.

[0049] A metasurface is a surface textured at the subwavelength scale, i.e. of an order of magnitude less than 500 nm, and exhibiting chromatism.

[0050] Advantageously, the spectral focusing optical group 9 is made of dispersive glass with an Abbe number of less than 45, preferably less than 30, even more preferably less than 20.

[0051] For example, thanks to the exacerbated chromatic properties of the front optical block 7 and / or of the spectral focusing optical group 9 and / or of the focusing optical block 11, the optical system 5 is configured to implement spectral defocusing along the 'optical axis X-X' of the refracted electromagnetic radiation.

[0052] The spectral defocus is greater than 100 micrometers along the optical axis for a difference of 100 nanometers in wavelength and for a wavelength between 500 nm and 1800 nm, preferably 1000 nm. Preferably, the spectral defocus is greater than 250 micrometers along the optical axis for a difference of 100 nanometers in wavelength.

[0053] In other words, the optical system 5 has a chromatism such that the focal point, located towards the sensor 3, of an electromagnetic radiation R700 with a wavelength of 700 nm and the focal point of an electromagnetic radiation R800 with a wavelength of 800 nm are separated by at least 100 micrometers.

[0054] Furthermore, the axial displacement system 13 makes it possible to make the optical spectral focusing group 9 movable between a focusing position on the sensor 3 of electromagnetic radiation R with a wavelength of 500 nm and a focusing position on the sensor 3 of electromagnetic radiation R with a wavelength of 1800 nm, preferably 1000 nm.

[0055] Thus, despite the significant chromatism of the optical system 5 and the significant defocusing between each wavelength, the multispectral imager 1 makes it possible to acquire focused images for specific wavelengths thanks to the displacement of the spectral focusing optical group 9. In other words, despite the chromatism of the optical system 5, the latter has an optimized modulation transfer function for each particular wavelength, when the spectral focusing optical group 9 is positioned so that the electromagnetic radiation R at said particular wavelength is focused, the modulation transfer functions for the other wavelengths being degraded.

[0056] Each image acquired by the sensor 3 for a focused wavelength, understood to be focused on the sensor, is an image representing in sharp focus all the elements of the scene having a component in the focused wavelength, the other wavelengths being blurred because they are not focused on the sensor 3. In other words, the acquired image is, for each position of the spectral focusing optical group 9, the algebraic sum of a range of sharp wavelengths, namely a range a few nanometers wide without defocus, and of all the other defocused wavelengths. The acquisition of several images over an entire wavelength range, for example from 500 nm to 1000 nm, therefore makes it possible to highlight a detail, in particular a laser spot, in at least one of these images.For example, the acquired image relating to the focusing on the sensor 3 of an electromagnetic radiation R700 of wavelength 700 nm makes it possible to highlight a laser spot of the observed scene produced by a laser of wavelength equal to 700 nm. The highlighting is carried out naturally because in practice, a reduced quantity of photons of the scene correspond as well to the focused wavelength as to the laser spot. A contrast in the image therefore appears naturally and can be identified by an operator, or by a recognition algorithm.

[0057] Figure 1 illustrates the multispectral imager in a case where the spectral focusing optical group 9 is positioned so that it is an electromagnetic radiation R700 of wavelength 700 nm which is focused on the sensor 3.

[0058] Similarly, Figure 2 illustrates the multispectral imager in a case where the spectral focusing optical group 9 is positioned so that it is electromagnetic radiation R800 of wavelength 800 nm which is focused onto the sensor 3.

[0059] Advantageously, the system 13 for axial displacement of the optical spectral focusing group 9 comprises a cam displacement system, and / or a stepping motor, and / or a continuous motor, and / or a piezoelectric motor.

[0060] Advantageously, the axial displacement system 13 is configured to axially move the spectral focusing optical group 9 with a minimum increment of 5 micrometers.

[0061] Thus, whatever the optical design of the optical system 9, such an increment allows precise movement of the spectral focusing optical group 9, from 5 micrometers to 5 micrometers, and therefore the acquisition of images regularly over a spectral range of interest, for example from 500 nm to 1800 nm.

[0062] Preferably, the axial displacement system 13 is configured to axially displace the spectral focusing optical group 9 so that image acquisition can be carried out for electromagnetic radiation R of wavelengths spaced 10 nm apart. A set of images can therefore be acquired, each image corresponding to a different focused wavelength, each wavelength being spaced 10 nm apart.

[0063] Figure 3 shows a method 17 for acquiring images of the same scene by the multispectral imager 1.

[0064] To implement the method, a step 19 is first carried out of positioning the optical spectral focusing group 9 in an axial position corresponding to a particular wavelength focused on the sensor 3. This is, for example, a wavelength located at the end of the spectral range of interest, for example at 500 nm or at 1800 nm.

[0065] A step 21 of acquiring an image by the sensor 3 is then carried out.

[0066] In order to obtain a set of images over the entire spectral range of interest, steps 19 and 21 are repeated for different positions of the spectral focusing optical group 9 corresponding to focused wavelengths spanning the spectral range of interest, for example from 500 nm to 1800 nm, preferably 1000 nm. For example, the set of images is acquired at a rate of 50 images per second.

[0067] Advantageously, a sufficient number of images is acquired during the method so as to obtain an image acquired for wavelengths focused at least every 20 nm, preferably at least every 10 nm, for a spectral range of interest of wavelength between 500 nm and 1800 nm, preferably 1000 nm.

[0068] Two images acquired at different focused wavelengths and within a spectral range of 50 nm are called adjacent images.

[0069] The method comprises a step 23 of obtaining a processed image for each image acquired at a particular focused wavelength, the processed image being obtained by subtraction, for example pixel by pixel, of the acquired image with an adjacent image or with an average of a group of adjacent images.

[0070] Thus, for an acquired image corresponding to a wavelength focused on sensor 3 of 800 nm, noted 1800, it is possible to subtract the two closest adjacent images 1810 and 1790, respectively at 810 nm and 790 nm, for example if an image acquisition is calibrated to be carried out every 10 nm, and to obtain the corresponding processed image IT800 according to the formula:

[0071] 1

[0072] 7T800 = 7800 - - (7790 + 7810)

[0073] This step 23 allows you to further highlight the details of the scene corresponding to a specific wavelength, such as a laser spot.

[0074] Advantageously, the subtraction, preferably pixel by pixel, is carried out by subtracting the gray intensities of each pixel. To do this, each acquired image is first converted into gray intensity.

[0075] In a particular embodiment, the method 17 further comprises a step 25 of processing the acquired images by applying an algorithm for stretching the histogram of each acquired image. This step 25 makes it possible to further highlight the differences in contrast in the imaged scene and to promote the detection of a laser spot.

[0076] Optionally, a step 27 is finally carried out to locate a laser spot in one of the acquired or processed images.

[0077] This step 27 is carried out either visually by an operator or by a detection algorithm scanning all of the acquired or processed images.

Claims

CLAIMS 1. Multispectral imager (1) comprising a sensor (3) and an optical system (5), characterized in that the optical system (5) comprises on an optical axis (X-X') a spectral focusing optical group (9), as well as a system (13) for axial displacement of said spectral focusing optical group (9), the optical system (5) being configured to refract electromagnetic radiation (R) incident on the sensor (3) and to implement spectral defocusing along the optical axis (X-X') of the refracted electromagnetic radiation (R), the spectral defocusing being greater than 100 micrometers along the optical axis (X-X') for a difference of 100 nanometers in wavelength and for a wavelength between 500 nm and 1800 nm, preferably 1000 nm,the axial displacement system (13) making the spectral focusing optical group (9) movable between a focusing position on the sensor (3) of electromagnetic radiation (R) with a wavelength between 500 nm and 700 nm, and a focusing position on the sensor (3) of electromagnetic radiation (R) with a wavelength between 1000 nm and 1800 nm, the axial displacement system (13) being configured to axially move the spectral focusing optical group (9) with a minimum increment of 5 micrometers., 2. Imager (1) according to claim 1, in which the spectral focusing optical group (9) is made of dispersive glass with an Abbe number of less than 45, preferably less than 30, even more preferably less than 20.

3. Imager (1) according to one of claims 1 and 2, in which the spectral focusing optical group (9) comprises a lens, or several lenses, or a plate with flat and parallel faces (15), at least one of the faces (15) comprising a metasurface.

4. Imager (1) according to any one of claims 1 to 3, in which the system (13) for axial displacement of the spectral focusing optical group (9) comprises a cam displacement system, and / or a stepping motor, and / or a continuous motor, and / or a piezoelectric motor.

5. Method (17) for acquiring images of the same scene by the multispectral imager (1) according to any one of claims 1 to 4, characterized in that it comprises the following steps, implemented repeatedly for different positions of the spectral focusing optical group (9) corresponding to particular focused wavelengths: Positioning the spectral focusing optical group (9) in an axial position corresponding to a particular sensor-focused wavelength (3) (step 19); and - Acquisition (step 21) of an image by the sensor (3).

6. Method (17) according to claim 5, wherein two images acquired at different focused wavelengths and included in a range of 50 nm are called adjacent images, the method comprising a step (23) of obtaining a processed image for each image acquired at a particular focused wavelength, the processed image being obtained by subtraction, preferably pixel by pixel, of the acquired image with an adjacent image or with an average of a group of adjacent images.

7. Method (17) according to claim 6, in which the subtraction is carried out by subtracting the gray intensities of each pixel.

8. Method (17) according to any one of claims 5 to 10. 7, during which a sufficient number of images are acquired so as to obtain an image acquired for wavelengths focused at least every 20 nm, preferably at least every 10 nm, for a wavelength range between 500 nm and 1800 nm, preferably 1000 nm.

9. Method (17) according to any one of claims 5 to 10. 8, further comprising a step (25) of processing the acquired images by applying an algorithm for stretching the histogram of each acquired image.

Images