Multispectral imager with spectral focusing optical group and associated acquisition method

Abstract

This multispectral imager (1) comprises a sensor (3) and an optical system (5) including a spectral focusing optical group (9) and an axial displacement system (13) for said spectral focusing optical group (9). 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), the spectral defocusing being greater than 100 micrometers for a wavelength difference of 100 nanometers. The axial displacement system (13) moves the spectral focusing optical group (9) 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. Figure for the abbreviation: Fig. 1

Description

Title of the invention: Multispectral imager with spectral focusing optical group and associated acquisition method. Technical field

[0001] The present invention relates to optical systems for multispectral imaging.

[0002] A particularly interesting application of the invention relates to the use of a multispectral imager for the detection of a laser spot of unknown wavelength in a scene observed by said imager. Previous techniques

[0003] In certain applications, particularly those mounted on binoculars or gyrostabilized 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.

[0004] 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 on an image of said scene.

[0005] Indeed, lasers classically have wavelengths between 700nm and Ipm, and coincide with the wavelengths of the visible domain present in the observed scene.

[0006] Lasers operate in this type of situation in continuous or quasi-continuous emission, the integration time of the sensors of conventional imagers not allowing the reliable detection of pulsed emissions.

[0007] Quasi-continuous emission means pulsed laser emission with a periodicity of at least one pulse every 10 ps, ​​and whose pulses are for example wide by 100 nanoseconds.

[0008] Solutions for detecting a laser spot exist and work, for example, using filters. However, these solutions require prior knowledge of the laser's emission wavelength in order to center the filters around that wavelength.

[0009] A laser spot of unknown wavelength cannot therefore be detected, and even less so in an almost instantaneous manner.

[0010] A technology based on a Fabry-Perot interferometer makes it possible to perform a scan of 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 with respect to the direction of the light, it is therefore impossible to image the whole scene. Description of the invention

[0011] 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, even for a laser of unknown wavelength.

[0012] The present invention relates to a multispectral imager comprising a sensor and an optical system including, on an optical axis, a spectral focusing optical group, and an axial displacement system for 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 mobile 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.

[0013] Thus, the multispectral imager has enhanced chromatic aberration properties and, by axial displacement of the spectral focusing optical group, allows for obtaining an image set of the scene at different observation wavelengths. If a laser spot is present in the scene, at least one image in this image set highlights said laser spot. Furthermore, the resulting imager is compact and can detect the laser spot without prior knowledge of the laser's wavelength.

[0014] Furthermore, the present invention goes against a classical optical design, which generally seeks to minimize the chromatic aberration of an optical system in the spectral band of interest in order to increase the optical quality of the images. Here, the pursuit of significant defocusing, and therefore strong chromatic aberration, is paramount.

[0015] Advantageously, the spectral focusing optical group is made of dispersive glass with an Abbe number less than 45, preferably less than 30, even more preferably less than 20.

[0016] In one embodiment, the axial displacement system is configured to axially move the spectral focusing optical group with a minimum increment of 5 micrometers.

[0017] In a particular embodiment, the spectral focusing optical group comprises a lens, or several lenses, or a plate with flat and parallel faces, at least one of the faces comprising a metasurface.

[0018] Advantageously, the axial displacement system of the spectral focusing optical group comprises a cam displacement system, and / or a stepper motor, and / or a continuous motor, and / or a piezoelectric motor.

[0019] 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:

[0020] - Positioning of the spectral focusing optical group in an axial position corresponding to a wavelength focused on the particular sensor; and

[0021] - Acquisition of an image by the sensor.

[0022] In a particular embodiment, two images acquired at different focused wavelengths and within 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 subtracting, preferably pixel by pixel, the acquired image with an adjacent image or with an average of a group of adjacent images.

[0023] Advantageously, the subtraction is carried out by subtracting the grey intensities of each pixel.

[0024] Advantageously, a sufficient number of images are acquired during the process 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.

[0025] In a particular embodiment, the method further includes a step of processing the acquired images by applying an algorithm for stretching the histogram of each acquired image. Brief description of the drawings

[0026] Other objects, features and advantages of the invention will become apparent from the following description, given solely by way of non-limiting example, and made with reference to the accompanying drawings in which:

[0027] [Fig.1] is a schematic view of a spectral imager according to the invention in a first position in which electromagnetic radiation with a wavelength of 700 nm is focused on the sensor;

[0028] [Fig.2] is a schematic view of a spectral imager according to the invention in a second position in which electromagnetic radiation with a wavelength of 800 nm is focused onto the sensor; and

[0029] [Fig.3] is a schematic view of the steps in the image acquisition process according to the invention.

[0030] Detailed description of at least one embodiment

[0031] A multispectral imager 1 according to the invention is schematically represented in figures 1 and 2.

[0032] The multispectral imager 1 is for example embedded in binoculars or a gyrostabilized observation ball.

[0033] The multispectral imager 1 comprises a sensor 3 and an optical system 5 comprising, aligned in that 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 an axial displacement system 13 for said spectral focusing optical group 9.

[0034] Sensor 3 is for example an FPA (“Focal Plane Array”) sensor, for example a Silicon or InGaAs sensor.

[0035] The optical system 5 is configured to refract electromagnetic radiation R incident on the sensor 3 through the front optical block 7, the spectral focusing optical group 9 and the focusing optical block 11.

[0036] The optical system 5 has, for example, a focal length of 40 mm and an aperture number of 2.

[0037] In particular, the front optical block 7 is configured to refract electromagnetic radiation R incident on the spectral focusing optical group 9.

[0038] The front optical block 7 comprises a lens, or a set of several lenses, depending on the optical design envisaged.

[0039] Optionally, the front optical block 7 has strong chromatic aberration. For example, the front optical block 7 is made of dispersive glass with an Abbe number less than 45, preferably less than 30, and even more preferably less than 20.

[0040] Equivalently, the focusing optical block 11 is configured to refract electromagnetic radiation R incident on the sensor 3.

[0041] The focusing optical block 11 comprises a lens, or a set of several lenses.

[0042] Optionally, the focusing optical block 11 has strong chromatic aberration. For example, the focusing optical block 11 is made of dispersive glass with a number d'Abbe less than 45, preferably less than 30, even more preferably less than 20.

[0043] The spectral focusing optical group 9 is configured to refract electromagnetic radiation R incident on the focusing optical block 11.

[0044] In various particular embodiments, the spectral focusing optical group 9 comprises one or more lenses.

[0045] In a particular embodiment as shown, the spectral focusing optical group 9 comprises a plate with flat and parallel faces 15, at least one of the faces 15 comprising a metasurface.

[0046] A metasurface is understood to be a textured surface on the sub-wavelength scale, namely of an order of magnitude less than 500 nm, and exhibiting chromaticism.

[0047] Advantageously, the spectral focusing optical group 9 is made of dispersive glass with an Abbe number less than 45, preferably less than 30, even more preferably less than 20.

[0048] For example, thanks to the enhanced chromatic properties of the front optical block 7 and / or the spectral focusing optical group 9 and / or 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.

[0049] 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.

[0050] In other words, the optical system 5 exhibits such chromatic aberration 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.

[0051] In addition, the axial displacement system 13 makes it possible to move the spectral focusing optical group 9 between a focusing position on the sensor 3 of an electromagnetic radiation R of wavelength of 500 nm and a focusing position on the sensor 3 of an electromagnetic radiation R of wavelength of 1800 nm, preferably 1000 nm.

[0052] Thus, despite the significant chromatic aberration 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 movement of the spectral focusing optical group 9. In other words, despite the chromatic aberration 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 other wavelengths being degraded.

[0053] Each image acquired by the sensor 3 for a focused wavelength, understood to be focused on the sensor, is an image representing in 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 of a few nanometers wide without defocus, and of all the other defocused wavelengths.

[0054] Acquiring several images over a wavelength range, for example from 500 nm to 1000 nm, makes it possible to highlight a detail, in particular a laser spot, in at least one of these images. For example, the image acquired relating to the focusing on sensor 3 of electromagnetic radiation R700 with a wavelength of 700 nm makes it possible to highlight a laser spot in the observed scene produced by a laser with a wavelength of 700 nm. This highlighting occurs naturally because, in practice, a small number of photons in the scene correspond to both the focused wavelength and the laser spot. A contrast in the image therefore appears naturally and can be identified by an operator or by a recognition algorithm.

[0055] Fig. 1 illustrates the multispectral imager in a case where the spectral focusing optical group 9 is positioned so that electromagnetic radiation R700 with a wavelength of 700 nm is focused on the sensor 3.

[0056] Similarly, [Fig.2] illustrates the multispectral imager in a case where the spectral focusing optical group 9 is positioned so that it is an electromagnetic radiation R800 of wavelength of 800 nm which is focused on the sensor 3.

[0057] Advantageously, the axial displacement system 13 of the spectral focusing optical group 9 comprises a cam displacement system, and / or a stepper motor, and / or a continuous motor, and / or a piezoelectric motor.

[0058] Advantageously, the axial displacement system 13 is configured to axially move the spectral focusing optical group 9 with a minimum increment of 5 micrometers.

[0059] Thus, whatever the optical design of the optical system 9, such an increment allows a precise movement of the spectral focusing optical group 9, in 5 micrometer increments, and therefore the acquisition of images in a regular manner over a spectral range of interest, for example from 500 nm to 1800 nm.

[0060] Preferably, the axial displacement system 13 is configured to axially displace the spectral focusing optical group 9 so that image acquisition can be performed for electromagnetic radiation R with 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.

[0061] Figure 3 shows a method 17 for acquiring images of the same scene by the multispectral imager 1.

[0062] To implement the method, a step 19 is first performed to position the spectral focusing optical group 9 in an axial position corresponding to a wavelength focused on the sensor 3 in particular. This is, for example, a wavelength located at the end of the spectral range of interest, for example at 500 nm or 1800 nm.

[0063] Next, an image acquisition step 21 is carried out by sensor 3.

[0064] 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 image set is acquired at a rate of 50 images per second.

[0065] Advantageously, a sufficient number of images are acquired during the process 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 with wavelengths between 500 nm and 1800 nm, preferably 1000 nm.

[0066] Two images acquired at different focused wavelengths and within a spectral range of 50 nm are called adjacent images.

[0067] The method includes a step 23 of obtaining a processed image for each image acquired at a particular focused wavelength, the processed image being obtained by subtracting, for example pixel by pixel, the acquired image with an adjacent image or with an average of a group of adjacent images.

[0068] Thus, for an acquired image corresponding to a wavelength focused on sensor 3 of 800 nm, denoted 1800, it is possible to subtract the two nearest adjacent images 1810 and 1790, respectively at 810 nm and 790 nm, for example if a Image acquisition is calibrated to be performed every 10 nm, and the corresponding IT800 processed image is obtained according to the formula:

[0069] / 7800 = 1800 4(^790 + 1810)

[0070] This step 23 makes it possible to highlight even more the details of the scene corresponding to a precise wavelength, such as a laser spot.

[0071] Advantageously, the subtraction, preferably pixel by pixel, is performed by subtracting the gray intensities of each pixel. To do this, each acquired image is first converted into gray intensities.

[0072] In a particular embodiment, the method 17 further comprises a step 25 for processing the acquired images by applying an algorithm for stretching the histogram of each acquired image. This step 25 makes it possible to highlight the differences in contrast in the imaged scene and to facilitate the detection of a laser spot.

[0073] Optionally, a step 27 is finally carried out to locate a laser spot in one of the acquired or processed images.

[0074] This step 27 is carried out either visually by an operator or by a detection algorithm scanning all the acquired or processed images.

Claims

1. Demands Method (17) for acquiring images of the same scene by a multispectral imager (1) comprising a sensor (3) and an optical system (5), the optical system (5) comprising on an optical axis (X-X') a spectral focusing optical group (9), as well as a system (13) for axially displacing 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) mobile 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, 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 of the spectral focusing optical group (9) in an axial position corresponding to a wavelength focused on the sensor (3) particular (step 19); - Acquisition (step 21) of an image by the sensor (3); and - Obtaining (step 23) a processed image for each image acquired at a particular focused wavelength, the processed image being obtained by subtracting, preferably pixel by pixel, the acquired image with an adjacent image or with an average of a group of adjacent images, two images acquired at different focused wavelengths and within a range of 50 nm being called adjacent images.

2. Method (17) according to claim 1, wherein the spectral focusing optical group (9) is made of dispersive glass with an Abbe number less than 45, preferably less than 30, even more preferably less than 20.

3. A method (17) according to any one of claims 1 and 2, wherein the axial displacement system (13) is configured to axially move the spectral focusing optical group (9) with a minimum increment of 5 micrometers.

4. Method (17) according to any one of claims 1 to 3, wherein the spectral focusing optical group (9) comprises a lens, or several lenses, or a plate with planar and parallel faces (15), at least one of the faces (15) comprising a metasurface.

5. Method (17) according to any one of claims 1 to 4, wherein the axial displacement system (13) of the spectral focusing optical group (9) comprises a cam displacement system, and / or a stepper motor, and / or a continuous motor, and / or a piezoelectric motor.

6. Method (17) according to any one of claims 1 to 5, wherein the subtraction is carried out by subtracting the grey intensities of each pixel.

7. A method (17) according to any one of claims 1 to 6, wherein 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.

8. A method (17) according to any one of claims 1 to 7, further comprising a step (25) of processing the acquired images by applying an algorithm for stretching the histogram of each acquired image.