DE102020133691A1 - Infrared thermal imaging camera and on-board system to assist in driving a motor vehicle having such a camera - Google Patents

Abstract

The subject matter of the present invention is an infrared thermal imaging camera (1') that can be integrated into a motor vehicle, the thermal imaging camera (1') having an optical block (2') and being characterized in that the optical block (2') has a front lens (L '1) forming an external part of the thermal imaging camera (1'), the front lens (L'1) being made of germanium or of silicon. Application: driver assistance systems

Description

technical field

The present invention relates generally to the field of motor vehicles, and more particularly to assisting in driving a motor vehicle by detecting objects in the vicinity of a motor vehicle based on images captured by an infrared thermal imaging camera.

General state of the art

To increase road safety, some motor vehicles, known as semi-autonomous motor vehicles, are equipped with partial automation systems or driver assistance systems (also known by the English abbreviation ADAS), in particular with systems that carry out lateral control and/or longitudinal control of the vehicle instead of the driver, or at least place the driver in front warn of a potentially dangerous situation in order to enable him to react in good time. It is also planned that vehicles will be fully autonomous, i. H. driverless to do.

Some ADAS systems use conventional camera-type vision devices that operate in the visible wavelength range to detect the presence of pedestrians, bicyclists, animals, and/or other motor vehicles in the vicinity of vehicles with these integrated ADAS systems . These vision systems rely on sunlight or streetlights, which reduces the effectiveness of the ADAS systems in driving situations with poor visibility, typically at night and/or in adverse weather conditions such as fog, heavy rain and snow.

To solve this problem, some ADAS systems use thermal imaging night vision devices, such as thermal infrared cameras, that have at least one wavelength in the range [8 µm; 14 µm].

As can be seen from the schematic representation in 1 As can be seen, an infrared thermal imaging camera 1 conventionally comprises an optical block 2 (also called a lens) and an infrared sensor 3 placed behind the optical block 2 and centered on the optical axis 4 of the optical block 2. In the application in ADAS systems, the infrared sensor 3 is generally a microbolometer forming a two-dimensional matrix of pixels with a ratio of 4/3 between the matrix width and the matrix height. For example, a VGA format sensor is used to obtain a resolution of 640 x 480 pixels. A sensor in QVGA format can also be used if a resolution of 320 x 240 pixels is desired. In all cases, the infrared thermal imaging camera 1 is capable of detecting infrared rays emitted, transmitted or reflected by an obstacle such as a pedestrian 5 in its field of view. More precisely, these infrared rays penetrate the aperture of the optical block 2, which concentrates them so that they hit some of the pixels of the matrix forming the microbolometer, thus modifying their electrical resistance by heating. The microbolometer then provides a thermal image (or thermogram) whose resolution corresponds to the resolution of the pixel matrix.

A typical optical block of a thermal imaging camera for an ADAS system is composed of two or three lenses, such as the lenses labeled L ₁ , L ₂ , and L ₃ in FIG 1 . The number and type of lenses used depends mainly on the requirements in terms of image quality and field of view. The lenses are conventionally made of chalcogenide glass. Chalcogenide glass has excellent transmission properties at wavelengths in the range [8 µm; 14 µm]. In addition, chalcogenide glass can be formed, so that any type of lens, in particular aspherical lenses, can be produced in a simple manner.

However, chalcogenide glass is not sufficiently impact-resistant. Consequently, since the camera is mounted on the outside of the vehicle, for example on the front bumper, the camera 1 also has a protective glass 6 which is arranged in front of the optical unit 2 . This protective glass 6 is generally made of germanium and is typically about 4 mm thick in order to ensure effective resistance to impacts from stones and gravel.

Thus, with the known thermal infrared cameras of the in 1 of the type shown schematically, the optical block 2 and the protective glass 6 are the most expensive components (after the bolome tersensor 3), which is due to the use of certain materials such as germanium or chalcogenide glass.

Summary of the Invention

The object of the present invention is to overcome the limitations of the prior art by proposing a solution that allows the protective glass 6 to be dispensed with.

Consequently, the subject of the present invention is an infrared thermal imager that can be integrated into a motor vehicle, the thermal imager having an optical block and being characterized in that the optical block has a front lens which forms an outer part of the thermal imager, the front lens being made of germanium or is made of silicon.

In one possible embodiment, the front lens is made of germanium with an average minimum thickness of 3 mm.

In this case, the germanium front lens preferably has an average maximum thickness of 4 mm.

In another possible embodiment, the front lens consists of silicon with an average minimum thickness of 1.5 mm.

The silicon front lens preferably has an average maximum thickness of 2 mm.

The outer surface of the silicon front lens is preferably coated with a DLC type hard coating.

In one possible embodiment, the front lens can be spherical.

In one possible embodiment, the optics block further includes at least one other aspheric lens made of chalcogenide glass located behind the front lens along the optical axis of the optics block.

The front lens is preferably provided with an anti-reflective coating.

For example, the infrared thermal imaging camera is configured to operate at a wavelength in the range [8 µm; 14 µm] with an average transmission rate of the optical block of at least 59%.

In one possible embodiment, the infrared thermal imaging camera further includes a VGA or QVGA format microbolometer centered on the optical axis and behind the optical block.

Another object of the present invention is a driver assistance system integrated into a motor vehicle, which is characterized in that it has a night detection device and an infrared thermal imaging camera according to the invention mounted outside the motor vehicle.

character list

• - 1, die bereits oben beschrieben wurde, in schematischer Weise ein Beispiel einer bekannten Infrarot-Wärmebildkamera;
• - 2 in schematischer Weise ein Beispiel einer Infrarot-Wärmebildkamera nach einer möglichen Ausführungsform der vorliegenden Erfindung;
• - 3 die Variationen der Übertragungsrate einer Siliciumlinse in Abhängigkeit von der Wellenlänge für zwei unterschiedliche Linsendicken;
• - 4 ein Prinzip zur Reduzierung des Erfassungskreises eines optischen Blocks auf ein Panoramaformat.

The invention will be clarified by the following description with reference to the accompanying drawings. Show it:

• - 1 , which has already been described above, shows in a schematic way an example of a known infrared thermal imaging camera;
• - 2 in a schematic way an example of an infrared thermal imaging camera according to a possible embodiment of the present invention;
• - 3 the variations in transmission rate of a silicon lens as a function of wavelength for two different lens thicknesses;
• - 4 a principle for reducing the detection circle of an optical block to a panoramic format.

Description of an embodiment(s).

2 Figure 12 shows diagrammatically an example of the infrared thermal imager 1' according to a possible embodiment of the invention, particularly adapted to an ADAS system.

The infrared thermal imaging camera 1 ', like the camera 1 in 1 Prior art shown essentially comprises an optical block 2' with the optical axis 4 and an infrared sensor 3 arranged behind the optical block 2 so that it is centered on the optical axis 4 of the optical block 2. The infrared sensor 3 is preferably a VGA or QVGA format microbolometer.

• - horizontales Sichtfeld (oder FOV): 40 Grad;
• - vertikales Sichtfeld (oder FOV): 30 Grad;
• - Blende: F/1,2;
• - Brennweite: 10,9 mm;
• - Effektiver Durchmesser: 15 mm;
• - gesamte optische Weglänge L: 21,3 mm.

For example, in a thermal imaging camera with a sensor 3 of VGA format (640 x 480 pixels), the optical block 2' is configured to have the following characteristics:

• - horizontal field of view (or FOV): 40 degrees;
• - vertical field of view (or FOV): 30 degrees;
• - Aperture: F/1.2;
• - Focal Length: 10.9mm;
• - Effective Diameter: 15mm;
• - total optical path length L: 21.3 mm.

The optical block 2' has at least one lens L' ₁ . By way of non-limiting example, the optical unit 2' here comprises three lenses L' ₁ , L' ₂ and L' ₃ , the lens L' ₁ being the so-called front lens.

In contrast to the camera 1, the camera 1' has no protective glass here. In other words, the front lens L' ₁ here advantageously forms an outer part of the thermal imaging camera. This is due to the fact that the front lens L' ₁ is made here, according to a first variant of the invention, from silicon, which has a high resistance to stone or gravel impact.

This choice of silicon contradicts the previously known teaching, which assumes that silicon does not have a sufficient transmission rate at wavelengths in the infrared range.

However, the Applicant has carried out various tests to simulate detection distances obtained with conventional ADAS systems for detecting pedestrians, cyclists and animals, which have surprisingly shown that a transmission of around 59% for the optical block of an infrared -Thermal imaging camera in the 8 - 14 µm band is sufficient to achieve the minimum detection distances (or ranges) generally required by these ADAS systems (typically between 50 and 100 m for a pedestrian).

• - eine erste Kurve C₁, die die Änderungen der Übertragungsrate einer Siliciumlinse mit einer mittleren Dicke von 1,5 mm in Abhängigkeit von der IR-Wellenlänge zeigt;
• - eine zweite Kurve C₂, die die Änderungen der Übertragungsrate einer Siliciumlinse mit einer mittleren Dicke von 2 mm in Abhängigkeit von der Wellenlänge IR zeigt.

Tests have also shown that transmission rates of over 59% can be achieved even with a silicon lens, depending on the lens thickness. So shows 3 :

• - a first curve C ₁ showing the variations in the transmission rate of a silicon lens with an average thickness of 1.5 mm as a function of the IR wavelength;
• a second curve C ₂ showing the variations in the transmission rate of a silicon lens with an average thickness of 2 mm as a function of the IR wavelength.

"Mean thickness" means the average of the various thicknesses of the lens measured parallel to the optical axis of the lens over the entire optical surface of the lens. In both curves, the silicon lens has an additional hard coating on the front and an anti-reflective coating on the back.

According to these curves, the average transmission rate T obtained for each of the silicon lens thicknesses (d=1.5mm or d=2mm) depends on the wavelengths, as summarized in the following table: T (d=1.5mm) T (d = 2mm) 8-12 µm 75.8% 71.1% 12 - 14 µm 59.6% 52.5%

In an embodiment of the invention, the average thickness of the silicon front lens L' ₁ is chosen to be at least 1.5 mm.

In another embodiment of the invention, the average thickness of the silicon front lens L' ₁ is chosen to be at most 2 mm.

In a preferred embodiment, the mean thickness of the silicon front lens L' ₁ is chosen to be in the range [1.5 mm; 2 mm] lies.

At the in 2 In the example shown schematically, the silicon lens L' ₁ is advantageously spherical and the average thickness of the silicon lens L' ₁ is chosen to be 1.5 mm and therefore, as seen above, a transmission rate of 75.8% for the wavelengths in the range [8 µm; 12 µm] and a transmission rate of 59.6% for the wavelengths in the range [12 µm; 14 µm]. The other lenses L' ₂ and L' ₃ are aspherical lenses made of chalcogenide glass, each of which has a transmission rate of 96.2% for the wavelengths in the range [8 μm; 12 µm] and a transmission rate of 90.6% for the wavelengths in the range [12 µm; 14 µm].

The outer surface of the silicon lens L'1 is advantageously provided with a hard coating, for example a coating of diamond-like amorphous carbon (known by its English acronym DLC) to resist stone chips. All lens surfaces preferably have an anti-reflective coating to maximize transmission.

$${\text{T}}_{\text{g}}=\mathrm{90,6}\%*\mathrm{90,6}\%*\mathrm{59,6}\%=\mathrm{48,9}\%\text{im Bereich}12-14\mathrm{\mu }\text{m},$$

was einem Durchschnitt von 63 % im Bereich 8 - 14 µm entspricht, was für die ADAS-Anwendungen ausreichend ist.In this case the total transmission T _g of the optical block 2' is given by the product of the transmissions of each lens, ie in our example: $${\text{T}}_{\text{G}}=96.2\%*96.2\%*75.8\%=70.1\%\text{in the area}\mathrm{8th}-12\mathrm{\mu }\text{m},$$

$${\text{T}}_{\text{G}}=90.6\%*90.6\%*59.6\%=48.9\%\text{in the area}12-14\mathrm{\mu }\text{m},$$

which corresponds to an average of 63% in the range 8 - 14 µm, which is sufficient for ADAS applications.

According to a second embodiment of the invention, the material used for the front lens L'1 is germanium. In this case, the front lens L' ₁ has an average minimum thickness of 3 mm. The average thickness of the germanium lens L' ₁ is preferably not more than 4 mm.

The optical block 2' according to the invention is not related to FIG 2 described three lenses limited. Other embodiments, in particular with a greater or lesser number of lenses, can be used without departing from the scope of the present invention, as long as the front lens L'1, which faces directly outside when the camera is mounted on a vehicle, is made of silicon or is made of germanium and the average total transmission rate T _g of the optical block is at least 59% at wavelengths in the range [8 µm; 14 µm].

In any case, not only the cost of the thermal imager is optimized, but also its footprint, since it is no longer necessary to use a protective glass.

As explained above, the sensor 3 used in an infrared thermal imaging camera for an ADAS system is generally rectangular and has a VGA or QVGA format. For its part, the lens (or optical block) is generally composed of circular lenses so that it focuses the incident infrared rays onto a circle, called the detection circle. Traditionally, the specifics of the optical block, in particular in relation to the number of lenses and the type of lenses, are calculated through complex design algorithms that make it possible to meet predetermined requirements in terms of the desired image quality and ensure that the rectangle, the represents the sensor contained in the circle associated with the optical block. This is in view (a) of 4 1, showing that the rectangular shape 7 of the sensor is included in the detection circuit 8 of the optical block. Furthermore, it has been seen that when the front lens L' ₁ is made of silicon, this lens should preferably be spherical for manufacturing cost reasons. In this case, however, it turns out that the design algorithms may not agree, especially when the predetermined requirements include a required field of view (FOV) of more than 40°. In other words, the design algorithms are not able to find an optical block composition that guarantees a field of view greater than 40° given a silicon spherical front lens.

This problem can be avoided if one realizes that a vertical field of view of 20° or less is sufficient when detecting objects for ADAS applications. This means that the 4/3 format traditionally chosen due to the use of a VGA or QVGA sensor actually seems oversized and that a panoramic format (16/9) would suffice. In order to obtain such a panoramic format while still using a 4/3 format sensor, the optical unit 2' is advantageously dimensioned so that its detection circle 8' (see view (b) in Fig 4 ) is not delimited by the rectangle 7 with the format 4/3, but by the rectangle 7' with the panorama format. By reducing the detection loop, the design algorithms can lead to a solution using a silicon spherical front lens. The four corners of the image would be blurred but would be removed from the final final image.

Claims (11)

Infrared thermal imaging camera (1') suitable to be integrated in a motor vehicle, the thermal imaging camera (1') comprising an optical unit (2') and being characterized in that the optical unit (2') has a front lens ( L' ₁ ) forming an external part of the thermal imaging camera (1'), the front lens (L' ₁ ) being made of germanium or silicon. Infrared thermal imaging camera (1 ') after claim 1 , characterized in that the front lens (L' ₁ ) is made of germanium with an average minimum thickness of 3 mm. Infrared thermal imaging camera (1 ') after claim 2 , characterized in that the front lens (L' ₁ ) is made of germanium with an average maximum thickness of 4 mm. Infrared thermal imaging camera (1 ') after claim 1 , characterized in that the front lens (L' ₁ ) is made of silicon with an average minimum thickness of 1.5 mm. Infrared thermal imaging camera (1 ') after claim 4 , characterized in that the front lens (L' ₁ ) is made of silicon with an average maximum thickness of 2 mm. Infrared thermal imaging camera (1 ') according to one of Claims 4 or 5 , characterized in that the outer surface of the silicon front lens (L' ₁ ) is coated with a hard coating of the DLC type. Infrared thermal imaging camera (1') according to any one of the preceding claims, wherein the front lens (L' ₁ ) is spherical. Infrared thermal imaging camera (1') according to any one of the preceding claims, wherein the optical block (2') further comprises at least one further aspheric lens (L' ₂ , L' ₃ ) made of chalcogenide glass arranged along the optical axis (4) of the optical Blocks (2 ') behind the front lens (L' ₁ ) is arranged. Infrared thermal imaging camera (1') according to one of the preceding claims, wherein the front lens (L' ₁ ) is provided with an anti-reflective coating. Infrared thermal imaging camera (1') according to any one of the preceding claims, further comprising a microbolometer (3) of VGA or QVGA format centered on the optical axis (4) and behind the optical block (2'). Driver assistance system which is integrated in a motor vehicle and is characterized in that it has a night detection device which has an infrared thermal imaging camera (1') according to one of the preceding claims mounted outside the motor vehicle.

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