Method for fog detection using different transmit / receive situations and associated fog detection device

Abstract

fog detection device (1), comprising a first optical transmitter (S1) that emits first light with a first spatial intensity distribution (I1) and a first direction of emission in the direction and around a first optical axis (1) generating a first beam of light (11); a second optical transmitter (S2) which, at a time offset from the first optical transmitter (S1), emits second light with a second spatial intensity distribution (I2) and a first emission direction in the direction of and around a second optical axis (2) which is not parallel to the first optical axis (1), generating a second beam of light (12); an optical receiver (E) which is oriented with its receiving direction in the direction of and around a third optical axis (3) which is not parallel to the first optical axis (1) and / or not parallel to the second optical axis (2), to receive reflected first light with a first spatial sensitivity distribution (E1) and reflected second light with a second spatial sensitivity distribution (E2) and to output a first output signal corresponding to the intensity of the received, reflected first light and a second output signal corresponding to the intensity of the received, reflected second light; an evaluation unit (8) configured to form a quotient of the first and second output signals and to assign a fog state to the quotient only if the quotient does not differ by more than a predetermined amount from a predetermined fog threshold value, which was determined or specified as the quotient of a first fog intensity and a second fog intensity prior to the calculation, wherein the first fog intensity corresponds to the first output signal obtained when the first light penetrates a substantially spatially uniform fog distribution and the second fog intensity corresponds to the second output signal obtained when the second light penetrates a substantially spatially uniform fog distribution.

Description

[0001] Optical fog detection devices are known and are regularly used in motor vehicles to automatically warn the driver of a fog situation by means of a signal light or a display, or other road users by means of a rear fog light, or to automatically adapt the vehicle lighting to the fog situation.

[0002] Optical sensors based on light barriers are frequently used today to detect objects located within the beam path. Such sensors can be used, particularly in motor vehicles, to increase safety standards, for example, for fog detection. These fog sensors are based on the principle of reflective light barriers. In DE 196 29 712 A1 and DE 103 02 970 A1, for example, light emitted by several transmitters and reflected from a spatially limited area is received and evaluated by a receiver. By comparing the intensities from the different, limited areas, the presence of fog or another object can be determined. However, defining the different areas requires either transmitters and receivers located far apart and / or expensive transmitters such as laser diodes and / or expensive optical components.Furthermore, these fog detectors are susceptible to dirt on the vehicle's windshield.

[0003] From EP 0 635 731 A1, an optical fog detection device without a further optical transmitter, which emits further light with a further spatial intensity distribution and a further direction of emission, generating a further beam of light, is known.

[0004] The object of the present invention is to provide a fog detection device and a method for its operation which can be implemented in a space-saving and cost-effective manner and is comparatively less susceptible to contamination, for example of a windshield located in the path of the beam.

[0005] The fog detection device according to the invention comprises an optical receiver, a first optical transmitter, a second optical transmitter, and an evaluation unit electrically connected at least to the receiver, preferably to both the receiver and the two transmitters. For example, the receiver is an infrared photodiode and the transmitters are each an infrared diode. The first optical transmitter emits light with a first spatial intensity distribution and a first direction of emission in the direction of and around a first optical axis, for example as a converging, preferably as a diverging beam of light. That is, the main direction of emission of the beam of light is defined by the respective optical axis.According to the invention, a second optical transmitter is provided which, at a time offset from the first optical transmitter, emits a second light with a second spatial intensity distribution and a first emission direction in the direction of and around a second optical axis that is not parallel to the first optical axis, for example as a converging, preferably as a diverging, light beam. For example, the first and second optical axes are skew to each other, preferably they lie in a common plane. Preferably, the first emission direction and the second emission direction are oriented away from the intersection or the point of greatest approximation of the optical axes of the transmitters.

[0006] The optical receiver of the fog detection device is aligned with its receiving direction in the direction of and around a third optical axis, which is non-parallel to the first optical axis and / or non-parallel to the second optical axis. It receives the reflected first light of the first light beam with a first spatial sensitivity distribution and the reflected second light of the second light beam with a second spatial sensitivity distribution, and outputs a first output signal corresponding to the intensity of the received, reflected first light and a second output signal corresponding to the intensity of the received, reflected second light. The first, second, and third optical axes are, for example, skew to each other; preferably, they lie in a common plane and intersect at different angles.According to the invention, the optical axis of a transmitter is defined by the direction of the main emission direction of the transmitter with anisotropic emission characteristics, i.e., by the direction of highest emission intensity of the transmitter, whereas the optical axis of a receiver is defined, according to the invention, by the main reception direction of the receiver with anisotropic reception characteristics, i.e., by the direction of highest reception sensitivity. According to the invention, the transmitter and / or receiver each comprise one or more optical elements, such as lenses, prisms, apertures, or the like, to align, for example, the emission or reception characteristics according to an optical axis. Optical means are understood to be means that can influence the path of the light rays or can block light, such as a lens, a prism, or an aperture.

[0007] According to the invention, the evaluation unit is configured to form a quotient of the first and second output signals and to assign a fog state to the quotient only if the quotient does not differ from a predetermined fog threshold by more than a predetermined amount. The fog threshold is determined or set as the quotient of a first fog intensity and a second fog intensity, wherein the first fog intensity corresponds to the first output signal obtained when the first light penetrates a substantially spatially uniform fog distribution, and the second fog intensity corresponds to the second output signal obtained when the second light penetrates a substantially spatially uniform fog distribution.An exclusively positive assignment means that the fog state is negated if the quotient differs from the predetermined fog threshold by more than a predefined amount. The inventive solution for a fog detection device achieves more reliable fog detection.

[0008] By generating output signals according to the invention based on reflected light which has been reflected from or into spatially different solid angle areas, the reliability of fog detection is increased, or deviations resulting from a strong deviation in the reflection behavior in one of the solid angle areas can be better detected by relating the output quantities in order to avoid future false detections.

[0009] For example, the fog detection device according to the invention is arranged in the interior of a motor vehicle and the transmitter(s) are aligned so that the light beam(s) penetrate a vehicle window, such as a windshield, and are directed into the vehicle environment.

[0010] The evaluation unit is designed, for example, to selectively activate the individual transmitters and the receiver. Generally, the first and second transmitters are not activated simultaneously. In another configuration, the first and second transmitters are activated simultaneously, and the light is, for example, frequency-coded.

[0011] The first transmitter emits light with a first intensity distribution. The second transmitter emits light with a second intensity distribution. The receiver receives light, regardless of the light source's position, with its own specific reception distribution. This reception distribution indicates how sensitive the receiver is to light from a particular area. The first intensity distribution and the reception distribution combine to form a first spatial sensitivity distribution, which incorporates the relative position and orientation between the receiver and the first transmitter. This first spatial sensitivity distribution indicates how sensitive the fog sensor is to reflective objects in a specific area when the first transmitter is activated. The second intensity distribution and the reception distribution combine to form a second spatial sensitivity distribution.The second spatial sensitivity distribution indicates how sensitive the fog sensor is to reflective objects in a specific area when the second transmitter is activated. The optical axis of the transmitters indicates the location and orientation of the main radiation direction; for example, its location is described by the longest extent of a lobe-shaped intensity distribution. Similarly, the optical axis of the receiver is defined by the receiver's location and the point of its greatest reception sensitivity in the spatial reception distribution.

[0012] From the first and second sensitivity distributions, a first and a second sensitivity can be determined along the optical axis of one receiver (in the embodiment with multiple transmitters) and along the optical axis of one transmitter (in the embodiment with multiple receivers), respectively, with a reflecting object located at a distance x from the receiver or transmitter. Distance x is understood here as the smallest distance between the reflecting object and the plane perpendicular to the optical axis of the receiver or transmitter, respectively, passing through the receiver or transmitter. The sensitivity is primarily responsible for the signal actually generated at the receiver, but the actual magnitude achieved is also determined by the type of reflecting object itself. For example, a concrete surface generates a stronger signal in absolute terms than a fog-like medium at the same distance x.

[0013] The two transmitters and the receiver are preferably positioned such that the maximum of the first spatial sensitivity distribution is closer to the receiver than the maximum of the second sensitivity. Both the first and second sensitivities are high enough to detect an optically dense object, such as a sheet of paper, at a distance of several centimeters (i.e., to achieve a corresponding signal-to-noise ratio).

[0014] Preferably, the transmitter(s) and receiver(s) of the fog detection device are arranged behind a window of the vehicle, for example, such that the light generated by the transmitter(s) passes through the window and the light reflected from the object passes through the window again to be received by the receiver(s). Preferably, the first sensitivity in the area just above the windshield is high enough to detect an optically dense object. Preferably, the second sensitivity in the area just above the windshield is low enough to prevent the detection of an optically dense object. Preferably, at least the second sensitivity is high enough to allow continuous detection of an optically dense object from a minimum distance (greater than the distance to the windshield) up to a distance of several meters.

[0015] Preferably, the transmitter(s) and receiver(s) are positioned such that the spatial profile of the quotient of the first and second sensitivity distributions increases monotonically with increasing distance from the receiver, at least along the third optical axis (of the exact one receiver or the exact one transmitter), starting at a predetermined minimum distance, preferably less than 10 cm, and preferably strictly monotonically. The minimum distance is, for example, the distance of the receiver to the surface of the windshield. The spatial profile of the quotient is defined as the distribution along the optical axis of the exact one receiver or transmitter.The variation of the quotient of the first sensitivity distribution and the second sensitivity distribution, determined by exactly one transmitter, is understood as such, which can be determined independently of each other, for example, using the same optically reflecting object at different locations along the optical axis of exactly one receiver or exactly one transmitter.

[0016] A corresponding variation can be achieved, for example, by placing the first transmitter further away from the receiver than the second transmitter. This can be achieved, for instance, by tilting the optical axes of the first and second transmitters relative to the receiver, preferably by tilting them differently.

[0017] Preferably, the transmitter(s) and receiver(s) are positioned such that the ratio of the first and second sensitivity distributions, determined spatially along the third optical axis (of the exact one receiver or the exact one transmitter), is less than one in the region of minimum distance, such as the distance to the windshield, where the first sensitivity is adjusted so that a layer of ice on the windshield produces a measurable signal. In other words, this means that the first intensity distribution has a non-negligible component in the region just above the windshield, while the second intensity distribution is very low in this region.

[0018] According to the invention, the evaluation unit is configured to calculate a quotient of the first and second output signals and to assign a fog state to the quotient only if the quotient does not differ from a predetermined fog threshold by more than a predetermined amount. This threshold is determined or defined as the quotient of a first fog intensity and a second fog intensity, calculated or set prior to the calculation. The first fog intensity corresponds to the first output signal, which is obtained when the light penetrates a substantially uniform fog distribution, while the second fog intensity corresponds to the second output signal, which is obtained when the light penetrates a substantially uniform fog distribution.

[0019] If an optically thin medium such as fog is present in the area in front of the vehicle, portions of the emitted light from the overall intensity distribution are reflected back to the receiver. The sensitivity ratio of the first sensitivity to the second sensitivity in the presence of fog or similar conditions therefore essentially corresponds to the ratio of the limit value x → infinity of the integrated first and second sensitivities, previously referred to as the fog threshold. If the ratio of the output signals lies within a predefined range around the fog threshold, the evaluation unit detects fog and, for example, sends a corresponding signal via the vehicle bus.

[0020] However, fog detection can also be triggered by an optically dense object located at a specific, fixed distance x_lim, thus leading to false detections. Therefore, the two transmitters and the receiver are preferably positioned so that x_lim is 1.5 m apart or less. This prevents false fog detections caused by oncoming or preceding vehicles, tunnels, or trees.

[0021] The two transmitters and the receiver are preferably mounted on a common circuit board, the circuit board having a maximum dimension of a few centimeters, for example 10 cm or 7 cm. Preferably, the evaluation unit is also located on this circuit board.

[0022] Preferably, the third optical axis intersects the first optical axis at a first intersection point, and the third optical axis intersects the second optical axis at a second intersection point, wherein the first and second intersection points are spatially separated and at different distances from the receiver. Preferably, one of the two intersection points is located at a maximum distance of 20 cm from the vehicle windshield, while the other intersection point is located at a distance of several tens of meters from the vehicle windshield.

[0023] The invention further relates to a method for fog detection comprising the following steps. In a provisioning step, a fog detection device is provided. This device comprises a first optical transmitter for emitting first light, for example infrared light, with a first spatial intensity distribution and with a first emission direction in the direction of and around a first optical axis, generating a first light beam. It further comprises a second optical transmitter for emitting second light, for example infrared light, with a second spatial intensity distribution and with a first emission direction in the direction of and around a second optical axis that is not parallel to the first optical axis, generating a second light beam.The provided fog detection device further comprises an optical receiver whose third optical axis is aligned with the third optical axis being non-parallel to the first optical axis and / or non-parallel to the second optical axis, in order to receive reflected first light with a first sensitivity distribution and reflected second light with a second sensitivity distribution, and to output a first output signal corresponding to the intensity of the received, reflected first light and a second output signal corresponding to the intensity of the received, reflected second light. The provided fog detection device further comprises an evaluation unit.

[0024] According to the invention, the first and second lights are then generated sequentially, i.e., by exclusively and successively activating the first and second optical transmitters. In a subsequent step, the evaluation unit determines the ratio of the first and second output signals. A comparison is then made to determine whether the ratio differs from a predetermined fog threshold by more than a specified amount. The fog threshold can be pre-stored as the ratio of a first fog intensity to a second fog intensity, i.e., it can be fixed before the determination process or determined in a prior step.In an evaluation step, the evaluation unit only detects a fog state positively if the quotient differs from the fog threshold by no more than the specified amount. The first fog intensity corresponds to the first output signal obtained when the first light penetrates a substantially spatially uniform fog distribution, and the second fog intensity corresponds to the second output signal obtained when the second light penetrates a substantially spatially uniform fog distribution.

[0025] Subsequently, light, for example infrared light, is generated. In a subsequent step, the evaluation unit determines the ratio of the first and second output signals. A comparison is then performed to determine whether this ratio differs from a predefined fog threshold by more than a specified amount. The fog threshold can be pre-defined as the ratio of a first fog intensity to a second fog intensity, i.e., it can be set before the evaluation process or determined in a prior step. A positive detection of fog by the evaluation unit occurs only if the ratio differs from the fog threshold by no more than the specified amount.The first fog intensity corresponds to the first output signal obtained when the first light penetrates a substantially spatially uniform fog distribution, and the second fog intensity corresponds to the second output signal obtained when the second light penetrates a substantially spatially uniform fog distribution.

[0026] Regarding further embodiments of the method according to the invention, reference is made to the above statements concerning the fog detection device.

[0027] The invention is explained in more detail with reference to the following figures. These figures are to be understood as examples only and represent merely preferred embodiments. They show: Fig. 1 a preferred embodiment of the fog detection device according to the invention 1; Fig. 2 a graphical representation of the quotient of the intensity I2 of the second transmitter by the intensity I1 of the first transmitter as a function of the distance of a reflecting object located on the third axis to the receiver as well as the graphical representation of the quotient of the sensitivity E2 of the second transmitter by the sensitivity E1 of the first transmitter as a function of the distance X of a reflecting object located on the third axis to the receiver.

[0028] The fog detection device 1 according to the invention comprises a first optical transmitter S1 and a second optical transmitter S2. It further comprises an optical receiver E and an evaluation unit 8 electrically connected to both the receiver E and the two transmitters S1 and S2. The receiver E is an infrared photodiode, and the transmitters S1 and S2 each comprise an infrared diode. The first optical transmitter S1 emits light with a first spatial intensity distribution I1 and a first emission direction in the direction of and around a first optical axis 1 as a diverging light beam 11. That is, the main emission direction of the light beam 11 of the first transmitter S1 is defined by the first optical axis 1.According to the invention, a second optical transmitter S2 is provided which, at a time offset from the first optical transmitter S1, emits a second light with a second spatial intensity distribution I2 and a first emission direction in the direction of and around a second optical axis 2, which is not parallel to the first optical axis 1, as a diverging light beam 12. The first optical axis 1 and the second optical axis 2 lie in a common plane. The first emission direction of the first transmitter S1 and the second emission direction of the second transmitter S2 are oriented away from the intersection of the two optical axes 1 and 2. In other words, the first transmitter S1 and the second transmitter S2 both emit light away from the intersection of their optical axes, namely away from the intersection of the first axis 1 and the second axis 2.

[0029] The optical receiver E of the fog detection device 1 is connected with its receiving direction 13, which is defined by a receiving distribution in Fig. The system, symbolically represented in Figure 1, is aligned in the direction of and around a third optical axis 3, which is non-parallel to the first optical axis 1 and non-parallel to the second optical axis 2. Its purpose is to receive the reflected first light of the first light beam 11 with a first spatial sensitivity distribution E1 and the reflected second light of the second light beam 12 with a second spatial sensitivity distribution E2, and to output a first output signal corresponding to the intensity I1 of the received, reflected first light and a second output signal corresponding to the intensity I2 of the received, reflected second light. The first axis 1, second axis 2, and third optical axis 3 lie in a common plane and intersect at different angles.

[0030] The third optical axis 3 of receiver E is defined by the main reception direction of the receiver E with anisotropic reception characteristics, i.e., by the direction of highest reception sensitivity. The first transmitter S1, the second transmitter S2, and the receiver E each have one or more optical elements O1, O2, O3, such as lenses, prisms, apertures, or the like, to align, for example, the emission or reception characteristics and thus the respective optical axes 1, 2, 3. Optical elements O1, O2, O3 are understood to be elements that can influence the path of the first or second light or that can block light, such as a lens, a prism, or an aperture. The first transmitter S1, the second transmitter S2, and the receiver E are arranged on a common circuit board 7, which has a maximum dimension of a few centimeters, for example, 10 cm or 7 cm.The evaluation unit 8 is also located on this circuit board 7.

[0031] The first transmitter S1, the second transmitter S2, and the receiver E of the fog detection device 1 are arranged behind a pane 6, such as the windshield, of the vehicle, so that the light generated by the first transmitter S1 and the second transmitter S2 passes through the pane 6, and the light reflected in spatially separated solid angle regions passes through the pane 6 again for reception by the receiver E. In particular, the points of intersection 4, 5 of the optical axes 1, 2 of transmitters S1, S2 with the third optical axis 3 of receiver E are located at different distances from the receiver E. Here, point of intersection 4 of the first optical axis 1 with the third optical axis 3 is closer to the windshield 6 than point of intersection 5 of the second optical axis 2 with the third optical axis 3, namely at a distance from the windshield that is no more than 20 cm.The distance between the intersection point 5 of the second optical axis 2 and the third optical axis 3, however, is several meters from the front window, for example, 20 m. This distance is measured, for example, along the respective optical axis with respect to the surface of the front window 6 facing the intersection point 4, 5. As shown, both intersection points 4, 5 are located on the side of the front window 6 facing away from the receiver E and the transmitters S1, S2.

[0032] According to the invention, the evaluation unit 8 is configured to form a quotient of the first and second output signals and to assign a fog state to the quotient only if the quotient does not differ from a predetermined fog threshold by more than a predetermined amount. This threshold is determined or set as the quotient of a first fog intensity and a second fog intensity, determined or defined prior to the calculation. The first fog intensity corresponds to the first output signal, which is obtained when the light penetrates a substantially spatially uniform fog distribution, while the second fog intensity corresponds to the second output signal, which is obtained when the light penetrates a substantially spatially uniform fog distribution.

[0033] Due to the inventive alignment of the optical axes 1, 2, and 3, the first sensitivity E1 in the area just above the disc 6 is so high that an optically dense object can be detected. Preferably, the second sensitivity E2 in the area just above the disc 6 is so low that an optically dense object cannot be detected. Preferably, at least the second sensitivity E2 is so high that an optically dense object can be continuously detected from a minimum distance (greater than the distance to the windshield) up to a distance of several meters.

[0034] From the first sensitivity distribution E1 and the second sensitivity distribution E2, a profile of the first and second sensitivity along the third optical axis with respect to a reflecting object can be determined as a function of the distance x to the receiver. Distance x is understood here as the smallest distance between the reflecting object and the plane perpendicular to the third optical axis of the receiver E, passing through the receiver. The sensitivity E1 and E2 are primarily responsible for the actual output signal generated by the receiver E, but the actual magnitude achieved is also determined by the nature of the reflecting object itself. For example, a concrete surface produces a stronger signal in absolute terms than a fog-like medium at the same distance x.

[0035] The two transmitters and the receiver are positioned such that the maximum of the first spatial sensitivity distribution E1 is a smaller distance from the receiver E than the maximum of the second spatial sensitivity distribution E2, which results, among other things, from the corresponding alignment of the first, second and third optical axes.

[0036] The magnitude of the first sensitivity distribution E1 is high enough in the region X_W just above disk 6 to detect an optically dense object. Preferably, the magnitude of the second sensitivity distribution E2 is so low in the region X_W just above disk 6 that an optically dense object cannot be detected. The second sensitivity distribution E2 has a curve as a function of X such that an optically dense object can be continuously detected from a minimum distance (which is greater than the distance to disk X_W) up to a distance of several meters.

[0037] How Fig.As shown in Figure 2, the transmitters S1, S2 and the receiver E are positioned such that the spatial variation of the quotient of the first sensitivity distribution E1 and the second sensitivity distribution E2, determined along the third optical axis, increases monotonically with increasing distance from the receiver, at least from a predetermined minimum distance X_W. The minimum distance X_W is the distance of the receiver E to the surface of the disk 6. This can be further achieved by appropriately aligning the optical axes 1, 2, and 3, for example, by positioning the transmitters S1 and S2 at different distances from the receiver E.

[0038] Transmitters S1, S2 and receiver E are positioned such that the ratio of the first and second sensitivity distributions is less than one in the region of the minimum distance X_W, with the first sensitivity being set so that an ice layer on disk 6 produces a measurable signal. In other words, this means that the first intensity distribution I1 has a non-negligible component in the region just above the disk, while the second intensity distribution I2 is very low in the region just above disk 6.

Claims

[1] Fog detection device (1), comprising a first optical transmitter (S1) that emits first light with a first spatial intensity distribution (I1) and a first direction of emission in the direction and around a first optical axis (1) generating a first beam of light (11); a second optical transmitter (S2) which, at a time offset from the first optical transmitter (S1), emits second light with a second spatial intensity distribution (I2) and a first emission direction in the direction of and around a second optical axis (2) which is not parallel to the first optical axis (1), generating a second beam of light (12); an optical receiver (E) which is oriented with its receiving direction in the direction of and around a third optical axis (3) which is not parallel to the first optical axis (1) and / or not parallel to the second optical axis (2), to receive reflected first light with a first spatial sensitivity distribution (E1) and reflected second light with a second spatial sensitivity distribution (E2) and to output a first output signal corresponding to the intensity of the received, reflected first light and a second output signal corresponding to the intensity of the received, reflected second light; an evaluation unit (8) configured to form a quotient of the first and second output signals and to assign a fog state to the quotient only if the quotient does not differ by more than a predetermined amount from a predetermined fog threshold value, which was determined or specified as the quotient of a first fog intensity and a second fog intensity prior to the calculation, wherein the first fog intensity corresponds to the first output signal obtained when the first light penetrates a substantially spatially uniform fog distribution and the second fog intensity corresponds to the second output signal obtained when the second light penetrates a substantially spatially uniform fog distribution. [2] Fog detection device (1) according to the preceding claim, wherein the specified amount is 0.5, preferably 0.

25. [3] Fog detection device (1) according to one of the two preceding claims, wherein the spatially determined variation of the quotient of first sensitivity distribution (E1) and second sensitivity distribution (E2) along the third optical axis (3) increases monotonically with increasing distance from the receiver (E) at least from a predetermined minimum distance (X_W), preferably from a minimum distance of less than 10 cm, preferably strictly monotonically. [4] Fog detection device (1) according to the preceding claim, wherein the spatially determined quotient of first sensitivity distribution (E1) and second sensitivity distribution (E2) along the third optical axis (3) at the minimum distance (X_W) is less than 1, preferably less than 0.

5. [5] Fog detection device (1) according to one of the two preceding claims, wherein the spatially determined quotient of first sensitivity distribution (E1) and second sensitivity distribution (E2) along the third optical axis (3) at a distance between 1m and 2m from the receiver (E) corresponds to the fog limit value. [6] Fog detection device (1) according to one of the preceding claims, wherein the first optical axis (1) and the second optical axis (2) intersect at a point of intersection (14) which is arranged on the side facing away from the first direction of emission with respect to the first transmitter (S1) and on the side facing away from the second direction of emission with respect to the second transmitter (S2). [7] Fog detection device (1) according to one of the preceding claims, wherein the third optical axis (3) intersects with the first optical axis (1) and the second optical axis (2) at different points of intersection (4, 5), wherein the points of intersection (4, 5) are spatially separated and have different distances from the receiver (E). [8] Arrangement comprising a vehicle window (6) and a fog detection device (1) according to the preceding claim, which is arranged such that first light, second light, reflected first light and reflected second light each pass through the vehicle window (6), and wherein one of the two intersection points (4) is arranged at a maximum distance of 20 cm from the vehicle window (6) and the other of the two intersection points (5) is arranged at a distance of several decades meters from the vehicle window (6). [9] Method for fog detection comprising the following steps: Providing a fog detection device (1), comprising: • a first optical transmitter (S1) to emit first light with a first spatial intensity distribution (I1) and with a first emission direction in the direction and around a first optical axis (1) generating a first beam of light (11); • a second optical transmitter (S2) to emit second light with a second spatial intensity distribution (I2) and a first emission direction in the direction of and around a second optical axis (2) that is not parallel to the first optical axis (1), generating a second beam of light (12); • an optical receiver (E) whose receiving direction is oriented towards and around a third optical axis (3) which is not parallel to the first optical axis (1) and / or not parallel to the second optical axis (2), to receive reflected first light with a first spatial sensitivity distribution (E1) and reflected second light with a second spatial sensitivity distribution (E2) and to output a first output signal corresponding to the intensity of the received, reflected first light and a second output signal corresponding to the intensity of the received, reflected second light; • one evaluation unit (8); Generating the first light and the second light at different times; Subsequent determination of a quotient of the first and second output signals using the evaluation unit (8); Positive detection of a fog state by the evaluation unit (8) only if the quotient does not differ from a predetermined fog threshold by more than a predetermined amount, wherein the fog threshold was pre-stored as the quotient of a first fog intensity and a second fog intensity in a temporally prior determination or determination, wherein the first fog intensity corresponds to the first output signal obtained when the first light penetrates a substantially spatially uniform fog distribution and the second fog intensity corresponds to the second output signal obtained when the second light penetrates a substantially spatially uniform fog distribution. [10] Method according to the preceding claim, wherein the specified amount is 0.5, preferably 0.

25. [11] Method according to one of the two preceding claims, wherein the spatially determined variation of the quotient of first sensitivity distribution (E1) and second sensitivity distribution (E2) along the third optical axis (3) increases monotonically with increasing distance from the receiver (E) at least from a predetermined minimum distance (X_W), preferably from a minimum distance of less than 10 cm, preferably strictly monotonically. [12] Method according to the preceding claim, wherein the quotient at minimum distance is less than 1, preferably less than 0.

5. [13] Method according to one of the two preceding claims, wherein the spatially determined quotient of first sensitivity distribution (E1) and second sensitivity distribution (E2) along the third optical axis (3) at the minimum distance (X_W) is less than 1, preferably less than 0.

5. [14] Method according to any one of the preceding claims 9 to 13, wherein the first optical axis (1) and the second optical axis (2) intersect at a point of intersection (14) which is located on the side facing away from the first direction of emission with respect to the first transmitter (S1) and on the side facing away from the second direction of emission with respect to the second transmitter (S2). [15] Method according to any one of the preceding claims 9 to 14, wherein the third optical axis (3) intersects with the first optical axis (1) and the second optical axis (2) at different points of intersection (4, 5), wherein the points of intersection (4, 5) are spatially separated and have different distances from the receiver (E). [16] Method according to the preceding claim, wherein a vehicle window (6) is further provided and the fog detection device (1) is arranged such that first light, second light, reflected first light and reflected second light each penetrate the vehicle window (6), and wherein one of the two intersection points (4) is arranged at a maximum distance of 20 cm from the vehicle window (6) and the other of the two intersection points (5) is arranged at a distance of several decades meters from the vehicle window (6).

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