JPH11295784A - Vehicle number plate photographing device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To surely photograph a luminous numeral and character type number plate without increasing the power consumption of a strobe light source. SOLUTION: A camera for recognition 1 photographs the number plate of a vehicle in the irradiating timing of the strobe light source 3. At such a time, if the number plate is the numeral and character type one, the light emitted from the luminous number plate becomes noise light, so that the number plate can not be surely photographed. The strobe light source 3 is an infrared laser diode and infrared rays having extremely large energy and extremely narrow wavelength half width are outputted from the laser diode in a short time. Since a wavelength selection filter 6 selecting from the laser diode is provided on the front surface of the camera 1, the luminous numeral and character type number plate is surely photographed without being influenced by the light emitted from the luminous numeral and character type number plate.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a vehicle license plate photographing apparatus suitable for photographing a character-type license plate.

[0002]

2. Description of the Related Art Conventionally, a vehicle recognizing device for recognizing a license plate of a vehicle is installed at an entrance of a motorway or a parking lot, and automatically recognizes a license plate number of a passing vehicle, or In the parking lot, the gate is automatically opened and closed when the recognized number is registered. As a light source for photographing a license plate used in this vehicle recognition device,
As shown in JP-A-7-296289, a strobe light source such as a tungsten ball or an LED is used.

[0003]

When the license plate of the vehicle is of the character type, the amount of light emitted from the number itself is larger than the amount of light emitted from the strobe light source and reflected from the character type license plate. There is a problem that the recognition cannot be performed due to the occurrence of the error, or the recognition rate is significantly reduced. To solve this,
The amount of light emitted from the strobe light source and reflected by the character-type license plate needs to be greater than the amount of light emitted from the character-type license plate.

However, in such a configuration, since the wavelength band of the light from the strobe light source is wide, the amount of light reflected from the license plate over the entire wavelength band is required to be larger than the amount of light emitted from the character-type license plate. However, there is a problem that the power consumption of the strobe light source increases.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a vehicle license plate photographing apparatus capable of reliably photographing a character-type license plate without increasing power consumption of a strobe light source. To provide.

[0006]

According to the first aspect of the present invention, when the vehicle is located in the region to be photographed, infrared rays are emitted from the strobe light source to the region to be photographed. Then, the infrared light reflected by the license plate of the vehicle passes through the wavelength selection unit and reaches the imaging unit, so that the imaging unit can image the license plate. In this case, when the license plate is of the letter type, although the energy of the light emitted from the letter type license plate is large over a wide wavelength band as shown in FIG. 8, only the wavelength of the infrared light from the strobe light source is determined by the wavelength selection means. By selecting, the influence of light emitted from the character-type license plate can be suppressed.

Here, the influence of the light emitted from the character light type license plate is smaller when the irradiation time of the strobe light source is shorter. This is because the photographing means such as a CCD camera or an image pickup tube is generally configured to accumulate and output the received light energy during the scanning period of one frame. This is because the light receiving energy in the scanning period of one frame becomes smaller when the scanning period is reached.
In addition, when the half-width of the wavelength of infrared light from the strobe light source is large, the amount of noise light received by the photographing means among the light emitted from the character-type license plate increases,
That makes it more susceptible to the light emitted from the character-type license plate.

Accordingly, the infrared ray from the strobe light source is set to have a wavelength half-width within a predetermined time and equal to or less than a predetermined value, and the passing wavelength band by the wavelength selection means is made substantially coincident with the wavelength band of the infrared ray from the strobe light source. The influence of the light emitted from the character-type license plate photographed by the photographing means can be suppressed, and the character-type license plate can be reliably photographed.

According to the second aspect of the present invention, the infrared laser diode can emit an infrared ray having a very narrow wavelength half width and a very large amount of light in a very short time, so that it is suitable as a strobe light source.

According to the third aspect of the present invention, the irradiation time of the infrared light from the strobe light source is set to a time sufficiently shorter than the scanning period of one frame of the photographing means (generally, 33 msec). Can reliably photograph a character-type license plate within a scanning period of one frame.

According to the fourth aspect of the present invention, the half-width of infrared wavelength of the strobe light source is set sufficiently smaller than the wavelength band of light emitted from the character-type license plate.
Further, since the amount of emitted light is large, the photographing means can suppress the influence of light emitted from the character-type license plate, and can reliably photograph the character-type license plate.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment in which the present invention is applied to a vehicle license plate photographing apparatus for entering a parking lot will be described below with reference to FIGS. FIG. 1 shows an arrangement relationship between a recognition camera and a strobe light source. This figure 1
1, a recognition camera 1 (corresponding to a photographing means) is provided so as to photograph a predetermined photographing target area corresponding to a gate 2 provided on an approach path to a parking lot.

The strobe light source 3 is mainly composed of an infrared pulse laser diode, and irradiates an area to be photographed by the recognition camera 1 with infrared rays. FIG. 2 shows the front of the strobe light source 3. In FIG. 2, a lens 5 is provided corresponding to the laser diode 4, and the laser light from the laser diode 4 is radiated to the photographing target area in a state converted into parallel light by the lens 5. In this case, although the peak output of the energy of the infrared light emitted from the laser diode 4 is large, the light emission time is extremely short. Therefore, the light amount of one laser diode 4 alone is not sufficient as the light amount of the image captured by the recognition camera 1. Therefore, about 100 laser diodes 4 are integrated and arranged in order to secure a sufficient amount of light for photographing by the recognition camera 1.

Here, an optical wavelength selection filter 6 (corresponding to wavelength selection means) is provided on the front of the recognition camera 1. The wavelength selection filter 6 has a half width of 20 to 20 in accordance with the center wavelength 850 nm of the laser diode 4.
A 30 nm narrow band wavelength selection filter is used. Such a narrow-band wavelength selection filter can be manufactured by stacking two dielectric multilayer filters of fluorine or the like.
That is, as shown in FIG. 3, a high-wavelength band cut filter that largely attenuates light in a wavelength band exceeding 850 nm, and as shown in FIG. 4, a low-wavelength band cut filter that largely attenuates light in a wavelength band below 850 nm. Are superimposed on each other to form an extremely narrow wavelength band (20 to 30 nm) having a center band of 850 nm as shown in FIG.
Can be manufactured.

Further, a vehicle sensor 8 (shown only in FIG. 6) is provided on the entrance road for parking in correspondence with the photographing target area, and this vehicle sensor 8 detects that the vehicle is located in the photographing target area. When it is detected, it outputs a detection signal.

FIG. 6 schematically shows the entire electrical configuration. 6, the recognizing device 7 drives the strobe light source 3 at a predetermined timing based on a detection signal from the vehicle sensor 8 and a one-frame scanning start signal from the recognizing camera 1, and recognizes the recognition result by the recognizing camera 1. The gate 2 is opened when it is determined that the vehicle is registered on the basis of the above.

Next, the operation of the above configuration will be described. FIG. 7 shows the operation of the recognition device 7. In FIG. 7, when the vehicle stops in the photographing target area just before the gate 2 to enter the parking lot, the vehicle sensor 8 detects the vehicle and outputs a detection signal. Then, the recognition device 7 determines that the vehicle has stopped in the shooting target area, and emits the flash light source 3. As a result, a very large output and a very narrow wavelength half-width infrared ray are emitted from the strobe light source 3 to the vehicle, so that the infrared light emitted from the strobe light source 3 is reflected by the license plate of the vehicle and transmitted to the recognition camera 1. To reach.
Then, the recognition device 7 recognizes the number of the vehicle by capturing an image from the recognition camera 1 in synchronization with the light emission timing of the strobe light source 3.

In this case, the light emission timing of the strobe light source 3 is a predetermined time (for example, 10 msec) from the timing when the one frame scanning start signal is output from the recognition camera 1.
) Is set to be the timing when the time has elapsed. This is to end the light emission period of the strobe light source 3 within one frame scanning period in the recognition camera 1.

Since the wavelength selection filter 6 is provided on the front surface of the recognition camera 1, the infrared light reflected by the license plate of the vehicle is converted to a wavelength selection filter 6 having a narrow half-value width corresponding to the wavelength of the strobe light source 3. And arrives at the recognition camera 1. Thereby, the influence of visible light which becomes noise light can be prevented.

When the recognizing device 7 recognizes that the number photographed by the recognizing camera 1 is registered, the recognizing device 7 executes the warehousing process after opening the gate 2 and passes the vehicle. In some cases, the gate 2 is closed. With the above operation, the gate 2 automatically opens and closes, so that the driver can enter and park the vehicle in the parking lot without getting off the vehicle.

When the license plate of the vehicle is a character license plate, the recognition camera 1 is affected by the light emitted from the character license plate.

A conventional strobe light source such as a xenon flash lamp has a spectral characteristic in a very wide wavelength band as shown in FIG. 8, and has a wide wavelength band of light emitted from a character-type license plate. Overlapping over the range. For this reason, when the character-type license plate is photographed by the recognition camera 1, halation occurs due to the light emitted from the character-type license plate, and the character-type license plate cannot be reliably photographed.

In this case, an LED having a narrow wavelength band is used as a strobe light source, and the recognition camera 1 captures light in the wavelength band of the LED to prevent the influence of light emitted from the character-type license plate. Although it is conceivable, as shown in FIG. 8, since the wavelength half-width of the light from the LED is as large as 20 to 30 nm, the light amount of the LED should be made larger than the light amount emitted from the character-type license plate over the entire wavelength band. Increases power consumption.

On the other hand, in the present embodiment, the infrared pulse laser diode 4 used as the strobe light source 3 has a half width of 1 to 2 nm, an emission time of 35 nsec, and an output of 30 W, as shown in FIG. Strong monochromatic light can be emitted in a pulsed manner. According to this characteristic, by transmitting an image through the wavelength selection filter 6 having the wavelength selection characteristic (laser light wavelength ± 10 nm) shown in FIG. Become. This principle will be described in more detail.

The total amount of light, which is emitted from the character-type license plate and becomes incident light, enters the recognition camera 1, the energy corresponding to the wavelength from the character-type license plate is f (λ), and the wavelength selection filter 6. Let h (λ) be the transmitted energy corresponding to the wavelength of

(Equation 1) Can be represented by

On the other hand, the amount of light emitted from the strobe light source 3 and reflected by the character-type license plate is
Let g (λ) be the energy corresponding to the wavelength radiated from and reflected by the license plate,

(Equation 2) Can be represented by

Here, when the laser diode 4 is used as the strobe light source 3, the peak light quantity of g (y) is very large, so that the light quantity irradiated from the laser diode 4 and reflected by the character-type license plate is smaller. The light amount is sufficiently larger than the light amount emitted from the optical license plate, and by capturing the light in the extremely narrow wavelength band from the laser diode 4 by the recognition camera 1, it is possible to capture an image that is strong against disturbance light. Therefore, the number recognition rate can be greatly improved.

Here, it is desirable that the half width of the wavelength of the laser diode 4 is 10 nm or less, more preferably 5 nm or less. In other words, the wavelength band of light emitted from the character-type license plate has a wide band characteristic, whereas the wavelength selection characteristic of the wavelength selection filter 6 is usually 20 to 3 in order to efficiently filter with a small amount of light.
Since it is about 0 nm, it needs to be narrower. However, the half width of the ordinary laser diode 4 is about 1 to 2 nm, and the half width of the wavelength does not matter.

The light emitting period of the laser diode 4 is set to one frame scanning period of the recognition camera 1 (about 33 msec).
In this case, the light emission time of the laser diode 4 is 10 ms because the light amount shortage in one frame occurs and the S / N ratio of the image decreases.
c The following is desirable. However, the light emission time of a normal pulsed laser diode is 1 μsec or less, and there is no problem with the light emission time.

The output of the laser diode 4 is: emission time × output = light quantity, but the emission time of the ordinary laser diode 4 is extremely short, 1 μsec.
To increase the amount of light in one frame, a large output is required. In this case, the center wavelength of the laser diode 4 is set to 850 nm and the output is set to 30 W in order to surely photograph the license plate of the vehicle with the recognition camera 1.

The center wavelength of the laser diode 4 is 8
It is not limited to 50 nm, and commercially available products can be widely used. At that time, the fill may be designed according to the wavelength.

According to the present embodiment, when photographing a vehicle with the recognition camera 1, the laser diode 4 for irradiating an infrared ray having a very narrow wavelength half width and a very large output as the strobe light source 3 is used. Since the wavelength selection filter 6 for selecting the wavelength from the laser diode 4 is provided on the front surface of the recognition camera 1, the power consumption can be increased even when capturing a character-type license plate. However, the effect of light emitted from the character-type license plate can be prevented, and the character-type license plate can be reliably recognized.

Further, the light emission timing of the strobe light source 3 is set to end within one frame scanning period of the recognition camera 1, so that a sufficient amount of captured image of the recognition camera 1 can be secured. Further, since the strobe light source 3 emits infrared light, the user does not feel discomfort that the image is being shot.

The present invention is not limited to the embodiment described above, but can be modified or expanded as follows.
As shown in FIGS. 10 and 11, the laser diode 4 may be incorporated in the gate bar 2a so that the vehicle is photographed when the vehicle stops in the photographing target area before the gate 2. In this case, since the license plate can be effectively irradiated with infrared rays from a position close to the vehicle, diffusion of light from the strobe light source 3 can be suppressed and the amount of light received by the recognition camera 1 can be increased.

Further, as the strobe light source 3, an infrared LED having a narrow half width (about 10 nm) other than a laser diode, a YAG laser, or a dye laser may be used.

[Brief description of the drawings]

FIG. 1 is a diagram showing an arrangement relationship between a recognition camera, a strobe light source, and a vehicle according to an embodiment of the present invention.

FIG. 2 is a front view showing a strobe light source.

FIG. 3 is a diagram illustrating transmittance characteristics of a cut filter having high wavelength cut characteristics.

FIG. 4 is a diagram illustrating transmittance characteristics of a cut filter having a low wavelength cut characteristic.

FIG. 5 is a diagram showing transmittance characteristics of a filter having a narrow band transmission characteristic.

FIG. 6 is a schematic diagram showing an electrical configuration.

FIG. 7 is a flowchart showing the operation of the recognition device.

FIG. 8 is a diagram showing the relationship between the wavelength and energy of each light source.

FIG. 9 is a diagram showing a relationship between a wavelength from a laser diode and a transmission wavelength band of a filter.

FIG. 10 is a diagram corresponding to FIG. 1, showing another embodiment of the present invention.

FIG. 11 is a front view of a gate bar.

[Explanation of symbols]

1 is a recognition camera (photographing means), 2 is a gate bar, 3 is a strobe light source, 4 is a laser diode, 6 is a wavelength selection filter (wavelength selection means), and 7 is a recognition device.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FIG06F 15/64 320D 325G

Claims (4)

[Claims] 1. A photographing means for photographing a predetermined photographing target area through which a vehicle passes, and when the vehicle is located in the photographing target area, within a predetermined time and a predetermined time within a scanning period of one frame of the photographing means. A strobe light source for irradiating an infrared ray having a wavelength half-value width equal to or less than the value to the photographing target area; A license plate photographing device for a vehicle, comprising: a wavelength selection unit that passes the light. 2. The vehicle license plate photographing apparatus according to claim 1, wherein said strobe light source is an infrared laser diode. 3. The vehicle license plate photographing device according to claim 1, wherein the irradiation time of the infrared light from the strobe light source is 10 msec or less, preferably 1 msec or less. 4. The license plate photographing apparatus for a vehicle according to claim 1, wherein a half-width at half maximum of infrared light of said strobe light source is 10 nm or less, preferably 5 nm or less.