Patent Publication Number: US-2006017656-A1

Title: Image intensity control in overland night vision systems

Description:
BACKGROUND  
      The present invention generally relates to an infrared night vision system. Specifically, the present invention relates to a near-infrared night vision system.  
      Despite technological developments in automotive safety during the past few decades, a driver still faces the danger of not seeing many hazards, such as pedestrians, animals, or other cars, after sunset that are easily avoided during the daytime. Recently, night vision monitoring systems have appeared in certain vehicles. These systems are based on a camera that detects far-infrared radiation with a wavelength of, for example, between of about 8 μm to 14 μm and displays the detected image at the lower part of the windshield. Such radiation provides useful thermal information of objects, which the human eye cannot detect. Far-infrared night vision system are passive systems since the illumination source is not necessary. These systems are capable of monitoring objects that are as far away as 400 m from the vehicle because the propagation path is a single trip. However, the cameras for these systems are quite costly.  
      More recently, near-infrared night vision systems have appeared in the automotive market. These systems are active systems in which a near-infrared source emits radiation with a wavelength, for example, between about 0.8 μm to 0.9 μm to illuminate objects in the road. Since this wavelength is invisible, the system can keep the illumination source in a high position even though there are on-coming vehicles. Thus, long range traffic conditions are visible to the driver as if the headlight is in high beam condition even though the actual leadlight is in low beam condition. A camera detects the reflection from the object, and the reflected image is displayed at the lower part of the windshield. The near-infrared night vision has a limited range of about, for example, 150 m, but the image is similar to that visualized by human eye, and the camera cost is much lower than that of the far-infrared night vision system. Similar to the aforementioned far-infrared system, the image is projected in a non-overlaid heads-up display, in which the driver has to compare the image in the lower part of the windshield with the actual image of the object.  
      To avoid the process of comparing the camera image with the actual image, which can reduce driver fatigue, an over-laid heads-up display is desirable, in which the camera image is overlaid on the actual image. However, there are several problems associated with over-laid heads-up displays. For instance, the positions of the images have to coincide with each other precisely, the images have to be similar to each other, and the camera image intensity has to be adequate. Although the positions of the images can be managed by the geometrical transformation of the camera, and the image similarities can be obtained in the near-infrared system since the wavelength between near-infrared radiation and visible light are similar, unfortunately, heretofore, there has been no effective method proposed to control the image intensity of the camera image, even though this control is critical for over-laid heads-up displays, since too strong or saturated image disturbs the actual image and too weak of an image is not effective.  
      In view of the above, it is apparent that there exists a need for a near-infrared night vision system that is able to suppress the saturation of the camera image in the over-laid heads-up display and keep the balance of the intensity between the camera and the actual images, since the saturation disturbs the actual image and may result in an accident.  
     SUMMARY  
      In satisfying the above need, as well as overcoming the enumerated drawbacks and other limitations of the related art, the present invention provides a near-infrared night vision system and method that controls the intensity of a reflected beam received by a camera in an over-laid heads-up display.  
      In a general aspect, an infrared source emits a near-infrared beam toward an object, and the infrared beam is reflected from the object as a reflected beam. The camera receives the reflected beam and generates an image signal in response to the reflected beam. An image processor receives the image signal, generates a distribution of intensities, compares the distribution to a threshold, and generates a display signal based on the comparison. A heads up display receives the display signal, generates a reflected image in response to the display signal, and overlays the reflected image over the actual image of the object.  
      In various embodiments, the image processor reduces the intensities received by the camera when the number of the cells having intensities exceeding the threshold is higher than a pre-determined value and increases the intensities received by the camera when the number is lower than the value. An attenuator may be employed to control the intensities received by the camera in response to the comparison between the distribution and the threshold. Alternatively, a power supply coupled to the infrared source may be employed. The power source modifies the power to the infrared source in response to the comparison between the distribution and the threshold.  
      Further objects, features and advantages of this invention will become readily apparent to persons skilled in the art after a review of the following description, with reference to the drawings and claims that are appended to and form a part of this specification. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1A  is a schematic view of a near-infrared night vision system in accordance with an embodiment of the present invention;  
       FIG. 1B  is a schematic view of the system of  FIG. 1A  implemented in a vehicle;  
       FIG. 2A  is schematic of an image at night without the use of a night vision system;  
       FIG. 2B  is a schematic of the image of  FIG. 2A  with the use of a near-infrared night vision system;  
       FIG. 3  is a schematic view of a far-infrared night vision system; and  
       FIG. 4  is a schematic of a near-infrared night vision system in accordance with another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION  
      Referring now to  FIGS. 1A and 1B , a near-infrared night vision system embodying the principles of the present invention is illustrated therein and designated at 10. As its primary components, the system  10  includes an illuminating source  12  with a power supply  14 , a camera  16 , an image processor  18 , and a heads up display  20 .  
      The system  10  resides in a vehicle  21 , and when in use, the source  12 , such as a halogen, laser diode or light-emitting diode, projects a near-infrared radiation beam  22  at one or more objects  26 , for example, a pedestrian  28  or a car  30 , or both. The radiation beam  22  has a power that is sufficient to illuminate the objects  26 . In certain embodiments, the beam has a wavelength between about 0.8 μm to 0.9 μm for a halogen source or has a bandwidth of about 3 nm for a laser diode.  
      The camera  16  detects a reflected beam  24  from the objects  26  and generates an image signal in response to the reflected beam. The image processor  18  processes the image signal (IS) from the camera  16  and provides a display signal (DS) to the heads up display  20 . The heads up display  20  generates a reflected image in response to the display signal and-overlays the reflected image over the actual image of the objects  26  as seen through the windshield of the vehicle  30 . The heads up display can be of common construction. In some configurations, the reflected image is displayed directly on the windshield. Alternatively, the heads up display  20  includes a semi-transparent glass on which the reflected image is displayed and through which the actual image can be seen.  
      For purposes of illustration,  FIG. 2A  illustrates the oncoming vehicle  30  on a road  31  as might be seen at night by the driver of the vehicle  21 , and  FIG. 2B  illustrates a view of the vehicle  30  and a set of poles  32  with the use of near-infrared illumination.  FIG. 2B  also illustrates the pedestrian  28  at a distance associated with the high-beam range (that is, beyond the low-beam range) that may not be seen without the use of the illumination system. The saturation of the camera image in the over-laid near-infrared night vision system caused by the headlamps of the vehicle  30  might disturb the view of the pedestrian  28 .  
      The camera  16  can be, for example, a CCD camera or a CMOS camera with a plurality of cells that captures the reflection from the objects  26 . Since the reflected beam  24  to the camera  16  has a distribution of intensities that may change significantly during the operation of the system  10 , certain cells may become saturated if the camera does not have a sufficient dynamic range. If saturation occurs, the reflected image in the heads up display will disturb the view of the actual image. For example, the reflected image of the poles  32  or the front of the car  30  in  FIG. 2B  may interfere with the actual image of the objects since this is an over-laid system.  
      The dynamic range of a reflected beam can be determined from the reflection coefficients of typical objects in front of the camera, output-power of the illuminating source, and the range between the objects and the camera. In particular, the intensity of the power received by the camera is inversely proportional to the 4 th  power of the distance between the object and the camera. For example, the reflection coefficient is usually in the range between about 0.1 to 1.0, and the effective operating distance of a near-infrared night vision system is between the camera and the object is usually in the range between about 5 m to 150 m. Thus, a camera needs a dynamic range of about 70 dB to view the object without saturation, as determined by adding the following two expressions 
 
10 dB=10 log (1.0/0.1) 
 
60 dB=10 log (150/5) 4  
 
      Thus, if the dynamic range of the camera is not sufficient, the saturation of the camera cells may occur, for example, as the object moves closer to the camera and the intensity of the source is high. However, the system  10  controls the intensity received by the camera  16  so that the reflected image is not saturated in a way that disturbs the view of the actual image when the reflected image is displayed in the over-laid heads up display  20 , and, therefore, the dynamic range of the camera can be used effectively. Hence, potentially fatal accidents associated with the disturbance of the actual image may be eliminated.  
      The system  10  controls saturation of the cells in the camera  16  by varying the power from the power supply  14  to the source  12  with a process  40  implemented as an algorithm, for example, in the image processor  18 . In essence, the system  10  controls the saturation by controlling the illumination power on the basis of an intensity histogram  42 , which represents a distribution of the number of camera cells exposed to a particular intensity.  
      Specifically, after the camera  16  captures an image, process  40  generates the histogram  42 . In some circumstances, the camera cells having the intensity larger than the threshold may be considered saturated cells. A decision step  44  determines if the number of the cells with intensities exceeding the threshold is larger than a pre-determined number. If so, then step  46  calculates a reduced power, and step  50  averages the value of the reduced power, for example, by integration to provide a smooth transition and an appropriate time delay that is compatible with human eyes. The averaged power value is sent to a power limiter  52 , which, in turn, reduces the power (P) from the power supply  14  to the source  12 .  
      Hence, step  44  determines whether the number of the cells with the high intensity exceeding the threshold is larger or smaller than the pre-determined value, and step  48  calculates an increased or decreased power and provides this value to the averaging step  50 , where a time delay is produced, before the power limiter  52  increases or decreases the power (P) from the power supply  14  to the source  12 .  
      Accordingly, the system  10  generates a reflected image overlaid with the actual image in a manner that does not disturb the view of the actual image by reducing the saturation of the camera cells. In this way, the dynamic range of the camera is fully utilized, and the requirement for the large dynamic range is reduced considerably, which reduces cost requirements, since cameras with large dynamic ranges are typically quite costly.  
      For the sake of comparison,  FIG. 3  illustrates a typical configuration of a far-infrared night vision system in which a far-infrared camera  60  is mounted on a vehicle  62 . The camera  30  detects a radiation beam  64  corresponding to thermal emissions of the person  24  or vehicle  26 . Referring to Table 1 below, near-infrared imaging systems, such as the system  10 , provides certain benefits over far-infrared systems. A particular drawback of far-infrared systems is their costs. With near-infrared systems, conventional devices such as halogen or laser diode sources and CCD or CMOS cameras can be used for the source  12  and camera  16 , respectively. Therefore, the cost of near-infrared systems are lower than that of far-infrared systems. Moreover, the image of the object appears more natural in near-infrared systems than in far-infrared systems.  
               TABLE 1                          Comparison between Far-infrared and Near-infrared systems                         Item   Far infrared (FIR)   Near infrared (NIR)               Basic:               Wavelength   8 to 17 μm   0.9 μm       Band   6 μm   2-3 nm       Active/passive   passive   active       Image resolution   low   high               (large number of cells)       System:       Azimuth angles   &gt;11 degrees   &gt;14 degrees           (limited by number   (with large cell number)           of cells)       Performance:       Range   &gt;400 m   150-200 m       Human detection   good   depends on clothes       Lane detection   difficult but possible   possible       Road side object   fair,   good       detection   necessary to process       Quality of image   not good,   good           necessary to process       Transmission       at 300 m:       Rain   good   fair       (medium 12.5 mm/h)       Fog (light)   fair   poor                  
 
      Referring now to  FIG. 4 , there is shown a system  100  in accordance with an alternative embodiment of the present invention. The system  100  eliminates the power limiter  52  for the power supply  14  of the aforementioned system  10  but incorporates an attenuator  102  positioned between the camera  16  and the objects  26 .  
      The system  100  controls the saturation of the camera cells by varying the attenuation of the reflected image  24  with an attenuator  102  with a process  104  implemented, for example, as an algorithm in the image processor  18  based on an intensity histogram  106  of the intensity received by the individual cells of the camera  16 .  
      Specifically, as the camera  16  receives the reflected beam  24  of the objects  26  through the attenuator  102 , the process  104  generates the histogram  106 , which indicates the number of cells at each intensity. The cells having an intensity larger than the threshold may be considered saturated cells. A decision step  108  determines if the number of the cells with an intensity exceeding the threshold is larger than a pre-determined value, and, if so, step  110  calculates an increased attenuation. The value of the increased attenuation is then averaged in step  114 , for example, by integration to provide an appropriate time delay that is compatible with human eyes. The averaged attenuation value is then provided to the attenuator  102  to further attenuate the intensity of the reflected image received by the camera  16 .  
      If step  108  determines that the cells at the highest intensity do not exceed the threshold value, then step  112  calculates a decreased attenuation value and provides this value to the averaging step  114 , where again a time delay is produced before the averaged attenuation value is provided to the attenuator  102  to decrease the attenuation of the reflected beam  24  received by the camera  16 .  
      In sum, the system  100  generates a reflected image of an object which is overlaid on the actual image in the heads up display  20 . The reflected image does not disturb the view of the actual image since the system  100  attenuates the intensity of the reflected beam received by the camera  16 . Again, the dynamic range of the camera is used effectively and the requirement for the large dynamic range is reduced remarkably, which reduces cost requirements. Moreover, the attenuation control operates independently from the power supplied to the source  12 , and the attenuator  102  itself may be a simple mechanism that is commercially available. This enables easy installation of the system  100  in a vehicle. Moreover, similar to the system  10 , the system  100  uses low cost hardware to minimize costs.  
      As a person skilled in the art will readily appreciate, the above description is meant as an illustration of various implementations of the principles this invention. This description is not intended to limit the scope or application of this invention in that the invention is susceptible to modification, variation and change, without departing from spirit of this invention, as defined in the following claims.