Abstract:
An improved camera for the acquisition of panoramic images is disclosed. The camera comprises a convex photochromic reflector directed towards an image capture element. Multiple incoming light sensors around the perimeter of the camera detect strong incoming light and cause the activation of a corresponding UV light. The UV light in turn activates the photochromic effect on a particular region of the photochromic reflector that corresponds to an area of strong incident light. The photochromic reflector then darkens in that area, providing compensation for a scene that has a wide range of lighting conditions. Therefore, the camera compensates for strong lighting indoors as well as outdoors.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation-in-part of U.S. patent application Ser. No. 12/035,404 (Panoramic Image Management System and Method), filed on Feb. 21, 2008. 

   FIELD OF THE INVENTION 
   The present invention generally pertains to surveillance systems. More particularly, the present invention pertains to imaging systems for the panoramic surveillance of a space using a single, stationary camera. 
   BACKGROUND 
   Panoramic cameras provide a convenient way to capture a 360 degree field of view with a single camera. Cameras of this type are very useful for surveillance and security applications. Such systems are disclosed in U.S. Pat. No. 7,071,964 (360-Degree Panoramic Scene-Storage Device), and U.S. patent application Ser. No. 12/035,404 (Panoramic Image Management System and Method). Both of these references are incorporated herein by reference, to the extent not inconsistent with this disclosure. Exposure control with cameras of this type can be quite difficult, as there are a variety of lighting conditions, and usage scenarios (e.g. indoor, outdoor, night use) that should be considered for optimal performance. As there is an ever increasing need for security and surveillance, it is desired to have an improved camera for the acquisition of panoramic images in the aforementioned usage scenarios. 
   SUMMARY OF THE INVENTION 
   The present invention provides for an improved camera for the acquisition of 360 degree panoramic images. The camera comprises a convex photochromic reflector disposed at one end of a generally cylindrical housing. The photochromic reflector is directed towards an image capture element (such as a CCD array) at the other end of the housing. The photochromic reflector darkens when exposed to light in the ultraviolet range. For example, sunlight during times where the sun is low in the sky (e.g. early morning or late afternoon) causes the photochromic reflector to darken in the area where the sunlight illuminates the reflector. On the opposite side, not exposed to sun, the photochromic effect does not substantially darken the reflector on that side. The net effect is to direct a light-compensated image into the image capture element. 
   The photochromic effect works well in outdoor situations, where sunlight is the primary light source that needs to be accounted for to maintain proper exposure. However, indoors, an artificial light source may not emit sufficient UV light to trigger the photochromic effect. Various indoor lights have UV filters. Furthermore, many home and building windows filter out substantial UV light. In the case of a 360 degree security camera mounted indoors, yet near a window, it is possible that ambient sunlight may enter the camera, yet the UV components may be filtered out, and therefore does not invoke the desired photochromic effect. 
   To accommodate these situations, the present invention provides a plurality of UV lights disposed around the photochromic reflector. There are also multiple light sensors around the exterior of the camera. When a light sensor detects light above a predetermined intensity level, the corresponding UV light is activated. This causes the photochromic effect to occur in the area where the incident light is received, and creates the desired photochromic effect in that area. A radial mirror (a mirror in the form of a ring that goes around the reflector, and is angled towards the reflector) serves to direct as much of the UV light as possible onto the photochromic reflector, to help maximize the photochromic effect. Therefore, the camera compensates for strong lighting indoors as well as outdoors. 
   The light sensors may include both visible light sensors, and infrared light sensors. For night operations, a plurality of IR (infrared) lights may be disposed around the perimeter of the camera, directed outward toward the area being viewed by the camera. Corresponding IR sensors detect IR light (either reflected light, or light from external IR sources) and activate the appropriate UV light, if too high in intensity, to cause the darkening of the photochromic reflector in the desired area. 
   To provide for additional darkening, the housing may also be comprised of a photochromic material. In this embodiment, there is a “double” photochromic effect, from both the housing and the reflector. This provides for more compensation in extra bright situations. 
   In another embodiment, heating elements are disposed to prevent icing of the camera during cold weather operations. In yet another embodiment, a dew point temperature is calculated from temperature, humidity, and barometric pressure sensors that are on the camera housing. The heating elements are activated to warm the photochromic housing and photochromic reflector when the ambient temperature falls below a predetermined value (e.g. freezing temperature, or a calculated dew point temperature). This maintains the camera temperature above the dew point temperature, and avoids condensation on the optical components such as the reflector or housing. 
   These advantages, along with others, will be further elaborated upon in the detailed description and drawings that follow. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The structure, operation, and advantages of the present invention will become further apparent upon consideration of the following description taken in conjunction with the accompanying figures (FIGs.). The figures are intended to be illustrative, not limiting. 
     In the drawings accompanying the description that follows, in some cases both reference numerals and legends (labels, text descriptions) may be used to identify elements. If legends are provided, they are intended merely as an aid to the reader, and should not in any way be interpreted as limiting. 
       FIG. 1  shows a side view of a camera in accordance with an embodiment of the present invention. 
       FIG. 2  shows a side view of the camera of  FIG. 1  during operation. 
       FIG. 2A  shows a detailed view of an embodiment of a photochromic reflector. 
       FIG. 3  shows a top-down view of a camera in accordance with an embodiment of the present invention. 
       FIG. 4  shows a cutaway side view of the bottom section of a camera in accordance with an embodiment of the present invention. 
       FIG. 4A  shows a cutaway side view of the bottom section of a camera in accordance with an alternative embodiment of the present invention. 
       FIG. 5  shows a view of the underside of the base of a camera in accordance with an embodiment of the present invention. 
       FIG. 6  shows an alternative embodiment of a camera of the present invention. 
   

   DETAILED DESCRIPTION 
     FIG. 1  and  FIG. 2  show a side view of a camera  100  in accordance with an embodiment of the present invention. The camera  100  comprises an optical housing  110  that is substantially transparent. A photochromic reflector  104  is disposed at the top of the housing, and configured to reflect the light from a  360  degree field of view around the camera to an image capture element  136  located within the base of the camera. Top cover  112  provides an overhang  113  that extends beyond the circumference of the optical housing  110  and circumscribes it, so as to block the overhead sun&#39;s rays from going directly into the camera. Radial mirror  108  serves to direct UV light from light sources  116  onto the reflector  104 . 
   Around the exterior of camera  100  are a plurality of UV (ultraviolet) lights  116 A- 116 C that are directed at photochromic reflector  104 . This enables the photochromic reflector  104  to be darkened in a particular direction as needed, providing additional control of the photochromic properties. Camera  100  further comprises a plurality of IR (infrared) light sources (indicated generally as  120 ) that emit infrared light towards the scenes that camera  100  captures. Camera  100  further comprises a plurality of light sensor modules  123 . Sensor module  123 A comprises a visible light sensor  124 A and infrared light sensor  128 A, and is configured and disposed to trigger an activation of a corresponding UV light source  116  when excess light (either infrared or visible) is detected. The other sensor modules (e.g.  123 B,  123 C, etc. . . . ) operate in a similar manner to sensor module 123 A. 
     FIG. 2  shows a side view of the camera  100  of  FIG. 1  during operation. In this figure, visible light rays L 1  illuminate one side of the camera. Light rays L 1  illuminate camera  100 , and are received by visible light sensor  128 A, causing UV light  116 A to emit light rays L 2 , which spread about 25° and are directed toward the reflector  104 . Any rays that miss the reflector are reflected off radial mirror  108  onto the reflector  104  so as to gather all the UV rays possible. The reflected light rays (indicated as L 3 ) illuminate a portion of the photochromic reflector  104 , which darkens accordingly, thereby maintaining appropriate exposure control for image capture element  136 . 
     FIG. 2A  shows a particular photochromic reflector embodiment, in which photochromic reflector  104  is hemispherical, and comprised of an acrylic “half-ball”  141  that has a mirror coating  143  applied to it, and a photochromic coating  145  applied on the mirror coating  143 . 
     FIG. 3  shows a top-down view of the camera shown in  FIG. 1 . In this view, the positional relationship of each UV light  116 , sensor module  123 , and infrared light  120  is shown. For example, if sensor modules  123 B and  123 C detect an excess amount of infrared or visible light, then UV lights  116 B and  116 C will be activated, reflecting off radial mirror  108 , and illuminating only a portion of photochromic reflector  104 . This will help maintain acceptable exposure levels in image capture element  136 . This is important in a 360 degree view camera, as there are many situations where there is intense light illuminating the camera from one side only (e.g. at sundown), which causes a disparity amongst different parts of the image captured by image capture element  136 . By employing a photochromic reflector  104 , and using the UV light sources to trigger the photochromic effect within the reflector  104 , this exposure problem is overcome. 
   Antireflection tube  140  surrounds image capture element  136  and shields it from stray reflections, to avoid corruption of the captured image. Antireflection tube  140  is preferably of a matte black finish to minimize any stray reflections that reach image capture element  136 . 
   Photochromic reflector  104  darkens when exposed to UV light. However, it is possible to have bright light in situations where the UV is largely filtered out. For example, many modern windows filter out UV light. A 360 degree panoramic camera mounted indoors near such a window may be exposed to bright sunlight, especially in early morning or late afternoon. However, as the UV light is filtered out by the windows, the photochromic effect would not be sufficient to improve the exposure. However, with the present invention, the UV lights  116  effectively “restore” the UV light that has been filtered out, thereby allowing the photochromic effect to occur sufficiently enough to darken photochromic reflector  104 , and maintain proper exposure in image capture element  136 . 
     FIG. 4  shows a cutaway side view of the bottom section of a camera in accordance with an embodiment of the present invention. Antireflection tube  140  shields image capture element  136  from stray reflection, to avoid corruption of the captured image. Image capture element  136  is mounted on camera body  148 , which houses additional electronics and/or optical components. Heating elements  144  serve to prevent frost from forming on the camera housing, reflector, and other components. In one embodiment, the heating elements  144  are activated by a temperature control circuit (not shown) when the ambient temperature falls below a predetermined level, such as 32 degrees Fahrenheit. The heating elements  144  serve to maintain the camera temperature above its lower operational temperature-limit and also serve as a deicer and defroster. In one embodiment, an adjustable bimetal strip is used as a controller for closing an electrical circuit. 
     FIG. 4A  shows an alternative embodiment, in which the heating elements  144  may also serve as part of an anti-condensation system. In this embodiment, in addition to considering the ambient temperature, the relative humidity and barometric pressure may also be considered, to calculate a dew point temperature. The heating elements are then activated as necessary to maintain the camera temperature above the dew point temperature, thereby preventing condensation on the housing  110  or reflector  104 . In one embodiment, a microcontroller  179  on board the camera  100  reads input from ambient temperature sensor  171 , ambient relative humidity sensor  173 , and ambient barometric pressure sensor  175 , and computes a dew point temperature value, and activates heating elements  144  to maintain the air temperature of the camera above the dew point temperature value. The internal implementation for the dew point calculation may be performed via formula, which is well known, or alternatively, the dew point may be approximated via a lookup table. In this case, it is possible to make an assumption about the pressure, and avoid the need for barometric pressure sensor  175 . 
   At the bottom of the camera is switch plate  152 , which is recessed slightly from the base, allowing for access to controls  158 A,  158 B,  164 , and  168 . These controls are explained further in the discussion of  FIG. 5   
     FIG. 5  shows a view of the underside of the base of a camera in accordance with an embodiment of the present invention. In this view, visible sensor enable control  158 A, IR sensor enable control  158 B, visible sensitivity control  164 , and IR sensitivity control  168  are shown. The visible sensor enable control  158 A is a switch that enables the triggering of the UV lights  116  based on visible light detected. Visible sensitivity control  164  allows the setting of an activation point which corresponds to the amount of visible light required to trigger the UV lights. The IR sensor enable control  158 B is a switch that enables the triggering of the UV lights  116  based on IR light detected. IR sensitivity control  168  allows the setting of an activation point which corresponds to the amount of IR light required to trigger the UV lights. 
     FIG. 6  shows an alternative embodiment of a camera  200  of the present invention. This embodiment is similar to camera  100  that is illustrated in  FIG. 1 . The additional element in this embodiment is a photochromic housing  210  in place of the optical housing  110  that is used in the camera  100  shown in  FIG. 1 . In this case, the photochromic housing  210  provides an additional level of darkening than with using only a photochromic housing or photochromic reflector. Furthermore, the placement of UV lights  116  allows them to illuminate both photochromic reflector  104 , and photochromic housing  210 . Thereby providing more exposure compensation capability, for situations where the incident light rays L 1  are particularly intense (e.g. very bright sunlight, or artificial lights). 
   Although the description above contains many specific details, these should not be construed as limiting the scope of the invention, but merely as providing illustrations of some of the presently preferred embodiments of the present invention.