Abstract:
Cameras such as electronic video surveillance cameras use an iris to adjust the total amount of light reaching the sensor element. An electrically adjustable iris composed of a magnetic fluid disposed within an aperture construction assembly allows the optical aperture to be adjusted dynamically and allows for more rapid change to optimum aperture settings for varying external lighting conditions.

Description:
TECHNICAL FIELD 
       [0001]    This invention relates to an electrically adjustable optical iris for use in optical devices such as cameras. Cameras such as those used in electronic video surveillance systems use an iris to adjust the total amount of light reaching the sensor element. An electronically adjustable iris allows the optical aperture to be adjusted dynamically and allows a light sensitive medium or sensor to operate within its optimum conditions with varying external lighting conditions. 
       BACKGROUND 
       [0002]    Currently available electrically adjustable iris assemblies are mechanical assemblies with moving and sliding parts. These assemblies use a servo controlled solenoid or motor and moving parts to provide an adjustable aperture through which the light passes. A significant disadvantage of existing devices is mechanical durability and ruggedness. Ruggedness and mechanical reliability are particularly important for use in mobile applications or environments exposed to vibration, shock, or extreme temperatures. In addition, overcoming mechanical latency in the movement of currently existing iris assemblies places a limit upon how quickly an iris can respond to changes in light conditions. With no moving parts, and, therefore, no mechanical latency to overcome, this invention can be manufactured to be more sensitive to changes in light conditions, responding more quickly and with greater reliability than an equivalent mechanical assembly. 
       SUMMARY OF THE INVENTION 
       [0003]    This invention relates to an optical iris with in which the mechanical moving parts are replaced by an optically opaque homogeneous magnetic fluid, such as magneto-rheological fluids that are known in the art, confined within a transparent aperture construction forming the iris of an optical device. This aperture construction is enclosed within a ring magnet. Through the application of an additional electrically induced magnetic field, the magnetic fluid can be caused to deflect and adjust the diameter of the optical aperture, forming an electrically adjustable iris assembly. 
     
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0004]      FIG. 1 : Transparent aperture construction assembly 
           [0005]      FIG. 2 : Iris assembly: magnetic field not applied 
           [0006]      FIG. 3 : Iris assembly: magnetic field applied 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0007]    In the present invention, a transparent aperture construction consists of at least two clear plates ( 240 ,  242 ) and a cylindrical side wall  245  of a transparent medium such as glass, acrylic, or other optically clear material. In the preferred embodiment the clear plates ( 240 ,  242 ) and cylindrical side wall  245  are optically clear glass plates which are configured to form an enclosed cavity  225 . An optically opaque homogeneous magnetic fluid  110  is confined within the optically clear cavity  225 . In the preferred embodiment, a second, optically clear material  220  that does not mix with the homogeneous magnetic fluid  110  is confined within the same cavity  225 . The second, optically clear material  220  may be a liquid, or a gas such as dry nitrogen. In another embodiment, the homogeneous magnetic fluid  110  is the only fluid present in the cavity which has been evacuated to form a vacuum and fills only a portion of the evacuated cavity space  225 . 
         [0008]    The aperture construction  100  is enclosed within a ring electromagnet  130  that is disposed about the entire circumference of the aperture construction  100  and the integral cavity  225 . The ring electromagnet  130  acts with magnetic force on the homogeneous magnetic fluid  110  within the integral cavity  225  such that the homogeneous magnetic fluid  110  forms an annular ring with an optically clear center aperture  120  through the redistribution of the homogeneous magnetic fluid  110  within the cavity  225 . In this way, a well defined circular aperture  120  is formed within the aperture construction cavity  225 , the aperture  120  serving to form an iris for the admittance of at least the visible portion of the electromagnetic spectrum. 
         [0009]    A coil of electrically conductive wire such as a cylindrical coil placed axially along the side of the aperture construction assembly  100  can apply a secondary magnetic force on the magnetic fluid  110  when an electric current is passed through the wire. The force on the magnetic fluid  110  is proportional to the number of turns and the electrical current applied to the coil. The addition of the secondary electromagnetic force deflects the homogeneous magnetic fluid  110  and causes the diameter of the optical aperture  120  to change, either increasing or decreasing the diameter of the aperture at the center of the annular ring of magnetic fluid  110 , in proportion to the current applied to the cylindrical coil  130 . 
         [0010]    With an external electrically induced magnetic field applied the additional magnetic field, exerted upon the homogeneous magnetic fluid  110 , causes the optical aperture  1120  diameter to be reduced. The redistribution of the homogeneous magnetic fluid  110  is in reaction to the applied electric power increasing the magnetic field of the electromagnet  130  and pulling the homogeneous magnetic fluid  110  in toward the ring electromagnet  130 . By adjusting the strength of the electric current in the coil through the action of an electronic control circuit the diameter of the aperture  120  of the iris can be electrically controlled. The electronic control circuit is also connected to a feedback loop to the ring electromagnet  130  such that the current applied may be monitored and dynamically changed to maintain a desired aperture  120  opening diameter. 
         [0011]    The electronic control circuit may also be electrically connected to an external optical sensor. This external optical sensor may be used to detect lighting conditions external to the aperture assembly construction and send light level data to the electronic control circuit. The light level data is used by the electronic control circuit to test against threshold values previously stored within the electronic control circuit. When threshold values are detected, the electronic control circuit will dynamically modify the amount of electrical current supplied to the ring electromagnet to modify the diameter of the aperture opening to achieve a pre-set aperture  120  diameter setting for each sensor threshold level. 
         [0012]    In addition, a default aperture opening diameter may be achieved by the introduction of a second ring magnet  210  with a known magnetic force level. Placing the second ring magnet  210  axially separated from the first ring electromagnet  130  produces an aperture  120  opening calibrated to be proportional to the separation distance between the two ring magnets ( 130 ,  210 ). The use of the manual configuration of two ring magnets ( 130 ,  210 ) can be used to configure the default aperture  120  setting when no other magnetic field is present or no electric current is applied to the electric coil of the ring electromagnet  130 . 
         [0013]    The present invention is compatible with input power signals from existing power supplies such as those motor inputs that may be used to stimulate mechanical iris assemblies and are well known in the art. 
         [0014]    While this invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.