Abstract:
An aperture unit includes a transparent cylindrical member, a magnetic fluid, and a magnetic field generator. The cylindrical member includes a cylindrical chamber. A light input end portion of the cylindrical member is transparent, and an opposite light output end portion of the cylindrical member is transparent. The magnetic fluid is received in the cylindrical chamber. The magnetic fluid includes a transparent solvent, a surfactant, and nano-magnetic particles dispersed substantially evenly in the solvent. Each of the magnetic nano-particles is enveloped by a surfactant. The magnetic field generator is positioned outside the cylindrical chamber. The magnetic field generator is configured for generating a magnetic field applied to the chamber and attracting and clustering the nano-particles, such that an annular nano-particle opaque layer is formed on an inner surface of the cylindrical member.

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure relates to optical imaging and, particularly, to an aperture unit and an imaging system using the aperture unit. 
     2. Description of Related Art 
     In imaging technologies, an aperture admits light into a system for a distinct period of time, to expose photographic film or a light-sensitive electronic sensor to the amount of light required to capture an image. As the value of the aperture increases, the amount of light admitted increases accordingly. Imaging systems often employ apertures of fixed value. This is inconvenient when the imaging system is applied in different environments. 
     Therefore, what is needed is an aperture unit and an imaging system using the same, which can overcome the limitations described. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, all the views are schematic. 
         FIG. 1  is a cross-section of a first embodiment of an imaging system, according to the disclosure. 
         FIG. 2  is a view of a cylindrical member of an aperture unit of the imaging system shown in  FIG. 1 , showing the cylindrical member in a first state. 
         FIG. 3  is similar to  FIG. 2 , but showing the cylindrical member in a second state. 
         FIG. 4  is a cross-section of a second embodiment of an imaging system, according to the disclosure. 
         FIG. 5  is a cross-section of a third embodiment of an imaging system, according to the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a first embodiment of an imaging system  100  according to the disclosure includes a lens unit  20 , a holder  30 , an aperture unit  40 , an image sensor  50 , a control unit  10 , and a circuit board  60 . The aperture unit  40  and the image sensor  50  are arranged in the holder  30 . The control unit  10  is electrically mounted on the circuit board  60 . 
     The lens unit  20  includes a barrel  21 , a lens  22 , a spacer ring  23 , and a filter  24 . The lens  22 , the spacer ring  23 , and the filter  24  are arranged in the barrel  21  in that order from the object side to the image side of the lens unit  20 . The barrel  21  is threadedly engaged with the holder  30 . 
     The aperture unit  40  includes a cylindrical member  41 , magnetic fluid  45 , a heater  460  and a magnetic field generator  43 . The cylindrical member  41  includes an upper plate  410 , a lower plate  420  parallel to the upper plate  410 , and a hollow cylinder  430 . The hollow cylinder  430  connects the upper plate  410  to the lower plate  420  to cooperatively define a cylindrical chamber  440 . The magnetic fluid  45  is received in the chamber  440 . The heater  460  is embedded in the hollow cylinder  430 , and operable to heat the magnetic fluid  45  in the chamber  440 . The upper plate  410  and the lower plate  420  are transparent and the hollow cylinder  430  can be opaque. The cylindrical member  41  with the magnetic fluid  45  and the heater  460  is positioned between the lens unit  20  and the image sensor  50 . The circuit board  60  is positioned between the image sensor  50  and the magnetic field generator  43 . 
     Further referring to  FIG. 2  and  FIG. 3 , the magnetic fluid  45  includes a solvent  452 , a plurality of magnetic nano-particles  454  dispersed therein, and a surfactant (not shown). The solvent  452  is transparent. The solvent  452  is water, alcohol, methanol, hexamethylene, or octane. The nano-particles  454  are small enough to permit transparency of the solvent  452  when evenly dispersed therein. In the present embodiment, the nano-particles  454  are ferrosoferric oxide nano-particles or manganese zinc ferrite nano-particles. Diameters of the nano-particles  454  are in the range from about 10 nanometers (nm) to about 100 nm. A weight percentage of the nano-particles  454  in the magnetic fluid  45  is in the range from about 1% to about 4%. Each of the nano-particles  454  is enveloped by the surfactant to allow even dispersal thereof in the solvent  452  when no magnetic field is applied. In  FIGS. 1 and 2 , the aperture unit  40  is shown in an initial (fully transmissive) state. The surfactant is polyvinyl alcohol, oleic acid, linoleic acid, or olive oil. Light passing through the lens unit  20 , the upper plate  410 , the magnetic fluid  45 , and the lower plate  420  arrives at the image sensor  50  in this exemplary embodiment. 
     The heater  460  is a resistance heater in this exemplary embodiment. When the heater  460  is activated, it enhances the diffusion speed of the nano-particles  454  in the solvent  452 . Accordingly, the transparent magnetic fluid  45  with the nano-particles  454  uniformly dispersed in the solvent  452  is quickly generated. The heater  460  is electrically connected to the circuit board  60  and the control unit  10  and activated by the control unit  10 . 
     The image sensor  50  can be a charge-coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) device, and is mounted on and electrically connected to the circuit board  60 . In the present embodiment, the circuit board  60  is a printed circuit board. 
     The magnetic field generator  43  magnetizes the magnetic fluid  45 . The magnetic field generator  43  includes an annular electromagnet  431  and a coil  433  wound therearound. The coil  433  is electrically connected to the circuit board  60  and the control unit  10  and activated by the control unit  10 . A magnetic field is generated by the electromagnet  431  when the coil  433  is electrified, and the magnetic field is absent when the coil  433  is not electrified. 
     In operation of the imaging system  100 , in an initial state, the nano-particles  454  are dispersed evenly in the solvent  452 , as shown in  FIGS. 1 and 2 . The magnetic fluid  45  is, at this time, substantially fully transparent, and light passes through the lens unit  20 , the upper plate  410 , the magnetic fluid  45 , and the lower plate  420  substantially unimpeded to arrive at the image sensor  50 . At this time, the aperture unit  40  has a large value. 
     Referring to  FIG. 3 , in a second state, the coil  433  is electrified by the control unit  10 , and the electromagnet  431  of the magnetic field generator  43  generates a magnetic field. The nano-particles  454  are attracted by the magnetic field to form an annular nano-particle layer  490  on an inner surface of the lower plate  420 . As a result, light passing from the upper plate  410  is partially blocked by the nano-particle layer  490 , and arrives at the image sensor  50 . At this time, the aperture unit  40  has a small value. 
     Referring to  FIG. 1  again, when the coil  433  is not electrified by the control unit  10 , the magnetic field is absent, and the nano-particles  454  of the nano-particle layer  490  are substantially uniformly diffused in the solvent  452  by random diffusion. Further, the heater  460  is activated by the control unit  10  to speed the diffusion of the nano-particles  454  in the solvent  452 . In summation, the imaging system  100  can controllably achieve two aperture values in the illustrated embodiment. Furthermore, the imaging system  100  can achieve a desired variety of discrete aperture values by the control unit  10  appropriately controlling the strength of the magnetic field generated by the magnetic field generator  43 . 
     Referring to  FIG. 4 , a second embodiment of an imaging system  100   a  is shown. The imaging system  100   a  differs from the imaging system  100  basically only in the structure and arrangement of a magnetic field generator  43   a  thereof. 
     The magnetic field generator  43   a  includes a first annular electromagnet  431   a , a first coil  433   a  wound around the first electromagnet  431   a , a second annular electromagnet  431   b , and a second coil  433   b  wound around the second electromagnet  431   b . The first electromagnet  431   a , the first coil  433   a , the second electromagnet  431   b  and the second coil  433   b  are arranged coaxially. 
     When the first coil  433   a  and the second coil  433   b  are electrified by a control unit  10   a , the first electromagnet  431   a  and the second electromagnet  431   b  generate a magnetic field. Accordingly, nano-particles  454   a  are attracted by the magnetic field, and a double annular nano-particle layer  491  is formed on an inner surface of a lower plate  420   a . The double annular nano-particle layer  491  includes a first annular nano-particle layer  490   a , and a second annular nano-particle layer  490   b  coaxially within the first annular nano-particle layer  490   a.    
     Advantages of the imaging system  100   a  of the second exemplary embodiment are similar to those of the imaging system  100  of the first exemplary embodiment. Furthermore, the imaging system  100   a  can achieve a desired variety of discrete aperture values by the control unit  10   a  appropriately controlling the strength of the magnetic field generated by the magnetic field generator  43   a . In particular, the strength of the magnetic field generated by the first electromagnet  431   a  can be controlled independently of the strength of the magnetic field generated by the second electromagnet  431   b , or in unison with the strength of the magnetic field generated by the second electromagnet  431   b , and vice versa. Thereby, a wide variety of formations of the double annular nano-particle layer  491  can be obtained. 
     Referring to  FIG. 5 , a third embodiment of an imaging system  100   c  is shown. The imaging system  100   c  differs from the imaging system  100  basically only in that a magnetic field generator  43   c  further includes a second annular electromagnet  431   c  and a second coil  433   c  wound around the second electromagnet  431   c  coaxially. The second electromagnet  431   c  surrounds a hollow cylinder  430   c . The second coil  433   c  is electrically connected to a control unit  10   c . The control unit  10   c  is configured for electrifying the coil  433  and the second coil  433   c.    
     When only the second coil  433   c  is electrified by the control unit  10   c , the second electromagnet  431   c  of the magnetic field generator  43   c  generates a magnetic field to which nano-particles  454   c  are attracted. A second annular nano-particle layer  490   c  is formed accordingly on an inner surface of the hollow cylinder  430   c . The inner diameter of the second annular nano-particle layer  490   c  is different from that of the annular nano-particle layer  490  formed only by the electromagnet  431 . That is, the second annular nano-particle layer  490   c  is formed differently from the annular nano-particle layer  490 , under control of the control unit  10   c.    
     Advantages of the imaging system  100   c  of the third exemplary embodiment are similar to those of the imaging system  100  of the first exemplary embodiment. Furthermore, the imaging system  100   c  can achieve a desired variety of discrete aperture values by the control unit  10   c  appropriately controlling the strength of the magnetic field generated by the magnetic field generator  43   c . In particular, the strength of the magnetic field generated by the electromagnet  431  can be controlled independently of the strength of the magnetic field generated by the second electromagnet  431   c , or in unison with the strength of the magnetic field generated by the second electromagnet  431   c , and vice versa. Thereby, a wide variety of formations ranging between the nano-particle layer  490  and the second annular nano-particle layer  490   c  can be obtained. 
     It is to be understood, however, that even though numerous characteristics and advantages have been described with reference to particular embodiments, the present invention is not limited to the particular embodiments described and exemplified, and the embodiments are capable of considerable variation and modification without departure from the scope and spirit of the appended claims.