Abstract:
A variable focal length light source is constructed using a tunable focus length lens which can be adjusted in synchronization to the focal length of a camera lens. In one embodiment, a deformable lens is used in conjunction with an LED light source to provide the tunable focal length required for a camera flash device.

Description:
TECHNICAL FIELD 
     This invention relates to light sources and more particularly to light sources for use with cameras and even more particularly to such light sources which require variable focal lengths. 
     BACKGROUND OF THE INVENTION 
     It has become standard practice to use light emitting diodes (LEDs) as flash modules in mobile applications that have camera functions. For example, mobile phones or PDAs are increasingly equipped with camera modules for image capture and a flash module serves as an illumination source in low ambient light situations. These flash modules must produce a large amount of light each time they are activated and in some cases the light from the LED is not sufficient for a camera flash. This is particularly true if the image to be captured is far from the camera in a total dark (Zero lux) condition. 
     Mobile device cameras (such as cameras in phones, PDAs, etc.) have zoom functions for bringing distant images into closer focus. These zoom lenses allow the user to zoom in on small objects located far from the camera. If the light from the flash is unable to properly illuminate the subject, particularly under dark conditions, the resultant picture may not look good. 
     Some prior art flash arrangements have a moveable optical lens to channel the light from the flash to the desired viewing angle (focal length) to match the viewing angle (zoom focal length) of the camera. 
     BRIEF SUMMARY OF THE INVENTION 
     A variable focal length light source is constructed during a tunable focus length lens which can be adjusted in synchronization to the focal length of a camera lens. In one embodiment, a deformable lens is used in conjunction with an LED light source to provide the tunable focal length required for a camera flash device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  shows one embodiment of a light source with a tunable lens; 
         FIGS. 2 ,  3 A and  3 B show embodiments of flash devices with tunable lens arrangements; 
         FIG. 4  shows one embodiment of a light source used in a camera device; 
         FIG. 5  shows a prior art moveable optical lens for changing the focal length of a light source; 
         FIGS. 6A and 6B  show prior art tunable focal length lens; and 
         FIGS. 7A and 7B  show a prior art design of a liquid crystal immersed microlens. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Before beginning a discussion of the detailed description of the invention, it might be helpful to review some existing structures.  FIG. 5  shows one prior art flash structure  50  having LED  51  as a light source with reflector cups  52  forcing the light ( 54 ) upward through movable optical lens  53 . Lens  53  is mechanically moved closer or farther from LED  51  in order to change the focal length of light  54 . This changes the viewing angle of light as well. In order for the lens to move, the width of the device must be such as to accommodate the full range of lens motion, typically on the order of 10 mm. 
       FIGS. 6A and 6B  show prior art tunable focal length lens  60 . Lens  60  is a plastic device that contains two liquids. One liquid ( 61 ) is based on a water soluble formulation, while the other ( 62 ) is oil based. The non-polar water based formulation is negatively biased. The curved interface between the oil and water layers acts as a lens. With no applied voltage, the lens focuses on objects at infinity as shown in  FIG. 6B . However, when voltage is applied to the electrodes the lens is altered and the curvature of the liquid-liquid interface changes. This is shown in  FIG. 6A  where the focal length of the lens has changed (closer) so that the printing on sheet  63  is now readable. These lenses are versatile in that they can change their shape from concave to convex in a matter of milliseconds. Such a lens is shown in the November 2003 issue of Opto &amp; Laser Europe, which reference is hereby incorporated by reference herein. 
       FIGS. 7A and 7B  show light structure  70  having microlenses  73  which have been fabricated and immersed in nematic liquid crystal to give an electrically controllable focal length. The liquid crystal material is uniaxially birefringent and the effective birefringence can be controlled since director  74  (the average direction of the molecules) reorients towards an applied electric field as shown in  FIG. 7B  when voltage is applied. Thus, for light polarized parallel to the liquid crystal slow axis, the refractive index can be voltage controlled and, consequently, when a lensing interface is formed between a refractive material and the liquid crystal, the overall lens focal length can be voltage controlled. In this arrangement, structures  71  and  72  are glass. Such a structure is shown in Electrode Designs for Tunable Microlenses, L. G. Commander, S. E. Day and D. R. Selviah, Dept. of Electronic and Electrical Engineering, University College London which is hereby incorporated by reference herein. 
       FIG. 1  shows one embodiment of light source  10  with tunable lens  11 . Device  10  combines the concepts shown in  FIGS. 6A ,  6 B,  7 A and  7 B and uses voltage  15  to tune the focal length of the lens. In this embodiment, the light source is LED  12  held by substrate  14 . Reflectors  13  reflect the light to impact on tunable lens  11 . As discussed, lens  11  changes its focal angle (length) quickly and can be synchronized with a camera lens, such as lens  42  of camera device  40 ,  FIG. 4 . Camera device  40  is typically part of another device, such as a telephone, and has, for example, screen/keypad  44 . Battery  43  can provide the voltage for tuning lens  11 . 
       FIG. 2  shows one embodiment of light source device  20  where LED chip  24  is directly immersed in the tunable lens. Isolative layer  25  is used to protect LED junction  28 . The LED chip is mounted in this embodiment on substrate  22 . The structure has sides  23  and top cover  21 . The actual tunable lens  26  is confined between the top, the substrate and the sides. 
       FIG. 3A  shows one embodiment where tunable lens  26  is the interface between dissimilar materials, such as liquids. In the embodiment, a first liquid (for example, oil) and a second liquid (for example, water) is used and voltage is applied by voltage source  43  ( FIG. 4 ). 
       FIG. 3B  shows one embodiment where tunable lens  26  is constructed in liquid crystal lenses  33 . Voltage  43  is applied to the liquid crystal to change the refraction index. The overall focal length will change when the light from the liquid crystal interfaces with lens  42 . 
       FIG. 4 , as discussed above, shows one embodiment of a light source  20  mounted in a camera device and is coordinated, for example with processor  45 , with lens  42 . 
     Note that while the tunable lens is shown herein as fixed in position (i.e. the transverse axis of the lens does not move with respect to the light source) it can, in fact, move if desired thereby combining the features of tunable lens with features of a moveable lens. In this way, the movement (in and out) can be reduced while still providing adequate light on a subject. 
     The optics tunable system discussed herein is faster, more compact, more robust and less expensive to manufacture than motorized systems.