Abstract:
A lens system has an optical path through a lens that is in contact with a liquid. The focal length of the lens system can be changed through introduction of a bubble in the optical path at the lens-liquid interface. The bubble changes the refractive index difference at the lens surface and thereby changes the focal length of the lens system. In a camera, the lens system can operate in a macro mode corresponding to a shorter focal length or a normal mode corresponding to a longer focal length.

Description:
BACKGROUND 
     Photographers often want to take close-up photographs of small objects. This is commonly referred to as “macro” photography and generally involves taking photographs of objects that may be less than a few inches from the camera. In contrast, “normal” photography generally involves taking a photograph of relatively large objects such as landscapes or people that are no closer than several feet away from the camera. Designing a camera lens capable of both normal and macro photography can be difficult because of the range of focus and light collection capabilities required for acceptable image quality. 
     Expensive cameras typically have lens mounts that permit a photographer to change lenses as required for the type of photography being done. Alternatively, an additional lens element could be added to a fixed lens system. A magnifier, for example, can be added in front of the normal lens of a camera for macro photography. A drawback of these types of cameras is the need to have separate lenses or lens elements, which may be inconvenient for a casual photographer and/or undesirable for compact cameras. Another alternative is to provide a camera with a mechanical system that can shift an additional lens element into or out of the optical path of the camera for different types of photography. Such mechanical systems generally have moving parts that can add to the cost and complexity of a camera and may be an additional reliability concern. 
     A lens system that is capable of both normal and macro photography is thus desired that is suitable for inexpensive and compact cameras. 
     SUMMARY 
     In accordance with an aspect of the invention, a lens system contains a liquid in the optical path of the lens system. Through creation or movement of a bubble adjacent to a lens surface, the lens system can be changed, for example, from a normal mode providing a longer focal length for normal photography to a macro mode providing a shorter focal length for macro photography. 
     One specific embodiment of the invention is an optical system including a first optical element, a second optical element, a liquid, and a control system. The liquid is generally in a gap between the first and second optical elements. The control system is operable in a first mode and a second mode. In the first mode, the liquid is in an optical path through the first and second optical elements, and in the second mode, a bubble is in the optical path. Either optical element can have a convex or concave side in contact with the liquid. The optical system is particularly suited for a camera lens, where one of the modes is for macro photography and the other mode is for normal photography. 
     One implementation of the control system includes a heating element, and the control system switches between the first mode and the second mode by controlling a current through the heating element. Another implementation of the control system switches between the first and second modes by moving the bubble into or out of the optical path. 
     Another specific embodiment of the invention is a method for operating an optical system that includes a first optical element, a second optical element, and a liquid between the first optical element and the second optical element. The method includes: forming a first image using an optical path through the first optical element, the liquid, and the second optical element; and forming a second image using an optical path through the first optical element, a bubble in the liquid, and the second optical element. The bubble in the liquid can be created or controlled using a variety of techniques. For example, the liquid can be heated to create the bubble in the optical path for the second image. Alternatively, the bubble can be moved into the optical path for the second image. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a cross-sectional view of a camera with a lens system in accordance with an embodiment of the invention in a mode where a fluid is in the optical path of camera. 
         FIG. 2  shows a cross-sectional view of the camera of  FIG. 1  in a mode where a bubble is in the optical path. 
         FIG. 3  is a plan view of a lens system in accordance with an embodiment of the invention that is capable of changing focal length through movement of a bubble. 
         FIG. 4  shows a cross-sectional view of a camera with a lens system in accordance with an embodiment of the invention where a fluid is adjacent to a concave lens surface. 
     
    
    
     Use of the same reference symbols in different figures indicates similar or identical items. 
     DETAILED DESCRIPTION 
     In accordance with an aspect of the present invention, a lens system switches between a first focal length and a second focal length through control of a bubble in a liquid layer that is between optical surfaces. The lens system can be used in a camera, where the shorter focal length corresponds to a macro photography mode of the camera and the longer focal length corresponds to a normal photography mode of the camera. The change between the camera modes can be achieved without requiring a removable lens and without a complicated mechanical system. Further, the lens system can be easily fabricated at size scales associated with digital camera integrated circuits. 
       FIG. 1  shows a camera  100  in accordance with an embodiment of the invention including a transparent liquid  120  that is between the optical surfaces of two elements  110  and  130 . In the illustrated embodiment, element  110  is a plano-convex lens having a convex surface in contact with liquid  120 , and element  130  is a plate having a planar surface in contact with liquid  120 . More generally, in alternative embodiments, at least one of the surfaces of elements  110  and  130  that are in contact with liquid  120  should be concave or convex to permit focal length changes as described further below. 
     Parallel rays  190 , which are perpendicular to the planar surface of lens  110 , illustrate an optical path through lens  110 , liquid  120 , and plate  130  and show a focal point  192  of a lens system including lens  110 , liquid  120 , plate  130 , and an additional lens element  140 . Additional lens elements  140  can be part of a camera lens of conventional design including a mounting  142  that permits movement of lens  140  for focusing. The focal point  192  of the lens system is on an image plane  144  when camera  100  is focused on an object that is a long distance from camera  100  (e.g., an object at infinity). In a camera, image plane  144  includes film (not shown) or an electronic image sensor (not shown) such as a CCD image sensor or a CMOS image sensor to capture an image formed on image plane  144 . Other conventional camera elements such as a shutter system, an adjustable aperture, and electrical circuits for digital image capture are not shown in  FIG. 1  but are well known in the art and may be included in some embodiments of the invention. 
     In the illustrated embodiment of  FIG. 1 , liquid  120  has a refractive index that matches the refractive index of lens  110 , causing the combination of lens  110  and liquid  120  to act as a plate with parallel sides. The focal point of the lens system of camera  100  is therefore the focal point of lens  140  when liquid  120  is an index matching fluid for elements  110  and  130 . More generally, liquid  120  has a refractive index greater than 1 and therefore increases the effective focal length of plano-convex lens  110 , relative to when lens  110  is surrounded by air. 
       FIG. 2  shows camera  100  after a bubble  125  or air gap in liquid  120  is created in or moved into the optical path of the lens system.  FIG. 2  shows that bubble  125  in the optical path causes plano-convex lens  110  to act as a magnifying lens that shortens the focal length of lens system. Light rays  194  originating from an object close to lens  110  thus focus at a point  196  on image plane  144 . Accordingly,  FIG. 2  can correspond to a macro photography mode of camera  100 , while  FIG. 1  corresponds to a normal photography mode of camera  100 . 
     In an exemplary embodiment of the invention, the focus distance of the lens system in camera  100  is about 50 mm in the macro mode and greater than about 300 mm in the normal mode. These focus distances can be when lens  140  has a focus distance of about 300 mm or more, lens  110  has a focal length of about 5 mm, and liquid  120  has a refractive index that is about the same as the refractive index of lens  110 . For example, when lens  110  is made of an optical quality glass such as BK7 having a refractive index of about 1.52, liquid  120  can be oil, which similarly has a refractive index of about 1.52. 
     A variety of techniques can create or place bubble  125  in the optical path. The technique illustrated in  FIG. 2  uses an electrical heating element or resistor  135  that can be formed in or on plate  130  or lens  110 . For example, a material such as TiN can be deposited on plate  130  and patterned to create a ring-shaped heating element or resistor  135 . A current through resistor  135  generates heat that causes formation of a vapor bubble in liquid  120  adjacent to resistor  140 . 
     Liquid  120  in the gap between lens  110  and plate  130  is in fluid communication with a reservoir  150  containing a gas cushion  155  that allows creation of bubble  125  without a large increase in the fluid pressure of liquid  120 . Bubble  125  will generally remain stable at a size such that the pressure in liquid  120  and the surface tension of bubble  125  balances the internal pressure in bubble  125 . Bubble  125  can be confined to the desired location adjacent to lens  110  through use of relatively narrow apertures for fluid flow away from the gap between lens  110  and plate  130 . 
     In one embodiment, heating creates bubble  125 , and the pressure and temperature of liquid  120  are maintained at levels such that continuous heating is required to prevent bubble  125  from collapsing. If desired, the current through resistor  135  can be reduced to maintain bubble  125  after initial creation. In this embodiment, resistor  135  creates a temperature gradient that confines bubble  125  to the area adjacent resistor  135  since portions of the bubble outside the area where resistor  135  applies heat collapse. When current through resistor  135  is turned off, bubble  125  collapses, and the lens system of camera  100  automatically switches back to the lens mode illustrated in  FIG. 1 . 
     Instead of creating bubble  125  in the optical path, bubble  125  can alternatively be moved into and out of the optical path to change the lens mode of camera  100 .  FIG. 3  shows a plan view of an embodiment of the invention in which a cavity  310  that is between optical elements  110  and  130  can either be filled with a liquid  120  or contain a bubble  125 . Cavity  310  is in fluid communication with an activation cavity  360  and reservoir  150 . Cavity  310  preferably has a minimum dimension that is large enough to allow a stable bubble to remain in cavity  310  at the operational temperature and pressure of liquid  120 . Activation cavity  360  contains a heating element  362 , and the height, width, or other minimum dimension of activation cavity  360  is preferably too small to allow a stable bubble in activation cavity  360  unless heating element  362  is activated. 
     Activation cavity  360  is used to move a bubble into or out of cavity  310 . To move a bubble into cavity  310 , heating element  362  is activated with sufficient power and with sufficient duration to create a bubble that extends from activation cavity  360  into cavity  310 . The bubble from activation cavity  360  can thus expand into cavity  310  while heating element  362  is active. Heating element  362  is turned off when the bubble in cavity  310  reaches the desired size. The bubble in activation cavity  360  collapses when heating stops, but a stable bubble remains in cavity  310 . Necks  364  and  366  in the fluid paths leading out of cavity  310  have minimum dimensions that are too small to allow the bubble to extend significantly into either neck  364  or  366  at the nominal liquid pressure and temperature. 
     A fluid flow from activation cavity  360  can be used to push the bubble out of cavity  310 . To move a bubble out of cavity  310 , heating element  362  is rapidly activated so that an expanding bubble in cavity  360  pushes liquid  120  out of activation cavity  360  into cavity  310 . A relative size of cavities  310  and  360  are such that the fluid flow from activation cavity  360  is able to push a bubble from cavity  310  through neck  366  into reservoir  150 , where the bubble is absorbed into gas cushion  155 . 
     In an exemplary embodiment of the invention, cavity  310  has a diameter of about 500 μm and a minimum height of about 45 μm. Activation cavity  360  has a volume greater than cavity  310  but a narrow width of about 12 μm. Each neck  364  or  366  is preferably about 10 μm long, 9 μm wide, and 45 μm high. 
     U.S. Patent Publication No. 2004/0067012, entitled “Latching Bubble For Fluid-Based Optical Switch”, which is hereby incorporated by reference in its entirety, describes some structures and techniques for movement or control of bubbles in optical switches that could also be applied to lens systems as described here. 
     In yet another variation of the invention, liquid  120  can be mechanically drained from the gap between lens  110  and plate  130  to switch camera  100  from the mode of  FIG. 1  to the mode of  FIG. 2 . Refilling the gap between lens  110  and plate  130  with liquid  120  switches the camera back to the mode of  FIG. 1 . 
       FIG. 4  illustrates a camera  400  in accordance with an embodiment of the invention that includes a liquid  120  between a plano-concave lens  410  and a plate  130 . The other elements of camera  400  are substantially the same as described above with reference to camera  100  of  FIGS. 1 and 2 . Use of a lens  410  having a divergent lens surface adjacent to liquid  120  causes the lens system of camera  400  to have a longer focal length when a bubble  125  is in the optical path of camera  400 . Accordingly, camera  400  can have a normal photography mode when a bubble  125  or air gap is between lens  410  and plate  130 , and a macro photography mode when liquid  120  is in the optical path between lens  410  and plate  130 . An advantage of the concave surface of lens  410  is that the concave surface tends to center bubble  125  on lens  410  and therefore center bubble  125  on the common location of the optical path. 
     Although the invention has been described with reference to particular embodiments, the description is only an example of the invention&#39;s application and should not be taken as a limitation. For example, although the above embodiments employ a liquid or a bubble between a lens and a plate, similar liquid/bubble modes could be implemented between other optical element combinations such as two lenses, and the lens system could include multiple liquid filled gaps that can be operated individually or simultaneously to provide the lens system with a series of different focal lengths. Various other adaptations and combinations of features of the embodiments disclosed are within the scope of the invention as defined by the following claims.