1. Field of the Invention
This invention relates to fast, electro-optically switched zoom lens systems.
2. Description of the Related Art
To achieve zoom capability, conventional imaging systems generally utilize a lens component that must be mechanically moved to adjust the magnification of the image viewed. Mechanically moving a lens component in relation to other fixed lenses alters the focal length of the optical system, thereby altering the magnification of an image.
There are several disadvantages to using a mechanical system for zooming. The mechanical movement required to adjust the focal length, and hence magnification of a light source, requires space along its optical axis. Additional space is also required for the motors, gears and power supply that physically move the lens itself. Finally, space is needed for any potential user interface, such as a knob on a riflescope, that may be used to initiate the lens movement. Additionally, moving mechanical objects consumes excessive amount of power and time.
The use of mechanical zoom systems in applications lacking space or power supply—such as Unmanned Aerial Vehicles (UAVs), riflescopes and satellites-introduces significant inconvenience and operational cost. The motors, gears, guide track and other parts of a mechanical system add weight, which in turn increases cost and reduces the applications' operational range. Mechanical zoom systems also suffer from slow operation. The physical movement of the lens takes time, so that achieving the desired magnification incurs a delay. This delay can result in severe consequences in applications where timing is paramount, such as riflescopes and UAVs.
Mechanical zoom systems also cannot be used to zoom mid-wave infrared radiation (“IR”) and beyond because the gears, motors and power supply radiate heat during operation, thereby interfering with IR detection. Finally, the repairs and adjustment required for a mechanical system, whose parts wear out over time, increases the downtime for any application in which it is used and thereby increases the costs of operating such a system.
An important requirement of many imaging systems with zoom capabilities is the reduction or elimination of optical aberrations, which blur and distort the image displayed. Optical aberrations of different types are caused by imperfect ray directing introduced by the system lens. In conventional single refractive lens systems, several known aberrations arise. Techniques for correcting these aberrations are well known. Birefringent lenses, on the other hand, produce a new aberration type, herein referred to as “birefringent aberration,” because the crystal material out of which they are made results in an extra-ordinary refractive index that varies with a ray's propagation angle through the lens. This aberration manifests itself in a number of different ways, depending upon whether the ray is on-axis or off-axis. A technique for correcting this type of aberration is desirable, so that birefringent lenses can produce an unaberrated image when used in place of isotropic lenses.
The operation of zoom systems across broad temperature and spectral ranges would also be desirable for many applications. For example, zoom capabilities in the infrared spectral range would be beneficial for use by applications that operate at night.
It would also be desirable for a zoom lens system to be capable of switching between zoom states almost instantly, even at extreme operational temperatures.
Furthermore, the use of an imaging system with zoom capabilities in a targeting application (such as a riflescope, for example) requires a reticle to provide a visual cue that helps a user aim at a particular target. For these applications, it would be desirable to provide a reticle that is clearly visible in all zoom states.
U.S. Pat. No. 5,052,791 to Kikuchi discloses the use of lens elements having variable refractive powers (birefringent lens components) made of crystalline materials, coupled with a controller that changes the polarizing direction of light by 90 degrees through either a physical rotation or by an electrically driven twisted nematic liquid crystal layer.
U.S. Pat. No. 6,888,590 to Nishioka et al. discloses an optical element which is capable of varying an optical characteristic thereof using a polymer dispersive liquid crystal. The element can be used as a vari-focal lens element, a vari-focal diffractive optical element, a variable declination prism or the like.
U.S. Pat. No. 6,437,925 to Nishioka discloses an optical apparatus that includes an optical element for use in cameras, microscopes and the like, whose optical properties can be changed by applying an electric or magnetic field or temperature to a liquid crystal.
U.S. patent application Ser. No. 11/238,262 discloses a polarizer, affixed quarter wave-plate and free quarter wave-plate forming a rotation-invariant linear polarizer.
Although the above approaches eliminate mechanical movement and its associated problems, they do not adequately address providing broad spectral and thermal range operation, fast zooming, or displaying an in-focus reticle for all zoom states. U.S. Pat. No. 6,888,590 utilizes birefringent lenses, but fails to provide any technique for removing aberrations inherently caused by such lenses.