Why confine LCD technology solely to displays? CRL Opto of Hayes, England, believes that there is no reason to. This company has extended LCD use to 3D-image capture. In combination with a single-lens camera, a fast-switching LCD can act as a shutter to capture and store both video and still types of stereo images. The technology can be integrated into a wide variety of applications where 3D-image capture is required, including camcorders and TV cameras.
Transmissive LCDs used conventionally as displays basically shutter light from a source on a pixel-by-pixel basis. To transform the display into a shutter, CRL Opto first divides the circular image area into shutter halves (Fig. 1). Next, engineers tie the pixels in each half to a single driver. The two-element ferroelectric liquid-crystal shutter (FLCS) is then placed in the iris plane of the optics of a camera lens. A sensor collects the light passing through the iris plane.
The two shutter halves switch alternately on and off, so the viewer perceives a 3D image. The FLCS selects either the right or left view. Off-axis light (diagonal rays) from the right part of the scene predominantly passes through the left half of the iris plane, and vice versa (Fig. 2). On-axis light (parallel rays) from both the right and left portions of the scene is collected over the entire aperture plane.
When employed in an interlaced video camera, the shutter has one half open during the odd frame. The other half is open during the even frame. Right and left images are stored in successively odd and even frames of the video signal. The stored composite stereo image can then be replayed on a conventional VCR and viewed using liquid-crystal shutter glasses. Or, the images can be processed and viewed through red/green glasses.
Because the shutters employ a fast-response, ferroelectric liquid crystal, the FLCS can switch from one image to another in less than 100 µs. Switching between left and right images can be achieved at frame rates of 25 Hz or higher, which is ideal for many camera-scanning formats. By using more than two elements, or multiple vertical strips, the stereo separation (i.e., the stereo depth) of the right and left images can be adjusted. A multi-element shutter can even operate with two different stereo separations (Fig. 3).
The number of possible separations is limited to the number of elements in the shutter. Yet 30 to 40 elements are quite possible. Designers also can link this stereo separation function to a zoom capability on a zoom lens, ensuring that a realistic stereo depth is maintained during zoom operation.
An increased aperture can be ob-tained while maintaining a variable stereo depth (Fig. 4). Of course, there is a tradeoff between the stereo depth and the amount of light entering the aperture. But the flexibility to be able to choose the appropriate tradeoff as governed by the application is quite useful. The tradeoff is analogous to the interplay between depth of focus and aperture inherent in most camera lens designs.
To learn more about CRL Opto's 3D shutters, go to www.crlopto.com.