A proposed charge-coupled-device (CCD) camera would be mechanically shuttered by a planar array of micromachined, electromechanically actuated shutters. This proposal has arisen as part of the solution to the problem of designing a visible/near-infrared imaging spectrometer using a commercial off-the shelf CCD as the image sensor at the focal plane. The need for mechanical shuttering arises because the desired exposure time clashes with the readout times of available CCDs. In particular, what is needed is an exposure time shorter than the readout time. By use of the array of micromachined mirrors, the image could be deflected off the focal plane during the readout cycle to prevent contamination of the captured image with light after the desired charge-integration (exposure) time.
According to the proposal, an object would be imaged on the focal plane via a folding mirror. In this case, the folding mirror would be the array of micromachined mirrors. Such arrays have been fabricated before for other purposes and are examples of what are now denoted generically as microelectromechanical systems (MEMS).
The elements of the array would be of the order of 10 by 10 *m. The mirrors would be operated in a binary mode, in which they would be switched between extreme angular positions 10° apart at megahertz rates. In the normal or unactuated state (mirrors at one of the extreme angular positions), the mirrors would reflect the image light onto the focal plane. In the fully actuated state (mirrors at the other extreme angular position), the mirrors would deflect the image light away from the focal plane and onto a beam dump, which would absorb the light. The exposure time could be set by setting the duty cycle of the two mirror states.
Unlike traditional iris- and leaf-type mechanical shutters, the proposed MEMS-type shutter would be capable of closing off the entire image at once, and would operate without appreciable jitter. Even at submillisecond exposure times, the proposed shutter would not pose any timing or jitter problems.
The proposed shutter could be used with any image sensor, including a 100-percent-fill-factor sensor, which is typically a high-end progressive-scan CCD. Heretofore, fast cameras have typically contained interline-transfer image sensors, which are less sensitive to red and infrared than standard CCDs. Thus, the user can obtain the advantage of the increased signal and increased red and infrared response of a 100-percent-fill-factor, progressive-scan focal-plane device, relative to an interline-transfer device.
The proposed shutter could also be utilized in time-resolved spectroscopy. This would involve (1) imaging a spectrum onto the array of mirrors, with the spectrometer slit oriented along the columns of mirrors and the spectrum along the rows of mirrors and (2) re-imaging the spectrum from the mirror plane onto the CCD array. The spectroscopic cycle would start with all mirrors in the "off"position, so there would be no image on the CCD. Then the mirrors would be switched momentarily to the "on" position, a few rows at a time, in succession across the array, yielding a succession of time-resolved spectra on the CCD.
This work was done by Gregory Bearman, Robert Green, Michael Eastwood, and Thomas Chrien of Caltech for NASA's Jet Propulsion Laboratory.
This Brief includes a Technical Support Package (TSP).
Snapshot CCD camera with microelectromechanical shutter
(reference NPO20396) is currently available for download from the TSP library.
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Overview
The document presents a technical support package for a Snapshot CCD Camera equipped with a microelectromechanical (MEMS) shutter, developed by a team from NASA's Jet Propulsion Laboratory, including Gregory Bearman, Robert Green, Michael Eastwood, and Thomas Chrien. The invention addresses challenges in imaging systems, particularly in achieving high frame rates without compromising image quality due to readout contamination.
The core innovation involves a MEMS mirror array that can rapidly deflect light away from the focal plane during the readout cycle, effectively shuttering each pixel. This mechanism allows for precise control over exposure times, which can be adjusted by varying the duty cycle of the mirror states. The mirrors operate in a binary mode, switching between two extreme angular positions at megahertz rates, enabling the entire image to be shuttered simultaneously without the jitter associated with traditional mechanical shutters.
Key advantages of this MEMS-based shutter include the ability to use a 100% fill factor CCD, which enhances sensitivity, particularly in the red and infrared spectrum, compared to interline-transfer sensors. The system is capable of very short integration times, making it suitable for applications requiring rapid imaging, such as transient event imaging and time-resolved spectroscopy. In this mode, the mirrors can be selectively activated to capture successive time-resolved spectra, enhancing the capabilities of spectroscopic analysis.
The document also discusses the commercialization potential of the invention, indicating that while it is not yet ready for market, it can be assembled from available components with some mechanical integration and custom software. The technology is positioned for various applications, including scientific research and biological imaging.
Furthermore, the report highlights the competitive landscape, noting that traditional mechanical shutters and electro-optic devices have limitations that the MEMS shutter overcomes, particularly in terms of timing and image integrity. The document concludes with a call for interest from potential commercial partners and government agencies, emphasizing the innovative nature of the MEMS shutter technology and its implications for future imaging systems.
Overall, this technical support package outlines a significant advancement in imaging technology, promising enhanced performance and versatility for a range of scientific applications.
