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.

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Snapshot CCD camera with microelectromechanical shutter

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This article first appeared in the February, 1999 issue of NASA Tech Briefs Magazine.

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