American Cinematographer - June 2011 - 75

Mahrer. “Although global shutters are available in CMOS imagers, they are much more complicated devices and more expensive to make.” Partial-frame exposures can also occur with rolling shutters when there is a fast-changing lighting event in the scene — a flash of lightning, for instance. With the rolling shutter, this fast lighting change might be captured between two partial frames of exposure, with half of the bright, light-flashed frame appearing at the bottom of one frame and the second half of the flash captured at the top of the next frame. An experienced eye can detect these partial frames. Why not use global shutters for all imagers? The fabrication techniques required to include global shuttering in a sensor’s function is fundamentally different between CCDs and CMOS imagers. By design, all CCD sensors are able to deliver global electronic shuttering with little impact to the imager’s fill factor. To implement the same electronic-shuttering scenario in a CMOS imager, you must introduce several more transistors per photosite, and this greatly impacts the fill factor and, in turn, the size of the photosensitive area of the photosite. (The additional shuttering transistors must be placed in what would otherwise be an optically sensitive area of each photosite.) “Rolling shutters [on CMOS sensors] cause the integration period for each row of photosites to happen at a slightly different period in time,” says Jeff Zarnowski, chief technology officer of Panavision Imaging. “Row 0 is read and reset to start integration for reading the next frame. Row 1 is then read and reset to integrate for reading the next frame, and the process is repeated for all rows. Global shuttering allows the collected image-signal charge for all pixels to be stored at once, but this requires an extra transistor to independently reset the pixel separately from the sense node, and this separate reset also causes an increase in noise of at least 3dB.”

Color Balancing By design, a digital-imaging sensor captures light differently than a film negative does. Film stocks are manufactured to produce a neutral color response for a specifically balanced illumination source (daylight or tungsten), whereas a digital imager can be adjusted electronically to match its spectralresponse output characteristics to a variety of illumination sources. For example,

in order for a digital imager to display a gray card as gray, it must balance the gain applied to the red, green and blue output signals from the sensor. Depending on the color balance of the light illuminating the scene, the amount of gain applied to each signal can vary greatly. The spectral response, sensitivity and dynamic range of a sensor are also greatly influenced by the color proper-

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American Cinematographer - June 2011

Table of Contents for the Digital Edition of American Cinematographer - June 2011