New-Tech Magazine - Europe | January Digital edition

light sensitivity when compared to 5T layouts or other designs.

Time-Delayed Integration Time-delayed integration (TDI) imaging is another clever way to better capture moving objects. By synchronizing pixel exposure with the mo-tion of the camera or the object, the effective exposure time can be increased. TDI implementation in CMOS has traditionally been difficult because of the lack of a charge- addition circuit. The application requires the combination of a global shutter and a low-noise readout method. Recently TDI in the digital domain, enabled by high frame rates, has become more and more popular, even though the improvement signal- to noise ratio improvement is lower than for the traditional TDI opera-tion. High Dynamic Range (HDR) Another factor in improving global- shutter CMOS sensors is applying a specific method to achieve a high dynamic range (HDR). HDR expands the scale of the captured light and dark areas of an image to depict them in a satisfactory way. This is appropriate when capturing an im- age before a bright sky background or against very bright light sources, which tends to deliver overexposed images with blurred-out white areas, whereas the darkest shadows appear underexposed and recede into an unstructured black. Thus, the exposure levels for both light and dark areas have to be equalized across the image. The reason for the unbalanced treatment of light and dark areas is the linear response curve of CMOS image sensors as opposed to the

Figure 6: Different exposure times for odd and even lines achieve a higher dynamic range

archi-tecture patented by CMOSIS differentiates it from the traditional 4T rolling shutter or the 5T global shutter concept. This eight-transistor pixel design is now implemented in all global-shutter sensor types of the CMOSIS CMV Series sensor family. The crucial point is that the 8T architecture provides two storage ele- ments inside the pixel, rather than just one (as in the 5T structure). They separately store an image taken at the beginning of the exposure,

and another one at the end of the exposure period. Deploying a clever algorithm, both these images are subtracted during readout to lower the total noise account and increase the shutter efficiency. This way, noise levels below 10 electrons can be reached, and a shutter efficiency of 99.999 percent has been demonstrated. The technique, called correlated double sampling (CDS), enables the lowest fixed-pattern noise and low parasitic-

Figure 7: Piece-wise linear forming of the sensor response curve achieves a higher dynamic range

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