Advanced Imaging


Advanced Imaging Magazine

Updated: July 8th, 2008 05:26 PM CDT

Make Mine a Double


Machine vision applications constantly present sensor and camera designers with conflicting goals: higher speed (for higher throughput), higher sensitivity (to operate with less light) and lower noise (to maintain image quality). Nowhere are these demands stronger than in linescan imaging, where integration times are mere microseconds and budget and safety concerns limit the available light.

Through hard work, careful pixel design and process development sensor designers have delivered steady progress in amplifier bandwidth for greater speed, even as they reduced amplifier noise. They have also boosted sensitivity, both by improvements in basic charge conversion efficiency (CCE) and in pixel architecture (for example, in-pixel amplifiers for CMOS devices). But as images are formed from fewer and fewer photons, conventional designs approach fundamental limits. In high-speed, low-light situations, photon shot noise is becoming a limiter to image quality.

Photon shot noise is a statistical phenomenon resulting from the random variation in the number of discrete photons striking the photosensor. Unlike reset noise (which can be corrected by correlated double sampling); thermal noise (which can be reduced by cooling); or various forms of fixed pattern noise (deterministically corrected by subtraction) photon shot noise cannot be electronically or algorithmically corrected. And because it scales with the root of the collected charge, it becomes a progressively more dominant noise source as the available light decreases (e.g. halving the number of photons only reduces the shot noise by v2). In low-light applications, photon shot noise will eventually limit the sensor and camera noise floors.

DALSA’s novel dual linescan CCD breaks this trend. By doubling sensitivity with only a v2 increase in shot noise, the design delivers a greater signal to noise ratio, especially in light-starved situations. Functionally similar to TDI arrays, the design consists of two parallel arrays of photodiode pixels. Each pixel is connected to a selectable delay gate that either allows charges through or delays the charges by one scan line. Signal electrons from the two arrays (one of which has been delayed) are then combined on-chip into a single output (with no increase in amplifier noise). Because the delay is selectable, the sensor allows bidirectional scanning. The design also preserves powerful conventional linescan features such as exposure control and high blue response.

The design does increase dark signal from the doubled pixel area, but for high-speed applications (as most linescan applications are) the integration time is so short that the dark signal remains negligible.

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