The integration of photodetecting elements and processing circuits on the same chip, for obtaining better performance from sensors, or for making the sensing and processing system more compact, is not a new idea, but the concept of smart sensing, i.e. sensor information processing without redundant and unnecessary data acquisition, and with at-sensor-level processing is relatively new. The word ``smart-sensors'' sometimes has been applied to those sensors which have only tried to integrate the sensors and processing modules, without any regard to the low level interaction that can exist between the sensors and processors. With this meaning in mind, even large systems with a vidicon and a 100kg main frame computer could be called a smart-sensor. The only difference would be the size. Here I would like to use a more fundamental meaning for smart-sensors. ``Smart-sensors are those devices in which the sensors and circuits co-exist, and their relationship with each other and with higher-level processing layers goes beyond the meaning of transduction. Smart-sensors are information sensors, not transducers and signal processing elements. Smart sensors are not general purpose devices. Everything in a smart sensor is specificly designed for the application targeted for.'' With this meaning in mind we exclude any camera-processor combination, even if they are integrated on the same chip. However, sensors such as NSIP architecture described in [Åström 93, Forchheimer and Åström 94] and column parallel architecture in [Hamamoto et al. 96a, Hamamoto et al. 96c], although do not integrate the sensors and processors at the pixel level, still possess a tight relationship between the sensors and processors. In fact these architectures suggest that some of the drawbacks of vision chips, such as loss of resolution and fill-factor, may be relieved, while maintaining a semi-parallel processing (in one dimension).
Traditional photodetectors could only output an analog signal, which required further signal conditioning. Still in most imagers the main focus is on the quality of the imaging in terms of noise, resolution, speed, and so on. It is assumed that further signal and image processing stages can acquire the imager output and process it. In contrast, in vision chips the main focus is on the quality of processing. The implementation of a certain algorithm using existing components is given the priority and often some image characteristics, such as resolution, are sacrificed.