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For outdoor surveillance, sensitivity and dynamic range are important to deliver reliable images over widely changing illumination. However, constant monitoring with maximum awareness requires large power consumption and is not suitable for energy-limited applications such as battery-operated and/or energy-scavenging wireless sensor nodes. One of the ways to reduce power is voltage scaling [1-4]. However, it significantly reduces the SNR and results in poor image quality . The signal can be easily corrupted from the noise in dark conditions or be saturated in bright conditions. Most imagers with high sensitivity and wide dynamic range [5,6] consume large power >;50mW, unsuitable for wireless imager node applications. Therefore, it is imperative to implement a sensor adaptable to environmental changes: i.e., the sensor keeps monitoring at extremely low power operation and only turns into high-sensitivity or wide-dynamic-range operations when requested due to illumination changes or requested from the host for detailed image transmission. The sensor changes its operation back to the monitoring mode as a default or when enough operating energy is not available from the battery or energy-harvester. In this paper, we report an adaptive CMOS image sensor that employs four different modes: monitoring, normal, high-sensitivity and wide-dynamic-range (WDR) modes. This adaptable feature enables reliable monitoring while significantly enhancing battery lifetime for wireless image-sensor nodes.
Date of Conference: 19-23 Feb. 2012