Panasonic has developed a new technology, electrical control of the near infrared (NIR) light sensitivity of the same pixel in an organic CMOS image sensor.
The sensitivity of all the pixels in the image sensor, which has directly stacked organic films, is simultaneously controlled by changing the applied voltage to the organic films.
This technology enables to switch between modes of color imaging and NIR imaging frame by frame without a mechanical IR cut filter required for conventional image sensors. This contributes to miniaturizing a camera system and improving the robustness. This technology also enables global shutter operation. Therefore, it can be suitable for applications in various fields of industry such as machine vision or intelligent transportation system, in which fast and accurate inspection or recognition are necessary.
About the new sensor
The design technology of organic CMOS image sensors which enables to design the photoelectric conversion part and the readout circuit part independently.
The sensitivity control technology which enables frame by frame switching between color imaging mode and NIR imaging mode by changing the applied voltage to the photoelectric conversion part.
In the organic CMOS image sensor, photoelectric conversion is executed by an organic film while signal charge accumulation and the signal readout are executed by Si based circuits beneath the organic film. The film and the circuits can be designed independently, so the organic CMOS image sensor can achieve the following features: high sensitivity, wide-dynamic range, and global shutter function. The newly developed technology has been realized to extend the spectral sensitivity of the image sensor from visible light spectrum to near infrared (NIR) light spectrum. And also, it makes possible to control the NIR light sensitivity electrically without losing the unique features of organic CMOS image sensors.
The spectral sensitivity of an organic CMOS image sensor can be extended from visible light spectrum to NIR light spectrum by stacking two organic layers, one of which can absorb visible light and the other of which can absorb NIR light. However, just simply stacking or mixing those two layers does not allow to distinguish the NIR light signal from visible light signal. In order to distinguish those signals, conventional methods need extra components such as electrodes and thin-film transistors, that results in increasing the pixel size.
To solve this problem, Panasonic adopted the direct stacked structure of two organic layers with the high resistance ratio of those. In this structure, the applied voltage is distributed to those two layers unequally. In other words, relatively low voltage is applied to one organic layer while relatively high voltage is applied to the other layer using the same voltage source. Organic CMOS image sensors need to be applied the voltage at least higher than a threshold in order to get signal information from incident light. Due to the characteristic, it is realized by the structure that both of those layers have no sensitivity at the low voltage, only one layer has sensitivity at the middle voltage, and both of those layers have sensitivity at the high voltage. Namely, this structure enables electrical switching of color imaging mode, in which the sensor is sensitive only to visible light and NIR imaging mode, in which the sensor is sensitive to visible light and NIR light without increasing any component.
NIR imaging mode enables to capture clear images even in the too dark situation for human eye to see. This contributes to improve the visibility of captured images by surveillance cameras in the night. And also, color imaging mode enables to obtain RGB color information based on visible incident light with NIR sensitivity being OFF state. These modes can be switched in the same pixel by changing the applied voltage to it, so IR cut filter is not necessary for this image sensor.
The image sensor can be also used for machine vision use. Even through subjects move with high speed and in invisible spectrum range, the image sensor can capture the image clearly. By controlling the sensitivity of the image sensor electrically, frame by frame image capture of subjects in different spectra is realized. This feature makes it possible for machine vison to earn images in both the visible light spectrum and the NIR light spectrum alternately by only a frame.