by Richard Lackey | 4th February 2016
The new Panasonic OPF CMOS sensor technology promises some significant performance improvements over the current generation of CMOS sensors. We can expect wider dynamic range, higher sensitivity and improved global shutter from smaller, yet more efficient photosites, pushing our image sensors to higher resolutions. It is well known that the current generation of CMOS image sensors have hit some limits in terms of architecture and fabrication. These limits determine both the maximum performance potential we can expect from our sensors, as well as the maximum resolution we can expect from a given sensor size. Any image sensor comprises a large array of photosites (think pixel), and a color filter array (CFA) assigning one color (red, green, or blue) to each photosite. The most common CFA is the bayer CFA. There has always been compromise, a trade-off, larger photosites result in higher sensitivity, but fewer of them can be squeezed into a given sized sensor. Smaller photosites result in a higher resolution, but compromised sensitivity. For example, this is essentially the difference between the sensors of the Sony A7S (Sensitivity) and A7R (Resolution) range of cameras. The photosensitive component of each photosite (pixel) on a conventional CMOS sensor is a photodiode, and it competes for space with other circuitry (a number of transistors and often a storage capacitor) as well as metal conductors reducing the active photosensitive area of each photosite. The back-illuminated (BSI) CMOS sensor improves on this by orienting the conductors behind the photocathode layer, so that light doesn’t have to pass through these additional layers before reaching the photocathode layer. This improves efficiency substantially, but it is still a design that has limits. Panasonic has some solutions. Here’s what Panasonic’s new sensor tech actually means. Higher Sensitivity With a traditional global shutter CMOS sensor, part of the total potential surface area of each light sensitive photosite is dedicated to necessary circuitry to store an electrical charge and is masked, this means there’s less space left for detecting light. Panasonic employs OPF (Organic Photoconductive Film) rather than the traditional silicon photodiode for photoelectric conversion, this allows the circuitry necessary to store charge to be almost totally independent and located under the OPF layer instead of taking up space on the surface of the sensor. This allows the full surface area of each photosite (pixel) to be light sensitive. In addition the OPF layer realises a 1.2x increase in sensitivity over a conventional photodiode. Because the OPF layer is four to six times thinner than a conventional photodiode (only 0.5 microns) the range of incident angles of light that can be converted increases from 40 degrees to 60 degrees further increasing the light gathering potential at each photosite. The maximum surface area per pixel can be used to detect light (photoelectric conversion). The OPF layer is 1.2x more sensitive to light than a conventional photodiode. Each pixel is sensitive to light arriving from a wider range of incident angles compared to the current generation sensor architecture. These improvements all add up to a significant overall increase in sensitivity, and higher signal to noise ratio in low light conditions. Wider Dynamic Range By locating the circuitry below the OPF layer it is possible to have much higher capacity charge capacitor, resulting in much higher saturation value compared to a conventional sensor. Panasonic claims an increase of up to 100x in terms of sensitivity to maximum brightness compared to conventional sensors. This means a massive boost in overall dynamic range. Improved Global Shutter The use of an OPF layer for photoelectric conversion instead of a photodiode allows a global shutter to be implemented in a completely different way compared to traditional sensors. A conventional CMOS image sensor with global shutter requires charge storage near the photodiode. As explained above this reduces the area of each photosite (pixel) that can be sensitive to light, and allows only a limited capacity for charge storage, limiting the maximum saturation value (dynamic range). Panasonic have implemented global shutter functionality by controlling the photoelectric conversion efficiency of the OPF layer itself without any impact to sensitivity or saturation value. Variable Sensitivity Multiple Exposure Technology Because the photoelectric conversion efficiency of the OPF layer can be controlled by modulating the applied voltage to the OPF, it is possible to record multiple exposures of variable exposure time and sensitivity. This also allows the sensor to detect motion and direction. A Bright Future It seems that the new Panasonic OPF CMOS sensor technology has solved every major limitation facing conventional CMOS image sensors without introducing any new compromises. We will have to wait and see how quickly Panasonic can implement this new technology in a commercial product, but it seems that OPF CMOS sensor technology could be the most significant advancement since digital video shifted from the CCD.Read more
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