Ultrasensitive Imaging: Hunting Every Photon
As the demands for precise imaging in fields such as medicine, astronomy, and real-time machine vision in hostile environments continue to increase, so the demands placed on imaging equipment become ever more stringent. Every single photon counts, in such extreme low-light conditions, in order to gain the highest quality of image possible. At the same time, many applications such as medical imaging demand a rapid image turnover, with dozens or even hundreds of image slices taken every second.
The two competing demands maximum number of photons in the minimum time pose complex engineering challenges, which must be overcome if the quality of imaging available to our systems is to continue to improve.
Researchers at the Fraunhofer Institute for Microelectronic Circuits and Systems in Duisburg, Germany , together with the partners of the MiSPiA project consortium, have created a new type of photon detector which they believe may alleviate the bottleneck between those two competing demands, by pushing our imaging capabilities beyond the state of the art for traditional photo detectors.
The new technology is based on Single Photon Avalanche Photodiodes (SPAD). It uses a pixel structure that is capable of counting impacts from individual photons with an exposure time of just a few picoseconds, and is therefore a thousand times faster than any other technology we currently possess. It certainly has the potential to open up a great many new applications for imaging.
Since each individual photon is taken into consideration images can be taken with next to no illumination, and still produce a meaningful result. This opens up the possibility of real-time mapping of pitch-black locations for example.
The new image sensor uses the internal avalanche breakdown effect a photoelectric amplification effect. The number of avalanche breakdowns corresponds to the number of photons that the pixels hit.
In order to count these events, each of the sensors pixels comes with very precise digital counters. At the same time, the scientists have applied microlenses to each sensor chip, which focus the incoming beam in each pixel onto the photoactive surface. Another advantage is that processing the digital image signals is already possible directly on the microchip; therefore, additional analogue signal processing is no longer needed.
The image sensor is a major step toward digital image generation and image processing. It allows us to have the capability to use even very weak light sources for photography. The new technology installs the camera directly on the semiconductor, and is capable of turning the information from the light into images at a significantly faster pace, states Dr. Daniel Durini, group manager for optical components at the Fraunhofer Institute IMS.
IMS engineered the sensor under the European research project MiSPiA (Microelectronic Single-Photon 3D Imaging Arrays for low-light high-speed Safety and Security Applications). Altogether, seven partners throughout Europe from the fields of research and business are involved in the project.
In the next stage, the scientists from Duisburg are working on a process to produce sensors that are back-lighted, and in this regard, even more powerful.