FEATURE SENSING TECHNOLOGY
IT-EMCCD CHALLENGES LOW-LIGHT IMAGING
Michael DeLuca from ON Semiconductor investigates how interline transfer (EMCCD) technology is leading the way for high dynamic range sub-lux imaging
W
hen considering the performance of low-light imaging systems,
whether used in surveillance to monitor a dark alley or on a microscope to detect low-level fluorescence, two image sensor parameters are key. Sensitivity, which is actually a combination of several factors such as noise, responsivity, quantum efficiency, and capture time, specifies the lowest level of light that can be detected by the camera. For a 30fps image capture, a standard image sensor might have the sensitivity to capture down to single digit lux levels, corresponding to light from a full moon or at twilight. The second is dynamic range, which measures the range of light the sensor can detect at one time. To capture under uncontrolled lighting conditions such as a dark alley way at night, both sensitivity and dynamic range are needed – sensitivity to see deep into the shadows, and dynamic range to extend image capture to the brightest part of the scene without losing image detail. Interline Transfer EMCCD (IT-EMCCD)
devices meet the needs of challenging low-light imaging applications by combining two different technologies – Interline Transfer CCD and Electron Multiplication CCD (EMCCD) – in a way that preserves the best features of each. Interline Transfer CCD provides a high performance base for the technology, with high image uniformity and a global shutter pixel design to capture moving objects without the introduction of motion artifacts. EMCCD multiplies signals coming from very low light scenes so they rise above the intrinsic noise floor of the image sensor, driving the effective noise floor to sub-electron levels and enabling detection at very low light levels. By combining these two technologies, IT-EMCCD devices provide the flexibility for a single camera to image a scene where one portion is very dark – measured at sub-lux illumination levels – while a separate region is brightly lit, all at the same time. Figure 1 demonstrates the low-light performance of this technology, showing a scene captured at 0.01 lux
32 FEBRUARY 2018 | ELECTRONICS
(corresponding to illumination from a quarter moon on a clear night). Unlike a standard CMOS sensor, where detail in the image is obscured by background noise, the IT-EMCCD image preserves detail even under this very low light condition.
ENHANCED DYNAMIC RANGE The enhanced dynamic range available from this technology can be seen in Figure 2. Here, the scene is illuminated only by the bright light on the left, so that illumination decreases going from left to right in the scene. A standard CCD captures the brightest part of the image well, but noise obscures the image in the shadows at the right side of the scene. While a standard EMCCD image sensor would image in these shadows without issue, the bright light on the left first
Figure 1:
Standard CMOS and IT- EMCCD image capture. Both images captured at 0.01 lux using f2 lens at 33ms exposure
Figure 2:
Standard CCD, Standard EMCCD, and IT-EMCCD Image Capture
initiates an overflow cascade that destroys the integrity of the image. But Interline Transfer EMCCD combines the best of these technologies, selectively using a standard CCD output for the bright parts of the image and an EMCCD output for the dark parts of the image. This extends the dynamic range available from the device beyond what is available from either technology on its own, allowing the full range of the scene to be captured in a single image. This combination allows IT-EMCCD technology to provide features unmatched by any single imaging technology. The electron multiplication register enables exceptional low light imaging, capturing information at light levels well below one lux. The Interline Transfer CCD design provides high image uniformity in an architecture that is easily scalable to high resolution or to colour imaging. And together they provide wide linear dynamic range – such as the 92dB (40,000 to 1) dynamic range available from ON Semiconductor’s KAE-04471 image sensor. This flexibility makes devices based on this technology an excellent match to light starved applications such as surveillance and intelligent transportation systems, medical imaging such as ophthalmology or fluoroscopy, or scientific microscopy – any application requiring capture of images with very low signal levels, or extension of that capture from low light through to bright light. Today, there are almost 30 different orderable parts available using Interline Transfer EMCCD technology, with resolutions ranging from 1080p to 8 megapixels, options for large pixels and enhanced NIR sensitivity, integrated cooling, and more. Since specific imaging requirements can vary greatly from application to application, the breadth available in this portfolio is critical to allow the most appropriate image sensor to be matched to the needs of a specific application.
ON Semiconductor
www.onsemi.com T: 00421 33 790 2910
/ ELECTRONICS
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