New-Tech Europe Magazine | August 2017

is illuminated only by moonlight or starlight. A traditional image sensor (the left image in Figure 2) images the bright part of the image well, but doesn’t have the sensitivity to “see” in the very darkest part of the image. A traditional EMCCD (center) can be configured to image in the very darkest part of the scene, but when the gain is turned up to enable this low light imaging, artifacts from the bright part of the scene destroy the image integrity. Interline Transfer EMCCD technology (right) allows the scene to be imaged continuously from the brightest to the darkest part of the image, where “dark” can extend all the way down to illumination only by moonlight or by starlight. Having been moved forward from the research labs to use in production devices, Interline Transfer EMCCD technology is being used today in a growing family of products. ON Semiconductor’s KAE 02150 image sensor uses Interline Transfer EMCCD technology to enable low light image capture at 1080p (1920 x 1080) resolution while operating at 30 fps, making this device well suited to security, surveillance, and situational awareness applications that require high sensitivity image capture with video frame rates. For higher resolution needs, the 8 megapixel (2856 x 2856) KAE 08151 image sensor is designed in a square aspect ratio with a 22 mm diagonal, aligning with the native optical format of many scientific microscopes and other medical equipment. By leveraging the advances available with Interline Transfer EMCCD technology, these devices are the first in a new class of image sensors that achieve high levels of performance under low lighting conditions.

Figure 1: Intra-scene Switchable Gain Output

image) are routed to a standard CCD output for conversion to voltage, while pixels with low charge levels (corresponding to dark parts of the image) are routed to the EMCCD output for additional amplification before conversion to voltage. These two datasets are then merged to generate the final image. Since the charge from pixels with high charge levels does not enter the EMCCD register, this output architecture allows both very low light levels and bright light levels to be detected while avoiding the image artifacts associated with overflow of the EMCCD output register. The power of this technology can be seen in Figure 2, which shows image captures of a single scene that includes both a bright light as well as very dark shadows, where the darkest portion of the image

designs that can range to multiple megapixels in resolution. Key to the performance of this technology is an Intra-Scene Switchable Gain feature, which avoids overflow in the EMCCD output register under bright illumination conditions by selectively multiplying only those portions of the scene that require it. This output design is shown in Figure 1, where charge from each pixel passes through a non-destructive sensing node which can be read by the camera control electronics to provide an initial measurement of the signal level for each pixel. This information is used to drive a switch in the sensor that routes charge packets to one of two outputs based on a camera-selected threshold. Pixels with high charge levels (corresponding to bright parts of the

Figure 2: A scene with both bright and very dark components, imaged by a standard IT-CCD (left), a standard EMCCD (center), and an Interline Transfer EMCCD device (right)

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