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Rolling Shutter vs. Global Shutter - 9 Pages

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Catalogue excerpts

TECHNICAL NOTE Rolling Shutter vs. Global Shutter Serial Register Output Node Single Digitization Point is Rate Limiting Step Figure 1. Basic schematics of CCD (above) and CMOS (left) sensor architectures. CCD and CMOS Sensor Architecture and Readout Modes Two image sensor types widely used in cameras for microscopy are scientific grade Charge Coupled Devices (CCD) and scientific Complementary Metal Oxide Semiconductors (CMOS or sCMOS). There is a number of similarities between the two technologies, but one major distinction is the way each sensor reads the signal accumulated at a given pixel. This Tech Note will explain how the differences in readout modes impact the exposure timing, illumination, and triggering of cameras and light sources in microscopy experiments. While many readout modes exist, CCD cameras popular in microscopy often use interline transfer CCDs in a Global Shutter mode. In Global Shutter mode, every pixel is exposed simultaneously at the same instant in time. This is particularly beneficial when the image is changing from frame to frame. The CCD however has an inherent disadvantage when it comes to frame rate. When the exposure is complete, the signal from each pixel is serially transferred to a single Analog-to-Digital Converter (A/D). The CCD’s ultimate frame rate is limited by the rate that individual pixels can be transferred and then digitized. The more pixels to transfer in a sensor, the slower the total frame rate of the camera. A CMOS chip eliminates this bottleneck by using an A/D for every column of pixels, which can number in the thousands. The total number of pixels digitized by any one converter is significantly reduced, enabling shorter readout times and consequently faster frame rates. While there are many parallel A/D’s sharing the workload, the entire sensor array must still be converted one row at a time. This results in a small time delay between each row’s readout. ©2014 QImaging. All rights reserved.

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Rolling Shutter vs. Global Shutter TECHNICAL NOTE Rather than waiting for an entire frame to complete readout, to further maximize frame rates, each individual row is typically able to begin the next frame's exposure once completing the previous frame's readout. While fast, the time delay between each row's readout then translates to a delay between each row's beginning of exposure, making them no longer simultaneous. The result is that each row in a frame will expose for the same amount of time but begin exposing at a different point in time, allowing overlapping exposures for two frames. The ultimate...

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TECHNICAL NOTE Rolling Shutter vs. Global Shutter Rolling Shutter Challenges While rolling shutter offers the advantage of fast frame rates, the overlapping behavior and time delay between each row’s exposure may also be a disadvantage under some conditions. Rolling shutter’s challenges can be summarized by two categories: 1. Rolling shutter spatial distortions Imaging moving objects requires consideration for the object’s size and velocity in order to properly sample and avoid motion blur. This is true for both global and rolling shutter readout, irrespective of whether a CCD or CMOS sensor is...

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TECHNICAL NOTE Rolling Shutter vs. Global Shutter 2. Complexity when synchronizing with a light source When the imaging application requires synchronizing with a triggered light source or switching between multiple excitation wavelengths for every exposure, the overlapping exposure of frames with a rolling shutter sensor can add a layer of complexity. a. Single Excitation Wavelength One common application of hardware triggering is to shutter a light source and synchronize the sample’s excitation with a camera’s acquisition. By doing this, light is only exposed to the sample when the camera is...

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Rolling Shutter vs. Global Shutter TECHNICAL NOTE b. Multiple Excitation Wavelengths For multi-channel fluorescence imaging, where the light source alternates between excitation wavelengths for every exposure, the gradient will appear consistently in each channel as long as the channels are slowly acquired so that each color capture is completely exposed before the next channel’s exposure begins. Figure 5. Two 10ms exposures with the optiMOS and the X-Cite® XLED1 from Excelitas Technologies. With the standard rolling shutter exposure and with a triggered high speed light source, a single acquisition...

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Rolling Shutter vs. Global Shutter TECHNICAL NOTE When this occurs, the light source is again triggered and the second excitation channel turns on while both the second half of the first frame and the first half of the second frame are exposing. The resulting Frame 1 image with a 5ms exposure then has a channel 1 (red) gradient running top to middle and a channel 2 (green) gradient running bottom to middle. The resulting Frame 2 image is then just the inverse of frame 1, with a channel 2 gradient running top to middle and a channel 1 gradient running bottom to middle. One way to avoid these single...

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Rolling Shutter vs. Global Shutter TECHNICAL NOTE All Rows Mode is described in Figure 7. In this example, the user enters a 10ms exposure, selects snap, and the camera receives a software or hardware trigger to begin acquisition. Row 0 on the sensor begins exposing, but initially the “Expose Out” hardware trigger signal from the camera stays low. Each row subsequently begins its exposure with the Expose Out signal remaining low until the very last row of the frame begins exposing. Once the last row starts its exposure, the Expose Out line goes high. This rising edge of the Expose Out signal is...

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TECHNICAL NOTE Rolling Shutter vs. Global Shutter With the All Rows Mode from the QImaging optiMOS, in combination with a high speed triggered light source, such as the X-Cite® XLED1 from Excelitas Technologies, it is possible to achieve over 90fps full resolution while maintaining low read noise and avoiding any rolling shutter artifacts. While it is possible to achieve much higher frame rates with this custom triggering mode over traditional global shutter, there is still a limitation to the achievable frame rates. The time required to digitize all rows within a single frame of the optiMOS is...

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