Global shutter vs rolling shutter
Most of the Complementary Metal Oxide Semiconductors (CMOS) cameras on the market today offer a trade-off between their speed and the image quality. However, QImaging can now offer the ‘optiMOS’ sCMOS camera, which is able to offer high-speed image capture without reducing the frame-rate or increasing the noise of an image.
CMOS cameras generally function in ‘Rolling Shutter’ mode. In this mode, the image is captured row-by-row, and each row is transferred to its own dedicated Analogue-to-Digital (A/D) converter. The frame rate is determined by how quickly the process can be completed, which in turn is determined by the number of rows and the speed of the A/D converters in the array.
Some CMOS cameras can also operate in ‘Global Shutter’ mode – this means that every pixel is exposed simultaneously. This is great for capturing images of objects which are constantly moving and changing, but the frame rate of the camera is decreased and the noise higher compared to rolling shutter.
Rolling shutter mode, on the other hand, generally offers higher quality images and very fast frame rates for high-speed objects/processes.
While rolling shutter mode seems like a great option, it still isn’t perfect. There are several problems that can occur due to the row-by-row processing of images:
- Spatial distortions.
If you’re trying to capture a large and fast-moving object/process, you may see motion blur in your image. This is true whether we use global or rolling shutter modes. However, specific spatial distortions/artifacts can occur with rolling shutter mode due to the time delay between exposures of each row. Although this can be remedied by using global shutter modes, we’re back to the same problem of speed and image quality!
- Complexity when synchronizing with a light source.
A layer of complexity can be added when using rolling shutter mode when switching between multiple excitation wavelengths or when using a triggered light source.
- Single excitation wavelength
To reduce bleaching and phototoxicity, a common technique is to synchronize the camera with the excitation of fluorophores. A trigger from the camera turns the light source on at the beginning of the image capture and off at the end. In rolling shutter mode, however, the light source is generally triggered on/off by the exposure of the first row, which finishes exposure before all the other rows. As a result, each row after the first one has a smaller portion of exposure time – what you’ll see is a decrease in intensity from the top to the bottom of an image.
- Multiple excitation wavelengths
For multi-channel excitation imaging, each channel is usually completely exposed before the next channel begins. However, due to the overlap of rolling shutter, this means that the second frame (or wavelength) begins exposure whilst the first frame (first wavelength) is still finishing.
- Single excitation wavelength
So, let’s use a CMOS camera in global shutter mode – will that not solve these issues? Unfortunately, no! Most CMOS cameras on the market can’t even operate in this mode, but when they do, there is a significantly reduced frame-rate (up to 50%) and a subsequent increase in noise.
This is where the optiMOS camera and its unique imaging mode come in.
The best of both worlds
The ever-innovative QImaging has developed their optiMOS sCMOS camera, which is able to achieve global exposure with rolling shutter readout. The unique ‘All Rows Mode’ allows for the rapid shuttering of high-speed light sources only when all rows in a single frame are capturing pixels at the same time. This achieves global shutter readout while the camera is in rolling shutter mode, thereby reducing noise and maintaining a high frame rate.
In ‘All Rows Mode,’ the ‘Expose Out’ hardware trigger turns on the excitation light source only after all rows in the camera have begun exposing and then turns off the light source after a user-defined exposure time has elapsed. Thus, even though the camera captures the entire frame row-by-row, triggered excitation light ensures light reaches all the camera’s rows for the same length of time and at the same instant. This is particularly useful when using multiple excitation wavelengths, as all rows are capturing an image simultaneously before the sample is exposed to the next wavelength, thus eliminating channel cross-talk while maximizing speed.
This special ‘All Rows Mode’ implemented in the optiMOS camera enables it to achieve high speed exposure/capture whilst at the same time eliminating spatial distortions, single channel gradients and channel bleed-through when using multiple excitation wavelengths.