In this post, we consider a simple example to illustrate the inter-frame and intra-frame effect of imaging fast moving particles.
A simple example
It is necessary to estimate whether the frame rate of the camera is sufficient to capture the movement of particles from one frame to the next: for example, a camera with a max frame rate of 60f/sec provides an inter-frame time of 16.6ms apart. A faster-framing-camera may provide frames at up to 200f/sec, so the inter-frame time is only 5ms. Multiplying the distance moved by the particle in the inter-frame time (either 16.6ms or 5ms in our example) by the magnification (a function of the focal length of the lens and its distance from the sample plane) and dividing by the pixel size, we can translate the displacement in the sample plane translates a number of pixels on the camera.
As described above, one could reduce intra-frame “motion blur”, by reducing the duration of exposure. Otherwise, a particles may only be imaged as a multi-pixel blur. By setting the camera shutter to (for example) 1/1000 sec = 1ms, each exposure captures a much smaller movement of the particle, which significantly reduces the pixel blur. Note that a shorter exposure is only possible if there is sufficient light in the FOV.
In a real-world case, one would do a calculation with the ACTUAL particle speed as projected by the optics onto the image sensor. This will help determine if the inter-frame effects of motion are within an acceptable range.
The next step is to estimate the duration of exposure that is long enough to produce an image with acceptable SNR but not so long as to make the motion-caused blur to be above a desirable threshold.
In the next post, we take on a slightly more complex example in which we compare two different cameras being considered for a particle tracking application.