Author Topic: frame straddling  (Read 208 times)

PIVer

  • Newbie
  • *
  • Posts: 1
    • View Profile
frame straddling
« on: July 17, 2019, 10:02:39 AM »
Hello,

I am trying to set up a Chronos 1.4 for particle image velocimetry (PIV) measurements. Many details about PIV are not important in this context, but two are: 1) images are taken in pairs, and 2) you should have control of the time between frames. The latter is, of course limited by the frame rate, i.e. you can't have a 500us inter-frame time if your recording at 1000 kHz.

One trick to decrease the effective inter-frame time is to use frame straddling, that is, use pulsed illumination at the end of the first image and at the beginning of the second image (see attached figure). While this works very well if you don't have no light apart from your pulsed illumination, the image quality is deteriorated when you have a long exposure time and there is significant ambient light.

To minimize the effect of ambient light, I would like to reduce the exposure using Shutter Gating (as also shown in the figure). By limiting the exposure to only a few microseconds before and after the illumination pulse, the ambient light that makes it to the CMOS would be negligible. My question is: can such a timing sequence be achieved?

Thanks for any help.

foobar

  • Krontech
  • Newbie
  • *****
  • Posts: 41
    • View Profile
Re: frame straddling
« Reply #1 on: July 17, 2019, 11:50:45 AM »
I'm afraid that this particular pattern of exposure and frame timing will not work. When the camera is operating in gated burst mode the rising edge of the trigger signal starts the exposure for a frame and the falling edge ends the exposure and begins the frame readout. During frame readout, the exposure for the next frame can be started but the exposure must continue for long enough that the falling edge happens after the readout of the first frame completes. A falling edge of the trigger signal while frame readout is in progress will be ignored because the pixel storage nodes on the image sensor are in use and are not able to accept another frame.

Because the image sensor does not have a physical shutter, there is no way to stop or pause the exposure of a frame, the exposure control is actually a signal that just continuously resets the state of the photodiodes to an unexposed state. At the end of exposure, the charge is transferred from the photodiodes into storage capacitors that hold the charge during sampling and readout. Therefore, the end of exposure must be concurrent with the start of the frame readout and only one frame can be held in the storage nodes at a time.