Frame straddling for PIV

[fzigunov]

Hi, everyone,

I think this question has already been asked but nobody has answered it previously with an actual number (from my research). For a particle image velocimetry application, we typically use a double-pulse laser to capture two consecutive frames at an arbitrary interval to measure velocity of a cloud of particles. These lasers typically have two individually-controlled heads that each can send a short (20-200ns long) pulse of light. In these applications, we typically expose the camera frame for as long as possible and control the illumination timing to get velocity measurements. But all cameras have a finite time required to download the frame from the sensor to a buffer before starting exposing the next frame. This information is not provided anywhere I can find for the Chronos camera (both versions).

As an example, if you sample at 1000fps the time between frames is 1ms but the exposure has to be (1ms - download time) at most. Therefore, the time between laser 1 and laser 2 has to be at least (download time + pulse length). This technique is called frame straddling in the field and enables the use of "low speed" cameras for high speed flows where the inter-laser time can be as low as 1us for some flows (depending on speed and magnification).

Anyone that has this experience with the Chronos, could you inform what is the minimum "download time" possible? Have you done any tests? I just wanted to know that info before committing to purchasing it.

Thanks!

[hummingbirdography]

The duration required for transferring frames from video memory to a storage device for both the 1.4 and 2.1 camera models depends upon a combination of factors:

• The recording resolution & quantity of frames
• The storage device selected (SD Card, USB/eSATA, Windows/SMB or NFS network storage)
• The output format selected (H.264, Raw 12bit, Raw 16bit, CinemaDNG, TIFF, or TIFF Raw)

Recording H.264 to an eSATA SSD is typically the fastest option. The Chronos 2.1 factory default settings for 1920x1080 resolution H.264 video @60fps and 0.7 bits per pixel will encode at about 59fps, so in other words would require approximately the same amount of time to save the video as playing it from start to end. The other output formats typically average saving this resolution at 10fps.

Note: For the use case you described, you might want to use an external trigger in combination with the "Segmented" or other recording modes described in the user manual.

There's no delay between recording segments, however all of the above still applies when saving the segments.

[fzigunov]

Hi, I'm sorry but these answers are not what I'm looking for. I am not interested in the time to save from camera memory to storage or encoding/processing times.

I'm asking about the time required to transfer a frame from the sensor to the internal camera RAM while it is recording. During this time the camera cannot start exposing the next frame because it hasn't downloaded the current frame yet. This time is the most limiting factor in the particle image velocimetry application and is called "frame straddling time". High-end cameras like Phantom cameras provide this info in their datasheet. Since Chronos advertises themselves as suitable for PIV, I was hoping they would provide this info.

And yes, I sent them an e-mail a month ago and have not heard yet.

[fzigunov]

Ok, I was able to get a response from Mike Tymofie at Krontech. Here's the frame straddling times for the two Chronos cameras:

Chronos 1.4 - 5.56 us
Chronos 2.1 - 6.67 us

These are calculated times, according to him. I am not sure what that means but I'd like to try it in practice. At least now I can plan my PIV experiments, and I believe these times are acceptable for moderate flow speeds (say, up to ~100m/s or so depending on magnification and other aspects). Say, at 100m/s a particle would move 0.7mm in 7us. If we're doing cross-plane Stereo PIV, that is as much you would want (about 25% of the ~3mm light sheet). So, unfortunately, no supersonic flows 

I'll let you know more when I get my hands on one and I have a laser available to test it.

Hope this helps somebody in the future!

[dellslender]

geometry dash lite

It would take roughly the same amount of time to save the video as it would to play it from beginning to end using the Chronos 2.1 factory default settings for 1920x1080 resolution H.264 video @60fps and 0.7 bits per pixel. This is really important to learn and understand more about it.

[moonlight]

Getting that download time is crucial, especially for high-speed flows. I'd recommend checking the user manual or reaching out to the manufacturer directly.