Hello dear RG community.
I have a Dantec PIV setup consisting of, in particular, an iNanoSense camera and a SoloPIV 120 NewWave Research laser.
I'm getting the first image less bright and nonuniformly lit as oppose to the second image in a PIV pair of images.
First thing I did is I tried to determine whether it is the laser's or the camera's fault. I swapped the laser heads and, still, got the first image less bright and nonuniformly lit.
Hence, I made a conclusion that it is the camera's fault.
The only fix I could think of was to play with the timing diagram to try and make the camera getting the first image right. I didn't manage to come up with such a timing diagram.
Now, I'm out of ideas.
I'm wondering if anybody had the same issue, came up with a working fix and could share it with me, please.
Attached, is my timing diagram.
Thank you in advance.
Ivan
P.S. I did talk to Dantec's support to no avail. Dantec did help me a lot overall, but we couldn't solve this particular issue.
P.P.S. I don't want to play with the attenuators to try and get the first laser head more powerful to compensate for the camera's error (if that's possible at all ...).
Ivan Nepomnyashchikh There are a few things you may attempt to reduce the lighting mistake in your PIV images:
1. As far as feasible, ensure that the lighting conditions are constant between the two photographs in the PIV pair. Using a consistent lighting setup or capturing the photographs at the same time of day can help to reduce the influence of fluctuating ambient light levels.
2. Adjust your camera's exposure settings to try to equal out the lighting throughout the photographs. To obtain the ideal balance, you may need to experiment with different exposure periods or aperture settings.
3. To try to compensate for uneven illumination, use image processing tools. This can include adjusting the overall brightness and contrast of the photos with operations like levels or curves or correcting color imbalances with targeted color modifications.
4. If the uneven lighting is caused by reflections or other external influences, you may need to move your camera or the items in the image to reduce their impact.
I hope you find these tips useful! Please let me know if you have any more inquiries.
Thank you Qamar Ul Islam .
With regards to the first suggestion, like I mentioned, I made sure that's not the laser's fault.
With regards to the second suggestion, I don't have a motorized lens. I'm using a manual one. Thus, the suggestion is not feasible obviously. Moreover, I'm not sure if such fast motorized lens exist at all.
The third suggestion is interesting. I was thinking about it, but I couldn't find the proper methods. I'm wondering if you can elaborate more on the tools you listed please.
With regards to the fourth suggestion, that's not the case.
I am not familiar with the cameras you are using, but I am assuming they are double-shuttering and slaved to the laser frequency? It is quite common in double-shuttered cameras for one of the two images to be brighter due to the way the exposure times are handled. Most of the time, one of the two exposures (the first) is quite long until the trigger signal is received. That sets into motion the transfer of the contents of the CCD array and the start of the second exposure. This process has typically been accomplished in a couple of different ways, but both ways result in unequal exposures between the first and second images in an image pair on a double-shuttered camera. Less exposure -> fewer photons captured -> less bright images. The newer cameras do a much better job of evening this out, but the bit-depth of the cameras we use still capture a measurable difference.
Thank you very much Aaron Altman !
That explains a lot.
You see, my PIV analysis of the images would result in an, approximately, 50% of correlations being in the range of 0...0.25 signal to noise ratio.
That's a pretty big percent of invalid correlations. I've been trying to reduce it without much success.
I thought one of the possible reasons of such a big error could be uneven brightness of the images. That is because if the same particle in frame_A is way less bright than in frame_B, the algorithm may not pick it up from frame_A at all and won't be able to correlate it in frame_B resulting in an error. That was my reasoning. That's why I've been trying to equalize the brightness of the images.
But you are saying that this is a common thing. And people have been getting valid results despite of it.
Hence, I conclude that my error is, primarily, due to something else and I can stop worrying about uneven brightness.
Thank you very much for the explanation. The uneven brightness has been driving me crazy since I began doing PIV (more than a year already), all this time I've been trying to mitigate it (unsuccessfully). And now I know the cause of it.
Thank you.
Ivan
Just by accident, I have found out a trick to brighten-up the first image in a pair.
I increased the exposure time of frame 1 a lot, to 180000 microseconds. And I've got both images in a pair equally lit.
My timing diagram is looking now as the one I attached to my post but with the dark green hashed bar extended way to the left.
I don't know why increasing the exposure time, extends the dark green bar to the left and not to the right and why it brightens up the first frames.
As a note, I had tried increasing exposure time of frame 1 to brighten-up the first image before. But I tried doing it within the sensible limits and it had never given me the desired result. I had no idea I had to increase the exposure time to as much as 180000 microseconds.
We have just figured out another trick to do that.
For the case when the first laser pulse is aligned with the beginning of the dark green hashed bar on the timing diagram, you need to go to the camera's setting, change to the Expert option, change "Delay to open" from the default 0.01microsecond to 0.5microsecond. That helps to make the both images equally lit.
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