On camera resolution and sensitivity for a PIV experiment

[Erich's Lab]

Hi all, I hope you are all having a good day. Below is a problem I am currently facing before I construct a low cost, decent accuracy planar PIV system for under $2,000 USD.

TL, DR:

I was wondering what resolution a camera should be for 200 +/- 100 μm helium-filled soap bubbles and a 15 x 10 cm area of interest. The following resolutions are of question:

 - 14040 x 1080 px @ 220 fps, ~104.2 μm/px  x ~92.6 μm/px resolution

 -  1920  x 1200 px @ 120 fps, ~78.1 μm/px x ~83 μm /px resolution

 -  2600 x 2160 px @ 60 fps,     ~57.7 μm/px x ~46.3 μm /px resolution

Detailed version:

Recently, I started researching into double and triple exposure, color PIV to circumvent the limitations of high interframe times in industrial and machine vision cameras. This requires a color image sensor which is known to be less sensitive compared to monochromatic sensors. This is due to smaller wells in the sensor along with a color filter (often a bayer filter) which rejects photons of a specific wavelength range. In essence, less photons are converted into electrons and as a result, the quantum efficiency, signal to noise ratio, and dynamic range suffers especially for blue and red wavelengths. This means that a higher powered light source is needed to illuminate the seed particles for a given exposure,. In my case, I will simply increase the amount of LEDs to 18 CBT 90 LEDs and collimated them into a 0.5 - 1 mm light sheet with moderate divergence using fiber optics and a cylindrical lens (overall optical efficiency is estimated to be ~15%, but the light output would be significantly more powerful than efficiently coupling a single LED).  However, this begs the question on how sensitive the camera sensor has to be to properly detect and resolve seed particles. In my current and ongoing experiment, two seed generators have been designed based on cavitation and helium-filled soap bubbles. The first method generates approximately 50 μm +/- 20 μm soap bubbles using cavitation via rapid and controlled depressurization of a soap-water mixture. The second method uses a nozzle constructed through additive manufacturing to controllably generate helium-filled soap bubbles of ~200 μm +/- 100 μm. The latter method was chosen due to concerns governing required resolution and the higher ability to scatter light. 

Moving forward, 3 industrial cameras were selected as possible candidates for this experiment. These cameras were selected based on cost, resolution, and apparent sensitivity. Their characteristics are detailed below.

Basler daA1440-220uc

  resolution: 1440x 1080 px

  pixel well size: 3.45 μm

  fps: 220

  monochromatic quantum efficiency (uncooled): ~62.7%

  monochromatic dynamic range: ~71.5 dB

  monochromatic s2n ratio: ~40.2 dB

Basler a2A1920-51ucBAS

  resolution: 1920 x 1200 px

  pixel well size: 3.45 μm

  monochromatic quantum efficiency (uncooled): ~62.2%

  monochromatic dynamic range: ~71.7 dB

  monochromatic s2n ratio: ~40.2 dB

Basler a2A2600-64ucBAS

  resolution: 2600 x 2160 px

  pixel well size: 2.5 μm

  fps: 64

  monochromatic quantum efficiency (uncooled): ~53.6%

  monochromatic dynamic range: ~71 dB

  monochromatic s2n ratio: ~36.7 dB

Honorable mentions:

Basler acA1920-40uc

  resolution: 1920 x 1200 px

  pixel well size: 5.86 μm

  fps: 41

  monochromatic quantum efficiency (uncooled): ~70%

  monochromatic dynamic range: ~73.5 dB

  monochromatic s2n ratio: ~45.2 dB

Basler acA2440-35uc

  resolution: 2448x 2048 px

  pixel well size: 3.45 μm

  fps: 35

  monochromatic quantum efficiency (uncooled): ~68%

  monochromatic dynamic range: ~73.4 dB

  monochromatic s2n ratio: ~40.2 dB

Note:

Monochromatic characteristics are used to give a rough idea of how sensitive the camera could be. For color sensors, these metrics do not apply..

Another honorable mention would be the Optolution OptoCam 2.3 mp camera with a 65 μs interframe time. This is the most optimal solution as it would remove the need for multiple color exposures as well as obtain a much higher sensitivity to light, but that device is sadly outside of my current budget for a camera ($850 USD).

My biggest concern is the resolution needed for properly resolving seed particles of 200 μm +/- 100 μm. Ignoring effects of light scatter enlarging sensed particles, a resolution of ~95 μm/px seems appropriate for resolving 200 μm +/- 100 μm particles as this should generate particles of 3 to 4 pixels. However, the particles are more likely to appear larger due to effects of scattered light which could double or even triple the imaged particle size, requiring larger areas of interest to keep the particles around 3 pixels (which is fine for this experiment). Another option is to use the scattered light of 50 μm particles (most optimal as they have a Stokes number of around 0.2) and rely on the camera's ability to detect particles below its resolution, but that would require possibly higher camera resolutions. This leaves me conflicted if I should use a lower resolution camera with larger seed particles or a higher resolution camera with smaller seed particles (that more faithfully follow fluid trajectories) and hope that they can be properly resolved by the camera. Of course, there are several algorithms to increase the particle size if needed, but I would assume it would be more beneficial to have particles in the range of 2 to 4 pixels before applying DSP algorithms.

All contributions are very welcomed.

Kind Regards,

Erich

[Alex Liberzon]

Hi Erich, 

You have a complex question here and a lot of helpful information. 

Regarding the bubbles - as far as I remember their images are not one bright blob (like the solid particles or small aerosol droplets), but rather a double dots (glare points?) view, like soap bubbles - you see where the light enters and leaves the bubble thin film. This means that the bubble diameter is a distance of two glare points, and the resolution has a different meaning here. also, the laser scattering pattern is not what we typically know from the book. Take a look at some recent works on HFSB to see what they recommend. 

From our side - I'd be very happy to learn about your two bubble devices, especially if these could be replicated - the price for the commercial HFSB generator is quite high. Please share (in private or publicly) what you've done, maybe a blog post with some drawings and tips? This would be a great source, I'm sure like all the other things you do. 

My experience is that color CMOS sensors are about 60% less sensitive than the same model monochromatic sensor. These are double effects—the size of the well and the filter itself. Maybe the situation has improved recently; I haven't checked the new models for a few years. 

I'd go for -  2600 x 2160 px @ 60 fps,     ~57.7 μm/px x ~46.3 μm /px resolution - the 40 fps or 60 fps versus 200 fps are meaningless if you speak of flows you do not plan to use them at this frame rate, right? I'd vote for the sensitivity first and then for resolution. 

By the way, I found a manufacturer, Viewworks, that uses OEM sensors sams as TSI's with a double-shutter option. Custom synchronization makes it possible to drive them in the PIV staggered mode. I'm working with the local machine vision company, and they prepared me a "homemade" PIV system with an 8Mp sensor. It is, however, more expensive than the Optolution option. 

Last idea: Come over for your experiment in my lab. Maybe it's cheaper than building your own lab. Of course, travel is not cheap either. 

Best regards, 

Alex

[Erich's Lab]

Hi Alex,

I finalized my camera selection to the following two cameras:

Basler acA1920-40uc

  resolution: 1920 x 1200 px

  pixel well size: 5.86 μm

  fps: 41

  monochromatic quantum efficiency (uncooled): ~70%

  monochromatic dark noise: 6.9 e-

  monochromatic saturation capacity: 32,800 e-

  monochromatic dynamic range: ~73.5 dB

  monochromatic s2n ratio: ~45.2 dB

FLIR BFS-U3-27S5C-C

  resolution: 1946 x 1464 px

  pixel well size: 4.5 μm

  fps: 95

  monochromatic quantum efficiency (uncooled): ~71%

  monochromatic dark noise: 5.8 e-

  monochromatic saturation capacity: ~25,000 e-

  monochromatic dynamic range: ~72 dB

  monochromatic s2n ratio: Not Specified

Both sensors have high quantum efficiency, low dark noise, and high well capacitance. To be honest, I am leaning towards the FLIR camera for its slightly larger resolution.

On the seeding generators, they are extremely simple to create, just hard to operate and maintain. For the micro bubble generator, it was made out of a used pressure washer with a nozzle and side discharge valve to control cavitation and fluid pressure. The nozzle is 3D printed and casted into aluminum using the lost-foam casting method. the nozzle hole is drilled using a .25 mm unbranded drill bit from an unknown origin. The system is quite crude, but is functional for the most part. I stopped investigating this method for generating seed particles as it requires a resolution of at least 50 μm to properly resolve the seed particles (e.g., a 5 mp sensor with smaller pixel sizes and well capacity). For the helium-filled soap bubble (HFSB) generator, I simply followed the current state-of-the-art modular 3D-printed HFSB nozzle designs and modified them to create slightly smaller seed particles. This method of generating seed particles is attractive to me mainly because of the use of low-pressure systems. However, it is quite finicky to setup since I have no flow meters a hand and the system is set up by observing the dispersion of the HFSB particles.

On your concern governing the halo effect on soap bubbles, this is really not an issue until the particles becomes at least 6 pixels in diameter. Below 6 pixels, they appear as a solid mass akin to micro spheres commonly used to seed water flumes.

Additionally, your remark on the sensitivity of color sensors seem correct. There is a rather significant luminosity difference between each color channel and a monochromatic sensor. Although this is not ideal, the main attractive advantage of color sensors is the ability to color-code three exposures enabling acceleration and pressure measurements regardless of the camera's interframe time. Beyond this, I would have otherwise focused on monochromatic sensors and request numerous evaluation cameras to analyze interframe and blind times for the monochromatic versions of the aforementioned cameras.

Finally, I would love to visit your lab :D

However, I would not be able to do so for some time since I work full-time and am a full-time student.

Best Regards,

Erich

[Erich's Lab]

Hi all, here is an update to my situation.

Due to some circumstances, I decided to snatch a Basler aca1920-40gc camera (used on eBay for $200 USD) for triple color, PIV. The camera has good quantum efficiency for the blue and green wavelengths, but suffers a little on the red wavelength. This effectively limits the PIV system to  two color  PIV experiments since approximately 20% or more red LEDs are needed to mostly balance the histograms of each segregated particle image pairs. However, I ran into an interesting find; the Basler aca1920-155um (~ $680 USD) supports interframe times as low as 150 microseconds with a chance of interframe times being as little as 100 microseconds. As such, I'll be forwarding some research into this camera as well as the two/three color PIV. I think comparing the multi-colored single exposure setup and the standard monochromatic setup could bring some insight into low-cost, high accuracy PIV systems for undergraduate-graduate research in limited funding environments.

Just for kicks, here are the specifications for the camera according to the EMVA1288 specifications

Basler acA1920-150um

  resolution: 1920 x 1200 px

  pixel well size: 4.8 μm

  fps: 150 (75 double-frames/second)

  interframe time: supposedly as low as 80μs, but I have not yet tested this camera

  quantum efficiency: ~50.45%

  dark noise: 10.21 e-

  saturation capacity: 5756 e-

  dynamic range: ~54.6 dB

  s2n ratio: ~37.6 dB

  linearity: horrible; do not recommend the color version of this sensor for TE-CPIV

These specifications look pitiful when compared to something like the Sony IMX series sensors  and PCO Edge cameras (I think they use GPixel GSense sensors), but keep in mind that this camera costs ~$670 USD compared to the PCO cameras (~$3900 to ~$13,000 USD each). Additionally, some commercial PIV suppliers also use this camera for specialized use cases (one used it for 4D lagrangian PTV and supposedly reported interframe times as low as 80 microseconds), so all can't be that bad ;)

To clarify the pixel resolution for the field of view, I used 15cm x 10cm as a standard to compare the different pixel resolutions, albeit ignoring the fact that there are two different resolutions for the x and y axis. The real field of view for experiment would be around 14.4 cm x  9 cm, giving a pixel resolution 75μm/px, but may change later on when I fully design and perform an experiment taking seed particle size into account. 

Best Regards,

Erich

[Erich's Lab]

Hey Alex,  what camera sensor is that "custom" camera solution using? I recently stumbled upon an interesting find when designing my own camera for fun where Sony's 4th gen sensors apparently have a double shutter capability with interframe times as low as 5 microseconds and resolutions between 5mp and 24mp. On the market, I found some camera distributors (including Vieworks) that sell cameras using these sensors and advertising double shutter capabilities. Additionally, these cameras range from $800 USD (~5mp models) to $2,400 USD (~24mp models) making them a possible low-cost alternative to the PCO edge double shutter cameras (the 26mp DS model cost in excess of $13,000 USD) for high resolution planar and volumetric PIV. However, I do find the trigger to exposure start delay a possible nuisance since it can range from 35 microseconds to 120 microseconds. 

By the way, bonus points if your custom camera is peltier cooled :)

[Alex Liberzon]

Hi Erich

We also have. chosen Viewworks, but probably a more expensive option VA-8M (because of a framegrabber compatibility I was looking for)

The time offset between the 2 frames in double exposure mode is 0.2 us (from the FrameA exposure end to FrameB exposure start), based on the following table from Vieworks:

 

VH Series(Camera Link and GigE)				VA Series(Camera Link and GigE)
VH-4M	VH-5M	VH-11M	VH-16M	VA-8M	VA-29M
0.4us	0.3us	0.3us	10us	0.2us	0.3us

The rest of VH and VA models would have around 0.4us.

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[Erich's Lab]

I finally got a low power PIV setup designed for air flow diagnostics. I am not really happy with it, but I think it will get the job done. Here are the parts that I am using:

 - Lucid Vision Labs 8MP camera (~$1000 USD, 5 microsecond interframe time)

- Nichia numb06 blue laser diode (it can be overdriven to 10W without noticeable damage, ~$35 USD each)

 - Custom laser driver and teensy 4.1-based synchronizer (~$70 USD, 15 microsecond interframe time, but much lower is possible)

 - 160 micron helium/air filled soap bubble generator ($1.25 USD each if you have a resin 3D printer capable of printing at 10 micron layer heights, produces ~200k bubbles per second)

 - mass flow meters (~$220 USD each, you can get away with two of them in a pinch)

Total cost of this project: $1,901.15 USD

This setup allows for PIV in air for volumes of around 15cm^2 depending on seeding concentration and pulse length. However, if a thicker laser sheet can be used (~1.5mm thick), I am very slowly working on a high power fiber-coupled laser array in ZEMAX so relatively high pulse energies could be obtained (at least for diode lasers) . However, there is one issue for my use case: I do not have access to helium cylinders for my seeding generators. This means that I would be producing 160 micron soap bubbles that are heavier than air and would have higher response times (perhaps around 60 microseconds but hopefully no more than 90 microseconds in a controlled environment; this has yet to be quantified). For my use cases, the higher response times would have an acceptable amount of error from delayed particle responses, but that may not be applicable to everyone. I'll also have to get a much stronger physics background to help better understand the errors at play so I can minimize the amount of incorrect conclusions if I want to compose a paper about my findings. Nonetheless, hope this helps someone who wants to overcome the financial barrier to PIV and all its glory.

[Alex Liberzon]

Dear Erich 

These are great news. We started creating our own bubble generator but not so successful so far. 

Is it possible to get your STL files and instructions on flow rates to create 220k bubbles? 

On our side we would like to help you with the development. If you can create a system for our lab, we would be able to purchase it from you.

Alex 

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[Erich's Lab]

Hi Alex,

Originally, I tried designing my own coaxial bubble generator, but I had a hard time generating bubbles that are small enough to have acceptable response times (e.g., 0.5mm air filled soap bubbles with high response times). So, I decided to stop reinventing the wheel and use the bubble generator designed at Delft University in the Netherlands. Their associated paper, "On the scalability of helium-filled soap bubbles for volumetric PIV", goes into details on the characteristics of the bubbles generated in their apparatus. In their supplementary information, a STL of their bubble generator is visible for those who want to use their design under Supplementary File 3. Although I am using air instead of helium, it is important that the inner air flow to bubble fluid solution is higher than 800 or the response times of the bubbles would be noticeably higher (presumably due thicker bubble films). It appears that air and helium provide similar bubble characteristics when generating the bubble, although I am not completely certain since I do not have a high focal length lens to view the micro-bubbles. It should be noted that the bubble generator setup to produce 160 micron bubbles has a probability to exceed 300k bubbles per second, according to the aforementioned paper. However, after some qualitative thought processing, it appears that the 160 micron bubble cloud is approximately 4 times as dense as the ~300 micron bubble cloud, which is where I made my (hopefully) conservative estimate.

For the mass flow flow meters, I am using the PFM710 mass flow meter to regulate air pressure and a borrowed mass flow meter for liquids because I ran out of money. The air pressure is maintained using a portable air compressor and a regulator. This means that once the system is calibrated, it is practically plug and play.

[Erich's Lab]

Hey Alex,

Here is an updated version of William's Optolution camera spreadsheet. I added a few more cameras that were considered applicable for PIV studies. I also kept the minimal interframe time of 10 microseconds for the sake of consistency, although some cameras have significantly lower experimentally derived minimal interframe times.