The following application note from Photron will help users to select the right camera for the right slow motion imaging task
Two sample imaging examples are discussed and two different Photron high speed cameras are chosen to fulfill the requirements.
Example A: You wish to image a mobile phone during a drop test.
Determine frame rate.
Decide image size in metres [ X ].
Enter (if known, otherwise estimate) the velocity in metres per second [ V ].
Decide how many times you wish to see the object before it moves out of the field of view [ N ].
Example X=0.2m V=6m/s N=100.
Frame rate [ f ] required = (1/X)VN or (1/0.2)x100x6=3000 frames per second (fps).
Select a camera that can provide your desired resolution at the calculated frame rate - bear in mind that image resolution gets lower as the frame rate is increased, ie, Photron's Ultima APX RS =1024x1024 pixels@3000fps or, Photron's Fastcam-X 1024 PCI =640x512pixels@3000fps.
For this example we will select Photron's Ultima APX-RS.
Calculate the required exposure duration.
The default exposure duration of any high-speed camera will be calculated as 1/ frame rate.
Enter Frame rate [ f ] (from above).
Therefore: Default exposure [ E ]= 1 / f or 1/3000 =0.000333s (333microseconds).
To calculate the blur with this exposure time.
Blur B =VEmetres or 6x0.000333 =0.001998m (1.998mm).
To put blur into context you will need to know how many pixels movement on the image this blur will create; for most applications a blur of five pixels or less is not significant, however if the magnification of the image is such that the blur equates to 50 pixels you are going to want to select a shorter exposure time.
To calculate the blur in pixels you will need to calculate the metres/pixel calibration.
Enter: image size [X] (from above) and resolution [R] of camera at required frame rate.
Pixel calibration [P] =X/Rmetres or 0.2/1024 =0.0001953m/pixel (0.1953mm).
Blur in pixels =B/P or 0.001998/0.0001953 =10.2 pixels.
So to completely eliminate blur (less than one pixel) we will need to reduce the exposure time by a factor of at least ten, ie, 333microseconds/10 =33.3microseconds or 1/30,000.
So to summarise, for this application we could use Photron's Ultima APX RS operating at 3000fps with a 1/30,000 exposure.
Example B: You wish to trouble shoot a machine tool that has an active area of 180mm and the process has a cycle time of 250ms or four cycles per second.
Determine frame rate.
The number of steps that you break this cycle time down into will determine the required frame rate.
In examples of this type it may not be clear at what speed any failure may occur - you may find that initially you have too low a frame rate and the fault is over in less than a frame - all you can do is keep trying increasingly greater frame rates.
Breaking the movement down into 50-100 steps is a good place to start.
Enter the number of cycles per second [C].
Decide how many steps you wish to break the movement down into [S].
Required frame rate [f] = CS or 4x100 =400fps.
With this frame rate we could select for example the Fastcam PCI as this will offer full resolution of 512x480 pixels at 500fps; however bear in mind that should you need to go to a higher speed, the resolution would start to drop.
In this case a higher specification camera could be more suitable, alternatively you could choose initially a camera with the potential to record at a higher frame rate with more resolution.
Calculate the required exposure duration.
Default exposure time [E] =1/f or 1/400 =0.0025s (2.5ms).
Unless you actually know the velocity of a particular moving part within the machine you could take the cycle time and use this as the basis for a velocity calculation ie, Cycle time [C] =0.25s.
Image size [X] =0.18m.
Velocity [V] estimated =(1/C)X or (1/0.25)x0.18 =0.72m/s.
Blur [B]=VE metres or 0.72x0.0025 =0.0018m (1.8mm).
To calculate the predicted blur in pixels.
Resolution [R] =512pixels.
Pixel calibration [P] =X/Rmetres or 0.18/512 =0.0003515m/pixel (0.3515mm).
Blur in pixels =B/P or 0.0018/0.0003515 =5.12pixels.
Now calculate exposure time.
To completely eliminate blur (less than one pixel) we will need to reduce the exposure time by a factor of at least five ie, 2.5milliseconds/5 =0.5milliseconds or 1/2,000, To summarise, for this application we could start with the Fastcam PCI operating at 400fps with a 1/2000 shutter.
Photron offers a wide selection of high speed video cameras for slow motion analysis.
These high speed cameras are used for applications as diverse as vehicle impact testing, military weapons testing, production line fault finding, biomechanics, slow motion sports playback, golf Swing-Vision, and research and development.
Photron's product range includes PCI digital imagers providing mega pixel slow-motion for the personal computer, stand alone high-G systems for use as onboard crash test cameras or ultra high-speed ballistics cameras.
