Analysing products with a high-speed camera

How high-speed equipment can be used to accurately assess the performance of fast-moving items and components.

High-speed photography has been used for scientific research for over 140 years. Ever since Eadweard Muybridge carried out his studies of galloping horses in the 1870s, scientists and photographers alike have been fascinated with the potential of high-speed cameras and photography.

Principles of high-speed videography

The principle behind high-speed videography is the recording of video at a higher rate of frames per second (fps), so that when the footage is played back at the standard 25 to 30 fps, it appears to be in slow motion. However, as the frame rate increases, the exposure time for each frame decreases, which lessens the amount of light entering the camera. This creates a demand for powerful lighting to be used with high-speed filming.

SATRA has high-speed cameras capable of recording in colour at up to a resolution of 1,280 x 1,024 pixels at a frame rate up to 2,000 fps, or at a reduced resolution up to 150,000 fps.

Each camera accepts interchangeable lenses, giving a choice of fields of view for recording different subjects. The cameras are controlled from a controller/display unit – a small hand-held screen that shows a live image of what the camera is viewing, and which allows for control and adjustment, as well as instant playback of the recorded footage. This means that several people, not just the operator, can see the footage and confirm that the correct details were recorded.

As mentioned above, lighting is extremely important for high-speed videography. SATRA therefore uses a range of light sources to suit the test being recorded, including cooler light-emitting diode (LED) lighting for illuminating samples that would be sensitive to the heat produced by conventional lighting.

It can be especially challenging to illuminate a test where the sample to be recorded is behind an opaque safety screen, a mesh guard or a similar safety barrier. However, with the facilities and expertise available at SATRA, lighting solutions can usually be found.

Two cameras can be synchronised to record the same event from different angles. This allows for a much greater understanding of the test being recorded, as measurements such as distance and velocity can be assessed from two viewpoints. These cameras can be triggered to start recording at the same time, either with a simple push-button start or coordinated by an external trigger or event marker. They can both be set to run at the same frame rate, or one can be set to run at a standard speed to record the full length of the test while the second records at a much higher frame rate to capture greater detail during a critical portion of the test duration.

Careful analysis of high-speed footage is key to its usefulness. In-camera software allows the operator to quickly measure distance, velocity, angle and angular velocity from a recorded video, and to create screenshots of critical moments within the test, overlaid with measurement data. Post-recording video analysis software also allows measurements to be taken later, and provides motion tracking and plotting capabilities, in addition to the other measurements. Both units are portable, so they can be used offsite, as well as in the laboratory.

Once the camera is set to record, its memory starts to cyclically buffer the output from the equipment’s sensor, so that it is constantly ready to commit a section of footage to long-term memory storage. This means that at the moment the camera is ‘triggered’ (when the critical part of the test takes place), either by the operator or a signal from another piece of equipment, it instantly stores the contents of its buffer memory. This enables the memory to capture several frames of footage from the moments before the camera was triggered and several frames from the moments afterwards. This ensures that the event to be recorded always appears within the duration of any captured footage and can be readily analysed.

SATRA utilises a variety of measuring equipment, including load cells, to obtain quantitative data from tests. The output from load cells or other test equipment can be connected to the controller of a high-speed camera so that its output appears on-screen in graphical format and can be overlaid on subsequent recordings. This can be very useful as a means of providing a definite event marker for critical moments during the test, and to more closely describe the product’s performance.

The high-speed cameras that SATRA uses can also record interval timer footage, where one frame is recorded every few seconds, thereby collecting footage of items that are moving or changing only slowly. This ‘time lapse’ imaging is very useful for tracking long-term changes, or gradual deterioration in a sample.

Applications for high-speed videography

The human eye has a remarkable range of visual acuity, and is capable of observing and registering brief incidences of movement and activity. However, when carrying out testing that lasts for only a fraction of a second or when the standard testing itself is only part of an overall assessment of the safety of a product, the use of high-speed videography is an excellent way to get considerably more information and see in greater detail what actually happened during the test.

For example, did safety features incorporated into the product activate or function correctly during the test? Did elements of the product become detached while they were being assessed? Did the product remain in place during the test, or was it displaced after an impact? How quickly did safety features or cushioning respond? How far did debris eject from the product during an impact test? What were the bounce and deflection characteristics from a dropped object? How many times did it bounce? How far did it rebound? Was it in a straight line? Did anything unusual happen during the test that might have been responsible for an unexpected result? Information like this can be used in the first instance to establish exactly what happened during a test, but can also be used to enhance and develop future iterations of the product – raising its performance beyond that required to merely meet the relevant standard.

High-speed videography is especially useful when the sample to be tested is a prototype, or a one-off production model, and it is important to get the maximum amount of information from the limited testing possible with just a single sample.

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