High-speed photography widely used in motion study

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September 2006 Maintenance, Test & Measurement, Calibration

High-speed photography has many applications today. It can be used to measure shells emerging from barrels of any calibre weapon, it can be used to study high-speed machinery as used in automated industry, and it can be used in biometrics. Other applications include its use in the study of vehicle impact testing, seat belt effectiveness, and the study of the expansion of car air-bags. In terms of the latter, manufacturers are required to test a bag to destruction after so many have been produced in a production run. Presumably if the air-bag is not seen to be unfolding in the right way and at the right speed the whole of the batch being produced would be destroyed. In biometrics high-speed photography is used extensively in sports science and golf is only one of many sports covered. The image of a golf ball encountering the club at very high speeds (greater than 100 000 fps) shows an amazing compression of the originally round golf ball. High-speed photography is used to study the launch of all space vehicles from the Ariane to the Soyuz and a large number are now deployed on the space shuttle launches to look for various pieces of foam coming off.

The recently released Photron FASTCAM APX-RS high speed digital camera is capable of imaging at 3000 fps with a resolution of 1024 x 1024 pixels. This imagery can be produced in full colour or monochrome. At higher speeds the number of pixels used decreases but at 512 x 512 pixels the frame rate is 10 000 fps. The frame rate can be pushed to 250 000 fps for 128 x 16 pixels resolution. The standard memory supplied with the camera is 2 GB, but this can be increased to 16 GB, allowing 4,1 seconds to be recorded at full frame at the speeds of 3000 frames per second. As the camera operates continuously, the trigger button can in most cases be pushed after the event has visually happened and it will still be recorded. The camera software can be used to cut this clip of the event out of the full image and stored for later study.

A coldrink tin being pierced by an arrow photographed by the APX camera at about 1000 fps

The FASTCAM APX-RS is easy to deploy and as it runs continuously alignment with the object under test is simple. The cameras are rugged and after a very large order from White Sands Test Range in the USA, an option is to have the camera completely sealed and to include a heating element to deal with the very cold temperatures encountered there.

An image of the same tin a short time later shows it falling. An interesting observation is that the tin is expected to fall forward but as a result of the gas and liquid expelled from the coldrink, the tin falls back into the direction it was hit by the arrow

An advantage of digital high speed cameras over the traditional film camera is that what has been recorded can be viewed within minutes of the test. Film usually requires hours of processing. If the digital camera has not recorded the event correctly, another test shot can be made available after altering say the exposure time of the camera if the previous imagery was too dark. Digital imagery is also much easier to study in the aftermath of the event and there are many software programmes available to make calculations and operation easier. Film cameras are of course still the answer where very high framing rates (up to several million) are required at high resolution.

Photron manufactures a full range of digital cameras with varying operating speeds and number of pixels. Some are made with PCI cards so that a computer can be used to directly record the imagery. Quite a few of these cameras have been sold in South Africa over the last few years and purchase of the higher-cost units includes installation (if required) and training.

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Maintenance, Test & Measurement, Calibration

High-speed photography widely used in motion study

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