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In the “old” days when high speed cameras used film, there was a very definite period of time in which the event one wished to record had to occur. This was between the point at which the film had ramped up from a standing start to the desired frame rate, and before the film magazine (which held up to a thousand of feet of 16mm, 35mm or 70mm film) would run out.

Figure 1. Example of Digital Image Correlation (DIC) studying stress on a car tire driving over wooden block. (Image courtesy of Correlated Solutions, Inc.)
High speed digital imaging systems have come a long way from their film camera predecessors. For example, today all-digital video high speed cameras are equipped with massive amounts of onboard, solid-state memory which can be continuously overwritten until the event of interest occurs, making them ideal for non-destructive testing applications. In theory, the event could happen days after the cameras are placed in record mode.

Trigger Modes

Modern high speed cameras offer a wide variety of trigger methods often used for NDT applications, including:

START trigger, where the camera can be entered into the record mode using the events trigger, as in the case of an explosive event, with all frames recorded after the trigger signal at T0.

END trigger, with all frames saved before the trigger signal is received. Again, the trigger is defined as T0 and all preceding frames shown as negative, i.e., occurring before T0.

CENTER, where frames are evenly split either side of T0.

MANUAL mode lets the end user select how many frames are recorded before and after T0. This is frequently used in vehicle impact tests, where typically ten or so frames are recorded before the impact to confirm the vehicle’s approach speed.

RANDOM mode enables a user-selected number of frames to be recorded every time a trigger signal is received. Multiple short recordings are usually combined into a single video sequence. This can be very useful when recording cyclical events, such as engine combustion where only one cylinder is recorded with the camera remaining inactive while the other seven or so cylinders operate.

DUAL SPEED – The camera starts recording, as in START mode, but then changes the recording frame rate by a factor ranging from two to eight times (e.g. from 125 frames per second (fps) to 1,000 fps with the application of a second trigger signal being applied. This can be useful to record a subject’s dormant state at a lower frame rate, say 125 fps before application of a current to the subject, then at the 1,000 fps rate when the subject is being tested, then returning to the slower frame rate to record the resultant condition.

High Speed and Resolution

Today’s high speed cameras utilize very specialized CMOS sensors that enable them to operate at full resolution to speeds as fast as 20,000 frames per second. Additionally, reducing the horizontal and/or vertical pixel resolution makes it possible to push the recording speed to over one million frames per second. However, the results at these very high frame rates typically look better on a data sheet than they do when viewed on a monitor or display.

Figure 2. Fastcam SA-X2 with built-in delay generator, SD Cards, and superior light sensitivity. (Photo courtesy of Photron USA, Inc.)
Many commercial digital single-lens reflex (DSLR), or point-and-shoot cameras, are venturing into the high speed arena. In reality, however, this is more of a marketing ploy than a usable tool because there is little to no control over the triggering. This makes capturing any event (other than an event that is guaranteed to occur within a few seconds of the trigger being applied) nearly impossible. In addition, the resolution is often so low that the imagery is unusable when replayed on a display larger than the one on the back of the camera.

Advanced high speed imaging systems provide megapixel resolution at speeds as high as 13,500 fps, and usable resolutions (defined here as being a minimum of 128 pixels wide by 128 pixels high) at speeds of more than 280,000 fps. Many of today’s cameras can provide true high definition (HD) at 1,920 pixels wide by 1,080 pixels high images in full 36-bit color, as fast as 2,000 frames per second. At present, most televised sporting events include at least a few shots recorded with a high speed video camera.

High speed cameras have traditionally been used to record destructive testing. We’ve all seen the wonderful images of a blade coming off a turbine, a missile exploding in glorious slow-motion detail, or vehicle impact testing where every nuance of an innocent dummy’s demise can be studied in high definition, slow-motion splendor. But there are several areas where high speed cameras are used to great effect in non-destructive testing (NDT) applications, particularly in digital image correlation (DIC).