Most of us have seen still images or slow-motion video of fast-moving objects, such as a missile in flight or a humming bird flapping its wings. Both scenarios are possible with high-speed visible image cameras using ultra-short exposure times and triggered strobe lighting. Video images of this sort generally require high frame rates to ensure smooth playback.
Newer infrared (IR) camera technologies, such as fast pixel clocks, detector readouts, and A/D converters, allow the same type of high-speed imagery. The core technology is in the detector Read-out Integrated Circuit (ROIC) that has an integration time (analogous to visual camera shutter speed) as short as 500 ns. These new ROICs maintain linearity all the way to the bottom of their integration time limits. Today’s high-end commercial R&D cameras also use digital imaging techniques that increase dynamic range without compromising the number of frames per second.
A classic application is using a high-speed IR camera to capture and measure the temperature of a .30-caliber rifle bullet in flight. At the point of image capture, the bullet is traveling at supersonic speeds (800-900 meters per second) and is heated by barrel friction, the propellant charge, and aerodynamic forces on the bullet. Due to this heat load, the IR camera can easily see the bullet even at the very short integration time of 1μs.
An acoustic trigger from the rifle shot starts the camera’s integration period and ensures the bullet is in the camera’s Field of View (FOV) for frame capture. Figure 1a shows a close-up IR image of the bullet traveling at 1900 miles per hour (mph). Figure 1b is an image of an identical bullet captured with a visible light camera set up with a 2-μs integration time. The glow seen on the bullet is a reflection of bright studio lights needed for proper exposure.
High-Speed IR Imaging for Fast Transients
Not all IR cameras function with both short integration times and high frame rates. Many IR cameras have one but not the other. Still, both functionalities are critical for properly characterizing targets with rapidly changing temperatures. A good example is IC overload characterization. The objective is to monitor maximum heat loads as the IC is forward and reverse biased at currents exceeding design limits. Without high-speed framing and a short integration time, under-sampling may result in data that does not accurately depict transient temperatures (Figure 2).
Pixel Clock and Multiple A/D Taps
High-speed framing requires a fast pixel clock and multiple A/D converters, commonly called channels or taps. Newer cameras offer 16 channels and clock rates around 205 megapixels/second. Generally, a minimum requirement for a high-speed IR camera is at least four A/D channels with 14-bit resolution and a clock speed of at least 50 mega-pixels/second. This results in digital frame rates of over 120 Hz with a detector focal plane array (FPA) window size of 640 x 512 pixels. Using a window smaller than the full FPA pixel size (an option with some cameras) will increase the frame rate, since fewer data per frame are digitized and transferred.
By using a data capture method known as superframing, an IR camera’s dynamic range can be increased from 14 bits to around 18 to 22 bits per frame. This is especially beneficial for imaging scenes with both hot and cold objects in the same field of view. Otherwise, the hot object is overexposed or the cold one is underexposed.
In superframing, the camera is typically cycled through four different integration times (presets), capturing one frame at each preset. Data are then combined by using off-the-shelf ABATER software. This software selects the best resolved pixel from each unique frame to build a resultant frame with a wider temperature range.
The down side is a reduction in the frame rate by a factor equal to the number of presets. This reinforces the need for a high-speed camera. For example, a superframed 305-Hz camera with two presets has a capture rate of over 150 Hz per preset frame, well above the minimum needed for high-speed IR imaging.
The latest IR thermography cameras allow ultra-high-speed image capture of hot, fast-moving objects without the need for strobe lighting required in visible imaging. In addition, superframing can substantially improve IR image resolution and increase the range of temperatures that can be captured in a single frame.