Author: Hannu Eloranta, Pixact Ltd
The imaging of high-speed objects, such as bullets, constitutes several technical challenges for the imaging equipment. Besides accurate timing of the imaging sequence, either a very short exposure time or pulsed illumination is required to freeze the motion of objects. The combination of high velocity and high magnification favors pulsed light sources since typically exposure times less than 1µs are required to avoid motion blur. Pulsed light sources also typically provide higher light output than cw light sources. Since the energy output is concentrated on short pulses, also the heating effect of the illumination is minimized.
CAVILUX diode lasers offer several attractive features for high-speed imaging. They can be programmed rather freely to deliver pulses or bursts with desired frequency and pulse energy. The laser head is coupled with fiber optics, which allows easy alignment of the illumination. Spot illumination is easily optimized by adjusting the lens system attached to the fiber light guide. In contrast to other laser light sources, the light provided by CAVILUX diode lasers is essentially incoherent. Thus, any special optical elements are not needed for speckle-free illumination. In some cases also the monochromatic nature of the laser illumination is important, because optical bandpass filters can be used to block radiation generated by explosions and hot objects.
This paper presents the visualization of bullets impacting with metal, plastic and wood plate structures. Image sequences are recorded with a high-speed camera and CAVILUX HF diode laser.
2 The imaging setup
Bullets are shot from a distance of 5 m to a test piece with a .375 Magnum handgun. The bullet speed varies between 220 and 300m/s. The impact of the bullet with the test piece is visualized from various directions. The camera and the illumination are located on single stand at a distance of 3 m from the test piece. A schematic drawing of the setup is presented in Fig. 1.
The camera used to record the high-speed sequences of the impacts is Photron FASTCAM SA1.1 equipped with Nikon 300mm f/#4 lens. The lens aperture is adjusted to f/#8. The imaging frequency of 50 kHz is chosen. At this high framing rate the camera resolution is limited to 512 x 208 pix2. The original bit depth of the images is 12bit, but the images are stored in 8bit mode with 2 bit shift lower. CAVILUX HF is driven in the slave mode triggered by the camera exposure start signal. Standard CAVILUX lens optics is used to produce a spot illumination matching with the camera field of view. The lens optics is located on top of the camera. Due to the laser duty cycle limit of 2%, the pulse duration at 50 kHz frequency is limited to 0.4µs, which yields 0.2mJ energy/pulse. In this example only one laser head is used. To further optimize the illumination, to minimize shadows etc., several laser heads can be combined. However, one laser head is enough to create high-quality images of the bullet even though the lens aperture is set to f/#8 increasing the depth of focus.
Figure 1. Imaging setup.
In Fig. 2 examples of frames captured after the impact are shown. In Figs 2a and 2b the bullet deformation and the type of splinters after impact can be seen. In both images the illumination is from the same direction, but in the upper image the plate is painted black in order to obtain better contrast between the bullet and the background. In this way small splinters are easier to detect. In Fig. 2c a three image sequence from the back side of the wood test piece is shown. In this sequence a pressure wave moving away from the point of impact in the wood plate can be seen. Fig. 3 presents image-by-image sequences of selected impacts.
Figure 2. Examples of images acquired after the impact.
Figure 3. High-speed image sequences of impacts.
A jitter-free timing of the light pulses enables the analysis of bullet speed. Image analysis is used to determine the bullet location in each frame. After scaling the camera to world coordinates, the displacement of the bullet between two frames can be used to evaluate the bullet speed before and after the impact. Vertical lines in Fig. 4 represent the location of the bullet tip in the image series. The impact with the plate clearly decelerates the bullet.
Figure 4. Examples of images acquired after the impact.
In high-speed imaging the design and properties of illumination is the key to achieve high quality image data. The role of illumination is pronounced for small objects requiring high magnification. Exposure times below 1µs are typically required in order to freeze the motion of bullets and other high-velocity targets. Even if there are cameras with sub-µs shutter speeds, a more convenient solution is to utilize pulsed light sources since high power illumination is needed anyway. Especially lasers can provide a very high light output over a short period of time. The pulse duration is typically adjusted in the ns range. However, a common problem related with most lasers is the coherence of light, which results in nonuniform beam profile with hot spots and other defects (speckle). In this respect CAVILUX diode lasers offer a highly attractive alternative. They provide a high-quality beam which is easily converted to spot with extremely uniform intensity profile. The high-quality beam combined with very flexible programming of the laser pulse frequency and pulse duration makes these diode lasers the obvious choice for most challenging high-speed imaging applications.
About the author
Hannu Eloranta (Dr.Tech.) has been working as a researcher in the Group of Experimental Fluid Dynamics at Tampere University of Technology. His research topics include several aspects of flow dynamics, such as turbulence, fluid-structure interactions and flow control. In addition he has been developing optical measurement techniques and analysis methods for fluid mechanics. Currently he is working in Pixact Ltd – a company specialized in developing optical measurement techniques.