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duced, which allows less of the bright
light from the subject to reach the sensor. Second, a filter can be used to remove
the majority of the broadband emission
light. Using a high-power pulsed laser as
a bright, monochromatic light source allows for a significant improvement in the
clarity of images of this type of event.
The monochromatic light from the laser allows the choice of a narrow-band-pass filter, which significantly reduces
the amount of emission light entering the
camera. Similarly, reducing the camera
integration time to match the duration
of a laser pulse means that the comparatively bright laser light overpowers the broadband emission
and provides a clear view of the internals of the process, as
well as its surroundings.
The use of high-speed cameras in the viewing of fast, bright
events requires a laser with equivalent repetition rate. At the
repetition rates common to contemporary high-speed cameras, which are typically between 1 and 10 kHz, several different types of pulsed lasers are suitable.
Near-infrared (NIR) laser diodes (such as Oxford Lasers’
Firefly laser), with output power up to 1 k W and minimum
pulse duration on the order of 250 ns, match well with the
minimum shutter times available on modern high-speed cameras. Furthermore, their low cost compared with equivalent
repetition-rate diode-pumped solid-state (DPSS) lasers makes
them attractive for those on restricted budgets.
For higher-speed events, for example a high-energy explosion or ballistics experiment, a shorter minimum pulse duration may be required to freeze the motion of the particles or
fragments within the fireball. In this case, frequency-doubled
DPSS lasers with pulse durations of between 70 and 200 ns or
copper-vapor lasers with pulse durations of 25 ns can be used.
Accelerate new process development
High-speed imaging was used to understand Surfi-Sculpt, a
new surface-modification process developed at The Welding
Institute (Cambridge, England). 1 In this process, which is analogous to laser welding, a laser beam from an ytterbium fiber
laser is rapidly scanned in a radial pattern to build up a stack
of material on the surface. This stack of material has the effect of increasing the surface area of the bulk surface and can
be used to increase the heat transfer from the surface or aid
bonding of material to the surface.
The light source was a copper-vapor laser that produced
pulses with a 25 ns duration and repetition rates up to 10 kHz,
which froze the motion of the fast-moving molten metal droplets (see Fig. 1). A bandpass filter was used to allow the light
from the copper-vapor laser into the camera, at the same time
rejecting the fiber-laser light and plasma emission.
Look inside fireballs
The explosion caused by the impact of projectiles onto armor
plate can generate a significant quantity of light, which obscures the dynamics of the impaction process. In Figure 2, the
effect of filtering can be clearly seen. The short pulse from a
copper-vapor laser freezes the motion of the projectile, which
is traveling at several hundred meters per second, but the fireball blinds the camera.
FIGURE 2. Two images of a projectile impacting an armor plate show the effect of
including a bandpass filter. Projectile velocity is approximately 715 m/s and both images
are illuminated with a fiber-delivered copper-vapor laser, in this case, running at a 22 kHz
repetition rate. The bright fireball from the impact completely obscures the position of the
projectile because there is no filter present (a), while an additional bandpass filter was fitted
to the camera to exclude the light emitted by the impaction of the projectile (center of image),
allowing it to be seen clearly (b). (Courtesy of Oxford Lasers)
