HIGH-SPEED IMAGING
Laser illumination reveals
high-brightness processes
TIM STEPHENS
The ability to make a slow-motion movie of a high-speed
event is a powerful diagnostic tool for scientists and engineers during process development. Events that emit light,
however, are difficult to image because the emission can
blind the camera and hide the details of what’s happening.
A good example is arc welding, where the bright plasma
caused by the arc from the torch obscures the metal trans-
fer that is going on within it. The traditional method of us-
ing dark neutral-density filters (just like welding goggles) al-
lows some information about the innards of the plasma to
be obtained, but the large difference in intensity between
the arc and the background means that it is often not pos-
sible to see both simultaneously.
Where cameras are used for monitoring a process, for ex-
ample in high-value welding processes in the nuclear and
oil industries, this lack of image fidelity means that weld
defects may not be spotted when they are introduced and
can be easily corrected. Instead, inspection must be car-
ried out after the welding equipment has been removed
from the part, introducing expensive production delays.
In the development of combustion processes, such as rock-et-engine or explosives, the bright fireball can prevent any
information whatsoever being obtained about the behavior of the components within until much time has passed.
Here, computer modeling must be used as an alternative to
observing the event.
The design of electrical contacts requires that the erosion
by arcing as the switch is opened and closed is fully understood. Since no information can normally be gathered
from within the arc, the only
way to examine the process is
by looking at eroded contacts
after many operations, which
may not give the full picture.
Laser illuminators see
through the glare
Being able to look within
these types of events means
needing to reduce the amount
of light emitted from them
arriving at the camera sensor.
The emission from hot events
is often broadband and continuous (compared with the
integration times for high-speed cameras), so two complementary techniques are
available.
First, the camera integration (shutter) time can be re-
High-brightness events such as arc welding
and explosions are normally difficult to
image due to obscuring glare, but pulsed
laser illuminators cut through the glare to
reveal the processes’ inner workings.
