SAMPE Journal - May/June 2020 - 46

FEATURE / NONDESTRUCTIVE INSPECTION
typical configuration, 2 flashtubes housed in reflectors are used to heat an area on the order of 1
sq. ft. Each lamp is powered by its own capacitor
discharging electrical energy in the 4-6 kJ range.
However, significant losses occur in the electrical
to optical energy conversion process, and through
resistance in cables and instrumentation, so that
the energy density delivered to the sample is on the
order of 25 J/in^2. Further losses are likely to occur as the result of intermediate optics, scattering
and the optical reflectivity of the sample. The duration of the flash pulse is normally characterized
by its Full Width Half Maximum (FWHM) duration,
which is in the range of 3-5 msec for most thermography system. The rise time of the flash pulse
is nearly instantaneous; however, the decay of the
flash pulse persists for tens of milliseconds (the decay component is primarily in the near-IR, extending into the mid-IR spectral range). Systems using
flashlamps with variable duration are sometimes
used for specialized applications, with FWHM durations as short as 200 nsec.
Results from flash thermography systems are
relatively easy to interpret and analyze. The flash
excitation is a reasonable approximation of a delta function input for most applications, so a pixel
time history from a given sample closely follows
the numerical or analytical model result closely.
Typically, the IR camera is saturated during optical
excitation, resulting in a blind period early in the
sequence. However, the brief duration of the flash
pulse minimizes the blind period so that nearly the
full response of the part can be observed.
Large Area Inspection
The field of view of an IR camera is determined by
its optics and detector, however, the field of view a
thermographic NDT system must also consider the
number of pixels that will cover the smallest defect that is to be detected (ASTM E2582-07 recom-

mends a minimum of 9 contiguous pixels on the
smallest defect1). Furthermore, as the inspection
field of view increases, the excitation energy density must be maintained over the entire area. These
considerations limit the extent to which the field
of view a flash system can be extended, in terms of
minimum detectable flaw size and the footprint,
safety and power requirements of the apparatus.
A flash system covering a 2' x 2' inspection area
and maintaining the energy density of a commercial 1' x 1' system would require approximately 50
kJ (8 power supplies). For stationary or analytical
applications, the typical 1 sq. ft. inspection area is
relatively large, compared to NDT methods using
single point or small array detectors that must be
scanned over the part. However, for large structures, the thermography head (camera and flashlamps) is typically either manually moved over
the part by an inspector, or mounted on a mobile
fixture, gantry or robot and scanned in a start-stop
manner over the full inspection area. The net result
is a stitched image of the entire inspection zone
showing subsurface features and anomalies.
LASLAT
Flash systems are typically operated in close proximity to the part under test. An alternative approach to thermographic large-area NDT has been
introduced. LASLAT (Large-Standoff, Large-Area
Thermography) is based on step heating using halogen lamps and IR camera placed at a large working distance from the part. From its remote working
distance, the LASLAT field of view can be scanned
over the test part surface while the apparatus remains stationary, using only pan and tilt degrees
of freedom, and eliminating the need for gantry or
robot scanning equipment (Figure 1). From a working distance of 3.7 m, the LASLAT single shot field
of view is ~ 51 cm x 38 cm, larger than the typical
flash field of view, but limited by the ASTM E2582-

Figure 1. Large-Standoff, Large-Area Thermography (LASLAT) concept
drawing.

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SAMPE Journal - May/June 2020

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