NDT
REQUIREMENTS FOR
UV-A-LEDS IN NDT
LEDs in NDT inspection devices can now
produce the ultraviolet light necessary to
excite the fluorescence of the penetrant or
the magnetic particles
// NATHANAEL RIESS
The advantage of UV-A-LEDs instead of
the formerly used Mercury High
Pressure lamps are manifold. Among
others, this is their much longer lifetime,
shorter readiness for operation and
multiple design possibilities. In addition, by
proper technical realization active cooling
of the diodes even for the high-power units
is not necessary.
The most essential parts are the
semiconductor itself, the silicon
encapsulate and the plastic lens for optimal
optical efficiency. The semiconductor
circuit integrates an ESD device
(Electrostatic Discharge Protection Diodes),
which is essential to limit the LED reverse
current before breakdown. The cathode
lead and gold wire supply the forward
voltage to the p-n-junction.
The temperature management is very
important for reliable operations of the
LED for the intended lifetime in the order
of 10.000-20.000 hours. Therefore the heat
sink of the LED and its connection to
the bearing construction are very
important. Heat transfer to the ambient
environment has to be very effective to
prohibit overheating of the LED. With
appropriate design even high-power LED
can be reliably operated using only
passive cooling.
Using AlGaIn and AlInN semiconductor
compositions nearly each wavelength
between 200 and 1200nm can be achieved.
For this development I. Akasaki, H. Amano
and S. Makamura received the Nobel Prize
in Physics in 2014.
Since the introduction of UV-A-LEDs the
requirements in the standards and
specifications for the use in NDT have
been much more detailed. This is because
Mercury lamps have a very defined and
strong wavelength in the UV-A at 365nm.
The wavelength as well as the temperature
properties of UV-A-LEDs however depend
on the relative composition of their
elements Aluminium, Gallium, Indium
Periodic System III, and Nitrogen Periodic
System V within the semiconductor.
Therefore, rather stringent specifications
are described in the standards and
specifications especially within the
aerospace industry: ASTM E3022, Rolls-
Royce RRES 9061: 2014 and Airbus AITM6-
1001: 2016.
According to the standards and
specifications, the LEDs have to be typeproofed
for ASTM and RRES and
individually certified in the case of AITM
136 SHOWCASE 2020 \\ AEROSPACETESTINGINTERNATIONAL.COM
1 // Helling Inspector 150,
certified according to
AITM6-1001: 2016
2 // Schematic drawing of
a LED
1
including the corresponding certificate by
an authorized body.
As an example, the specification of the
Airbus standard AITM6, Section 7 are
presented.
In Figure 1, the requirements (set value)
with respect to the emitted UV-A-LED
spectrum and the measured results (actual
value) for a UV-A-LED (type Helling
UV-Inspector 150, Serial No. 1508) are
shown. The wavelength of the maximum
of the spectrum must be at 365+-5nm. The
Full Width Half Maximum and the
spectrum width at 1/10 of the maximum
intensity are specified. In addition, the
radiation intensity at a distance of 48mm
must be minimum 1200 MicroW/cm2 and
maximum 5000 MicroW/cm2.
The intensity at the inspected area must
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