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Fig. 1. The lock-in thermography principle (LIT). (a) Schematic description of a typical LIT setup. The thermal radiation emitted by the surface of the stimulated
sample is captured by the infrared camera objective and converted into temperature maps or thermogramms T(x, y). The resulting images are processed to calculate
an amplitude A(x, y) in Celsius or Kelvin and a phase image ϕ(x, y) in Radian from in-quadrature S90
(x, y) and in-phase images S0
(x, y). For best performances, camera
acquisition and thermal stimulation are synchronized by a trigger signal. (b)Transient temperature signal of a single pixel of the infrared images. The temperature
oscillates at the same frequency as the stimulation signal but exhibits a particular amplitude A and phase difference ϕ. At the beginning of the stimulation, the
signal experiences a non-steady-state phase called thermal relaxation where it starts at temperature Ti
and steadily increases before oscillating around a mean
value Tf in a quasi-steady-state phase. Using mathematical modelling, the amplitude and/or phase information can be converted to valuable quantitative data.
Various lock-in thermal imaging instruments have been
developed, depending on the application and sample being
investigated. Fig. 2 presents a picture of the two laboratory setups
specifically developed to investigate stimuli-responsive
NPs. Magnetic NPs are stimulated using an AMF. We used a
commercial generator (MagnethermTM V1.5, nanoTherics)
that comprises a function generator (SFG-2004, GW Instek), a
water-cooled coil and a laboratory power supply (EA-PS 303220B,
EA Elektro Automatic) (Fig. 2a). For photothermal NPs,
we designed and built a custom LED-based light source (see
Fig. 2b and the paragraph below dedicated to non-homogeneous
stimulation). Infrared radiation of the sample surface
was captured by an infrared camera (Onca-MWIR-InSb-320,
XenICs). The Onca camera is based on an InSb array (320 ×
Fig. 2. (a) Picture of the lock-in thermal imaging apparatus developed to investigate magnetic NPs, such as SPIONs. (b) Picture of the lock-in thermal imaging
apparatus developed to investigate plasmonic and other photo-thermal NPs.
4
IEEE Instrumentation & Measurement Magazine
June 2021

Instrumentation & Measurement Magazine 24-4

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