  We present a novel characterization of mixing events associated with the
propagation and overturning of internal waves, studied thanks to the
simultaneous use of Particle Image Velocimetry (PIV) and Planar Laser Induced
Fluorescence (PLIF) techniques. This combination of techniques had been
developed earlier to provide an access to simultaneous velocity and density
fields in two-layer stratified flows with interfacial gravity waves. Here, for
the first time, we show how it is possible to implement it quantitatively in
the case of a continuously stratified fluid where internal waves propagate in
the bulk. We explain in details how the calibration of the PLIF data is
performed by an iterative procedure, and we describe the precise spatial and
temporal synchronizations of the PIV and PLIF measurements. We then validate
the whole procedure by characterizing the Triadic Resonance Instability (TRI)
of an internal wave mode. Very interestingly, the combined technique is then
applied to a {precise} measurement of the turbulent diffusivity K_t associated
with mixing events induced by an internal wave mode. Values up to K_t=15
mm^2\cdot s^{-1} are reached when TRI is present (well above the noise of our
measurement, typically 1 mm^2\cdot s^{-1}), unambiguously confirming that TRI
is a potential pathway to turbulent mixing in stratified flows. This work
therefore provides a step on the path to new measurements for internal waves.
