Simultaneous measurement of whole field velocity and temperature distribution is carried out using laser-induced phosphorescence (LIP) based imaging technique, namely Thermographic Particle Imaging Velocimetry (TPIV). Temperature sensitive seeding particles exhibit a spectral shift in luminescence with temperature, which is partially temperature-dependent. The present experiments employ an Nd: YAG laser (355 nm) for exciting the phosphorescence particles. The intensity ratio of two LIP cameras' images has been taken to determine the temperature distribution. These cameras are fitted with two optical bandpass filters. For simultaneous determination of velocity field, a dual pulsed laser light sheet coupled with a double frame camera that records the shift in the position of the seeding particles with time has been used. To demonstrate the potential of the TPIV technique in the context of natural convection heat transfer regime, experiments are performed in a back wall heated open square cavity to map the flow and thermal field in the cavity simultaneously. Results are presented in the form of whole field temperature distribution and velocity field. The TPIV-based observations are compared with those reported in the literature. The dependence of velocity boundary layer thickness and inlet/outlet of fluid flow on the applied temperature potential has been investigated.