Traditionally, photothermal spectroscopy (PTS) has been exploited for the detection of nanoparticles [2, 3]. With the advancement of broadband quantum cascade lasers (QCLs), optical photothermal infrared spectroscopy (O-PTIR) has gained interest for imaging applications due to its capability to combine IR specificity and submicron optical resolution [4, 5]. However, little has been reported on imaging artifacts. Based on a commercial confocal microscope, a MIR PTS instrument was developed at TU Wien to investigate photothermal image quality. Beside edge effects [1], standing wave patterns were observed in a thin-film sample. This phenomenon will be explained by investigating the effect of the detected signal on the photothermal image. [1] Aamont, L.C. and Murphy, J.C. (1982) “Effect of 3-D heat flow near edges in photothermal measurements.” Applied Optics 21(1):111-115. [2] Adhikari, S., Spaeth, P., Kar, A., Baaske, M. D., Khatua, S. and and Orrit, M. (2020) “Photothermal Microscopy: Imaging the Optical Absorption of Single Nanoparticles and Single Molecules”. ACS Nano 14:16414–16445. [3] Berciaud, S., Lasne, D., Blab, G. A., Cognet, L. and Lounis, B. (2006) “Photothermal heterodyne imaging of individual metallic nanoparticles: Theory versus experiment”. Physical Review B 73(4):045424. [4] Furstenberg, R., Crocombe, R. A., Kendziora, C. A., Papantonakis, M. R., Nguyen, V. and McGill, R. A. (2012) “Chemical imaging using infrared photothermal microspectroscopy”. Proc. of SPIE 8374.837411. [5] Zhang, D., Li, C., Zhang, C., Slipchenko, M. N., Eakins, G. and Cheng, J.-X. (2016) “Depth-resolved mid-infrared photothermal imaging of living cells and organisms with submicrometer spatial resolution”. Sci. Adv. 2(9):e1600521.