Context : Aims : We performed a detailed photometric analysis of the lensed system UM673 ( Q0142-100 ) and an analysis of the tentative lens models . Methods : High-resolution adaptive optics images of UM673 taken with the Subaru telescope in the H band were examined . We also analysed the J , H and K-band observational data of UM673 obtained with the 1.3m telescope at the CTIO observatory . Results : We present photometry of quasar components A and B of UM673 , the lens , and the nearby bright galaxy using H-band observational data obtained with the Subaru telescope . Based on the CTIO observations of UM673 , we also present J- and H-band photometry and estimates of the J , H and K-band flux ratios between the two UM673 components in recent epochs . The near-infrared fluxes of the A and B components of UM673 and their published optical fluxes are analysed to measure extinction properties of the lensing galaxy . We estimate the extinction-corrected flux ratio between components A and B to be about 2.14 mag . We discuss lens models for the UM673 system constrained with the positions of the UM673 components , their flux ratio , and the previously measured time delay . Conclusions :