We propose a new independent thermometry method, line strength ratio thermometry (LRT), based on optical spectroscopy measurement of the line strength intensity ratio R between pairs of molecular transitions. Due to strong dependence of R on kT, a given measurement uncertainty delta R for R reflects in a small uncertainty of kT determination. By assuming experimental uncertainties of R and T to be those reported in literature, we foresee a k determination at the 5 ppm level, which is better than the most precise k determination by using Doppler broadening thermometry (DBT). In the frame of a new definition of the SI Kelvin unit, based on k as fixed constant, once the k constant is exactly established, LRT is proposed as a high resolution noncontact thermometry technique for absolute temperature measurements of gas samples at the ppm level.