Exposing living cells to a certain level of Electromagnetic Field (EMF) might induce some biological effects including temperature elevation. In this paper, we show the dosimetry of exposure systems such as an Open Transverse Electro-Magnetic (TEM) cell allowing the study of the effect of EMF on biological samples exposed to 1.8 GHz signals. Temperature measurements are carried out with a fluorooptic probe to extract specific absorption rate (SAR) values that are compared to numerical dosimetry, based on a FDTD method. To investigate dosimetry at a microscopic level the fluorescence of the temperature dependent dye Rhodamine B was measured with fluorescence microscopy. The results are confirmed by measurements and simulations with a SAR of 13.9 and 11.8 W/kg for 1 W incident power, respectively. Results evidence that the objective working distance of the microscope strongly influence SAR values. After calibration, the fluorescence fits well with the temperature variation measured by the probe.