Mendi does not measure brainwaves, since it is not EEG technology. Mendi hardware is instead based on technology called functional near-infrared spectroscopy (fNIRS), which among other things, measures blood flow and oxygenation in the prefrontal cortex (PFC). By employing targeted instructions in our software, we can guide brain activity in the PFC and overtime measure, control and enhance overall brain functioning. If you would like to learn more about the technology and its use, here is a systematic review of fNIRS based neurofeedback, In it you find a summary of all past relevant fNIR research in neurofeedback that has been published and peer-reviewed.
If you are looking for device to measure the brain waves, you should look for neurofeedback device based on EEG technology.