How non-invasive blood glucose concentration measurement work?
I'm not exactly sure how it is done non-invasively in the manner you describe but suspect that it has to do with IR absorption spectroscopy. Glucose has distinct bands in the near-IR frequency range. So by illuminating the ear lobe and the capillaries in the surface, picking off the refracted light, and spectrally analyzing you should be able to determine the amount of glucose relative to some reference calibration.

I worked for a start-up company that did this semi-invasively for glucose measurements in the ICU. We drew a small blood sample and optically analyzed it in a glass cell using a scanning interferometer - then returned the blood to the patient.

The optical method that draws blood probably is the best approach with regards to accuracy - where you would like to get within 10 mg/dL - but it also has its problems including response to 'interferents' in the blood that can pose as glucose not to mention high cost for the interferometer and fouling of the blood access system.