GLACIER® utilizes cavity ring-down (CRD) spectroscopy to measure the losses of advanced optical coatings down to 5 ppm, allowing the characterization of highly-reflective mirrors with up to 99.9995% reflectivity. This method provides unrivalled sensitivity compared to conventional absorption and reflection measurements, limited to the >1000 ppm range (<99.9% reflectivity). CRD loss meters repeatedly measure the losses within a cavity with a longer intra-cavity dwell time for smaller losses, resulting in increased measurement precision the lower the losses are. The device features high-speed data acquisition, enabling quick measurement recording and analysis.