The attenuation and other optical properties of a medium are exploited to measure a thickness of the medium between a sensor and a target surface. Disclosed herein is a device comprising: a luminescent layer of a material adapted to form a conforming layer when placed over a target surface of an object, the luminescent layer configured to emit light at a first wavelength and a second wavelength； a sensor positioned to capture an intensity of the first wavelength and an intensity of the second wavelength in a direction of a location on the target surface； an absorbing medium, the medium positioned between the sensor and the luminescent layer and the medium absorbing the first wavelength more than the second wavelength； and a processor programmed to calculate a thickness of the medium in the direction of the location based upon a function of the intensity of the first wavelength and the intensity of the second wavelength.
