Transparency measurement of thin films with one-sided optical access using fluorescence imaging

Optical transparency can be determined by passing light through an object and comparing the intensity of the transmitted light with the intensity of the incident light. This measurement needs optical access to both sides of the object for illumination and measurement. Optical access to both sides of the object may be restricted in some situations due to the presence of an opaque obstacle, lack of physical access, etc. A novel technique of transmittance measurement is presented that is able to determine the transparency of thin-film objects with optical access limited to just one side. This method involves mounting the object on a fluorescent substrate, illuminating the object at an excitation wavelength, and observing the light radiated from the object at the fluorescence wavelength. The observed intensity of the light at the fluorescence wavelength is directly related to the transmittance of the object at the excitation and fluorescence wavelengths. This optical configuration eliminates the spurious effect of reflection of the incident light by filtering the excitation wavelength before reaching the optical detector. The technique was used to measure the transmittance of neutral density filters, which were also measured using a conventional transmittance configuration. The difference between the transmittance measured using one-sided optical access and the conventional two-sided configuration was 2.4% or less. As an example of the utility of the one-sided measurement technique, the transmittance of paper was measured during drying, while the paper sample was sandwiched between a woven dryer fabric and a heater. The relationship between the optical transmittance of paper and its moisture content has been determined previously, and this relationship was used to infer the moisture content of the paper during drying. The moisture content distribution during the drying process is shown to be spatially correlated with the structure of the dryer fabric.