Measuring the spatial distribution of the absolute transmittance of optical components at NPL
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The result of a measurement of absolute transmittance corresponds to the average transmittance of the test sample over the area illuminated by the probe beam. This is usually sufficient when the test sample has spatially uniform transmittance.
However, we have found that a number of optical components, such as non-linear crystals, bandpass filters and anti-reflection (AR) coated optical components, exhibit large spatial variations in transmittance and therefore cannot be adequately quantified by a single, average, transmittance value.
NPL has developed a facility which measures the spatially resolved absolute transmittance of optical components as large as 50 mm by 50 mm with a spatial resolution down to 8 μm. Measurements can be performed anywhere in the 300 nm to 20 μm wave length range. This contribution describes the facility and provides examples of its use.