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The primary usability assessment of ZnS for multispectral-infrared window applications is its transmission of light in the visible and near-infrared portion of the electromagnetic spectrum (0.4–2 mm wavelength). For this reason, measurements are routinely taken from the band edge in the ultraviolet to the limit of standard laboratory monochrometer equipment (frequently 2.5 mm). Infrared transmission is also taken due to the presence of extinction in transmission in standard as-deposited CVD ZnS out to at least 7-mm wavelength. Additionally, the association of the 6-mm absorption with visible color has been repeatedly stated in the literature (e.g., Ref. 1), and this feature is the most-prominent one in the infrared spectrum of red ZnS and elemental ZnS particularly. It should be noted that transmission measurements that are to be interpreted quantitatively require careful preparation and measurement protocol. It has been found that surface roughness and wedge in the samples to be measured must be carefully controlled to get reproducible results, especially in the visible and near-infrared. Wedge in the part being measured is routinely kept below five arcmin to avoid these issues. Additionally, direct comparison of two samples where several-percent transmission difference is of interest may require polishing the samples all on the same block throughout each step of the polishing process. The effect of a poor surface polish has been shown to reduce the transmission at 1-mm wavelength as much as 10% in 0.14"-thick samples. It has also been shown that re-polished parts had significantly improved transmittance without having appreciably changed thickness. In the long-wavelength infrared, the effect of unintentional tilt (i.e., the effect of slight differences in how the sample was held in the fixture) has been shown to vary the transmission measurement by 4% and sometimes more across the entire spectrum. In the visible, tilt was readily identified by the periodic oscillations in the transmission due to the etalon effect.
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