Cooling optical clarity

Fig. 3.9. Enhancement of transmission Raman signal using a standard dielectric band-pass filter (BF) placed within the laser beam into the proximity of sample. The Raman spectra are those of a standard paracetamol tablet measured with (with band-pass filter) and without (bare) the filter. The spectra are offset for clarity. The acquisition times were 1 s in both cases with a laser power of 250 mW (827 nm). The spectra were detected with an Ocean Optics spectrograph equipped with a detector array cooled to -15° C...

TV cameras have been developed for a wide variety of hostile environments and are applicable to refueling. They represent an dtemative optical meftiod but will not provide the clarity of view of precision metrology. Specialized TV cameras such as fiberscopes have been developed with high radiation resistance and enable the electronics to be positioned in cooled low-radiation zones. 

Processing problems that result in poor optical properties include die lines, low cooling rates, melt fracture, and, for coextrusion, interfacial instabilities. Die lines, as discussed previously, are bothersome because they can reduce both mechanical and optical properties. The cooling rate has a significant effect on polymer crystallization. Because crystallinity in polymers generally reduces optical properties, cooling rates are often increased to improve film clarity. Melt fracture will usually have an effect similar to die lines with respect to optical properties. Any occurrence that creates texture on the film surface will reduce film transparency. Similarly, if the texture occurs at the boundary of two layers in a coextruded film, as happens with interfacial instabilities, the same reduction in optical properties will result. 

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