Measurement of Optical Filter Spectra
Optical filters play an important role in enabling applications such as fluorescence microscopy and Raman spectroscopy. In these applications there are two distinct types of beams: the illumination (or excitation) beam and the signal (or emission) beam. Not only are these beams spectrally distinct, but also they differ significantly in their intensity – the signal beam can be a million times (or more) weaker than the illumination beam. Therefore, the ability of filters to selectively transmit desired wavelengths of light while blocking unwanted light is critical. The performance of such filters is determined by their spectral characteristics, including transmission efficiency of the signal and attenuation (or blocking) of the illumination light and undesirable emission wavelengths. In particular, often it is critical for filters to transition from deep blocking to high transmission over a very short wavelength range, leading to steep and deep spectral edges. However, due to limitations of standard metrology techniques, the measured spectral characteristics of thin-film interference filters are frequently not determined accurately, especially when there are steep and deep edges. As a result, it is difficult to ensure sufficient system-level performance without a lot of trial-and-error experimentation with the filters....
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