High Contrast Multipass Fabry-Perot Spectrometer

Although diffraction grating and Fourier Transform spectrometers are widely used in the infrared & far infrared, both techniques posses inherent disadvantages. It is well-known that the radiation throughput of the grating spectrometer is severely reduced as the resolution is increased, it is this lack of radiation energy and the accompanying need for long measurement times which has led to the rapid expansion of the Fourier technique as the basic FIR instrument. However the grating does have a number of advantages, namely it is real-time, resolution over a small spectral interval can be improved by closing the splits without a too prohibitively long scan time, it has high contrast, i.e., its spectral purity when used with good blocking filters is very good. This means that transmissions less than 1% can be measured with a high degree of confidence and small, < 0.5%, changes can easily be seen.