The ideal IR source should be monochromatic and tunable within a wide range of wavelength. According to T ∝d/λ, where T is the transmission coefficient, d the aperture diameter and λ is wavelength, the aperture-based NSOM/FTIR transmission is even more limited due to the long infrared wavelength; therefore, an intense IR source is needed to offset the low transmission through the optical fiber. The common bright IR light sources are the free-electron laser (FEL), color-center lasers, CO lasers and laser diodes. FEL is an excellent IR source, with 2–20 μm spectral range, short pulses (picosecond) and high average power (0.1-1 W). Alternately, a tabletop picosecond optical parametric oscillator (OPO) can be used which is less expensive, but has a limited tunability and a lower power-output.