The standard method for detecting absorption of light in the visibly and ultraviolet region is to disperse transmitted light using a monochromator. A monochromator is a device that has an input slit and one or more gratings that cause the light to reflect in a frequency dependent manner. Red and blue light reflect at different angles so that they are spatially separated. One may detect the different frequencies of dispersed light using three methods. A scanning method uses a second (output) slit and a single detector, such as a silicon photodiode. A second method is a photoarray, which is a set of photodiodes that can detect the entire dispersed range in one shot. The third way is to disperse light onto a charge-coupled device (CCD). A CCD chip is a fancy version of the chip you have in a cell phone that can take a picture. We can call all of these methods of detection, dispersion methods. There are dispersion spectrophotometers in the infrared. Actually, the grating technology works very well in the infrared, but detectors are more expensive. Instead of silicon the most sensitive detector is a mercury cadmium telluride detector. Array technology, such as photodiode arrays or CCDs is still pretty expensive in the infrared. Moreover, scanning methods are quite slow. An alternative method that is particularly suited to infrared spectroscopy uses an interferometer. An interferometer is a device that splits in the incoming light into two brances using a beam splitter. One path is fixed and the second path has a moving mirror. Those two paths interfere with each other in a way that depends on the position of the moving mirror. By scanning the moving mirror and detecting the intensity of all wavelengths simultaneously one can build up an interferogram. The optical signal is obtained from the fourier transform of the interferogram. The optical source in a Fourier transform infrared (FTIR) spectrometer is called a glowbar. The glowbar is a carbon filament that emits heat (infrared light) because of its moderately high resistance and a relatively large flow of electrical current.
The advantages of FT detection
Fourier transform methods are advantageous. The throughput of light is much greater in an interferometer than in a dispersive spectrophotometer. Scanning through the various positions of the FTIR can be very rapid. Using these methods one can obtain a spectrum with higher signal-to-noise ratio than is possible in typical dispersive detection apparati. The alignment of the moving mirror is crucial and the positional and angular accuracy must be significantly smaller than the wavelength of light. Since infrared light is in the 1-10 micron range (starting in the near-IR), this means that the position accuracy must be less than one micron. This can be achieved using a red laser (a HeNe laser). The HeNe laser beam path is the same as that of the IR light. The detector can determine the position of the mirror based on the fringes of the HeNe laser. Since the wavelength of HeNe is 632 nm the peaks and nulls of the beam are spaced are separated by 632 nm/4 or 158 nm. Detecting where the mirror is with respect to the peaks and nulls of the HeNe wavelength permits us to know the mirror position to the needed accuracy. If we wanted to design a visible FT spectrometer we would need an ultraviolet (UV)laser for alignment. That is both expensive and technologically challenging from a materials point of view since UV light causes ionization and damage to many materials.
