FTIR Analysis measures the infrared region of the electromagnetic radiation spectrum, which has a longer wavelength and a lower frequency than visible light, and is measurable in a sample when submitted to infrared radiation (IR). The basic theory is that the bonds between different elements absorb light at different frequencies.
A simple device called an interferometer is used to identify samples by producing an optical signal with all the IR frequencies encoded into it. The signal can be measured quickly.
interferogram graph consists of set of various intensities for discrete value of optical path difference of two arms of mirror(one mirror is stationary and another moving mirror at constant speed back forth)
fourier transformation(mathematical technique) to convert interferogram into spectral form.
For details please go through the video.