Fringe referencing

Obviously, this problem would be alleviated if ADCs of higher dynamic range were used. Although 18-bit ADCs are commercially available, they are typically slower and more expensive than their 16-bit counterparts. A relatively recent development has been the introduction of the sigma-delta ADCs (see Section 3.4). These devices have an effective dynamic range of about 21 bits but sample the signals from the infrared and laser detectors simultaneously at equal intervals of time rather 

For a Michelson interferometer, the rate of change of retardation is twice the mechanical velocity of the mirror. This parameter is often known as the optical velocity, V, and has the units of centimeters (retardation) per second. Some FT-IR vendors specify the velocity of the moving mirror in their interferometer in terms of its mechanical velocity and some in terms of its optical velocity. Note Users should make sure which of these two parameters is in fact specified.) Still others specify the optical velocity in terms of the corresponding modulation frequency of the laser interferogram although a few commercial interferometers no longer use HeNe lasers, we will denote the frequency of the laser interferogram as /neNe- 

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