A typical step-scan measurement might utilize a step rate of 1 laser fringe per second producing modulation frequencies 5000 times lower than the typical rapid-scan measurement. Thus, the infrared modulation frequencies would be in the range of 0.25 to 0.025 Hz. The frequency spacing between adjacent spectral elements is then only 0.24 milliHertz. It is the very low modulation frequencies and their close spacing that make step-scan measurements susceptible to low-frequency noise.