When selecting an interferometer, the most critical factor is measurement accuracy and resolution, typically measured in fractions of the wavelength (e.g., λ/20 or λ/100). You must also evaluate the light source stability (He-Ne lasers are standard for high precision) and the aperture size (ranging from 4 inches to 12 inches) to ensure it covers your test optics. Additionally, check the Phase-Shifting capability, as digital interferometers with specialized software provide much higher data density and repeatability than manual fringe analysis.