Gap thickness retrieval on air etalon by using a focused incoherent white-light beam

In a transmittance spectrum of air etalon composed of ITO-coated glass plates, stronger spectral modulation is observed by using a focused white-light incident beam compared with that by using a conventionally collimated beam. The light source is a single commercial light-emitting diode. An angular integrated multiple-beam Fabry–Perot model is proposed that is well fitted to the measured transmittance spectra. The gap thickness of the air etalon is sensitively retrieved and mapped at a lateral surface resolution of about 1 mm, and the unparallelness of the etalon angled as low as several microradians is thereafter estimated.