Coherence effects on the interference colors of soap films

Acquiring the thickness field of a soap film from interference colors requires an accurate relationship between color and film thickness. Throughout the literature, an interference relation derived using monochromatic waves is widely used to calculate the colors of soap films illuminated by light sources with significant frequency bandwidths by applying the relation at a number of discrete wavelengths in the source, which assumes that the interfering waves are perfectly coherent. However, since the coherence between waves is expected to decrease with increasing film thickness, it is poorly understood when interference relations derived using monochromatic waves can be applied. In this study, an interference relation incorporating the coherence between interfering waves is derived. The effects of coherence on the interference colors of soap films are then studied by comparing the colors computed using each of these two interference relations for light sources with different frequency bandwidths. As the frequency bandwidth of the light source increases, the difference in the colors computed using each interference relation increases, which implies that the accuracy of the method involving the monochromatic relation decreases with increasing frequency bandwidth of the source. The findings of this study will allow for more accurate measurements of the thickness of soap films from their interference colors. (c) 2023 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).