The Fourier number ( F o {\displaystyle {\mathit {Fo}}} ) (also known as the Fourier modulus), a ratio α t / d 2 {\displaystyle \alpha t/d^{2}} of the rate of heat conduction α t {\displaystyle \alpha t} to the rate of thermal energy storage d 2 {\displaystyle d^{2}} Fourier transform spectroscopy, a measurement technique whereby spectra are collected based on measurements of the temporal coherence of a radiative source