Further, the rules change dramatically if you are operating in the Superluminescent regime. The reason for often using the term coherence length instead of coherence time is that the optical time delays involved in some experiment are often determined by optical path lengths. One should be careful not to confuse the coherence time with the time duration of the signal, nor the coherence length with the coherence area (see below). The difference in the coherence length, L coh, is unimportant when thin layers of biological objects are irradiated, inasmuch as the longitudinal size of the irradiated object, Δ, is less than L coh for any source of monochromatic light (i.e., filtered lamp light, LED, laser). 1(a).We consider a point light source emitting waves of different wavelengths. What is your spectral shape, and how did you determine that, by the way? The effect of temporal coherence is discussed in Fig. The coherence length L c is defined as the distance the wave travels in time τ c.: 560, 571–573. The … This decrease in effective coherence length is due to the deviations from a Gaussian spectrum, effect of spatial coherence properties of the LEDs and dispersion [22] , [23] . For a LED source with a continuous spectrum, if we just consider a specific wavelength such as 570nm, how can I understand its spatial coherence or temporal coherrence？ $\begingroup$ To the best of my (ancient) knowledge, any formulae beyond the basic 1/bandwidth are estimates only, rather than strict derivations from the fundamental properties of a given LED material structure. They found that the value of the theoretical coherence length is larger than the experimentally obtained effective coherence length. Figure 1: Setup of an interferometer, where the coherence length of light is important.