if in chromatic confocal sensor, a led with low coherence and a multi-mode fiber are used. I wonder this imaging process should be the which one ? In the monograph about fiber optical confocal scanning microscopy by GuMin, he treat the imaging process as totally coherent imaging when a laser and single-mode fiber is used.
I do not know GuMin's work or much about lasers, scattering permitted in various waveguides, and so on; so I almost did not dare offer an opinion. But I think that the answer to your question is in part definitional, regarding just what is meant by "coherence": What is the error for Wavelength in a given setup, and how broad (how many nm) can the standard error be, before one says that the light is incoherent? Furthermore, even if the standard error is > 0.5nm, if it on average "does the job" in permitting a coherent mapping of related data to occur, it may be functionally "coherent" without being completely coherent in terms of pure theory. What is the bandwidth of coherence? See the work of Charles Proffer Saylor, 1935:
Saylor, C. P., 1935a. Accuracy of microscopical methods for determining refractive index by immersion. J. Res. Nat. Bur. Standards, Paper RP829, v. 15, pp. 277 - 294.
See also Lipson and Lipson, 1969:
Lipson, S. G., and H. Lipson, 1969. Optical Physics. Cambridge Univ. Press, 494 pp.
especially their treatment of anomalous dispersion: Their Fig. 10.9 (p. 310) is entitled "Anomalous dispersion by a strong spectral line, which can lead to evanescent propagation. They write (Ibid.): "This absorption is, of course, the absorption line corresponding to an emission line in the atomic spectrum" and (p. 31): "The classical theory cannot explain this multitude of resonances, but takes them into account by assigning a number Ni of model atoms to the jth resonance so that the refractive index is written ...", about which I don't know much. :-) But I do pay attention to the tangent function related to boundary conditions at the edges of Saylor's "Zone of Match", because that seems strongly to affect some of my own results.
I hope this helps, Cheng Chen. -Steve-
Yeah,there is some relations with the detector bandwidth as the spectrometer can only detect finite width of wavelength.That's really annoying me. In GuMin's work, when a laser( i think he means single wavelength) is used, if a finite detector was used, he think it is partially coherent imaging, while a point detector means totally coherent imaging. If a single mode fiber was used as detector, the fiber will lead a totally coherent imaging.I think in GuMin's work, he may have made some assumptions which is not explicit.
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