I agree that it is the size only. The individual emitters on the small area are still independent of each other, but they are so close that the phase delays during propagation of the light are quite similar for all emitters within the small area.

The poor spatial coherence of light from a bulb has relatively little to do with the process (thermal emission) which generates that light. The actual point is the large emitting surface. If one could reduce the emitting area to the order of a micron squared, the resulting spatial coherence would surpass that of various lasers. There are only two difficulties with a strategy of reducing the emitting area:

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We have often read that laser light is coherent, light from a bulb is incoherent, therefore you can well focus laser light but not light from a bulb. The matter deserves some more thoughts, however.

by DH Sliney · 2016 · Cited by 284 — Today we recognize that light possesses both a wave and particle nature. It is also clear that the limits of visibility really extend from about ...

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When placed in the beam path of a lighting fixture, diffusion material modifies the harsh quality of the light by spreading or dispersing the beam. This ...

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Abstract: The article discusses the coherence properties of light from incandescent lamps (light bulbs). It shows that it is important to distinguish between spatial and temporal coherence, and that both aspects of coherence are not totally excluded for bulbs.

At least in one dimension, the filament of a bulb may be fairly small. If the filament diameter is 0.1 mm and the filament is kept precisely straight, the beam quality of the emitted light at least in the direction perpendicular to the filament axis is roughly comparable with that of the slow axis direction of a high-power diode bar. Of course, in the other direction the beam quality is extremely poor, whereas the diode bar exhibits a fairly high beam quality in the fast axis direction.

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This article is a posting of the Photonics Spotlight, authored by Dr. Rüdiger Paschotta. You may link to this page and cite it, because its location is permanent. See also the RP Photonics Encyclopedia.

Ref.: encyclopedia articles on coherence; F. Kusonoki et al., "Narrow-band thermal radiation with low directivity by resonant modes inside tungsten microcavities", Jpn. J. Appl. Phys. Part 1, 43 (8), 5253 (2004)

How can laser light be spatially coherent even if the emitting area is large? This results from the laser process, which kind of coordinates the emission over the whole beam area. The laser resonator defines a set of resonator modes, and stimulated emission means that excited laser-active atoms or ions are forced to emit exactly into one mode or at least some limited number of modes.

However, it is physically possible that even a solid-state surface exhibits strongly wavelength-dependent absorption and emission characteristics, so that the spectral shape of thermal radiation exhibits significant peaks and thus deviates strongly from a Planck-type shape. As an example, some Japanese researchers (see the above reference, Kusonoki et al.) have investigated microstructured tungsten surfaces (with a periodic array of microcavities), the emission of which exhibits strong peaks with a bandwidth well below 1 μm. Although this is still far from reaching the temporal coherence of a laser, it demonstrates that at least substantial increases of temporal coherence are possible for thermally generated light.

201598 — Unpolarized light is a mixture of light of all different polarization. Polarized light is light with its electric field in a precise direction.

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The temporal coherence of light is intimately related to its optical bandwidth. Typically, thermal radiation is very broadband. If it is blackbody radiation, its spectrum is described by Planck's formula. At least for solid-state devices, one always expects a broadband spectrum, so there seems to be no way that a bulb could generate light with a high temporal coherence.

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Thank you for an interesting article. You wrote “If one could reduce the emitting area to the order of a micron squared, the resulting spatial coherence would surpass that of various lasers.” My question is: why does the smaller size lead to higher coherence? My thought is that it is ONLY due to its small size. Are you suggesting that individual regions of light emission are coupled? Is there stimulated emission if the radiating atoms are close together, or is it in fact, as I conjecture, just a geometric thing?

Why actually is light from a bulb (incandescent lamp) incoherent? Does that property apply to any thermally generated light? This is actually a bad question, since one should make clear whether we talk about temporal or spatial coherence. Let us look at both aspects.

If the electric field vectors are restricted to a single plane by filtration of the beam with specialized materials, then light is referred to as plane or ...

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