Coherence is about whether scattered waves stay in step
Picture light as a train of ripples passing through a tissue full of tiny particles or fibers. In coherent scattering, each particle re-emits its ripple with a fixed timing relationship to the others, so the outgoing ripples can line up and reinforce each other in some directions and cancel in others. In incoherent scattering, that timing relationship is lost, so the re-emitted ripples add up more randomly and the result is a broad, even glow rather than a sharply directed signal.
This distinction is a general property of scattering, not something unique to biology. Experiments on single atoms show that the split between coherent and incoherent scattering can be analyzed through entanglement and which-way information, and that the coherence properties of the scattered light do not depend on whether the atom was held in a trap [1]. The same underlying physics — whether phase relationships survive — is what separates the two scattering regimes.
Why either kind of scattering can leave a blue color
Small scatterers affect short wavelengths more strongly than long ones, so white light passing through a tissue rich in tiny structures loses its reds and yellows and what reaches the eye is dominated by blue. Both coherent and incoherent scattering can therefore generate blue, but they do not generate the same kind of blue. Coherent scattering, because the waves reinforce at selected angles, tends to give colors that shift or brighten as the viewing angle changes. Incoherent Tyndall scattering, with its random addition of waves, tends to give a more uniform blue that looks similar from many directions.
A review of structural coloration in nature places scattering — including coherent and incoherent types — alongside thin-film interference as a physical route to color, and cites blue coloration in butterflies and bird skin among the examples [5]. The traditional term Tyndall scattering is used in that literature for the incoherent, particle-based case [5], which is why the two labels are often contrasted when explaining natural blues.
What controlled scattering experiments show about the split
Work on X-ray and gamma-ray scattering shows that coherent and incoherent contributions can be treated as two components of a single total intensity, and that their ratio changes with the scattering material and the angles observed [2]. That is a useful reminder that the coherent/incoherent split is not a fixed property of a beam but depends on the scatterers and the geometry — the same lesson that applies when comparing a densely packed array of biological structures with a looser, more disordered one.
Multiple-scattering studies add a further caution: in a dense, disordered medium, light can scatter many times, and models that keep all interference effects can be compared with simpler models that treat the light as a random walk [3]. The random-walk, incoherent picture reproduced most scattering features well in that comparison, while coherence effects such as backscattering required the fuller treatment [3]. For a biological tissue, this suggests that whether coherent effects are visible depends on how ordered and how densely packed the scattering structures are.
What this means for a dragonfly, a butterfly, or a bird
The practical reading is that a blue produced by coherent scattering should look more directional and change with angle, because the scattered waves reinforce only in certain directions. A blue produced mainly by incoherent Tyndall scattering should look flatter and more diffuse, because the waves add randomly. The structural-coloration review supports scattering as a route to blue in butterflies and bird skin [5], but it does not settle, for any particular species, how much of the color comes from the coherent versus the incoherent channel.
That last point is where the evidence runs out. The scattering experiments describe coherence in atoms, X-rays, and cold-atom clouds [1][2][3], and the structural-color review describes the biological examples at a general level [5]; none of these sources measures the coherent-to-incoherent balance in a specific dragonfly, butterfly, or bird-skin structure. So the distinction is well grounded as physics and plausible as an explanation of why some natural blues are angle-dependent and others are not, but assigning a particular animal's blue to one mechanism or the other remains an open question rather than a settled result.
Sources used in this answer
Coherent and Incoherent Light Scattering by Single-Atom Wave Packets
Single-atom experiments show that coherent and incoherent light scattering can be analyzed via atom-photon entanglement and which-way information, and that the coherence properties of the scattered light do not depend on the presence of a trapping lattice.
Ratio of incoherent and coherent scattering intensities of X‐ray and γ‐radiation
The Klein–Nishina–Tamm equation treats coherent and incoherent X-ray and gamma-ray scattering as components of a total intensity whose ratio changes with the scattering material and the range of scattering angles.
Coherent and Incoherent Multiple Scattering
In multiple scattering from a disordered cloud of scatterers, an incoherent random-walk model reproduced most scattering features, while coherence effects such as backscattering required a coupled-dipole model that keeps interference.
On the Possibility of Separating Coherent and Incoherent (Anti)neutrino Scattering on Nuclei
Coherent and incoherent neutrino-nucleus scattering can be mixed in the same kinematic region, and the paper argues that the two cross sections could in principle be separated experimentally.
Structural coloration in nature
A review of structural coloration in nature identifies scattering, including coherent and incoherent types, as a physical mechanism of color and cites blue coloration in butterflies and bird skin among its examples.