Abstract
Using light to biostimulate a cell is a common approach in low-level laser therapy (LLLT). 1,2 The growing acceptance of incoherent light sources (such as LEDs) in phototherapy continues debate on the value of coherence in achieving beneficial results with light. Smith argues that the spatial coherence of lasers is not useful in LLLT, 3 as according to the first law of photochemistry, light must be absorbed to induce a chemical reaction, and therefore the intensity of the illumination rather than its phase (which determines the coherence of light) plays the critical role. Hode claims that coherence of laser light is not lost when the light enters tissues, and thus it affects the measured outcome. 4
The purpose of this editorial is to clear up those issues for non-professionals while addressing the meaning of speckle and of coherence of light.
Coherence of light occurs when all the light waves are “in phase” with one another along time, i.e., the crests of one wave are aligned with the crests of all the other waves, and similarly for the troughs of the waves. Therefore, coherence of light is basically related to how randomly or how often the electric field of a light wave (by its amplitude and phase) is changing within the time of observation. In other words, coherence relates to the extent of similarity or synchronization between time varying distributions of the electric field that can be measured at different spatial or temporal locations.
One should distinguish between spatial and temporal coherence. Spatial coherence is related to how the changes of the field of light (usually caused by changes of its phase) in two spatial locations are temporally correlated (mathematical measure that characterizes similarity) to each other. 5 Temporal coherence is related to examining the temporal rate at which the optical field is changing in a given spatial location (usually the change is in the phase of the optical field). The inverse of the maximal rate of temporal change is called the coherence time. This coherence time can be translated into coherence length, simply by multiplying it by the speed of light.
Because temporal coherence is inversely proportional to the rate of temporal changes of the field, it is directly related to precise extent of the monochromaticity of the beam. The more polychromatic the light is, the larger is its rate of temporal change. Therefore, a monochromatic beam is temporally coherent, because the change of its electric field over time is fully correlated and anticipated. The more polychromatic the illumination is, the shorter is its temporal coherence length.
Therefore, one may, for example, have spatially coherent light that is temporally incoherent. In this case, the light is polychromatic with a large temporal rate of changing (in the value of its optical field) and in all the spatial positions along the beam of light, the field of light is rapidly oscillating with time in the same manner. On the other hand, one cannot have an optical beam that is temporally fully coherent and spatially fully incoherent, because if it is temporally coherent, its field has a harmonic (sinusoidal) oscillation and its amplitude and phase do not change with time beyond this defined harmonic relation. Therefore, one cannot get two spatial positions along the beam that are differently changing with time.
Coherent light that is being propagated through a biological tissue may eventually lose its spatial coherence because, as previously stated, there are temporal changes in the value of the optical field that are being involved with the tissue medium. Those temporal changes that eventually break the spatial coherence are related to flow of fluids through the biological tissue. In cases there is no flow, the spatial coherence will not be lost. In case there is a flow, the rate at which the spatial coherence is lost is directly related to the volumetric flow rate of the fluid. 6
Speckle patterns are spatially random self-interference spots of coherent light that are generated when spatially coherent light is reflected off or transmitted through a rough surface. Speckle distributions accumulate themselves as spots, 5 and therefore they are basically a locally generated nonuniformity of laser beam intensity (spatially coherent source) obtained along the plane perpendicular to the direction of propagation. The average power density remains the same, but the local power density is not uniform, having higher power density within the speckle spot, and lower power density around it. 7 The contrast of those random spots is directly related to the spatial coherence of light (100% for highly spatial coherent and 0 for fully incoherent light).
It is important to distinguish between primary and secondary speckle patterns. Primary speckle patterns are generated by projection when the light passes through a ground glass or a diffuser, and then illuminates the detection system. Secondary speckle patterns are self-generated random patterns created because of the roughness of the illuminated surface from which the light is reflected toward the detection system. A biological tissue acts as such a diffusive and scattering medium as well. The speckle statistics depend upon the ground glass or the diffuser that is used to project the patterns (primary speckle), and on the surface characteristics on which they shine (secondary speckle). The parameters of the optical system can determine the dominance of each type (primary or secondary) of speckle patterns.
Following the explanations given previously, we now aim to focus on the relevance of coherence and speckle in photomedicine and photobiology. Coherence and speckle are related to the temporal/spatial variation of the distribution of field of light (and its phase). In photomedicine light must be absorbed before photochemical reactions can occur (according to the first law of photochemistry), and therefore its intensity (the absolute value square of the field) rather than its field plays the major role. Therefore, the phase of light, which is a fundamental issue in coherence-related effects such as speckle, is irrelevant when irradiating tissues (e.g., in LLLT). The only benefit of using spatially coherent sources in this case is higher efficiency in their collimation and beam forming.
Indeed Hode et al. 8 present their simulations suggesting that intensities of up to five times the mean intensity occur in a speckle field together with the fact that polarization patterns also occur in speckle fields (the photon absorption cross section is polarization dependent and up to two times more effective if the polarization vector is aligned with the dipole moment of the target photoreceptor molecule). Nevertheless, the contrast of the speckle pattern is reduced when it penetrates through the tissue. This reduction is related to the fluids flowing through biological tissues. The amount of loss of the spatial coherence of the illuminating beam, and the reduction in the contrast of the secondary speckles, strongly depend upon the volumetric flow rate of the fluid through the tissue. 6 In the experimental measurements of Fixler et al. 6 we measured the contrast of speckle patterns at the output of a constructed phantom as well as a real tissue, as a function of flow rate. We generated the flow by inserting needles having different diameters of 0.8128 and 0.7112 mm into the phantom tissue. Two flows of water at of 0.1 and 0.2 ml/sec were injected into the generated tunnels. In our experiment, we saw that for 0.1 ml/sec velocity, the contrast is reduced by 50% for an integration time of <1 sec and at 0.2 ml/sec velocity the spatial coherence is almost fully destroyed and the contrast was reduced to only 10% at an integration time of <0.2 sec.
An important hypothesis is often presented about the possibly important role of speckle pattern in laser photobiology. It is related to the fact that it is widely accepted that mitochondria are principal sites of the chromophores that are responsible for many of the biological effects of LLLT. Mitochondria in mammalian cells are 0.2–1.2 μm in diameter and 1–4 μm in length. The dimensions of the speckles are related to the wavelength of the light used (somewhat under 1μm). Therefore these two sets of dimensions match well. However, it is noteworthy that even when the light is not coherent it may still be focused into a spot related to the wavelength of illumination and therefore have localized effects over the mitochondria. 9 In addition, as we have experimentally demonstrated, 6 the spatial coherence of light is lost when the light is propagated through the tissues (because of the existing liquid flow through these tissues) the contrast of the speckle patterns is significantly reduced upon further penetration. Therefore, as the contrast of the speckle patterns may be significantly reduced, the speckle's photobiology effect may be negligible.
It is noteworthy that although, as previously stated, the spatial coherence is probably not useful for phototherapy, which is related to light absorption and depends upon the absolute value square of the electrical (and magnetic) field but not on its phase, coherence of light has its usefulness in the fields of optical diagnostics; e.g., when measuring flow rates or in optical coherence tomography (OCT) used for performing high resolution three-dimensional mapping of tissues. 10