Lasers in Surgery and Medicine 43:339–343 (2011) Determination of Coherence Length in Biological Tissues Dror Fixler, PhD , Hamootal Duadi, BSc , Rinat Ankri, MSc , and Zeev Zalevsky, PhD School of Engineering, Bar Ilan University, Ramat Gan 52900, Israel Background and Objective: Lately in phototherapy the use of diodes instead of lasers was suggested for econ- omical and practical reasons.It has been argued that lasers have no preference over diodes since they lose their coherence once penetrating biological tissues. However, this point has never been experimentally proven. In this work we, for the first time, have experimentally validated the conditions affecting the spatial coherence ofa laser illumination going through a biological tissue. Study Design/Materials and Methods: In our exper- iments we measured the spatialcoherence ofthe light passing through phantoms containing intralipid and ink component as well as through uncooked turkey meat. We do this measuring the changes of the contrast of the speckle patterns generated due to laser illumination. Flow tunnels inside the phantoms were generated by nee- dles in two different diameters. The measurements were performed for varied integration time, varied thickness of phantoms,and for varied flow rates.The measurement system included two excitation sources: a green doubled Nd:YAG laser at wavelength of 532 nm and an ultra high power green LED at a wavelength of 520 nm. Results: It was experimentally validated that the thickness of the tissue does not change the coherence while there is no flow. Furthermore, the flow velocity and the flow volumetric rate highly affect the coherence length. Previously developed mathematical expression, in which the contrast depends on the correlation and the exposure time, was found to be com- patible with the obtained experimental results. Conclusions: We found that the coherence of the laser is not lost when the light goes through a static tissue but it is partially lost when there is a flow of fluid through the tissue. The volumetric flow rate is directly correlated to the loss of spatialcoherence.Higher flow rate produces shorter coherence length. Lasers Surg. Med. 43:339–343, 2011. ß 2011 Wiley-Liss, Inc. Key words: coherence; speckle; tissues; phantoms; flows INTRODUCTION Light–tissue interaction is common in clinical treat- ments and medicalresearch while different light source are widely used in the visible and the near infra red (NIR) spectral range especially at therapeutic field, such as photo dynamic therapy that uses light to damage tumor cells, and at low level laser therapy (LLLT) that uses visible-NIR light to biostimulate cells [1–4]. One of the characteristics of laser illumination is its spatial coherence. The growing acceptance of incoherent light sources (such as light emit- ting diodes)in phototherapy continues to debate on the value of coherence in achieving beneficial results with light. In Ref. 5 Kendric Smith argues that lasers are not magical and writes that four of the laser-specific properties of light are not useful. One of those properties is their spatial coherence. On the other side in Ref.6 Lars Hode claims that coherence oflaser light is not lost when the light enters tissue and only the length of coherence is reduced. Since spatial coherence is the basic property distinguish- ing lasers from light emitting diodes (LEDs), while lasers are highly coherent sources of light and LEDs are not, it is importantto finally answer the question ofhow spatial coherence is affected while light interacts with biological tis- sues. In this work, for the first time, we have experimentally validated the conditions affecting the spatial coherence of a laser illumination going through a biological tissue. We pro- duced a phantom that has characteristics similar to human tissue and simulated blood flow by entering a needle through the phantom and injecting water. This setup was then illu- minated under differentflow conditions.The method we used for evaluating the spatial coherence involved measur- ing the contrast of the speckles generated within the trans- mitted laser beam spot. The speckle patterns were captured by a CCD camera having varied the integration time. It is well known that a rough surface that scatters laser illumination causes,due to self interference,the gener- ation of a random pattern created inside the light spot and which is called a speckle pattern [7]. The properties of this pattern can only be described statistically. A detailed theory was developed [8–10] while it was demonstrated that the contrast of the speckle pattern highly related to the spatial coherence of the light beam. At highly coherent light the contrast is 100% but it is reduced when the spatial coherenceis reduced.In our experiments,we measured the spatial coherence of light passing through the tissue by measuring the contrast of the speckle pattern generated inside the spot of light. Those measurements were performed for varied integration times. In second section, we present the theory of speckles and in the third section we describe the materials and methods in the measurement system. In section four, we perform *Corresponding to: Zeev Zalevsky, PhD, School of Engineering, Bar Ilan University, Ramat Gan 52900, Israel. E-mail: zalevsz@eng.biu.ac.il Accepted 11 January 2011 Published online 15 April 2011 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/lsm.21047 ß 2011 Wiley-Liss, Inc.