<title>Measurements of retinal blood flow using biospeckles: experiments with glass capillaries and in the normal human retina</title>

Coherent Optical Techniques for Diagnostics of Retinal Blood Flow

“…By reason of the complexity of biospeckles, the theoretical background has not yet been established, including the proportionality between the object mean velocity and biospeckle fluctuations. However, a variety of in vitro experiments [29][30][31] verified that the width of broadening in the autocorrelation function or power spectrum of speckle fluctuations is linearly related to the mean flow velocity, consequently, relative measurements of the velocity being realized. Therefore, the relative velocity of the retinal blood flow can be obtained by measuring, for example, a correlation time that is defined as the delay time needed for the correlation function to decrease to the half.…”

“…These properties on the correlation time should be compensated in order to make the technique available for metrology. For this purpose, experimental studies 30,31 were conducted to calibrate the correlation time to the absolute mean velocity in the glass capillary model having a corresponding diameter.…”

“…To obtain the well-converged correlation function needed for precise measurements, the diameter of the detecting aperture should be twice as large as that of the speckle grains. 30 It may provide the best convergence in the correlation while it causes an acceptable small integration effect. Examples of the evaluated correlation time are shown in Figure 10 for four different measuring points in the human optic nerve head, which is known as the area having high reflectance.…”

“…Since the biospeckle technique evaluates such an effect as highly randomized or modulated intensity fluctuations, the blood flow in this area can be measured in the same way as in the other retinal areas. The measured correlation time is calibrated to an absolute mean velocity in the corresponding glass tube model by using the equation 30,31 obtained experimentally:…”

Two channel laser speckle instrument for biological microflow localization and velocity measurements

“…In the blood or lymph flow studies, the speckle field intensity fluctuations are detected at one point, and the flow velocity is estimated from the width of power spectrum of these fluctuations or from the width of their autocorrelation function 2,[5][6][7][9][10][11] . Measurements of this type cannot be used to determine the direction of flow in blood or lymph vessels because the speckle field fluctuations recorded at a point do not depend on the direction of the scattering object motion 2, 4, 9 . Another method of measuring the velocity of object motion is based on the recording of intensity fluctuations of the speckle field at two points separated in space and on an analysis of their mutual correlation.…”