Evaluation of activity through dynamic laser speckle using the absolute value of the differences

Abstract

When a material is illuminated with a laser beam, it is possible to verify a phenomenon known as dynamic speckle or biospeckle. It exhibits an interference image that contains lots of information about the process being analyzed, and one of its most important applications is determining the activity quantity from the materials under study. The numerical analysis of the dynamic speckle images can be carried out by means of a co-occurrence matrix (COM) that assembles the intensity distributions of a speckle pattern with regard to time. An operational method that is widely used on the biospeckle COMs is the inertia moment (IM). Some studies demonstrate that IM is more sensitive on analyzing processes that involve high activities or high frequencies if considering the spectral analysis of the phenomena. However, when this variation is not so intense, this method is less efficient. For low variations on the activity or low frequencies, qualitative methods such as wavelet based entropy and cross-spectrum analysis have presented better results; however, processes that are in the intermediate range of activity are not well covered for any of these techniques mentioned earlier. The contribution of this research is to present an alternative approach, based on the absolute value of the differences (AVD) when handling the biospeckle COM. By using AVD on the seed-drying process, was found that it is efficient on verifying the behavior of the intermediate frequencies. Accumulated sum test (Coates and Diggle) showed that AVD and IM are generated from the same stochastic process. Thus, AVD is useful as an alternative method in some cases or even as a complementary tool for analyzing the dynamic speckle, mainly when the information of the activity is not present on high frequencies.

Introduction

The research on biospeckle or dynamic laser speckle been carried out on wide areas for analyzing many biological or non-biological features, such as drying paint process [1], cerebral, skin, and eyeblood flow [2], [3], [4], sperm motility [5], seed analysis [6], [7], [8], [9], and many others [10]. Biospeckle image patterns return reliable information about the phenomenon in study, which can be achieved by numerical and graphical approaches [3], [4], [7], [11], [12], [13], [14].

The inertia moment (IM) [13] is a routine method to verify the activity on biospeckle images, because it returns a numeric value that is related to the activity of the illuminated material. Despite the reliable results presented by the IM in many applications, there are some limitations with respect to its use, in particular, related to the answer limited to higher frequencies in the time history observed in the speckle patterns [6]. The time history speckle pattern (THSP), which is the base of an IM calculation, can be analyzed under the spectral content, verifying how some techniques act in the presence of low, intermediate, and high frequencies. The frequency of the process was found to be directly proportional to the activity, indicating that low frequencies are related to a low activity process, while high frequencies are related to the presence of high activities. The methods IM, wavelet based entropy and cross-spectrum analysis were analyzed in the spectral domain [15], [16] showing that IM is more accurate on working with high frequencies, while wavelet based entropy and cross-spectrum were more sensitive to lower frequencies. In addition, none of these techniques was found to be efficient to work with the whole spectrum.

This work intends to present an alternative method to generate a value that indicates the activity from biospeckle laser data. The new method was based on the absolute value of the differences (AVD) and was applied to biospeckle laser images in seeds. A statistical test and analysis in the spectral domain were applied to the data to evaluate the reliability of the new approach.