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The article considers the topical problem of non-destructive filament defectoscopy for 3D printing. The subject of the research is the process of determining the refractive index of the filament material for 3D printing taking into account the reflections from sample opposite walls, which is studied by terahertz spectroscopy in the time domain. Reflections from opposite walls are called the Fabry-Perot effect, and interference members resulting from reflections from walls are traditionally taken into account by summation and represented as a series. The disadvantage of the model in the form of a simple summation is the rejection of the members of the series above the fourth, which leads to inaccuracies in the model. The main problem with terahertz spectroscopy and this study in particular is the contradiction between the rapid development of terahertz spectroscopy and the slow development of models used in terahertz spectroscopy, while the adjacent microwave region has a set of ready-made models. Models based on the description of a standing wave in the microwave tract with refinements, transferred to a new region of terahertz spectroscopy in the time domain. The scientific novelty lies in increasing accuracy by taking into account previously unaccounted for interference members. The analogy between the Fabry-Perot effect used in terahertz spectroscopy and the reflections in a microwave multiprobe multimeter suggested the following recommendations. First, because the phase distance between the sensors in the microwave multimeter is similar to the thickness of the sample in terahertz spectroscopy, therefore, there was choosen such a sample thickness that the interference members are compensated, and secondly, instead of simple sum up it is possibility apply algorithmic processing, the condition for this is the existence in addition to the main signal in the time domain of the recorded echo signals of much smaller amplitude, therefore, one can build a system of equations and by solving it to determine the desired refractive index parameters of the filament sample material.

The paper considers solutions to a scientific and practical problem of applying covariance analysis to determine the factor influence on the functional transformation of the control parameter in colorimetric study. The study implies determining the factor influence on the additive and multiplicative components of the measurement error of the colorimetric control parameter to assess the validity of conclusions about the factor influence on the transformation of the control parameter. The limitations on the number of basic levels (control parameters) and the factors influencing the result of colorimetric control are determined. During the study, the equations to assess the validity of statistical conclusions about the informational significance of the colorimetric control indicators for a simplified model of cross-classification were obtained. The need for the study is due to the fact that in colorimetric control of cereal grains, the measurement uncertainty of the results of measuring the values of the controlled indicators at given levels of the control parameter is quite high. In modern industry, colorimetry has a number of advantages over other methods, such as weight analysis. Colorimetric determinations are performed much faster. In the case of weight analysis, the chemical reaction is at the beginning of the determination followed by a series of long operations, while in the case of colorimetry, the colors are compared immediately after the chemical reaction. Therefore, the colorimetric method belongs to the methods of express control. The main task of colorimetric express control (in our case) is to determine the quality of cereal grains. The task is important and relevant because grain, namely flour, is the main ingredient of many foods, e.g. bread, pasta, pastries, and cookies. Quality products can be obtained only from quality raw materials. Thus, at present time, it is relevant to determine the quality of cereal grains, and flour as the derived product, in Ukraine.