## 1 Introduction  
There is no doubt that the droplet size spectra produced by different nozzle types (e.g., air induction or conventional) [1] and also by liquid atomization (e.g., hydraulic or pneumatic) [2] affect the efficiency of spray application and the efficacy of chemical plant protection products (PPP). An efficient spray application is the main strategy to simultaneously increase the benefits of PPP, reduce the risk of environmental and human contamination, and produce high-quality and safe food in a sustainable way [3, 4]. Therefore, the efficiency of spray applications depends primarily on the proper functioning of the nozzles and on the work parameters selected [5, 6]. Conventional flat fan nozzles, characterized by hydraulic atomization, are the most commonly used in spraying of arable field crops [6, 7, 8] and for herbicide applications in 3D crops [9, 10]. The selection of nozzle type according to the needs of the treatment specifications should aim to achieve the required total application volume rate, taking into account the spray quality [11], to both maximize the efficacy of treatments and minimize the drift risk. The PPP label recommendations should also be taken into account to select the best working parameters. The assessment of the work performance agricultural nozzles is based on the testing macro- and microparameters of the atomized liquid. The most important of these parameters are the unit flow rate and the uniformity of the transverse volume distribution, characterized by the CV coefficient. Another important criterion is also the qualification of the nozzle for the appropriate droplet class (spectrum of droplets produced) according to ANSI/ASAE S572.1 in terms of volume diameters Dv0.1, Dv0.5 (VMD), and Dv0.9 (\(\upmu\)m) [12; 13]. The acceptable values of the listed parameters and criteria for assessing the operation of nozzles are: tolerance of the unit flow rate (\(\pm\)5%) of the nominal value according to ISO 10625-2005 [14]; CV, with a maximum allowable value of 10% (ISO 5682-1-1996) [15]; and the droplet spectrum, ranging from extra-fine (XF) to ultra-coarse (UC), according to ANSI/ASAE S572.1, 2009. Therefore, the effectiveness of the application process of plant protection products is influenced by the spraying characteristics, which depend, among others, on the droplet size distribution, the nature of the volume distribution, and the spraying structure [16], which determines how well the plants are covered with working liquid [17]. From the above, various consequences arise regarding the spray quality of the utility liquid and the optimal droplet sizes [16; 17; 18]. The suitability of individual droplet fractions, and thus the efficiency and effectiveness of the chemical application, depends on their intended use (fungicide, herbicide, insecticide, etc.) and the conditions of use [19]. The droplet fraction (class) produced by the nozzle is given by its manufacturer, together with the nominal unit flow rate and the CV coefficient value. The listed parameters usually fall within their allowable values, which is confirmed by the results of numerous tests. However, the authors would like to draw attention to the fact that there may be significant local deviations of the droplet fraction (with an unknown mechanism of formation), which in many cases cause negative consequences related to the quality and effectiveness of the procedure. Such diversity of the droplet spectrum in the spray liquid stream (droplet sizes and their number of occurrences) causes an unfavorable volume distribution, which negatively affects the coverage uniformity of the sprayed surface [20]. Analysis of the spectrum characteristics for the entire stream, especially when spatial measurement methods are used to obtain them, may not reflect this significant variation in the droplet spectrum in individual stream bands. This fact is the genesis of undertaking research in the field of spraying spectra assessment in selected liquid stream bands using the laser Doppler anemometry method.