## 3 Results  
To understand which particles we are dealing with, we tested their composition, structure and size. XRD for the 600 \({}^{\circ}\)C-treated nanoparticles are shown in Figure 1. The data confirms that the chosen technique of synthesis of nanopowders leads to the formation of fluorite structure particles (JCPDS #06-0262, a = 5.800 A). The monophase and purity of the obtained samples are also confirmed. The scanning electron microscopy image for Sr\({}_{0.955}\)Yb\({}_{0.020}\)Er\({}_{0.025}\)F\({}_{2.045}\) samples shown in Figure 2A made it possible to determine the shape and diameter of the particles. Two fractions of spherical particle were identified: primary particles were approximately 75 nm, and agglomerates with a 300 nm diameter. The dynamic light scattering method confirmed the formation of both primary particles and their agglomerates (Figure 2B). The energy-dispersive X-ray analysis clarified the elemental composition of the particles: Sr\({}_{0.948}\)Yb\({}_{0.024}\)Er\({}_{0.028}\)F\({}_{2.052}\). Figure 3A presents the spectra of the semiconductor laser-induced photoluminescence of nanoparticles measured in visible range. Under 976 nm excitation (curve 2), the nanoparticles emit spectra with maximum wavelengths of 660 nm, 545 nm and 525 nm (curve 1). Very weak blue (410 nm) and down-shifted (1520 nm) luminescence are not shown. It must be emphasized that the amplitudes of both the excitation light and the luminescence are given in arbitrary units and should not be used to calculate the luminescence efficiency. Difference spectrum (\"light spectrum under photoconversion film\" minus \"light spectrum under common film\") allowed the visualization of changes in the spectral composition of growth light passed through photoconversion glasses, are shown in Figure 3B. It was shown that the photoconversion glasses we used were capable of increasing PAR intensity and decreasing intensity of UV and far-red. Thus, the luminescent nanoparticles are capable of optimizing the illumination spectrum (increasing PAR intensity and changing the ratio of red and far-red light) and can be used in photoconversion materials for agricultural applications. Since our study is aimed at optimizing the conditions for growing agricultural plants with a shortage of natural light, we imitated a cloudy day in temperate regions using UV and incandescent lamps. UVA and PAR intensities were \(\leq\) 10 \(\upmu\)mol photons s\({}^{-1}\) m\({}^{-2}\) and \(\approx\) 70 \(\upmu\)mol photons s\({}^{-1}\) m\({}^{-2}\). The overall light intensity (350-800 nm) was greater than 140 \(\upmu\)mol of photons s\({}^{-1}\) m\({}^{-2}\). The results of morphometric studies are presented in Table 1. It was shown that photoconversion film has a moderate, but statistically significant, effect on the leaf formation. Under control films, the appearance of a new leaf was observed every sixth day, whereas under photoconversion films, new leaves appeared every fourth day. At the same time, the number of leaves in the control samples increased by 55% and by 85% in the experimental conditions. Figure 1: XRD for the Sr\({}_{0.955}\)Yb\({}_{0.020}\)Er\({}_{0.025}\)F\({}_{2.045}\) nanoparticles (1) and JCPDS #06-0262 for the strontium fluoride (2). In comparison to other parameters, the area of leaves showed the greatest increase. Before the experiment, the average leaf area was 23 cm\({}^{2}\); after 25 days, control samples grew by 290% under common film and by 435% under photoconversion film. Thus, the acceleration of leaf growth under photoconversion films was 50%. Note that PCF contributes to acceleration in the length of the stem, while it practically does not affect the length of the internodes. It is interesting to mark that the chlorophyll concentration in the tomatoes leaf remained relatively unchanged under the common film, while PCF increased the amount of chlorophyll in the leaf. Thus, it is shown that plants grow and develop faster under the films containing photoconversion nanoparticles.