Measurement of the Effect of Luminescent Layer Parameters on Light and Communication Properties

This publication describes the measurement of the effect of selected luminescent layer parameters on the light and communication characteristics of light generated by a luminescent layer using a light-emitting diode (LED) as a radiation source. The luminescent layers were implemented based on a combination of polydimethylsiloxane (PDMS) polymer and YAG:Ce and CaS:Eu phosphors by spin coating. For the chosen weight ratios of PDMS and phosphors, a set of test samples of luminescent layers was implemented whose thicknesses differed from each other in the order of units to tens of micrometers. Experimental measurements have clearly shown that even a slightest change in the thickness of the luminescent layer has a nonnegligible effect on the change of the luminous and communication parameters of the light generated by the luminescent layer. The colorimetric parameters of the color colorimetry temperature (CCT), color rendering index (CRI), and color coordinates <inline-formula> <tex-math notation="LaTeX">$x$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">$y$ </tex-math></inline-formula> [Commission Internationale de l’Eclairage (CIE) 1931] were recorded for the investigated luminescent samples. The communication parameters were evaluated by measuring the error vector magnitude (EVM), modulation error ratio (MER), phase error, bit error rate (BER), and normalized signal-to-noise ratio <inline-formula> <tex-math notation="LaTeX">$E_{b}/N_{0}$ </tex-math></inline-formula>.

In terms of lighting applications, it is very important that the light source that is used creates a "quality" lighting environment that ensures the visual comfort for the user. To achieve this, several requirements regarding the used light source (illuminance, color colorimetry temperature (CCT), color rendering index (CRI), and so on) must be fulfilled. The standard EN 12464-1 [20] states that a CRI value of less than 80 should not be accepted in permanent work locations. In the case of artificial light sources, higher CRI values can only be obtained for those that generate broadband white light with a suitable spectral distribution. Using only narrowband light-emitting diodes (LEDs) of a single color (e.g., blue) is not possible due to the very low CRI values.
It is known that the use of a blue excitation LED and luminescent layer with YAG:Ce phosphor is a well proven and inexpensive way to produce white light [1], [2], [3], [4]. However, the white light generated in this way is insufficient in the red spectral region and therefore has a lower CRI value. To increase the CRI value, it is necessary to increase the red spectral component in the generated white light. This problem can be solved through various methods. For example, Chen et al. [5] created a white light-emitting diode (WLED) in which YAG is the phosphor: Ce coated some of the so-called perovskite materials, which have the same type of crystal structure as CaTiO 3 . The combination of phosphors with these materials increases the red content of the spectrum, making this method suitable for producing WLEDs emitting light with a warm white tone. Red-emitting phosphors based on Mn 4+doped BaGe 1−x Si x F 6 (0 ≤ x ≤ 1) are described in [6]. It is also shown here that a WLED diode based on a blue chip, a YAG:Ce phosphor, and a red BaGe 1−x Si x F 6 phosphor:Mn 4+ exhibits a CRI, Ra > 80, and CCT < 4500 K. In [7], another approach leading to an increase in the CRI value is shown, namely, the use of three layers of phosphors emitting light with the color "red + cyan + yellow" (R + C + Y). After irradiation of this three-layer structure (R + C + Y) with a laser diode (LD) with a wavelength of ∼450 nm, white light with CCT = 5988 K and CRI = 90 was obtained. Quantum dots are also used to increase the CRI value. For example, Lan et al. [8] used red light-emitting carbon dots (R-CDs), combining a blue chip, a YAG:Ce phosphor, and R-CDs to create a WLED with a CRI = 90.9 at color This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/ coordinates CIE x,y = (0.344, 0.333). The use of InP/ZnS green-emitting and InP/ZnSe/ZnS red-emitting quantum dots in combination with a blue chip and a YAG:Ce phosphor is presented in [9], and the WLED prepared in this way exhibited CRI = 91 and CCT = 5313 K for color coordinates (0.3365, 0.3334).
The luminescent layer with YAG:Ce phosphors is also widely used in the field of VLC [10], [11], [12], [13]. For example, in [10], the effect of YAG composite materials is discussed: Ce phosphor and CdSe/ZnS quantum dots on the modulation bandwidth (BW), and the experimental values of the modulation BW show high consistency with the results of the simulation models. In [11], a white light source based on a combination of YAG is presented: Ce phosphor and a blue superluminescent diode (SLD)-type InGaN/GaN, where the generated light has CRI = 68.9 and CCT = 4340 K and the SLD itself has 3-dB BW = 560 MHz. In [12], white light VLC communication with a transmission rate in the order of gigabits per second using YAG is shown: Ce phosphor excited SLD based on InGaN.
The IEEE 802.15.7 standard describes the use of VLC for wireless personal area networks (WPANs). Three types of modulation are standardized for VLC: ON-OFF keying (OOK), variable pulse position modulation (VPPM), and color-shift keying (CSK) [21]. The luminescent layer with YAG:Ce phosphor is mainly used for OOK modulation. For example, in [14], a 700-μm-thick single-crystal YAG:Ce phosphor luminescent layer modulated by a blue LD using the OOK modulation format is described. For the VPPM, there is no known publication that addresses the influence of the parameters of the luminescent layer with YAG:Ce phosphor on the parameters of this modulation. CSK modulation is only designed for systems with colored light sources such as red, green, and blue (RGB), and therefore, it is not possible to use a luminescent layer with YAG:Ce phosphor for CSK modulation. In addition to OOK, VPPM, and CSK modulation, orthogonal frequency-division multiplexing (OFDM), discrete multitone modulation (DMT), and quadrature amplitude modulation (QAM) have been used for VLC for many years. The use of OFDM for VLC was first mentioned by Tanaka et al. [22]. There are a large number of publications that address this topic. For example, Wu et al. [17] showed a data rate of 513 Mbit/s using a commercial WLED and using a combination of DMT and QAM modulation. Retamal et al. [16] described data transmission over a distance of 0.5 m using a blue LD, a luminescent layer containing a YAG:Ce phosphor, and a blue filter. Using 16 QAM OFDM modulation, a rate of 4 Gbit/s was achieved at bit error rate (BER) = 9.7 × 10 −5 . The thickness of the luminescence layer used is not specified and the resulting white light for one of the results has a value of CCT = 6409 K and color coordinates CIE x,y = (0.32; 0.32). Wu et al. [17] verified the transmission of data over a distance of 0.5 m at a rate of 1.6-4.4 Gbit/s using 16 QAM OFDM modulation. The experiment used three samples containing Lu 3 Al 5 O 12 :Ce 3+ and CaAlSiN 3 :Eu 2+ phosphors with thicknesses of 0.7, 0.85, and 1 mm. Depending on the sample used, the transmission rate, CCT, and color coordinates were varied. Chi et al. [18] described the transmission at a distance of 0.5 m for one sample containing a YAG:Ce phosphor and two samples with Lu 3 Al 5 O 12 :Ce 3+ /CaAlSiN 3 :Eu 2+ phosphors with the thicknesses of 0.75 and 0.85 mm.
Using the QAM-OFDM modulation, a speed of 4.4 Gbit/s was achieved for the sample with YAG:Ce phosphor and the speeds of 2.8 and 2.4 Gbit/s were measured for 0.75-and 0.85-mm-thick samples with Lu 3 Al 5 O 12 :Ce 3+ /CaAlSiN 3 + :Eu 2+ phosphors. The thickness of the sample with YAG:Ce phosphor is not specified. An overview of related works is given in Table I. Software-defined radio (SDR) [19], [23], [24], [25] and virtual instrumentation [26], [27] are slowly gaining on importance in the VLC area. The results presented in these publications [19], [23], [24], [25], [26], [27] are based on measurements of commercial LED and automotive lights for which the luminescent layer parameters are not specified. Table I shows that most of the authors are investigating either the lighting parameters or the communication parameters, and only some of them are addressing the issue more comprehensively for both lighting and communication characteristics. Parameters of the luminescent layers used in the related works are not always given and the thicknesses of the luminescent samples used differ by at least 50-100 μm (except for one author's publication). In several cases, only 1-3 samples were used.
The research presented in this article is partly related to the author's publication [13], but in many respects, it extends it and introduces new findings. In the author's study [13], ten luminescence samples with two mass ratios of phosphor and polydimethylsiloxane (PDMS) are investigated, and these were small-area samples deposited on a microscope slide. The thickness of these samples was measured at 1 measurement point using the two-plane focusing method and could be biased by error due to subjective focusing. Colorimetric parameters were measured for the samples, but the process of optimizing the luminescent layer with respect to the colorimetric parameters was not addressed. From the communication parameters, only leading and trailing edges are measured. In contrast, in the research presented in this new publication, 21 luminescence samples were fabricated and investigated for three different phosphor:PDMS mass ratios, involving large-area luminescent layers deposited on a 100 × 100 × 2 mm glass substrate. The thickness of these layers was measured using a calibrated WENZEL LH65 instrument at 25 measurement points, with a guaranteed maximum measurement error of 2 μm. Furthermore, this new publication addresses the optimization process of luminescence samples with respect to colorimetric parameters, particularly the color coordinate parameters. In addition, this new publication addresses the complex influence of luminescence layer parameters on selected QAM modulation parameters (error vector magnitude (EVM), modulation error ratio (MER), phase error, E b /N 0 , and BER).
In the research presented in this article, the original contribution of this publication is the measurement of the effect of very subtle changes in the thickness of the luminescent layer (in the order of units to tens of micrometers) on the observed colorimetric and communication parameters. At the same time, the author's method of producing large-area luminescent layers based on phosphors and PDMS deposited on glass is described. A total of 21 luminescence samples were produced with selected weight ratios of phosphors and PDMS, where the thickness of the luminescence layer varied in the range of 58-95 μm. A robust configuration, including a low modulation order 4-QAM, a narrow BW of 500 kHz, and carrier frequencies of 2, 3, and 4 MHz, was used to measure the communication parameters. Due to this, it was possible to monitor the changes of the communication parameters and their dependency on the change of the luminescent layer parameters. The presented results clearly confirm that the change of the thickness of the luminescent layer in the order of micrometers has a significant effect on the values of colorimetric parameters and a nonnegligible effect on the change of communication parameters of the light generated by this luminescent layer. In addition, it is shown here that the choice of the selected weight ratios of phosphors and PDMS has a significant effect on the observed colorimetric and communication characteristics.

II. METHODOLOGY
The objective of the presented research was to specify the influence of selected luminescent layer parameters on the light and communication properties for the given experimental setup by measurement. The ongoing results of the light characteristics measurements were used in the process of optimizing the colorimetric parameters of the produced luminescent layers, and subsequently, their communication parameters were measured. In the process, it was found that the thicknesses of the studied luminescence layers varied in the order of units to tens of micrometers. Colorimetric and communication parameters were measured at a uniform distance of 1 m from the luminescent layer under investigation. The colorimetric parameters monitored included CCT, CRI, and CIE x,y color coordinates, which were measured with an AvaSpec-HS2048XL-EVO spectrometer. The monitored communication parameters were EVM, MER, phase error, BER, and normalized signal-to-noise ratio E b /N 0 . The measurement of communication parameters was carried out using a universal software radio peripheral (USRP), specifically USRP-2950R, which has a frequency range of 50 MHz-2.2 GHz. Due to the use of lower carrier frequencies, the original transmitter (Tx)/receiver (Rx) board was replaced by the LF TX/RX Daughterboards operating at 0-30 MHz. The USRP typically runs on the LabVIEW platform [28], [29]. In the presented research, two LabVIEW applications were created for the USRP-2950R to set and measure communication parameters. The USRP was connected to the PC using a PCIe 4.0× interface (named MXIe by NI).

A. Manufacture of Luminescent Layers
The luminescent layers presented in this research were fabricated from a homogeneous mixture of PDMS and YAG:Ce   ratio of YP:PDMS of 1:2 was chosen, to which mass ratios of RP:PDMS of 0, 1:20, and 1:10 were added. The mixture of PDMS and phosphors at the selected weight ratios was accurately weighed into tubes using a KERN EWJ 300-3 balance with a resolution of 0.001 g. Tubes with the weighed mixture were placed in a Multi Bio RS-24 rotary shaker (Biosan). The slowest rotational mode (1 r/min) interspersed with vibrational pauses was used to mix the luminescent mixture. To remove larger luminophore clusters, an ultrasonic bath DSA100-GL1 was used. Most of the processes were carried out in an Indel B TB74 Steel refrigerator at a temperature of approximately 5 • C to delay the thermal crosslinking process of the PDMS. The total homogenization time of this luminescent mixture exceeded 10 h.
After homogenization, the luminescent mixture was deposited on the substrate glass by the spin coating method using spincoater WS-650Mz-23NPPB (Laurell Technologies Corporation); 2.5 g of homogenized luminescent mixture was applied to a 100 × 100 × 2 mm base glass, and then, the glass was placed in a spincoater with this mixture (see Fig. 1). Subsequently, it was rotated for the first time at a speed of 540 r/min for 60 s. Due to this, the luminescent material was spread over a larger part of the substrate glass surface. The parameters of the first rotation were maintained for all samples produced. A second rotation of the substrate glass with the liquid luminescent mixture was then performed, which always lasted 300 s, but its speed varied from sample to sample. The speed of this second rotation is v 2r . This procedure can be seen in Fig. 2.
After the application of the first and second rotations, the substrate glass with the liquid luminescent layer was placed in an electric kiln-type CONCEPT ET5050 at a temperature of about 150 • C for 60 min. The internal temperature of the kiln during curing was measured with a DT-613 thermometer  with a resolution of 0.1 • C. After curing and cooling to room temperature, the next layer could be applied. All the luminescent samples presented in this research were produced by the successive deposition of two layers on a substrate glass.

B. Measuring the Thickness of Luminescent Layers
One of the key parameters of the examined samples was the thickness of the luminescent layer. The measurement of the thickness of the luminescent layers was performed with the WENZEL LH65 instrument, which uses the Metrosoft QUARTIZ R6 software for recording and processing the measurement process. This instrument works on the principle of 3-D surface sensing by touch using a ruby bead placed on the measuring tip. Before the actual measurement, the luminescent layer was cut and a 5-mm-wide strip was removed from each side. This exposed the reference plane of the underlying glass around the perimeter of the luminescent layer. Adjusting the dimensions of the luminescent layer can be seen in Fig. 3 and the examples of fabricated luminescent layers are shown in Fig. 4.
For each sample, the reference plane of the underlying glass was first measured and deposited. The thickness of the luminescent layer was then measured at 25 measurement points evenly spaced over the entire surface of the layer. From the measured values, the instrument software determined the mean value of the layer thickness, with a guaranteed maximum  Tables II and III.

C. Measurement of Colorimetric Parameters
The CCT, CRI, and CIE x,y (1931) color coordinates were monitored as part of the colorimetric measurements. The AvaSpec-HS2048XL-EVO spectrometer (Avantes B.V.) using AvaSoft 8 software was used to measure these quantities. A schematic of the used measurement setup is shown in Fig. 5. The investigated luminescent layers 3) were excited by a blue LED type PM2B-3LBS-SD (PM2B) placed in a plug-in module 2), which was connected to a VLC prototype 1) with built-in bias tee and an adjustable current source. The measurements were performed at an excitation current of 500 mA. The light generated by the luminescence layer was collected at a distance of 1 m using an multimode (MM) optical fiber 5) of type M45L02-Ø400 μm, 0.50 NA, subminiature assembly (SMA)-SMA Fiber Patch Cable (Thorlabs), which was fed into the spectrometer 6). In order to ensure stable light conditions at the point of light incidence on the SMA connector of the optical fiber, a paper "tunnel" 4) with dimensions of 100 × 100 × 1000 mm was used, with its inner walls having a matte black coating. The spectrometer 6) was connected via USB cable 7) to a PC 8) with Avasoft 8 software installed, where the measurements were processed. Fig. 6 shows a color diagram of Commission Internationale de l'Eclairage (CIE) 1931 (x, y) with Illuminant E, which is a reference source with a uniform power spectral density distribution. In addition to the broader white light area (thin gray line) [30], a narrower white light area (thicker line) is indicated for the white light emitted by the vehicle [31]. The process of luminescent layer production was directed so that after the optimization of the luminescent layer parameters, the generated light would fall within this narrowly defined region and in the vicinity of Illuminant E, which is the point of spectral equilibrium in the CIE 1931 color diagram. Indeed, light with a location in the vicinity of Illuminant E is known to have CCT values corresponding to daylight white light and high CRI values [8], [9], [32], [33]. These are therefore desirable colorimetric parameters suitable even for demanding lighting applications.
The location in the CIE 1931 color chart is given by the CIE x,y (1931) color coordinates, which are determined from the tristimulus values of X, Y , and Z using the  known equations The AvaSpec-HS2048XL-EVO spectrometer used with Ava-Soft 8 software contains algorithms that not only determine the CIE x,y (1931) color coordinates but also the CCT and CRI quantities.

D. Measurement of Communication Parameters
Measurements of communication parameters were performed for 12 luminescence samples that were most optimized with respect to colorimetric parameters. The measurement setup is presented in Fig. 7.
Components 1-4 are identical to the colorimetric measurement setup. Other components used are given as follows: 1) photodetector-type PDA10A2 (Thorlabs company) with mounted aspherical condenser (5); 2) coaxial cables with SMA connectors (6); 3) capacitive isolator containing 220 nF capacitor (7); 4) USRP-2950R (National Instruments) with Ettus LF Daughterboard 0-30-MHz Rx and Tx (8); 5) amplifier MHz-1000MHZ 35DB 3W HF VHF UHF FM Tx broadband RF power amplifier (9); 6) CPX200D laboratory power supply (AIM-TTI) (10); 7) fan used to cool the amplifier (11); 8) PC with LabVIEW application for QAM modulation and analysis of light communication parameters (12). An essential part of this measurement setup was the USRP-2950R, for which the LabVIEW application was created. A highly robust configuration of input parameters was used for the communication measurements since the influence of the luminescence layer parameters on the communication chain was primarily investigated. The monitored communication parameters included MER, EVM, phase error, BER, and normalized signal-to-noise ratio E b /N 0 .
It is known that the communication characteristics of the light generated by the luminescent layer are closely related to the luminescence decay time of the luminophores used. This is due to the fact that the decay time of luminescence affects the rising and falling edges of the transmitted signals in VLC communication [13], [34]. The decay process of the luminescence intensity I (t) after the excitation is terminated at time t = 0 is generally represented by an exponential function where τ is the decay time constant of the emission. The luminescence layers presented in this publication contain YP and RP whose decay time constants are approximately in the range from τ YP ∼ 10 −7 to 10 −8 s and τ RP ∼ 10 −5 to 10 −6 s [35]. Thus, luminescent layers composed of RP and YP could be expected to affect the communication parameters more strongly than luminescent layers containing only YP.

A. Measurements for Optimization of Luminescent Layers With Respect to Colorimetric Parameters
The optimization process for the production of luminescence samples was based on the colorimetric properties of the produced light. The aim of this optimization process was to ensure that the light generated by the luminescent layers meets the requirements of EN 12464-1 [20] with respect to the CRI = 80, which is the minimum required value for permanent work locations. At the same time, according to this standard, a CCT in the range of 4000-6500 K is required for several types of workplaces. In doing so, it is known that within the CIE 1931 color chart, light with the required CRI and CCT values is in the vicinity of Illuminant E [8], [9], [32], [33]. Therefore, the optimization process for the production of luminescence samples was conducted in such a way that the CIE x,y 1931 color coordinates of the light generated by the luminescence layers were as close to Illuminant E as possible. This process was carried out for three phosphor and PDMS mass ratios, which are further specified in this article. It is known that for a given mass ratio of phosphor and PDMS, the resulting colorimetric quantities are primarily influenced by the thickness of the luminescent layer, which can be controlled by the rotation speed during the deposition of the layer. The whole optimization process is shown in Fig. 8.
First, based on previous experiments, a default second rotation speed of v 2r = 680 r/min was selected at which samples 1-3 were fabricated. Subsequently, the colorimetric parameters CCT, CRI, CIE x,y 1931 color coordinates, and layer thickness were measured and recorded for these samples. After entering the measured color coordinates into the CIE 1931 color chart, it was found that the locations of samples 1-3 in the color chart were relatively far from Illuminant E and had a low blue content. This was because samples 1-3 had such a thickness of the luminescence layer that most of the blue excitation light was converted to the broadband region in the range of about 500-800 nm, and only a small portion of the blue light passed through the luminescence layer and was not converted there. Therefore, a first optimization step was performed in which the rotation speed v 2r was increased to reduce the thickness of the luminescence layer and to increase the proportion of the blue component of the generated light. As a result, the light locations of the other samples produced shifted in the color diagram toward the left of Illuminant E. The gradual increase in the rotational velocity v 2r was applied to the samples with the three chosen mass ratios of phosphor and PDMS. After each increase in v 2r rate, a colorimetric measurement was performed and the thickness of the luminescence layer was recorded. Based on the measurement results and the color coordinates displayed in the color chart, the next increase in v 2r was estimated. This optimization step (setting the v 2r value, measuring the colorimetric parameters of the new sample and setting the new v 2r value) was repeated several times. The flow of the optimization process with respect to the color coordinates is shown in the color diagram in Fig. 9. At the same time, the values of the quantities measured during this process are recorded in Tables II and III. By comparing  Tables II and III and Fig. 9, it can be seen that the thickness of the deposited luminescent layer decreases with increasing v 2r . This affects the change of CCT, CRI, and color coordinate parameters. Due to the carried out optimization process, it was achieved that the color coordinates of the samples gradually approached the desired Illuminant E point (except for samples 17, 19, and 21, when they already started to move away from Illuminant E). Samples marked in yellow have a mass ratio of 1:2 (YP: PDMS), samples with mass ratios of 1:2 (YP: PDMS) and 1:20 (RP: PDMS) are marked purple, and the red color refers to samples with weight ratios of 1:2 (YP: PDMS) and 1:10 (RP: PDMS). Samples marked with the same symbol were produced with the same second rotation speed, while the first rotation parameters were identical for all samples 1-21. The graph in Fig. 9 shows the continuous adjustments to the second rotation speed, and the location of the light generated by the luminescent layers was gradually brought closer to Illuminant E. Adjustments to the manufacturing process, therefore, led to a gradual optimization of the location of the light in the CIE 1931 color diagram.
In addition to the location of the light in the CIE 1931 color chart (x, y), its important luminous characteristics include the correlated color temperature CCT and the CRI. The measured values of these variables for samples 1-9 and 10-21 are recorded in Tables II and III. For a general overview, their color coordinates (x, y, z), excitation current I , luminophore mass ratios, total thickness h of the luminescent layers, and second rotation velocities v 2r are also given during their production. The values of the correlated color temperature CCT, given in Table III, lie approximately in  the range 4000-6200 K. It is therefore the shade of daylight white. This table also shows that samples with nonzero RP phosphor content have a CRI significantly higher than 80 and for some samples even exceed 90. It is therefore a light with high-quality light parameters. For better illustration, Figs. 10 and 11 show the plots of the dependence of CCT and CRI on layer thickness h.
Since the excitation LED was powered by a stabilized current source, there was no fluctuation in light intensity, and therefore, the colorimetric parameters were highly stable during the measurement. The recorded sets of colorimetric parameters revealed that the maximum measurement inaccuracy is given as follows: for CCT measurements, it is in the order of units of kelvin; for CRI measurements, it is less than 0.2; and for CIE x,y color coordinate measurements, it is less than 0.0006.
In addition, the spectra of light generated by the luminescent layers in the wavelength range of 400-800 nm were recorded For a clearer comparison of the individual spectra, the measured values were converted to a uniform integration time. The colored graphic symbols indicating each sample are identical to those used in the graph in Fig. 9. Comparing the plots in Fig. 12 (left and right), it can be seen that the spectra of samples 10-21 generally have higher light intensities than those of samples 1-9. This is because the luminescence layers of samples 10-21 were deposited at higher speeds and thus have smaller thicknesses, as can be verified in Tables II and III. The thickness of the luminescent layers is then mainly reflected in the intensity of the blue light component, which is not absorbed by the sample. From the results of the following measurements, it can be concluded that the magnitude of the intensity of the unconverted blue light component significantly affects the communication parameters of the light generated by the luminescent layers.

B. Measurement of Communication Parameters of Light Generated by a Luminescent Layer
Before the communication parameters of the light generated by the luminescent layers were measured, an initial reference measurement was carried out with the excitation module itself containing a blue PM2B LED. The excitation current of the excitation LED was set to 500 mA. To obtain reference values, the measurement setup shown in Fig. 7  Subsequent measurements with luminescence samples 10-21 showed a deterioration of all these parameters relative to the reference values, with an increase in EVM and the phase error and a decrease in MER. The measurement setup shown in Fig. 7 was used again. The dc component of the excitation current of LED type PM2B was set to 500 mA. The LabVIEW application was used for the measurements and its optional parameters were set as in the case of the     In relation to the reference values given in Table IV, the values are EVM, and the phase error is added to them, whereas MER is subtracted from them. If we wanted to optimize these communication parameters, then we would need to achieve the lowest possible EVM and phase error values, while the optimal MER value is as high as possible. It follows that the optimum values are the minimum values of EVM, phase error, and MER.
The LabVIEW application used in these measurements also allows the recording of a constellation diagram. Fig. 13 on the right shows that the largest difference in EVM values is between sample 17 at a carrier frequency of 3 MHz and sample 14 at a carrier frequency of 4 MHz. An example of recording constellation diagrams of these samples at given carrier frequencies is shown in Fig. 16. By comparing them, it can be found that in the case of sample 14, the symbol points in the constellation diagram have a larger scatter.
With the unchanged measurement setup (according to Fig. 7), the LabVIEW application was then used, with which the BER and the normalized signal-to-noise ratio E b /N 0 were measured, with 50 values measured for each sample. In doing so, it was found that the measured BER values do not follow a Gaussian distribution, and therefore, the graphically recorded BER values were determined as the geometric mean of the measured values. The results from the measurements are shown in Fig. 17. For comparison, reference values measured for the excitation LED alone without embedded luminescence samples are also included.
From Fig. 17 on the left, it can be seen that under the used measurement setup and LabVIEW application with a      discerned. It is also clear from Fig. 17 that the excitation LED itself achieves a lower BER and a higher E b /N 0 ratio. This is because in this case, the process of interaction of excitation light with the phosphor is eliminated, and therefore, light converted by the phosphor whose temporal intensity is affected by the decay time of luminescence is not generated [13], [34].  [18][19][20][21][22] show the dependence of the EVM, MER, phase error, BER, and E b /N 0 on the thickness of the luminescence layer h at a carrier frequency of 4 MHz. For presented samples 10-21, the thickness of the luminescence layers ranged from 58 to 73 μm. Therefore, this very small change in thickness caused a subtle but observable change in the communication parameters EVM, MER, phase error, and E b /N 0 . As mentioned above, in the case of BER measurements, this small change in the thickness of the luminescent layer did not reveal a clear effect on the BER values for a given experimental setup. Besides, Tables V-VII show that the largest standard deviation was measured in the case of BER.
Individual mean values and sample standard deviations for 2-, 3-, and 4-MHz carrier frequency measurements can be seen in Tables V-VII, respectively. The recorded results represent the arithmetic mean of thousands of measured values. The platform was configured according to the most robust scenario, which included a 500-kHz channel width in combination with 4-QAM modulation format.

IV. CONCLUSION AND DISCUSSION
Section II describes in detail the process of producing luminescent layers for selected weight ratios of phosphors and PDMS. The luminescent layers were deposited by a rotary method on the substrate glasses using spin coater WS-650Mz-23NPPB. A strong correlation between rotational speed and coating thickness was found, which was used in the optimization process. The layer thickness was measured using a calibrated WENZEL LH65 measuring instrument with a maximum guaranteed measurement deviation of 2 μm. The thicknesses of the most optimized luminescence samples 16-21 (66-58 μm) varied in the order of units of micrometers, and therefore, the guaranteed error of the measuring instrument caused a nonnegligible error in their thickness measurement. This error caused that the measured light and communication parameters can be assigned not only to a selected luminescent layer of a given thickness but also to a luminescent layer whose thickness differs from this value up to a guaranteed deviation of 2 μm. This means that for a selected luminescent layer with defined parameters (thickness, mass ratio of phosphor, and PDMS), the possibility of the unambiguous assignment of light and communication parameters of the light generated by this layer is slightly limited. However, considering that the guaranteed inaccuracy of the luminescent layer thickness measurement is a maximum of 2 μm, it can be assumed that for common user applications (lighting and VLC), this accuracy will be quite sufficient.
In the experimental setup used for measuring the colorimetric and communication parameters (see Figs. 7 and 9), there is a free space filled with air between the excitation LED and the luminescent layer (deposited on the glass substrate), while the glass substrate has the shape of a planar layer with a thickness of 2 mm. In the measured wavelength range (ca. 400-800 nm), the refractive indices of air, glass substrate, and PDMS are approximately n air = 1, n glass = 1.53, and n pdms = 1.41, respectively. Different values of these refractive indices can lead to multiple reflections and resonances of some specific wavelengths of light. This can cause some error signals in the measurements. To minimize this measurement error, in future research, we plan to fill the free space between the excitation LED and the glass substrate-the used material must be transparent and index-matched to the glass substrate.
Measurements for optimizing the luminescent layers with respect to colorimetric parameters were performed using samples 1-21, which were successively produced and whose production parameters were adjusted according to the colorimetric measurements of the previous samples. The essential criterion was the location of the samples in the CIE 1931 color chart (x, y). The aim of the optimization was to achieve a location in the vicinity of Illuminant E that guarantees white light with high-quality lighting parameters. Samples 10-21 were particularly close to this location, with sample 18 being the closest approximation. All measured colorimetric parameters for samples 1-21 are presented in Tables II and III. These tables also show the mass ratios of phosphors and PDMS, the thicknesses of the luminescent layers, and the second rotation rates at their deposition. The range of correlated color temperatures for samples [10][11][12][13][14][15][16][17][18][19][20][21]  In addition, the spectra of luminescence patterns of 1-21 excited PM2B-type LEDs were investigated at a constant excitation current of 500 mA. Three characteristic spectral responses were found to be closely related to the mass ratios of phosphors and PDMS. By analyzing the measured spectra, it was found that the thickness of the luminescent layer affects the ratios of blue narrowband unconverted light and broadband converted light. This ratio has a significant effect on the CCT and CRI values. Samples with thinner luminescent layers generally transmit a larger fraction of blue unconverted light, which is advantageous in terms of communication parameters.
For the measurement of the communication parameters, the measurement setup as shown in Fig. 7 was used. Considering the results of preliminary experimental measurements, 4-QAM modulation was chosen for the measurement of communication parameters, and the settings of 4-QAM modulation parameters for both LabVIEW applications were identical. The excitation current of excitation LED type PM2B was set to 500 mA. Measurements of the communication parameters were performed for samples 10-21, which had already undergone a previous process of colorimetric parameter optimization. The graphs in Figs. 13-15 on the left show the deviations from the reference values EVM, MER, and phase error due to the effect of these samples. The analysis of the graphs showed that the smallest error variations were caused by luminescent samples containing only the YP. This is because these samples have a shorter luminescence decay time, i.e., τ YP < τ RP . At a carrier frequency of 3 MHz, the smallest EVM and MER deviations were measured for samples 16 and 17. The smallest value of phase error was measured for samples 10 and 11 at a carrier frequency of 4 MHz. The plots in Figs. 13-15 further showed that as the mass ratio of the RP and PDMS phosphors increases, the error deviations from the reference values increase, and hence, the communication parameters of the light generated by these samples deteriorate. Figs. 13-15 also show that the increasing thickness of the luminescence layer has a similar effect on the increase of the EVM, MER, and phase error deviations, which again causes a deterioration of the communication parameters. The BER error rate and normalized signal-tonoise ratio E b /N 0 measurements were performed using the LabVIEW application. The settings of the QAM modulation parameters were identical to the previous measurements. The plot in Fig. 17 (right) showed that the E b /N 0 value is affected by the luminescence layer parameters in a similar way to the deviations EVM, MER, and phase. In contrast, Fig. 17 (left) shows the measurement setup of Fig. 7 using the LabVIEW application. For a given setting of QAM modulation parameters, there was no clear effect of the luminescence layer parameters on the magnitude of the BER error rate.
The measurement of communication parameters was influenced by the chosen photodetector PDA10A2, whose peak wavelength is λ p = 730 nm and thus has a maximum sensitivity for the red spectral region, which is a typical characteristic of Si photodetectors. The spectra of samples 10-21 recorded in Fig. 12 (right) show that the red spectral region was mostly affected by the light generated by samples 14, 15, 20, and 21. Thus, these were the samples with the highest RP:PDMS = 1:10 mass ratio used. For these samples, the most pronounced effect was the relatively large value of the emission decay time constant τ RP , and at the same time, its effect was amplified by the photodetector used. It can be assumed that if a photodetector with a maximum sensitivity shifted closer to the blue spectral region was used, then smaller error deviations EVM, MER, and phase error would be measured; therefore, the system would show better communication parameters.
Measurements of the effect of luminescent layer parameters on the light and communication properties revealed that it is not trivial to produce a luminescent layer that generates light with good illumination and communication parameters. In the case of samples 1-21, it was observed that there are two basic parameters that affect the light and communication properties. These basic parameters are the thickness of the luminescent layer and the mass ratio of phosphor to PDMS. The mass ratios of phosphor and PDMS fundamentally affect the region of the color diagram in which samples with a given mass ratio can occur. Measurements have shown that luminescent samples with the same mass ratio lie approximately in a line, with the thickness of the layer affecting the "shift" along this line to the right or left. Thus, in other words, the mass ratios of phosphor and PDMS affect the "coarse" alignment of the area line (the achievable color coordinates) within the color diagram. Also, changing the thickness of the luminescent layer allows for "fine" tuning within that area line. The measurements further revealed that changing the thickness of the luminescent layer in the order of units to tens of micrometers changes the communication parameters only slightly (see Figs. 18-22), while the same thickness change alters the colorimetric parameters significantly, as shown in the graphs in Figs. 9-11. If we want to influence the communication properties in a significant way, there are two possible approaches based on the presented measurement results. In the case of using the layer thickness parameter, the changes would have to be in the order of higher tens to lower hundreds of micrometers. In the case of using the mass ratio parameter of phosphor and PDMS, a change of units of percent of this mass ratio would be sufficient to significantly change the communication parameters (see the plots in Figs. 13-15). This research has also shown that samples containing only YP have the best communication parameters, and unfortunately, their colorimetric parameters are significantly worse (e.g., a lower CRI value) in comparison to other tested samples. On the other hand, samples containing a combination of YP and RP have better colorimetric parameters (e.g., a higher CRI value) but at the same time worse The presented research has shown the necessity and importance of detailed measurements of the influence of luminescent layer parameters on illumination and communication properties. It can be assumed that the presented optimization process can find universal application for various applications in the field of production of white light sources and WLEDs for lighting and VLC. In subsequent research, we want to focus on the fabrication of small-area luminescent layers that would be optimized in a similar way and that could be directly placed on the blue LED chip. This would enable the production of highly optimized WLEDs that would have the desired parameters for specific consumer applications. His research is specializing in visible light communication systems, wireless communication platforms, and advanced signal processing.
Rene Jaros was born in Ostrava, Czech Republic, in 1992. He received the bachelor's degree and the master's degree in biomedical engineering from the Department of Cybernetics and Biomedical Engineering, VSB-Technical University of Ostrava, Ostrava, Czechia, in 2015 and 2017, respectively, and the Ph.D. degree in technical cybernetics from the VSB-Technical University of Ostrava, in 2019.
His research interests are focused on hybrid methods for advanced signal processing and mainly on fetal electrocardiography (fECG) and fetal phonocardiography (fPCG) signal processing.
Radek Martinek (Senior Member, IEEE) is currently a Full Professor of cybernetics at the Faculty of Electrical Engineering and Computer Science, Technical University of Ostrava, Ostrava, Czechia. He is also the Vice-Dean of Science and Research and the Deputy Head of the Department of Cybernetics and Biomedical Engineering, Technical University of Ostrava. The main priority of his research activities is high applicability of results and deployment of novel experimental algorithms in the field of cybernetics and biomedical engineering. He is the author of more than 300 publications with over 2000 citations and an H-index of 21. He holds ten Czech national patents and is a leader or co-leader of dozens of projects. His research is mainly focused on hybrid and bio-inspired methods for advanced signal processing. His research activities closely correlate with pedagogical practice.