Daily Light Integral (DLI) Mapping Challenges in a Central European Country (Slovakia)

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It was demonstrated how customized Daily Light Integral (DLI) maps with 1 mol·m⁻²·d⁻¹, 2 mol·m⁻²·d⁻¹ and 5 mol·m⁻²·d⁻¹ can support optimized lighting strategies in both greenhouse and open-field crop production. These maps help plan supplemental lighting, shading, and efficient use of photosynthetically active radiation (PAR) throughout the year. The results provide a practical tool for improving light management in horticultural and agricultural systems.

Abstract

The role of customized DLI maps in optimizing lighting strategies for controlled and open field crop production is gradually increasing, resulting in the creation of specialized DLI maps for more countries. Daily Light Integral (DLI) [mol·m⁻²·d⁻¹] is an accumulation or integration of quantum flux measurements per second over one day (24 h), its spatial distribution will be visualized on maps. Our research objectives are: (1) to create 1 mol·m⁻²·d⁻¹ resolution Slovakia DLI map and explore the seasonal and regional characteristics, (2) to create 2 and 5 mol·m⁻²·d⁻¹ resolution DLI maps to show how the spatial resolution capabilities change in a local (country) and regional (Europe) context, (3) to summarize and compare the seasonal patterns for mountainous and lowland areas with characteristic DLI values (minimum, maximum, average, range). The current study shows how much light was available at different times of the year using monthly DLI threshold maps for 1 mol·m⁻²·d⁻¹, 2 mol·m⁻²·d⁻¹, and 5 mol·m⁻²·d⁻¹. The data present a clear seasonal and regional pattern. In the seasons, the monthly total DLI maximum and minimum differences reached: 21 DLI units (38–17 mol·m⁻²·d⁻¹) in spring, 17 DLI units (46–17 mol·m⁻²·d⁻¹) in summer, 20 DLI units (26–6 mol·m⁻²·d⁻¹) in autumn, 9 DLI units (13–4 mol·m⁻²·d⁻¹) in winter. Slovakia is an East–West oriented country, which explains the use of the 1 mol·m⁻²·d⁻¹ DLI map. DLI maps are of particular importance for plant cultivation technologies that are sensitive to the amount of light and its temporal and spatial distribution, such as greenhouse vegetables or certain fruit species. Spatial DLI data support lighting strategy and design, supplemented by lighting, shading management, and photosynthetically active radiation (PAR) availability and efficient use.

1. Introduction

Light is a fundamental environmental factor required for plant growth and development, functioning not only as an energy source but also as a regulatory signal [1,2,3,4]. Phototropic responses direct plant growth toward the light source, a phenomenon governed by specific photoreceptors sensitive to light quality and direction [5,6,7,8]. Only a certain part of the radiation spectrum (qualitative) can be utilized by plants; at the same time, it is equally important how much photon flux (quantitative) reaches the surface of the plant’s leaves [9]. Qualitative components are characterized by spectral ranges such as photosynthetically active radiation (PAR 400–700 nm), extended photosynthetically active radiation (ePAR 400–750 nm) or photobiologically active radiation (PBAR 280–800 nm) [10,11,12,13] and dedicated wavelengths influencing other photobiological mechanisms, such as photosynthesis, photomorphogenesis, phototropism, stress responses, and metabolic processes [14]. The quantitative parameters are described by photosynthetic photon flux (PPF) [µmol s⁻¹], photosynthetic photon flux density (PPFD) [µmol m⁻² s⁻¹], or daily light integral (DLI) [mol·m⁻²·d⁻¹]. The photosynthetic photon flux (PPF) [µmol s⁻¹] is the total number of photons in the photosynthetically active radiation (PAR) range (400–700 nm) emitted by a light source per second. The photosynthetic photon flux density (PPFD) [µmol m⁻² s⁻¹] defines the number of photons in the PAR range that actually arrive at a specific surface area per second [10,15]. Optimal light intensity, photoperiod, and spectral composition can enhance cellular functions and metabolic activity in crops [16,17,18]. Each plant species exhibits a specific light intensity threshold that maximizes photosynthetic efficiency and biomass accumulation [19,20].

Daily Light Integral (DLI) [mol·m⁻²·d⁻¹] is an accumulation or integration of quantum flux measurements per second over 1 day (24 h). The standard unit for DLI is expressed in mol·m⁻²·d⁻¹, and it is calculated using the following formula: DLI (photosynthetic photon flux density) = (μmol·m⁻²·s⁻¹) × photoperiod (h·d⁻¹) × 3600 (s·h⁻¹) × 10⁻⁶ [21,22,23,24,25,26]. Using the search terms (‘daily light integral’ and ‘map’), entered into the Web of Science platform, the search returned a total of 10 publications (as of 11 November 2025). For some countries, DLI maps exist in the USA [22], China [27], Hungary, and Spain [25,26]. Methodologically, DLI maps are based on the concepts of Faust and Logan [22], Blonquist and Bugbee [28], who suggested that 45% of the total solar radiation falls within the PAR range, and they used a conversion factor of 4.484 μmol·m⁻²·s⁻¹. This concept is widely adopted by the scientific community and used in DLI mapping.

DLI maps represent crucial information layers in precision farming. A country-scale DLI map could be useful for understanding regional and seasonal light distribution. DLI maps are not commonly and widely used because of their limited availability and accessibility. Several researchers emphasized the importance of customized DLI maps to optimize lighting strategies in controlled and open-field crop production [21,22,26,29,30,31,32].

Factors that fundamentally influence DLI values include geolocation, seasonal variation, weather conditions, and topographic diversity. Slovakia, located in the continental temperate climate zone [33], exhibits considerable climatic variability shaped by these factors. Average annual temperatures range from 5 to 8.5 °C, with July being the warmest month (average 25 °C) and January the coldest (average 1 °C), resulting in an annual temperature amplitude of approximately 24 °C. The country receives an average annual precipitation of about 648 mm, with the highest rainfall occurring during June–August (approximately 213 mm in total) [34]. July is typically the wettest month, averaging 77 mm of precipitation. Annual sunshine duration ranges from 1600 to 2000 h, with the Southern lowlands receiving the most solar radiation. The Southern lowlands represent the most fertile agricultural zones, benefiting from higher solar radiation, longer growing seasons, and elevated Daily Light Integral (DLI) values [35]. Therefore, developing a detailed DLI map for Slovakia is a crucial step toward agricultural practices to support sustainable lighting strategy, resource efficiency, and climate-resilience. By accurately assessing the spatial and seasonal variability of light availability, farmers and growing consultants can make profound decisions about crop selection, supplemental lighting or shading, and production management.

Present research and studies show that there is an existing gap in DLI mapping for European countries; no publications related to this topic have been found for Slovakia. As one of the smallest countries in Eastern Europe, Slovakia serves as a prototype area for testing different DLI mapping scales and demonstrating their usability. Our research objectives are:

(i)

to create a 1 mol·m⁻²·d⁻¹ resolution Slovakia DLI map and explore the seasonal and regional characteristics,

(ii)

to create 2 and 5 mol·m⁻²·d⁻¹ resolution DLI maps to show how the spatial resolution capabilities change in a local (country) and regional (Europe) context,

(iii)

to summarize and compare the seasonal patterns for mountainous and lowland areas with characteristic DLI values (minimum, maximum, average, range).

2. Materials and Methods

2.1. Study Area

Located in Central Europe, Slovakia—total area 49,035 km², latitudes between ~47°44′ N and 49°36′ N, and longitudes between ~16°51′ E and 22°34′ E—has diverse geographical zones from the high-altitude Tatra Mountains in the North to fertile lowlands along the Danube River in the South, presenting complex topographic and climatic gradients that demand detailed light climate assessment. Approximately 50.2% of the country’s territory consists of woodland and scrubland, 24.7% of croplands, 17.9% of grassland, 4.1% of water areas and wetland, and 3.1% of artificial land [36]. Lowlands are concentrated mainly in the Southern and Southwestern regions, such as the Danube Lowland (area: 14,993 km², altitude ranges: 100–350 m), the Eastern Slovak Lowland (area: 6752 km², altitude ranges: 150–200 m), and the Záhorie Lowland (area: 570 km², altitude ranges: 100–300 m). These lowland regions stand in contrast to Slovakia’s mountainous highlands, shaping the country’s agricultural capacity and climate variation [37] (Figure 1).

2.2. Spatial Sampling, Data Acquisition, and Automation

A 30 m resolution grid was used for spatial sampling, generated from 1 arcsec global digital elevation data, which was derived from the Shuttle Radar Topography Mission (SRTM) for Slovakia [38]. The coordinates were saved (CSV format) for further processing, and these sampling points were referenced by the WGS 84 horizontal datum and the EGM96 geoid for vertical referencing. Because of the large amount of spatial data, manual access to a country-level DLI map is not available automatically.

To increase the efficiency of data retrieval, an automated script-based pipeline was implemented. This was developed using PHP (version 8.2). To send API calls and to retrieve DLI values to each grid point, the Sun Tracker remote server was remotely integrated [39]. This was made with Linux Mint 21 on an Ubuntu server 20.4 LTS hosted by Oracle Virtual Box. The acquired values were in JSON format, which were converted and stored as CSV files for further analysis. The script also included verification to handle missing or malformed data. Such responses were recorded with the “xxxx” string and created a secondary script (repeat.php) in case of running these incomplete entries again. Furthermore, the validation checks ensured that in the final CSV files, there is no incomplete, corrupt, or repetitive data without any loss of values.

DLI values retrieved were georeferenced, processed, and analyzed in QGIS 3.30 [40]. Three separate thematic maps were created from the raw monthly data. Specific thresholds such as 1 mol·m⁻²·d⁻¹, 2 mol·m⁻²·d⁻¹, and 5 mol·m⁻²·d⁻¹ were used to study the monthly DLI distribution in space. DLI data was acquired from the Sun Tracker DLI calculator (Sun Tracker Technologies Ltd.), which is an online website where we can retrieve the cumulative daily DLI values received over a 24 h period. Pyranometric data from 1991 to 2020 were used, following the standards of the World Meteorological Organization (WMO). This system builds hybrid data techniques that are based on the World Meteorological Organization’s (WMO) ground weather stations, as well as NOAA (National Oceanic and Atmospheric Administration) GOES (Geostationary Operational Environmental Satellite) satellite data. The geographic coordinate-based DLI calculations were made by internationally accepted methods using the following formula: DLI_PAR (photosynthetic photon flux density in PAR range (400–700 nm) = (μmol·m⁻²·s⁻¹) × photoperiod (h·d⁻¹) × 3600 (s·h⁻¹) × 10⁻⁶. The incoming PAR photon flux density (μmol·m⁻²·s⁻¹) is calculated by taking 45% of the solar spectrum (300–3000 nm) and multiplying by the factor of 4.484 [22].

3. Results

3.1. High Precision 1 mol·m⁻²·d⁻¹ DLI Map

The ranges of minimum DLI values in the spring months (March, April, and May) are in a moderate range, spanning 11 DLI units. It begins with a minimum value of 17 mol·m⁻²·d⁻¹ in March and 24 mol·m⁻²·d⁻¹ in April, and increases to a minimum value of 28 mol·m⁻²·d⁻¹ in May. The range of maximum DLI values in the same period spans 16 DLI units. It begins with a maximum value of 22 mol·m⁻²·d⁻¹ in March and 37 mol·m⁻²·d⁻¹ in April and increases to a maximum value of 38 mol·m⁻²·d⁻¹ in May. In the spring season, the monthly total DLI minimum and maximum differences reached 21 DLI units.

The ranges of minimum DLI values in the summer months (June, July, and August) are in the second narrowest range, spanning 8 DLI units. It begins with a minimum value of 33 mol·m⁻²·d⁻¹ in June and 29 mol·m⁻²·d⁻¹ in August, and increases to a minimum value of 36 mol·m⁻²·d⁻¹ in July. The range of maximum DLI values in the same period spans 7 DLI units. It begins with a maximum value of 39 mol·m⁻²·d⁻¹ in August and 44 mol·m⁻²·d⁻¹ in June, and increases to a maximum value of 46 mol·m⁻²·d⁻¹ in July. In the summer season, the monthly total DLI minimum and maximum differences reached 17 DLI units.

The ranges of minimum DLI values in the autumn months (September, October, and November) are in the broadest range, spanning 12 DLI units. It begins with a minimum value of 6 mol·m⁻²·d⁻¹ in November and 11 mol·m⁻²·d⁻¹ in October, and increases to a minimum value of 18 mol·m⁻²·d⁻¹ in September. The tendency is showing a shift to the winter months. The range of maximum DLI values in the same period spans 17 DLI units. It begins with a maximum value of 10 mol·m⁻²·d⁻¹ in November and 15 mol·m⁻²·d⁻¹ in October, and increases to a maximum value of 26 mol·m⁻²·d⁻¹ in September. In the autumn season, the monthly total DLI minimum and maximum differences reached 20 DLI units.

Analyzing the seasonal changes in DLI maps, there are some recognizable patterns to observe. The ranges of minimum DLI values in the winter months (December, January, and February) are the narrowest, spanning 5 DLI units. It begins with a minimum value of 4 mol·m⁻²·d⁻¹ in December and January and increases to a minimum value of 9 mol·m⁻²·d⁻¹ in February. The ranges of maximum DLI values in the same period also span 6 DLI units. It begins with a maximum value of 7 mol·m⁻²·d⁻¹ in December and January and increases to a maximum value of 13 mol·m⁻²·d⁻¹ in February. In the winter season, the monthly total DLI minimum and maximum differences reached 9 DLI units (Figure 2).

The DLI ranges show a characteristic pattern. The most variable are the spring months: on 5 March, DLI units; on 13 April, DLI units; and on 10 May, DLI units. Summer months are balanced: on 11 June, DLI units; on 10 July, DLI units; and on 10 August, DLI units. In autumn, a DLI range narrowing is observed, with September having 8 DLI units, October having 4 DLI units, and November having 4 DLI units. The smallest and most balanced DLI months are in the winter months; December has 3 DLI units, January has 3 DLI units, and February has 4 DLI units.

The Carpathian Mountain ridge includes the high peaks of the High and Low Tatras, the Western Carpathians, the Slovak Ore Mountains, and the Eastern Carpathians. The Slovak Ore Mountains border Northern Hungary, known for its mineral resources and caves. The higher parts of the Western Carpathians and Eastern Carpathians are characterized by forested areas and valleys. Generally, DLI tendencies are recognizable in Slovakia as well, with narrow DLI ranges and values in winter, and broader ranges and higher values in summer. Slovakia’s terrain and topography are dominated by mountains, which is a very influential factor and a powerful modifier of spatial and temporal DLI values. In spring, the DLI values are 17–20 mol·m⁻²·d⁻¹ in March 24–30 mol·m⁻²·d⁻¹ in April, and 28–34 mol·m⁻²·d⁻¹ in May. The mountain areas show an approximate DLI range of 17 units, with a peak of 34 mol·m⁻²·d⁻¹ in May and a minimum of 17 mol·m⁻²·d⁻¹ in March. In summer, the DLI values are 33–39 mol·m⁻²·d⁻¹ in June, 36–40 mol·m⁻²·d⁻¹ in July, and 29–35 mol·m⁻²·d⁻¹ in August. In the mountains, a very narrow DLI range of 11 units is observed, with a peak of 40 mol·m⁻²·d⁻¹ in July and a minimum of 29 mol·m⁻²·d⁻¹ in August. In autumn, the DLI values are 18–23 mol·m⁻²·d⁻¹ in September, 11–13 mol·m⁻²·d⁻¹ in October, and 6–8 mol·m⁻²·d⁻¹ in November. The mountainous DLI range in fall is 17 units, with a maximum of 23 mol·m⁻²·d⁻¹ in September and a minimum of 6 mol·m⁻²·d⁻¹ in November. In winter, the DLI values are 4–7 mol·m⁻²·d⁻¹ in December, 4–7 mol·m⁻²·d⁻¹ in January, and 9–12 mol·m⁻²·d⁻¹ in February. In the mountains, a DLI range of 8 units is observed, with a peak of 12 mol·m⁻²·d⁻¹ in February and a minimum of 4 mol·m⁻²·d⁻¹ in December and January. It is worth mentioning that in the mountains, DLI values are typically lower in the summer months (June, July, August), but in the winter months (December, January), DLI values can be relatively higher in the mountains within a narrow 2 DLI units DLI range with 5–7 mol·m⁻²·d⁻¹ lower values. In Summer, the DLI values are generally lower in the mountains, within a 6 DLI unit range, with 33–39 mol·m⁻²·d⁻¹. The winter DLI values are often higher in mountains than in lowlands because higher elevations receive more sunlight due to a thinner atmosphere, cleaner air, and snow reflection. In contrast, valleys frequently experience fog, haze, and cloud cover from temperature inversions, which block and diffuse light, reducing DLI (

Table 1. Summary of seasonal and regional DLI maps with characteristic DLI values (minimum, maximum, average, range).

		Mountains				Lowlands				Total
		Minimum DLI	Maximum DLI	Mean DLI	Range DLI	Minimum DLI	Maximum DLI	Mean DLI	Range DLI	DLI Range (Maxima)	DLI Range (Minima)	DLI Range (Max and Min Differences)
Spring	March	17	20	18.5	3	19	22	20.5	3	2	2	5
	April	24	30	27	6	30	37	33.5	7	7	6	13
	May	28	34	31	6	33	38	35.5	5	4	5	10
Summer	June	33	39	36	6	40	43	41.5	3	4	7	10
	July	36	40	38	4	40	46	43	6	6	4	10
	August	29	35	32	6	34	39	36.5	5	4	5	10
Autumn	September	18	23	20.5	5	22	26	24	4	3	4	8
	October	11	13	12	2	12	15	13.5	3	2	1	4
	November	6	8	7	2	8	10	9	2	2	2	4
Winter	December	4	7	5.5	3	4	5	4.5	1	2	0	1
	February	4	7	5.5	3	5	7	6	2	0	1	3
	January	9	12	10.5	3	11	12	11.5	1	0	2	3

).

The Slovak Lowlands include the Danube Basin (area: 14,993 km²), the Eastern Slovak Lowland (area: 6752 km²), and the Záhorie Lowland (area: 570 km²). The Danube Basin and Záhorie Lowland are geographically close to each other and have similar DLI values, while the Eastern Slovak Lowland is located on the other side of the country. In spring, DLI values in the lowlands range from 19 to 22 mol·m⁻²·d⁻¹ in March 30–37 mol·m⁻²·d⁻¹ in April, and 33–38 mol·m⁻²·d⁻¹ in May. This corresponds to an approximate DLI range of 19 units, with a minimum of 19 mol·m⁻²·d⁻¹ in March and a peak of 38 mol·m⁻²·d⁻¹ in May. In summer, DLI values are 40–43 mol·m⁻²·d⁻¹ in June, 40–46 mol·m⁻²·d⁻¹ in July, and 34–39 mol·m⁻²·d⁻¹ in August. The lowlands exhibit a DLI range of 12 units, with a minimum of 34 mol·m⁻²·d⁻¹ in August and a peak of 46 mol·m⁻²·d⁻¹ in July. In autumn, DLI values are 22–26 mol·m⁻²·d⁻¹ in September 12–15 mol·m⁻²·d⁻¹ in October, and 8–10 mol·m⁻²·d⁻¹ in November. The DLI range in the lowlands is 18 units, with a maximum of 26 mol·m⁻²·d⁻¹ in September and a minimum of 8 mol·m⁻²·d⁻¹ in November. In winter, DLI values are 4–5 mol·m⁻²·d⁻¹ in December 5–7 mol·m⁻²·d⁻¹ in January, and 11–12 mol·m⁻²·d⁻¹ in February. The lowlands show a DLI range of 8 units, with a minimum of 4 mol·m⁻²·d⁻¹ in December and a peak of 12 mol·m⁻²·d⁻¹ in February.

3.2. Comparison of DLI Maps for 1 mol·m⁻²·d⁻¹, 2 mol·m⁻²·d⁻¹ and 5 mol·m⁻²·d⁻¹

The Carpathian Mountain ridge includes the high peaks of the High and Low Tatras, the Western Carpathians, the Slovak Ore Mountains, and the Eastern Carpathians. The Slovak Ore Mountains border Northern Hungary, known for its mineral resources and caves. The spatial distribution and ranges of DLI values are basically defined by spatial-temporal (geolocation) and seasonal conditions (geometry), but they are influenced by other factors. From a meteorological point of view (cloud and precipitation forms, surface reflectance/albedo) and from a topographical point of view (altitude, slope, orientation, horizontal limitations, vegetation volumetric parameters, and human landscape interventions), the DLI values are additionally modified. These effects are accumulated and result in a complex and mosaicked pattern. Slovakia is situated between its Northernmost point at 49°36′48″ N and its Southernmost point at 47°44′21″ N, giving a North–South extent of about 1.87 degrees of latitude. This is an actual distance of approximately 208 km, from west to East, the country extends between 16°50′56″ E and 22°33′53″ E, a difference of about 5.72 degrees of longitude, so the East–West span amounts to nearly 420 km. In general, the North–South orientation has a stronger effect on DLI values than the East–West orientation. This is because the North–South axis influences the daily and seasonal solar path, resulting in more pronounced differences in the duration and angle of incoming radiation. While East–West orientation also contributes to DLI variability, its impact is typically weaker and more dependent on local topography and the specific time of year.

The three scales (1 mol·m⁻²·d⁻¹, 2 mol·m⁻²·d⁻¹, and 5 mol·m⁻²·d⁻¹) are based on the same meteorological field data and spatial sampling strategy, while the DLI resolutions are different. DLI maps with alternative resolutions are useful for different applications; neither one is better nor worse than the other, but each is good for different purposes. Various advantages can be achieved if maps with all three resolutions are created. Although technically feasible, maps above DLI map at 5 mol·m⁻²·d⁻¹ have not become widespread, and we have not found any published studies on this topic, because the visualization would be too homogeneous and uninformative. An appropriately broad scaling range (over 5 DLI increments) is masking the visual diversity of areas, while they typically have different actual DLI values.

Based on the monthly daily light integral (DLI) maps of Slovakia, comparing three resolutions simultaneously provides detailed monthly differences for a better understanding of DLI variability and complexity. The most detailed DLI map with 1 mol·m⁻²·d⁻¹ was calculated for Slovakia, which is the highest resolution map available with the current methodology. Data are based on the World Meteorological Organization’s (WMO) ground weather station measurements as well as NOAA (National Oceanic and Atmospheric Administration) GOES (Geostationary Operational Environmental Satellite) satellite values.

1 mol·m⁻²·d⁻¹ maps enable farmers to make decisions related to light utilization (plant species selection, supplemental lighting, shading, etc.) based on the needs of their crops using monthly DLI maps. Due to the high resolution, the DLI mosaic structure can be used during cultivation. This is particularly important in the Slovak lowlands (Danube Basin, Eastern Slovak Lowland, Záhorie Lowland), where the majority of cultivation takes place. The significance of DLI heterogeneity is reflected in the differences in monthly DLI values, which are highest in the following order: 7 mol·m⁻²·d⁻¹ (April), 6 mol·m⁻²·d⁻¹ (July), 5 mol·m⁻²·d⁻¹ (May, August), 4 mol·m⁻²·d⁻¹ (September), 3 mol·m⁻²·d⁻¹ (March, June, October), 2 mol·m⁻²·d⁻¹ (November, January), 1 mol·m⁻²·d⁻¹ (December, February) (Figure 3).

The raw DLI data behind the DLI maps are the same, but the display resolution varies. However, it is important to interpret the potential of visualization correctly using DLI ranges. For instance, in March, the visualization range is 17–22 DLI (1 mol·m⁻²·d⁻¹ resolution scale), 17–23 DLI (2 mol·m⁻²·d⁻¹ resolution scale), and 15–25 DLI (5 mol·m⁻²·d⁻¹ resolution scale). This difference arises not from the measured values themselves but from the rescaling applied during map adjustment. A DLI map with a resolution of 5 mol·m⁻²·d⁻¹ does not provide deeper spatial information about the distribution of lower resolution values, but if we also prepare maps with resolutions of 2 mol·m⁻²·d⁻¹ and 1 mol·m⁻²·d⁻¹, DLI resolution maps, these distributions can also be explored if necessary. In our case, all three maps have been created, so we can obtain more detailed spatial information monthly with the 1 mol·m⁻²·d⁻¹ and 2 mol·m⁻²·d⁻¹ maps. The Supplementary Materials contain the monthly DLI maps of Slovakia with 1, 2, and 5 mol·m⁻²·d⁻¹. Separate maps are also available for comparison in further studies (Figures S1–S3). For the highest resolution, it is recommended to use the 1 mol·m⁻²·d⁻¹ DLI map.

4. Discussion

The study area experiences four distinct seasons, with a climate predominantly characterized by continental conditions, particularly in the Eastern and Central regions. The Western areas exhibit a slight maritime influence, while alpine effects are observed in the high mountain regions. The Southern part of the country and the neighboring Danubian lowland are warmer and more arid than the more humid Northern regions [41,42]. The determining factors of the country’s climate are its geolocation, altitude above sea level, and exposure, which basically influence DLI values. The Daily Light Integral (DLI) [mol·m⁻²·d⁻¹], as the accumulation or integration of quantum flux measurements per second over one day (24 h), was visualized on DLI maps with spatial and seasonal aspects. For better understanding and a focused overview, a Summary of seasonal and regional DLI maps with characteristic DLI values (minimum, maximum, average, range) was provided. The analysis of monthly DLI values for Slovakia shows clear seasonal and spatial patterns.

One of the major contributions of this study is the development of a 1 mol·m⁻²·d⁻¹ resolution DLI map, as finer-resolution mapping provides new insights into high-resolution spatial and temporal light variability and enables more efficient, location- and region-specific agricultural lighting planning. A 1 mol·m⁻²·d⁻¹ DLI map has not yet been developed for Slovakia, and no related publications are currently available. This advancement also allows for improved temporal and spatial comparisons that support practical applications. The fine-scale mapping reveals microclimatic and topographic influences on light distribution that are not visible in coarser datasets. The new perspective offered by this study is both methodological, demonstrating how high-resolution DLI maps can be generated and what limitations should be considered, and application-oriented, providing tools for more accurate agricultural planning, including crop selection and supplemental lighting strategies. Moreover, this approach establishes a framework applicable to other Central and Eastern European regions. Currently, among the countries in this region, only Hungary has a similarly detailed DLI map available [25]. Slovakia lies North of Hungary and shares around 627 km of border, even though the DLI values show differences because its range varies from 3 to 10 mol·m⁻²·d⁻¹ in winter and 33–46 in the peak summer period from June to August. DLI in Hungary ranges from 4 to 5 mol·m⁻²·d⁻¹ in winter and 46–47 mol·m⁻²·d⁻¹ in summer. So, the availability of DLI in Slovakia in comparison with Hungary is lower [25,26].

All DLI calculations at different scales are based on the same input data sources. The comparison of DLI maps at three resolutions—1, 2, and 5 mol·m⁻²·d⁻¹—demonstrates the advantages of high-resolution mapping. While coarser 5 mol·m⁻²·d⁻¹ maps provide an overview of regional trends, the 1 mol·m⁻²·d⁻¹ maps capture fine-scale heterogeneity critical for local agricultural decision-making, allowing for targeted supplemental lighting, shading, or crop selection. This approach enhances productivity and ensures efficient use of resources. The creation of 5 DLI resolution maps was primarily motivated by the desire to make the results comparable with other DLI maps with the same classification (0–5 mol·m⁻²·d⁻¹, 5–10 mol·m⁻²·d⁻¹, 10–15 mol·m⁻²·d⁻¹, etc.) as this is the most common resolution and classification display [21]. The ideal DLI map resolution depends on the research or practical purpose. Spain’s study underscores that finer-scale increments, such as 2 mol·m⁻²·d⁻¹, better capture intra-regional gradients, also applicable to Slovakia’s topographically complex terrain. The findings give an insight that ultimately, these reinforce the necessity of regionally tailored lighting strategies in Central and Eastern Europe, considering not just DLI magnitudes but also their spatial-temporal stability. These studies validate that Slovakia, particularly its central and Northern parts, is more limited by low solar input in the winter and thus requires artificial lighting systems or light-assisted horticulture for reliable production during off-peak seasons.

According to the European commission’s data sources [36,43], 50.2% of the country’s territory consists of woodland and scrubland, 24.7% of croplands, 17.9% of grassland, 4.1% of water areas and wetland, and 3.1% of artificial land. Totally, 47% of Slovakia’s territory is used for agricultural purposes. Wheat is a major cereal produced in the region, root crops such as potatoes, sugar beet, and maize, and industrial crops such as red clover and pulp are also common in the area [36,44]. Winter has narrow DLI ranges, while spring and summer are more variable, with spring values spanning 17–38 mol·m⁻²·d⁻¹. This reflects transitional weather and changing cloud cover. Summer is relatively stable, and autumn shows gradually narrowing DLI ranges, consistent with mid-latitude continental climate patterns and highlighting periods of potential crop light stress or optimal growth. Topography strongly influences local DLI patterns. Mountains, including the Tatras and Carpathians, have lower summer DLI due to shading and elevation, but relatively higher winter DLI from clearer air and snow reflectance. Valleys may experience reduced winter light from fog and inversions. Lowlands, such as the Danube Basin and Eastern Slovak Lowland, receive higher summer DLI, making them suitable for high-light-demand crops, while shade-tolerant species are better suited to mountains and valleys. The North–South axis influences the daily and seasonal solar path, resulting in more pronounced differences in the duration and angle of incoming radiation. While East–West orientation also contributes to DLI variability, its impact is typically weaker and more dependent on local topography and the specific time of year. In such regions, where the North–South orientation has a stronger effect on DLI values than the East–West orientation, the higher DLI resolution is reasonable and necessary. This way, small DLI differences can also be determined and visualized. This study presents the first 1 mol·m⁻²·d⁻¹ high-resolution DLI map for Slovakia, revealing fine-scale spatial and seasonal light variability and providing a practical framework for precise, region-specific agricultural planning and resource management in Central and Eastern Europe.

The current approach has some limitations, including limited data sources, higher temporal resolution, or real-time data acquisition. DLI resolution can be improved by moving from current single-channel pyranometers (285–2800 nm, W·m⁻²) to high-resolution spectroradiometers (285–2800 nm in 1 nm steps, W·m⁻²·nm⁻¹·sr⁻¹), enabling more precise measurement of spectral light distribution. These limitations can be reduced with future methods that could improve accuracy by integrating spectral measurements, microtopographic information (slope, exposure, orientation), and automated supervision (online monitoring, IoT, and variety-specific plant light requirements). Such advanced approaches would enable more precise and adaptable DLI estimation for agricultural and research applications.

5. Conclusions

According to our first objective. The highest-resolution DLI map with 1 mol·m⁻²·d⁻¹ was created for Slovakia, offering detailed insight into the spatial and temporal variability of photosynthetically active radiation (PAR 400–700 nm). This resolution is necessary for medium-sized countries with an East–West orientation, such as Slovakia. The 1 mol·m⁻²·d⁻¹ maps capture fine-scale heterogeneity critical for local agricultural decision-making, allowing for targeted supplemental lighting, shading, or crop selection. High-resolution scale influencing conditions are characterized by cloud cover, precipitation forms, surface reflectance/albedo, altitude, slope, orientation, horizontal limitations, mountains, vegetation volumetric parameters, and human landscape interventions.

According to our second objective. The results present a clear seasonal and regional pattern. The 1, 2, and 5 mol·m⁻²·d⁻¹ DLI maps are based on the same meteorological data. The 2 and 5 mol·m⁻²·d⁻¹ resolution DLI maps serve different purposes. Coarser maps provide broader trends, which highlight changes in a local (country) and regional (Europe) context. Scale increments such as 2 mol·m⁻²·d⁻¹ are better suited to capture intra-regional gradients, also applicable to Slovakia’s topographically complex terrain. The findings give an insight that ultimately, these reinforce the necessity of regionally tailored lighting strategies in Central and Eastern Europe, considering not just DLI magnitudes but also their spatial-temporal stability. These studies validate that Slovakia, particularly its central and Northern parts, is more limited by low solar input in the winter and thus requires artificial lighting systems or light-assisted horticulture for reliable production during off-peak seasons. The creation of 5 mol·m⁻²·d⁻¹ resolution maps was primarily motivated by the desire to make the results comparable with other DLI maps with the same classification. Maps above 5 mol·m⁻²·d⁻¹ are uncommon, as larger increments mask spatial diversity and make visualization less informative.

According to our third objective. Our research gives a comparative evaluation of Daily Light Integral (DLI) patterns across Slovakia with 1, 2, and 5 mol·m⁻²·d⁻¹ DLI maps, showing a valuable understanding of seasonal and regional variations. The mountainous and lowland areas of Slovakia were compared and summarized with characteristic DLI values (minimum, maximum, average, range) and monthly DLI maps. The differences between monthly maximum and minimum DLI values reached 21 units in spring (38–17 mol·m⁻²·d⁻¹), 17 units in summer (46–29 mol·m⁻²·d⁻¹), 20 units in autumn (26–6 mol·m⁻²·d⁻¹), and 9 units in winter (13–4 mol·m⁻²·d⁻¹). Slovakia has an East–West orientation with a difference of about 420 km, while the North–South extent is approximately 208 km.

Future research could combine these high-resolution maps with crop growth models, assess potential shifts under climate change scenarios, and validate findings with in situ measurements, further enhancing precision agriculture and sustainable land management in Slovakia. An improved assessment of the spatial-seasonal variability in light availability can support decision-making on crop selection, lighting strategies, and enhancing resource efficiency and climate resilience.