In recent years, research on light emitting diodes (LEDs) has highlighted their great potential as a lighting system for plant growth, development and metabolism control. The suitability of LED devices for plant cultivation has turned the technology into a main component in controlled or closed plant-growing environments, experiencing an extremely fast development of horticulture LED metrics. In this context, the present study aims to provide an insight into the current global horticulture LED industry and the present features and potentialities for LEDs’ applications. An updated review of this industry has been integrated through a database compilation of 301 manufacturers and 1473 LED lighting systems for plant growth. The research identifies Europe (40%) and North America (29%) as the main regions for production. Additionally, the current LED luminaires’ lifespans show 10 and 30% losses of light output after 45,000 and 60,000 working hours on average, respectively, while the vast majority of worldwide LED lighting systems present efficacy values ranging from 2 to 3 μmol J−1 (70%). Thus, an update on the status of the horticultural LED sector, LEDs’ applications and metrics, and the intense innovation are described and discussed.
The use of artificial light for plant growth and development purposes has been known for more than a century [1]. However, until recently, horticultural lighting systems were based on the traditional industry and therefore not specifically designed for plant growth applications [2]. Particularly, only in the last few decades lighting technologies, such as fluorescent (FL), high-pressure sodium (HPS), metal halide (MH) and incandescent (INC) lamps, have started to be implemented for plant cultivation and research [3]. Light is the most important source of energy for photosynthesis; therefore, the lighting environment surrounding plant canopy can differently influence plant growth and development [4]. The above-mentioned artificial lighting sources (e.g., FL, HPS, MH and INC lamps) can present several drawbacks, as they are not spectrally optimal for crops nor energetically efficient, furthermore releasing a large amount of radiant heat [3]. The most recent light emitting diodes (LEDs) have replaced conventional lighting technologies in many indoor and protected environments, resulting in great and rapid technological evolution in the horticultural lighting industry [5].
Horticultural LED luminaires represent solutions more environmentally friendly and economically favorable than conventional lighting while having safer management and disposal practices [3]. In contrast with traditional HPS lamps, which convert only 30% of the energy into usable light, with significant radiation losses in the form of heat, LED lighting sources often convert about 50% of the electricity into light, resulting in an economically and energetically better solution [6]. Additionally, due to a non-uniform distribution of solar radiation across world regions, supplemental illumination has been used at higher latitudes in greenhouse crop production, allowing improvements in terms of productivity and quality, and enabling year-round cultivation [7]. In this sense, a comparative study with supplemental light from traditional overhead HPS lighting vs. red and blue LEDs reported that greenhouse tomato growers can get the same yield by using LEDs while consuming only 25% of the energy used for traditional lights [8]. Moreover, greenhouse or indoor cultivation relying on lighting is expected to play an important role in fulfilling the increasing food demand [9]. Renewable energy sources (e.g., wind and solar energy) can also contribute to covering LED energy consumption, therefore improving the economic and environmental sustainability of their application [10].
LEDs are solid-state semiconductor diodes with the capacity to release energy in the form of photons after the application of a proper amount of voltage [11]. The suitability of LED lighting systems for plant-growing applications is based on their potential features (e.g., small size, durability, long lifetime and cool emitting temperature) in combination with the advantages offered by the modularity in wavelength selection and light output, and the elevated energy conversion efficiency [12]. LED application for plant growth was first studied in the 1990s [13,14], when NASA-affiliated researchers performed much of early work as preparation for the development of plant-based regenerative life-support systems for future Moon and Mars bases [15]. However, at that time, only red (660 nm) LEDs were available. Therefore, it could be also asserted that the actual era of LED research started with the introduction of blue LEDs to the market [5]. Today, LED lighting systems have experienced wide evolution in terms of physical shapes and designs, waveband color availability, power use reduction per unit of light output, and cost decrease per unit of light output [2]. The technical development of LEDs is said to follow Haitz’s law [16], which was reformulated in 2011, stating that, every decade, the cost per unit of useful light emitted for a given waveband declines by a factor of 10, while the amount of light generated per LED package increases by a factor of 20 [17]. However, LED lighting applications may ultimately be limited by market forces, as, for instance, some reachable light levels may also be above commercial requirements [2].
During recent years, a large number of lighting companies have entered the horticultural sector in a joint collaboration with plant producers and academics, leading to extraordinary advances in both scientific and commercial application of LED technology for plant cultivation [18]. Meanwhile, scale economy has been significantly driving cost decreases, also thanks to the increasing opportunities to regulate plant growth, development, and concentration of phytonutrients through light spectral control [2]. Thus, the efficacy values of LED lighting systems are expected to continue on a growing trend for several years, which, together with improvements in intrinsic LED features (e.g., durability, longevity, fixture design, emission spectrum, etc.), could lead to increasing applications of LED lighting systems in horticulture [18]. The rapid evolution of the sector is, for instance, represented by lighting manufacturers that release smart LED control systems together with software applications, with which artificial intelligence using sensor feedback can automatically detect plant health issues and adapt the lighting environment for greater plant production efficiency [19,20].
Until recently, however, horticultural LED luminaires were not provided with any published indicator for testing, resulting in a lack of quality standards, ultimately driving confusion on industry performance metrics. Only in 2017, the American Society of Agricultural and Biological Engineers (ASABE) released the first of three expected series of standard tests for horticultural luminaires (S640 standard), establishing the quantities and units used to describe light in relation to plants [21]. Afterward, in 2018, the society focused on specifying the performance of horticultural LED luminaires with the S642 standard: “Recommended methods for measurement and testing of LED products for plant growth and development” [22]. In this context, it is very important to identify the optimal or minimal crop light requirements in protected and indoor cultivation in order to enhance the yield and quality of the produce [23]. A metric characterizing the luminaire, which describes how many photons a light source emits per second (expressed in μmol s−1), is the Photosynthetic Photon Flux (PPF), while the Photosynthetic Photon Flux Density (PPFD) characterizes light installation depending on luminaire position in relation to the illuminated area (in μmol m−2 s−1) [10]. Moreover, in LED lamps for plant growth, the proper electrical efficiency metric is measured in the units of micromoles of photosynthetic photons per joule of energy input (μmol J−1), a further and important parameter when comparing LED luminaires [24].
Due to continuous advances in LED technology and the promising opportunities for lighting regime optimization in controlled or closed plant-growth environments [25], together with the growing LED-related plant research, a continuous update on the status of the horticultural LED industry and its applications is essential [10]. The main scope of this research is to explore and characterize the global distribution of horticultural LED industry. Particularly, the study aims at identifying and categorizing LED lighting system manufacturers on a global scale and the typologies of horticultural LED lighting solutions offered in the market. Furthermore, the current potential of LED luminaires in horticulture through a compilation and evaluation of operational functions and features is addressed.
To achieve the stated objectives, the research builds on a comprehensive database compilation of LED manufacturers and luminaires from the available scientific literature, toward the identification of the main features and functionalities of LED luminaires for horticulture, opening the ground for discussion on the future trends and challenges for the sector. Within the paper, Section 2 presents the material and methods used within the review. Section 3 presents the research results and their discussion. This section is further divided in five subsections. The business growth trend is introduced together with the worldwide distribution of horticultural LED manufacturers. Moreover, LED lighting systems are grouped according to outer surface and classified by considering the largest production areas for each luminaire typology. The electricity aspects of current LED luminaires’ performance are evaluated, and a detailed global analysis of LED luminaires’ lifespans is included. The current efficacy of the global LED lighting industry (building on declared data) is defined. Finally, Section 4 presents the conclusion of this work, highlighting the future prospects in the LED horticultural field.
The research was implemented by the following consecutive steps: (1) definition of the research aim; (2) choice of keywords and database; (3) selection of LED grow-light manufacturing companies; (4) tabulation of the information compiled; (5) analysis and presentation of the results. Consequently, information regarding each of these stages is presented following the same sequence.
For a proper identification of the global LED grow-light manufacturing industry, different keywords were defined, while several online databases were consulted. The keywords used as the basis for manufacturer consultation were “LED horticulture”, “LED grow lights” and “horticultural LED lighting”, also adding the terms “manufacturers” and “companies” to perform a deeper investigation. The search was performed through online sources such as Google and LinkedIn databases. In order to perform a better search on the Google database, each country was investigated by changing the location setting and also translating the keywords into the local language. Databases of conferences, seminars, events, exhibitions or meetings databases related to the fields of LED lighting, horticulture technology, fruit and vegetable market and cannabis industry (e.g., LED Professional Symposium, GreenTech, Macfrut, HortiCann Light+Tech, etc.) were considered. Moreover, keywords were inserted in the reference journal databases for extensive research (Springerlink, Sciencedirect, Scopus and Google Scholar).
A global inventory of 301 LED lighting system manufacturers, ranging from multinational corporations to local companies, was collected. Distributors, wholesalers and retailers were excluded from the study. The global inventory of LED lighting system manufacturers was used to customize a map through the online tool “Google My Maps”, from which the worldwide distribution was developed.
Concerning the analysis of the features and operational functions of LED horticultural solutions, a database compilation based on the manufacturer global inventory was performed. Data collection was performed from March to June 2020 through the available manufacturers’ websites. The data of 1473 LED horticultural luminaires were obtained, representing the lighting solutions of 161 horticultural LED manufacturers. Luminaires’ data were collected by using a template format (spreadsheet), which included the following database entries, allowing an organization of the data in a tabular form: (a) company name, (b) company country, (c) website link, (d) contact email, (e) luminaire model name, (f) luminaire typology, (g) input voltage, (h) input frequency, (i) power use, (j) Photosynthetic Photon Flux (PPF), (k) efficacy, (l) Photosynthetic Photon Flux Density (PPFD), (m) ingress protection, (n) hours of rated life and (o) additional information. Concerning the mentioned entries, in the case of companies presenting branch offices in multiple countries, the headquarters’ location was deemed as representative of the company’s country. Moreover, LED horticultural luminaire typologies were listed according to outer appearance (e.g., rounded LEDs, linear LEDs, panel LEDs and others) in order to further classify them. The corresponding lamp metrics and/or performance values (e.g., input voltage, input frequency, power use, PPF, efficacy, PPFD, ingress protection and hours of rated life) were compiled through their technical data sheets (when available) or website information. Furthermore, some enterprises not reporting data within their website were contacted through email to obtain the corresponding datasheets of their products. If none of the technical data were available, the LED lighting system was excluded from the study. For those cases in which the company provided a range value for any of considered parameters, the highest value was selected as the refer
