LED Lighting Installations in Professional Stadiums: Energy Efficiency, Visual Comfort, and Requirements of 4K TV Broadcast

Nowadays, LED lighting technology reaches a higher Value of Energy Efficiency in Installations (VEEI) (W/m2*100 lux) than current luminaire lighting due to the number of lumens per watt that these are able to generate, as well as the directional nature of their emissions together with the adjustment capability through concentrator lenses with beam graduations that reach 5º. This achieves energy savings of up to 80%. Furthermore, also considering the substantial decrease in flickers, and that it noticeably improves usage of ultra-slow-motion cameras and the fading of switching-on or rearm times upon failure, LED technology stands out as the main solution for illumination of professional sports facilities. This article describes the evolution of regulatory requirements that are being imposed by the governing institutions of sports (FIFA, UEFA, FIBA, etc.) and professional leagues (LaLiga, Euroliga, etc.) in order to guarantee their competence as high-quality television products. In addition, the trends in requirements and specifications regarding lighting equipment and its installation, which are intended to convert stadiums into optimized centers for the celebration and dissemination of mass events, are analyzed (settings, photometry, etc.), particularly those concerning horizontal, vertical, and camera illuminance, average and extreme uniformities, glare, reduction of intrusive light in bleachers, flickers, color rendering index (CRI), correlated color temperature (CCT), and start-up times.


Introduction
In the current context, the celebration and TV transmission of sports competitions have become a way of global entertainment that moves significant sums of money and human masses [1]. There is high competitiveness among multiple sports and professional leagues to concentrate the greatest demand and the most possible economic resources based on large audiences [2] because these would raise prices [3] and revenues from advertising [4]. Of all the elements that foster or increase the perception of the event as a show, this factor makes improvement very important in all possible ways. The last transformation effect in mass sports was generated based on the growing interest of Asian countries [5] in acquiring the image rights to European football competitions [6], and this has created a scenario in which the demand levels for lighting parameters have increased considerably [7] by virtue of their direct influence on the TV image quality [8].
In general, based on the audiences, movement of people, and the money that these generate, football competitions are leading the sports broadcast updating process and its adaptation to the new format and emission requirements of ultra-high definition or 4K [9]. The lighting levels needed to obtain a video image that is as clear and realistic as possible in this format have caused the enclosures' lighting values to rise greatly from the previous conventional or high-definition (HD) (1080 p) formats [10,11]. Thus, the lighting installations of these stadiums are undergoing evaluations as well as rethinking and renovation projects.
Until the appearance of projectors based on LED technology, in a generalized way, the main tools used for the illumination of large installations and sports venues, both indoors and outdoors, were metal halide (MH) lamps [12]. MH use was based on the need to obtain a white light that was far from sodium vapor lamps' color rendering index (CRI), with typical efficiencies of higher than 100 lumens per watt (optimized in lamp models with higher powers), which is higher than those of other light sources, such as fluorescent lamps [13]. In this way, the lighting systems of these enclosures have usually been developed based on the concentration of high-intensity projectors (1, 2, and 4 kW) and the directional concentration of their light flow, both by direct projection and by reflection, to reduce possibly high values of glare [12].
The emergence and development of high-power LED luminaires have allowed, with the same white light generation requirements, the realization of higher efficiencies than with any other lighting technology [13]. Another aspect of these LED luminaires to highlight is the energy savings of up to 70% from direct electricity consumption, the contracted power, and the requirements of the necessary transformation centers. Consequently, its implementation has investment recovery ratios (i.e., payback period) based on low energy savings with average terms of three years [14]. In this work, other technologies, such as OLED [15][16][17] or QLED [18][19][20] were analyzed-technologies with high efficiency values in lm/W [16,21,22]. Since the main development of this technology is currently focused on screens, in this sense, it is a technology that has not reached the necessary maturity to represent a competitive alternative in the lighting of sports stadiums [23,24]. Proof of this is the need to develop standards to help establish trust in the lighting market. The Testing Procedures Committee (TPC) of the IESNA (Illuminating Engineering Society of North America) has not carried out activities for the development of standards for the photometric properties of OLEDs [25].
In addition to mere technical lighting parameters, the solid-state nature of the LEDs allows them to reach 100% of the equipment lighting intensity in less than a second, starting from either a cold or a hot start. Functionally, in cases of power failure, the lighting may be fully active when the electrical power is recovered. High-intensity discharge lamps have a starting time of 3-5 min for MH and up to 10-12 min for cold sodium vapor lamps [17]. However, if the lamps were on, a minimum cooling time is required before they are able to reconnect in order to lower the gases' starting voltage, so they adapt to the generating capacity of their auxiliary equipment (ballasts), which normally takes an additional 1 to 8 min for the latest generation of high-discharge-density lamps [26]. These times are totally unacceptable for the strict timetables set by the television networks that broadcast these events [27]. Throughout this work, the different prevailing normative changes for the adjustment of the stadiums' lighting systems to the new requirements of the television networks are detailed and analyzed. In addition, different processes and practical cases are shown that are being carried out in the assessment, analysis, remodeling/renovation, and adaptation of these facilities' management models using football stadiums related to high-level competitions as a specific control sector.

Lighting Requirements for Football Television Broadcasts
In this section, an analysis of the regulatory framework for high-level sports competitions will be carried out, particularly with respect to the lighting requirements currently demanded. Table 1 details the different current minimum lighting requirements demanded in high-level sports competitions in football by the different competition organizations [28][29][30][31]. The two official football control bodies on the European continent (UEFA and FIFA) and the two football leagues with the greatest global impact, which are in the middle of a commercial war to expand their markets for broadcasting rights around the world-LaLiga (Spain) and the Premier League (England)-are detailed.
One of the most significant analysis parameters is vertical illuminance, which is the value that directly affects TV cameras and sets the level of detail that they can show, especially in close-up shots of the players (field camera) and when capturing details of moving plays on the playing field (fixed camera). In this sense, it is proven that LaLiga is positioned as the sports competitor with the most stringent requirements.
Horizontal illuminance constitutes a measurement of the light that reaches a horizontal plane; in other words, it is the level of lighting that players will perceive, and will, therefore, directly affect their performance. Nowadays, FIFA occupies the first position and sets the highest minimum horizontal average illuminance requirements.
Uniformity indexes are an important part of a complete set of lighting criteria and have a determining effect on the lighting quality of the installations [32]. The directionality of the light emitted by the LEDs facilitates the elimination of losses due to reflection that are produced in conventional luminaires [33]. This is achieved by using lenses to obtain a higher functionality of the concentration degrees of the emitted light with minimal losses. In this way, a correct installation design eliminates shadows as well as dark and "hot" spots on the playing surface without damaging players' and spectators' vision due to glare [28].
High values of illuminance (vertical and horizontal) must be accompanied by proper uniformity; large variations in vertical lighting factors will produce low-quality video images, and horizontal illuminance variation will decrease the perceptual ability of both players and stadium spectators. Table 1 shows how LaLiga and FIFA have established the most restrictive uniformity values: LaLiga regarding horizontal illuminance and FIFA for vertical illuminance.
From a different perspective, an ultra-high-definition (UHD) or 4K television broadcast seeks the highest color quality to optimize the level of detail in its pixels. The unit of measure for this characteristic is the Color Rendering Index (CRI), which represents the ability of an artificial lighting source to reproduce the effect of natural light. This index establishes a measuring range of Ra0 to Ra100, where Ra100 is the illumination given by an incandescent bulb. This light source is used as the established color reference most similar to solar light of all developed technologies. It is to be noted that LaLiga demands a remarkably high CRI, with values equal to or greater than Ra90.
The new television requirements for UHD and 4K broadcasts adapt better to LED technology, since it provides a more controlled emission spectrum, totally eliminating ultraviolet and infrared radiation and reducing color correction and post-production in the mobile transmission unit. In this sense, the use of super-slow cameras requires the minimum possible flickering of light or "flicker" of the installations.
This "flicker" is the impression of instability as a result of the visual sensation induced by a light stimulus in which the luminosity or spectral distribution fluctuates over time [34]. The harmful physiological effects of this micro-flickering in humans are discomfort, distraction, eye fatigue, headaches, and migraines [35,36]. The UEFA requires a parameter for football clubs that does not exceed 5% for elite stadiums of level A (UEFA Champions League, UEFA Euro, and UEFA Europa League finals) [31]. On the other hand, the Premier League regulation requires that clubs have a flickering rate that does not exceed 6%. The new LED system tests have registered a rate of only 0.2% [30]. Given the previous assessments, LaLiga is shown to represent the competition with the highest lighting requirements overall, so in the subsequent sections, it will be the study reference.  [19]; LaLiga [20]; Premier League [21] & UEFA [22]).

Location of TV Cameras in Stadiums
When simulating sports stadiums, camera location represents another aspect to highlight, since small modifications have a significant impact on lighting results. Each sport competition establishes minimum configurations to ensure optimum event retransmission. Two reference cases are detailed in Figure 1: In the case of LaLiga, four types of coverage are established [29]. For its part, FIFA sets minimum requirements for stadiums used for the FIFA World Cup [28]. It is easily appreciated how the current trend is to considerably increase the number of cameras in the venues, with the aim of optimizing event broadcasting and being able to clearly capture any game detail or incident, as well as the direct environment.

Renovation of the Lighting Installations of Football Stadiums in Spain
In this section, the results of the inspections carried out to evaluate the playing field lighting and energy supply in LaLiga stadiums after the adaptation to LaLiga's present regulations will be summarized, in accordance with the "Television Broadcasting Regulation (TBR)" [37] requirements approved by the Board of Directors of the Higher Sports Council (Ministry of Sports of the Government of Spain) [29]. These reports are structured in four chapters: introduction, lighting, power supply, and data collection. Two examples of the performed audits will be summarized, an analysis of the evolution of the lighting installations will be included, and, finally, a review of successful renewal of the installations will be detailed.
The first example describes the audit conducted on 30 June 2016 at the Nuevo Los Cármenes stadium (Granada, Spain). In this stadium, the existing projectors used to light the playing field were installed on four towers located in the stadium corners and on the edge of the west tribune deck  The results obtained are much lower than those required by the TBR, since the minimum values of horizontal and vertical illuminance are not met. Other deficiencies are also present, such as the absence of an instantaneous reignition system or variable color temperature devices, among other shortcomings. The audit concludes that given the stadium's geometry, it would be feasible to install LED projectors to meet the TBR requirements regarding 4K TV broadcasting.
The second example summarizes the audit conducted on 31 May 2016 at the Mendizorrotza Stadium (Vitoria-Gasteiz). In this case, the installation consisted of 146 metal halide projectors of 2000 W located on the edge of the roof in a walkway structure that runs along both sides at the closest point to the vertical of the playing field. The results, as shown in Figure 3, are even worse than in Example 1, with values far from the TBR requirements. The following requirements are not met: horizontal illuminance (U1: 0.37) or vertical illuminance based on cameras, color rendering (Ra65), and correlated color temperature (CCT), which were met neither in uniformity nor in value (4200 K). The recommendations of the audit follow the line of the previous example, including the replacement of the current metal halide luminaires with LED luminaires. After detailing two examples of football stadiums, a review was performed on the evolution of football stadiums from the audit phase until December 2017 (Figure 4), where it was shown that most stadiums had already incorporated LaLiga's recommendations [38].
The lighting systems at the Wanda Metropolitano of Atlético de Madrid and at the Ramón Sánchez-Pizjuán stadium of Sevilla CF ( Figure 5) are worth mentioning because they are references in Spain, as they were the first stadiums in the world to have LED technology in 100% of the sport space [38]. However, the Premier League clubs are considered pioneers in the implementation of lighting with LED technology. These are the Arsenal FC, Chelsea FC, Southampton FC, Hull City FC, and AFC Bournemouth.

Distribution of Television Rights in Spain
The Royal Decree-Law 5/2015 of April 30 regulates the commercialization of audiovisual content rights of professional football competitions in Spain [39]. It is necessary to conceive football as a business and, therefore, as a source of income for public coffers. In 2013, professional football in Spain generated a Gross Added Value (GAV) of more than 7600 million EUR (M€), which represented 0.75% of the Spanish GDP, an income for the state coffers of 2896 M€, and the creation of more than 140,000 full-time jobs [40,41]. In the last report of 2017, this income increased to 1.37% of the Spanish GDP, a tax revenue for the state coffers of 4100 M€, and the creation of 185,000 jobs [35,36].
Television rights represent 40% of the football clubs' income in Spain. In addition, LaLiga occupies the world's second position in terms of competitions that receive the highest income from football match retransmissions, behind the English Premier League. The Spanish competition achieved an increase of 250 million EUR in its national income for the 2019/2020 season, reaching 1140 M€ per season from 2019 to 2022. The bulk of the auction was assumed by the telecommunication company Telefónica. As for the rights for the rest of the world, revenues have increased by 281 M€ per season, reaching a total of 821 M€ for the next five seasons in a contract awarded to Mediapro. It is worth mentioning that there are still four lots of the eight existing lots to be allocated [42][43][44]. The club with the highest income is Barcelona FC (154 M€), followed by Real Madrid CF (148 M€) and Atlético de Madrid (110 M€), with Girona FC and CD Leganés receiving the lowest income (43.3 M€) [45].

Results of Practical Case Studies Performed by the Authors of This Paper
Once the stadiums that must adapt their facilities to the new regulatory requirements (TBR) were identified as a result of the audit performed by LaLiga described in the previous section, the authors of this publication carried out lighting studies that justify compliance with the new requirements, highlighting the Nuevo Los Carmenes, Benito Villamarin, and Sanchez Pizjuan stadiums, respectively.
In the following sections, the main technical difficulties that have been associated with lighting design in high-level sports facilities will be detailed, given that not only players, but also the television cameras that broadcast the event, must have now optimal lighting conditions. In this situation, engineers have been forced to reinvent the guidelines for lighting design.
Depending on the technical specifications required, the usual procedure when designing sports facilities begins with the creation of a three-dimensional model that can be exported in 3D, gbXML, or DIALux Object formats. Subsequently, the 3D model is imported into DIALux, where our lighting study was carried out.

Lighting of Football Fields Where Non-Televised Events Will Take Place
To illuminate stadiums where non-televised matches will be played, vertical illuminance is not relevant, so each luminaire or group of luminaires directs their emissions to the nearest surface to minimize lighting losses and comply with horizontal illuminance. This can be seen in Figure 6, which corresponds to the design of the Haría municipal football field in Las Palmas, an installation with a Class II lighting classification according to the UNE EN 12193:2020 standard.

Televised Events, Second Spanish Division: New Los Cármenes Stadium
In the New Los Cármenes Municipal Stadium, the Granada CF matches are played as a local team. The stadium opened on 16 May 1995 and has a capacity of 22,000 spectators. In Figure 7, an aerial image of the stadium is presented, and it is verified that the luminaires are grouped into four identical towers and two linear groupings on the east and west stands [46]. When high-level stages are illuminated, vertical illuminance becomes the main requirement to be met, and it is a parameter that will be measured based on all the cameras in the stadium. In this case, it will not be enough to illuminate the area closest to the luminaire groups, as the opposite end of the field should also be illuminated. This may seem inefficient, since directing a luminaire towards a point located at a great distance entails high lighting losses. Moreover, it will be necessary to increase the inclination degree; therefore, the glare will be increased, thus decreasing visual comfort. In November 2017, the authors of this publication carried out a study of the lighting of the stadium to provide a simulation that satisfies the TBR requirements for the Spanish second division of LaLiga. During the simulation process, it was especially difficult to reach the minimum vertical illuminance values based on the cameras located behind the goals. Figure 7 shows how the luminaires that direct their emissions towards the goal area are located in the towers at the field corners. That is, there are no luminaires in the area before the goals, oriented towards the goal line, or coinciding with the position of the cameras located behind the goals (Figure 1).
In the study, the required values were reached by directing luminaires with less-concentrated lenses (25 degrees) towards the goal line, contemplating that this would worsen the installation's efficiency by illuminating areas outside the limits of the playing field. The ideal solution for achieving homogeneous and efficient lighting would be to locate two posts or groups of additional luminaires behind the goals.
To comply with the requirements of LaLiga's second division, 606 luminaires with a power of 240 W and with 15 • concentrator lens as well as 222 luminaires with a power of 240 W and with 25 • concentrator lens were needed-a total of 828 luminaires, which means an installed power of 198.72 kW. Figure 8 shows the results after the simulation.

Retransmissions in 4K, Spanish First Division: Benito Villamarín Stadium
The Benito Villamarín Stadium is the playing field of Real Betis Balompié and has a capacity of 60,721 spectators, ranking as the fourth largest stadium in Spain. Figure 9 shows one of the upper ends of the stadium, where the luminaires can be observed grouped along the upper perimeter, as can the preference for linear distribution [47].
In March 2017, the study of the lighting of the Benito Villamarín Stadium was carried out. Following the line of previous studies, a three-dimensional model of the stadium was created with SketchUp, and it was then inserted into DIALux to perform the lighting simulation. Firstly, it is noted that the luminaire distribution is apparently more homogeneous than in the previous case. Secondly, the projectors' color temperature is not uniform, thus causing a worse performance by the players and preventing high-definition reproductions.
To comply with the requirements of LaLiga's first division, 1,572,240 W luminaires were required, representing an installed power of 377.28 kW. Luminaires with 15 • concentrating lenses, an IRC (Chromatic Reproduction Index) of 90, a lighting efficiency of 136.5 lm/W, and a color temperature of 5000 K were used. The results of the lighting simulation are provided in Figure 10.

Retransmissions in 4K, Spanish First Division: Ramón Sánchez Pizjuán Stadium
The Ramón Sánchez Pizjuán Stadium, with a capacity of 43,883 spectators, is a playing field where Sevilla CF plays its home games [48]. Before the last remodeling, the luminaires were located in four luminaire groups in the northern and southern goal areas, a double luminaire line in the front, and a luminaire line in the east gallery. Figure 11 shows photographs taken in May 2016 and shows the luminaire distributions mentioned. In June 2016, a lighting study of the stadium was carried out by the authors of this communication.
The simulation results are shown in Figure 12. This is the best luminaire distribution of all the case studies analyzed, so higher lighting results are expected. In comparison with Figures 7 and 10, it is verified that the illuminance is concentrated almost entirely within the play field. Luminaires with a total of 1,136,240 W were used, representing an installed power of 272.64 kW, 25% lower than the simulated power for the Benito Villamarín Stadium and 31% lower than the simulated power for the Nuevo Los Cármenes Stadium if the demands were increased from the second to the first Spanish division.

Reference Football Stadiums
The most optimal lighting distribution would be one that takes an oval shape, with its major and minor symmetry axes coinciding with the bottom and band lines, respectively. In Spain, the San Mamés ( Figure 13) [49] and the Wanda Metropolitano ( Figure 5) [32] stadiums have this type of design. They were awarded as the best stadiums in the world in the "Industry Awards, World Football Summit" in 2017 and 2018, respectively. In this last edition, the Wanda Metropolitano won against the Mercedes-Benz Stadium ( Figure 14) [43]. Other stadiums worth mentioning are the ones that hosted the 2018 World Cup in Russia-among them, the Volgograd Arena, the Ekaterinburg Arena, and the Nizhni Novgorod and Luzhniki stadiums (Figures 15 and 16) [44]. A strong candidate to be the new best stadium of the world is the Tottenham Hotspur Stadium (Figure 17), recently opened on 3 April 2019 [49][50][51][52].

Conclusions
This research began with an analysis of the regulatory frameworks in high-level sport competitions, particularly with respect to the current lighting requirements, where it was verified that the Spanish LaLiga represents the competition with the highest demands in terms of lighting. For this reason, it is the competition that was chosen as a study reference.
After publishing the new regulations, and with the aim of verifying the degree of compliance with the new technical requirements, LaLiga began an audit process where it identified the stadiums that required adaptation of their facilities to the new lighting requirements. It is from here that the authors of this work participated directly by making the lighting designs of various stadiums and highlighting the Nuevo Los Cármenes, Benito Villamarín and Sanchez Pizjuan stadiums, respectively. After carrying out the lighting studies, it was concluded that there are two ways to comply with the new requirements. One is by carrying out a very precise and complex dimensioning that forces the energy efficiency of the installation to be reduced. In order to achieve the required vertical illuminance, it is necessary to illuminate each area of the playing field at least from the four sides of the stadium (north and south goal and side stands); it is also necessary to illuminate areas such as the goals from long distances, which means wasting a large amount of light, thus reaching, in most cases, the limits of the glare allowed. The other way is by redistributing the luminaires that, in an optimal case, occupy the entire perimeter of the stadium, forming an oval design like that seen in modern stadiums, such as the Wanda Metropolitano or the San Mamés Stadium.
It was concluded that it is unfeasible to illuminate high-level stadiums using only luminaires in the corners, as is the case of the Nuevo Los Cármenes Stadium. The location of the luminaires in the stadium is a determining factor. With a symmetrical design, the same point can be illuminated from multiple locations, reaching maximum horizontal and vertical uniformity. As described in the previous section, the most efficient lighting pattern is achieved with an oval-shaped distribution of luminaires.
Spectators have witnessed the transformation that the stadiums belonging to the Spanish firstand second-division clubs are undergoing. Since the beginning of the 2016/2017 season, the stadiums that are undertaking works have been in the news. The latest cases that have delayed the necessary renewal of their lighting installations are the Vallecas, Anoeta, El Alcoraz, and Riazor stadiums. Moreover, the Rayo Majadahonda team (Majadahonda, Madrid) was forced to play in the Wanda Metropolitano stadium during the 2018/2019 season [47] due to lack of fulfillment of the requirements and the impossibility of achieving them in a short period of time.
In the case studies presented, it stands out that this type of installation represents a challenge for the engineers in charge of the lighting design because the requirements are maximized and, therefore, the calculation difficulty is remarkably high, particularly in those cases where the stadium geometry does not allow a luminaire distribution that optimizes results. It is worth noting that most of the cases studied exceed one thousand points of light. Considering that each point of light has a location (x, y, and z), direction, and orientation, this means that each luminaire will have a coordinate in space, inclination, and rotation. That is why a symmetrical design of the stadium is so important; this would simplify both the simulation and the future installation.
Another aspect to highlight is the mentality of the engineer when addressing the calculations because, in this type of installation, significant sums of money need to be invested and aesthetic aspects are relevant. To comply with regulations will not be enough; the design must also have a margin for improvement, and it will have to offer the greatest technological advances possible. In other words, it will not be enough to present the most economical offer with the lowest installed power that meets the regulatory requirements. This type of installation must go one step further.
The near future of all professional stadiums seems to follow the trend marked by the most recent redesigns or constructions, which act as current references for remodeling projects either in their project phases or in the progress of other important sports venues, such as the new Stamford Bridge (Chelsea