The illuminated playing surface takes up a major part of the field of view for anyone in a sporting venue, whether players, officials or spectators. Horizontal illuminance (Eh) represents the illuminance on this horizontal plane at ground level. It serves primarily to create a stable visual background against which the eye can discern players and objects.
For non-televised lighting classes, an average horizontal illuminance of between 50-100 lux and 750 lux is required, depending on the sport in question and on the lighting class. For televised competitions, the vertical illuminance level is more important than the horizontal illuminance level; to ensure that the television picture has a well-balanced brightness, the ratio between the average vertical and horizontal illuminance should match as closely as possible, but shouldn’t exceed a 0.5 to 2 ratio. The horizontal illuminance shouldn’t be less than half the vertical illuminance or greater than twice the vertical illuminance.
The athletes in any particular sporting event, as well as the ball they’re using, can be understood as vertical surfaces. This means that we need to keep vertical illuminance (Ev) primarily in mind when we light them.
To guarantee an optimal view and make it possible for the human eye to identify players from every direction, we should generally measure Ev at a height of 1.5 meters, which corresponds approximately to the faces of the players.
Experience shows that there’s an intimate relationship between vertical and horizontal illuminance. For sports with no specific vertical illuminance criteria, vertical illuminance will be sufficient if the required horizontal illuminance is achieved, and if the lighting design rules are followed.
Televised events involve exceptions to this rule of thumb; vertical illuminance has a major influence on the quality of a final television or film picture. Television broadcasting generally calls for an average Ev of between approximately 1000 lux and 2000 lux.
Ensuring uniformity is important in avoiding adaptation problems for both players and spectators. If uniformity is inadequate, certain objects or player details might be difficult to see from certain positions.
Uniformity is expressed as
- the ratio of the lowest to the highest illuminance (U1 = Emin/Emax)
- the ratio of the lowest to the average illuminance (U2 = Emin/Eaverage)
In non-televised situations, the uniformity of the horizontal illuminance is generally specified as between 0.5 to 0.7 (Emin/Eaverage) depending on sport and lighting class.
In televised situations, high uniformity is necessary for smooth and natural-looking scenes, especially in this era of HDTV; horizontal illuminance is generally 0.8, whereas vertical illuminance in the direction of fixed cameras requires a uniformity value of 0.7 (Emin/Eaverage).
Even when the uniformity ratios as we’ve defined them are acceptable, changes in illuminance can be disturbing if they happen too quickly. This problem is most likely to arise when a television camera pans. The illuminance uniformity for TV/film coverage at a certain grid point thus has to be expressed as a percentage change from the average adjacent grid points. This is called the uniformity gradient.
A common uniformity gradient value for both horizontal and vertical illuminance in the direction of main cameras of ≤20% on a 4m calculation grid might ensure smooth panning between one area to another.
Glare is a subjective factor for which CIE has, on the basis of extensive field research, developed a practical evaluation system for use in outdoor sports applications (CIE 112 Glare evaluation system for use within outdoor sports and area lighting).
CIE 112 defines a so-called glare rating factor (GR) ranging from 10 to 90 on the assessment scale. The lower the glare value, the better the glare perception for the players in a sporting event.
A maximum GR value of 50 is generally specified for sports projects.
Modelling refers to lighting’s ability to reveal form and texture. Modelling ability is particularly important in providing a pleasant overall impression of the athletes and objects in the field of play, not to mention of the spectators in the stands. An installation where light comes from only one direction will result in harsh shadows and poor modelling.
The color properties of luminaires have two important aspects:
- The color appearance of the light. This is the color impression of the total environment that the light source creates.
- The color rendering properties of the light source used, or the CIE Color Rendering Index (CRI). This describes how faithfully a light source can reproduce a range of colors.
An indication of a lamp’s color appearance can be obtained from its correlated color temperature as measured in degrees Kelvin (K), which vary mainly between 2000 and 6500K. The lower the color temperature, the "warmer" the color impression of the light is; the higher the color temperature, the "cooler" or more bluish the impression of the light is.
Sports lighting generally requires a color temperature of between 4000 and 6500 K.
The color rendering properties of a light source can be indicated by its Color Rendering Index, expressed as a numerical value between 0-100. A light source with a CRI of 100 will represent scene colors faithfully, with daylight as the standard of comparison. Color perception is highly relevant in most sports applications.
While some of the color distortions that artificial lighting causes are acceptable for non-televised activities, TV broadcasting requires highly accurate color rendition.
The transition from conventional lighting to LED lighting gave rise to a discussion of whether CRI remains the correct color fidelity metric for television broadcasting. It was developed based on the human eye response curve and for a set of pastel colors, and isn’t necessarily appropriate for sports broadcast cameras that transmit images rich in saturated colors.
The European Broadcasting Union (EBU) has developed the Television Lighting Consistency Index (TLCI), which is based on camera response. TLCI is gaining popularity among broadcasters as a color metric specifically designed for their needs. Discussion of this topic is ongoing, but there’s a general consideration that it should be easy to get colors right with a TLCI>80.
A particular problem for super slow-motion cameras is a 50Hz flicker, due to the phasing of the light.
Cameras perceive light level changes due to the uneven ratio between the camera scanning frequency and the alternating amplitude of artificial lights powered by mains frequency.
This effect, which is visible only during slow-motion replay, is called the flicker effect.
Sports federations have started to incorporate a so-called flicker factor into their lighting recommendations. To avoid any visible slow-motion image flicker, a flicker factor of less than 3 percent is recommended.