Stadium lighting is specified to satisfy television rather than spectators. Cameras need considerably more light, and more consistent light, than the human eye does.
Cameras are less forgiving than eyes
Human vision adapts continuously to different light levels and compensates for uneven illumination without the viewer noticing.
A camera has a fixed exposure for a given shot, so a player moving from a bright area into a darker one appears to change colour and brightness.
Broadcast standards therefore specify not only how much light is required but how uniform it must be across the entire playing surface. Uniformity is expressed as a ratio between the darkest and brightest points, and meeting it is harder than meeting the brightness figure.
Vertical light is what matters
Lighting a pitch adequately for play means illuminating the ground, but a camera photographs players' faces and bodies rather than the grass.
That requires light arriving horizontally onto vertical surfaces, measured toward each camera position rather than downward onto the pitch.
Fixtures are consequently aimed at the players from multiple directions, which is why floodlights sit at several points around the rim rather than directly overhead.
Flicker and slow motion
Older discharge lamps pulse in time with the mains supply, which is invisible at normal speed but produces pronounced flicker in slow-motion replay.
Broadcasters replay incidents at high frame rates, and a light source that pulses makes those replays difficult to watch and to judge.
Modern installations use drivers that hold output steady between cycles, which is one of the main reasons venues have replaced otherwise serviceable equipment.
Glare and spill are constraints
Light aimed at players inevitably reaches spectators and, beyond the stadium, nearby housing, and planning conditions frequently limit how much may escape.
Goalkeepers and fielders looking upward are particularly affected, so fixtures are positioned to keep sources outside the normal upward field of view.
Meeting brightness, uniformity and spill limits simultaneously is the real design problem, and each additional constraint narrows where fixtures may be placed.
What the change to solid-state lighting altered
Newer sources reach full output immediately rather than requiring a warm-up period, so a power interruption no longer means a long delay before play resumes.
They can also be dimmed and switched instantly, which is why light shows before kick-off became feasible where they previously were not.
The colour of the light can be tuned as well, and broadcasters specify it so that shirt colours appear consistent across different venues in the same competition.
