Professional Engineering Series

Horizontal vs Vertical Foot-Candles: The Critical Metric Most Sports Lighting Designs Get Wrong

Horizontal vs Vertical Foot-Candles: The Critical Metric Most Sports Lighting Designs Get Wrong

Why Surface Brightness Does Not Equal Visibility—and How Vertical Illuminance Determines Real Performance

The Industry’s Core Mistake

Most sports lighting designs are built around one number: horizontal foot-candles. This measures light on the playing surface, and it is easy to specify, measure, and compare.

However, sports are not played on the ground. Players track balls, opponents, and motion in three-dimensional space. Designing lighting based only on horizontal illuminance creates systems that meet specifications—but fail in real gameplay.

The correct metric is not brightness—it is visibility.

What Horizontal Foot-Candles Measure

Horizontal foot-candles measure the amount of light hitting the ground plane.

They are used to define:

  • Surface visibility

  • General brightness

  • Minimum compliance thresholds

Typical use cases:

  • Walking paths

  • Parking lots

  • Basic recreational fields

In sports lighting, horizontal values are necessary—but not sufficient.

What Vertical Foot-Candles Measure

Vertical foot-candles measure light within the player’s field of vision.

This includes:

  • Ball trajectory

  • Opponent movement

  • Spatial awareness

Vertical illuminance is measured at specific heights and angles that simulate how players actually see the game.

This is the metric that determines reaction time and visual clarity.

Why Vertical Illuminance Matters More

In nearly all sports:

  • The ball travels above ground

  • Players look upward or across the field

  • Critical decisions are made based on moving objects

If vertical illuminance is insufficient:

  • The ball disappears against the background

  • Depth perception is reduced

  • Player performance declines

A system can meet 50 fc horizontal and still perform poorly if vertical levels are low.

Real-World Example (Tennis & Pickleball)

In court sports:

  • Most ball tracking occurs between 2 ft and 10 ft above ground

  • Players frequently look upward during volleys and lobs

If lighting is ground-focused:

  • The court appears bright

  • The ball appears dim or inconsistent

This is the most common complaint in poorly designed systems.

Real-World Example (Baseball & Softball)

In field sports:

  • Ball flight occurs at significant height and distance

  • Outfield visibility depends heavily on vertical illuminance

Common failure:

  • Bright infield

  • Poor outfield visibility

This is caused by insufficient vertical lighting—not low horizontal levels.

Real-World Example (Basketball & Indoor Sports)

In basketball:

  • Players track arcs, rebounds, and passes

  • Lighting must support mid-height visibility

Low vertical illuminance results in:

  • Missed shots

  • Reduced visual comfort

  • Increased glare sensitivity

Why Most Designs Ignore Vertical Illuminance

Vertical lighting is often ignored because:

  • It is harder to measure

  • It requires advanced photometric modeling

  • It exposes poor optical design

Many systems compensate by increasing wattage instead of improving distribution.

This increases cost and glare without solving the problem.

Indirect Asymmetric Optics (The Correct Solution)

Indirect asymmetric reflector systems:

  • Distribute light across the playing volume—not just the surface

  • Increase vertical illuminance without increasing wattage

  • Reduce high-angle glare

  • Improve uniformity across both horizontal and vertical planes

This is how high-performance systems achieve visibility, not just brightness.

Uniformity in Two Planes

True lighting performance requires uniformity in:

  • Horizontal plane (ground)

  • Vertical plane (player field of view)

Most designs only optimize one.

This creates inconsistent visual conditions and player fatigue.

Glare Interaction with Vertical Lighting

Poor vertical lighting often leads to increased glare because:

  • Designers increase brightness to compensate

  • High-angle light enters player sightlines

Correct vertical distribution reduces the need for excessive brightness and lowers glare simultaneously.

Photometric Modeling (Where the Truth Is Revealed)

Vertical illuminance must be validated through:

  • AGi32 modeling

  • Multi-point vertical grids

  • Aiming diagrams

Without this, vertical performance is unknown.

Any design that does not include vertical data is incomplete.

Specification Strategy (How to Control the Outcome)

To prevent poor systems, specifications should require:

  • Vertical illuminance targets

  • Horizontal + vertical uniformity

  • Photometric validation

  • Aiming diagrams

Without these, contractors will default to lowest-cost solutions.

Common Design Failures

  • Designing only to horizontal foot-candle targets

  • Using wide-beam floodlights

  • Ignoring vertical modeling

  • Increasing wattage instead of improving optics

  • No glare control strategy

These systems appear bright—but perform poorly.

Conclusion

Horizontal foot-candles measure brightness. Vertical foot-candles determine visibility. The difference between the two defines whether a lighting system performs in real-world conditions.

By prioritizing vertical illuminance, controlling glare through indirect asymmetric optics, and validating performance through photometric modeling, sports lighting systems can deliver true visual clarity—not just compliance.

For full standards, see IES RP-6-22 Explained. For class requirements, refer to Sports Lighting Classes I–IV.