Understanding the Three Most Confused Lighting Metrics in Outdoor 

Applications

In outdoor lighting, terms such as lumens, illuminance, and luminous intensity are often used interchangeably. In reality, they describe fundamentally different aspects of light. Confusing these metrics frequently leads to incorrect lighting design decisions, unrealistic performance expectations, and poor product selection.

This article clarifies what each metric actually represents, how they relate to one another, and why understanding the difference matters—especially in outdoor and solar-powered lighting systems.

1. Lumens: Total Light Output

Lumens (lm) measure luminous flux, which represents the total amount of visible light emitted by a light source in all directions.

In simple terms, lumens answer the question:

How much light does the light source produce in total?

Key characteristics of lumens

• Lumens describe quantity, not distribution

• They do not indicate how light is directed

• Higher lumens do not automatically mean brighter illumination on the ground

Two luminaires can have the same lumen output but produce very different lighting results if their optical designs differ.

Common misunderstanding

A common assumption is that a higher lumen value guarantees better lighting performance. In outdoor applications, this is rarely true. Without considering how light is distributed, lumen values alone provide limited insight into real-world visibility.

2. Illuminance: Light Reaching a Surface

Illuminance measures how much light actually arrives at a surface.
Its standard unit is lux (lx), defined as lumens per square meter.

Illuminance answers the question:

How much usable light reaches a specific area?

Why illuminance matters in outdoor lighting

In practical terms, illuminance determines how bright a road, pathway, or parking area appears to human observers. It is the metric most closely related to visual performance and safety.

Factors that influence illuminance include:

• Mounting height

• Beam angle and optical distribution

• Distance from the light source

• Surface characteristics

Lux vs foot-candles

Foot-candles are the imperial equivalent of lux and are commonly used in some regions.
1 foot-candle ≈ 10.76 lux.

While both units describe the same concept, lux is internationally standardized and more commonly used in technical documentation.

3. Luminous Intensity: Directional Brightness

Luminous intensity, measured in candela (cd), describes the amount of light emitted in a specific direction.

It answers the question:

How concentrated is the light in a given direction?

Why luminous intensity matters

Luminous intensity is critical for understanding:

• Beam shape and focus

• Glare potential

• Long-distance visibility

A light source with high luminous intensity concentrates light into a narrow beam, which can create strong brightness in a specific area while leaving surrounding areas underlit.

4. How These Metrics Relate (and Why They Are Not Interchangeable)

Although related, these three metrics describe different layers of lighting performance:

• Lumens: total light produced

• Luminous intensity: how that light is directed

• Illuminance: how much light reaches a surface

A high-lumen luminaire with poor optics may produce lower illuminance than a lower-lumen luminaire with well-designed light distribution.

Similarly, high luminous intensity can increase glare without improving overall visibility if not carefully controlled.

5. Why Confusion Leads to Poor Outdoor Lighting Design

In outdoor lighting projects—especially solar-powered systems—misinterpreting these metrics often leads to:

• Oversized systems based on lumen targets

• Insufficient ground illumination despite high lumen ratings

• Excessive glare from overly concentrated beams

• Inefficient energy use and reduced system autonomy

Designing based solely on lumens ignores how light behaves in real environments.

6. Practical Guidance for Outdoor and Solar Lighting

When evaluating or specifying outdoor luminaires, consider the following hierarchy:

1. Start with illuminance requirements

• Define how much light is needed on the ground for the application

2. Evaluate optical distribution and luminous intensity

• Ensure light is directed where it is needed, without excessive glare

3. Use lumens as a supporting parameter

• Lumens help estimate system scale, not final performance

This approach leads to more predictable results and better long-term performance.

7. Why This Matters More in Solar Lighting Systems

In solar-powered lighting, energy is limited. Every inefficiency in light distribution translates directly into:

• Higher power consumption

• Larger battery requirements

• Reduced autonomy during low-sun periods

Optimizing illuminance through effective optical design often delivers better performance than simply increasing lumen output.

Engineering takeaway

Lumens, illuminance, and luminous intensity describe three distinct aspects of lighting behavior. Treating them as interchangeable oversimplifies a complex system and often leads to poor outcomes.

Understanding their differences allows designers, engineers, and buyers to evaluate lighting solutions based on real-world performance, not just headline specifications.

In outdoor lighting, clarity of concepts is the foundation of reliable design.