Glare vs Brightness: Why Is a Glary Streetlight a “Bad” Streetlight?

Why Does It Feel Harder to See Even When It’s Bright?

Understanding Glare with Numbers

1️⃣ “Bright” Actually Means Different Things

When people say a light is “bright,” they usually mean just one thing. But in lighting, brightness is described using several different values.

Term	Unit	What it means
Total light output	lm (lumens)	How much light the fixture produces
Illuminance	lx (lux)	How much light reaches the road or ground
Luminance	cd/m²	How bright the light appears to the human eye

👉 Glare is not mainly a lumen problem — it’s a luminance problem.

2️⃣ The Key Number Behind Glare: Luminance

What is luminance (cd/m²)?

Luminance describes how intense a light source looks to your eyes.

Small, extremely bright sources create very high luminance.

For example:

• Bright wall surface: a few hundred cd/m²

• Smartphone screen: about 500–1,000 cd/m²

• LED chip surface: tens of thousands to hundreds of thousands cd/m²

  
  

👉 This is why LEDs can feel harsh or piercing even when they are small.

3️⃣ Why Glare Is More Noticeable with LED Streetlights

Even with the same power or lumen output:

• Light spread over a large area → lower luminance → comfortable

• Light concentrated in a small area → higher luminance → glare

In other words:

With the same lumens, luminance can differ by tens of times depending on design.

Glare is a design issue, not simply a brightness issue.

4️⃣ How the Human Eye Reacts (Numerically)

The human eye is very sensitive to contrast.

• When a light source is 10× brighter than its surroundings,→ discomfort glare often begins

• When the difference reaches 100× or more,→ disability glare occurs (objects actually become harder to see)

At night:

• Background luminance is very low

• LED streetlights can be extremely high in luminance

  
  

👉 That’s why glare feels much worse at night.

5️⃣ Why Adequate Lux Levels Still Aren’t Enough

Typical roadway lighting standards:

• Roadways: 10–30 lx

• Sidewalks: 5–20 lx

  
  

Even when these values are met, safety can still be compromised if glare is high.

Why? Because human vision responds more strongly to luminance contrast than to average illuminance.

6️⃣ Professional Glare Evaluation Metrics

In lighting design, glare is evaluated using specific indices.

• UGR (Unified Glare Rating)→ Commonly used in indoor lighting

• TI (Threshold Increment)→ Used in roadway lighting→ Expressed as a percentage of visual performance loss due to glare

Typical references:

• TI ≤ 5% → minimal glare

• TI ≈ 10% → noticeable loss of visibility

• TI ≥ 15% → potentially dangerous

  
  

👉 Good streetlight design aims to minimize TI, not just maximize brightness.

7️⃣ “Bad” vs “Good” Streetlights (With Numbers in Mind)

❌ Poor streetlight design

• LED source directly visible

• High luminance enters the eye

• High contrast → high TI

• “Bright, but hard to see”

  
  

✅ Good streetlight design

• LED source properly shielded

• Light directed onto the road surface

• Controlled luminance and contrast

• “Comfortable and clear visibility”

  
  

👉 Same lumens, completely different visual experience.

8️⃣ One-Sentence Technical Summary

A good streetlight meets illuminance requirements while keeping luminance contrast and TI low.