# Adjustable Street Light

How a street light's height, optic and tilt decide where its light lands

By greenlig

Topic: Physics & Chemistry

[Open the interactive Jig](https://ask.jig.com/j/kuy9za7JPYmq)

## Guide

### 1. A street light at dusk

As the sky darkens, a small sensor on top of the lamp switches it on\. Its light pours onto the road, spills toward the houses and fades softly at the edges\. Drag the street to look around it\.

That sensor is a photocell: it measures daylight and turns the lamp on when it gets dark\.

### 2. Inside the lamp head

The flat head holds a board of LEDs behind a clear lens\. Fins on top carry away heat, because LEDs dim and age faster when they run hot\. The lens bends the light into a chosen shape\.

A lamp like this gives about 4,000 lumens from roughly 33 watts\.

### 3. Raise and lower the pole

The pole slides like a telescope\. A low lamp makes a small, bright pool of light; a tall one spreads the same light wider and dimmer\. Use the slider, or drag the pole up and down\.

Brightness falls with the square of distance: twice as high spreads the light over four times the area\.

### 4. Shape the light

Engineers call the pattern a light distribution\. A narrow optic makes a tight spot, a wide one floods everything, and a road optic stretches the light along the street and across it\.

Road optics are rated by how much light goes backward, upward and into your eyes as glare\.

### 5. Tilt the head

Tilting the head swings the light off the road, onto the houses and finally into the sky\. Watch the three gauges on the front of the street: road, houses and sky\. Drag the head to tilt it too\.

Most street heads are mounted level or tilted only a few degrees\.

### 6. Turn on the lux map

Now the street becomes a map of brightness\. Each colour band is a level of light, measured in lux, from pale yellow under the lamp to deep violet where it fades\. The thin rings join places that are equally bright\.

A full moon gives about 0\.2 lux; a quiet street needs about 3 to 10 lux\.

### 7. Height on the map

With the map on, change the height again\. A low lamp makes a hot yellow centre and dark gaps\. A tall one gives wider, gentler rings, so the street looks more even\.

Planners space residential lamps about three to four pole heights apart\.

### 8. Light that goes astray

Tilt the head up and watch the colours climb the walls and the sky gauge rise\. Light in a bedroom window keeps people awake, and light sent upward is wasted and hides the stars\.

That faint dome of light over towns is called sky glow\.

### 9. A well\-aimed light

A road optic, a level head and the right height keep the light on the street, where people walk and drive\. The houses stay dark and the sky stays black\. Good lighting is about where the light lands, not just how bright it is\.