Monitor Light Pollution with Photodiodes

Shine a laser pointer or flashlight in a dark room, and you’ll see its beam highlighted by the glow produced by light scattered from molecules of air. This is known as Rayleigh scattering. Occasional sparkles in the beam are caused by dust, which is called Mie scattering. Brush a foot against the carpet and you’ll see many more dust sparkles. Both Rayleigh and Mie scattering contribute to light pollution, but Mie is worse.

Light pollution can be detected by the LED twilight photometer previously described in this column (which you can build for yourself). I’ve found that orange and red LEDs work best for detecting sky glow.

Alternatively, you can build a simpler version on a solderless breadboard using the circuit in the figure above, which uses a standard silicon photodiode or miniature silicon solar cell instead of an LED. This provides increased sensitivity, which means that resistor Rf can be much smaller than the huge resistance required for the LED photometer. The resistance in ohms of Rf equals the amplification of the circuit. Thus, a circuit with a 1,000-ohm (1kΩ) feedback resistor will amplify the signal from the photodiode by 1,000. A good start value for Rf is 10 megohms (10MΩ), but you’ll need to experiment since some photodiodes are more sensitive than others.

Place a tube over your photodiode so that it looks at the sky above through a cone of several degrees.

If you use a multimeter to see the output from the photometer circuit, power it with a 9V battery. If you plan to connect the circuit to a data logger, use a 5V USB supply or a 6V battery.

I live in a rural area near Seguin, Texas. You might think that the sky over my place provides excellent viewing, but it does not. While planets and well-known stars and constellations can be observed on moonless nights, the Milky Way is usually hidden in the glow of city lights a few miles away.

For this column I decided to go on a midnight light pollution survey much like the heat island survey previously described in this column (“Tracking Heat Islands,”). Instead of driving to and from San Antonio, I drove an 80-mile circuit across Seguin and then to San Marcos and back. San Antonio’s light pollution is well known, and I wanted to find out more about light pollution from much smaller cities. I used a pair of 4-channel Onset UX120-006M Analog Data Loggers to record the sky glow signal at 1-second intervals. You can use a digital voltmeter to see the data but — and this is important — only if a friend is doing the driving.

I put the plastic ammo box that holds my two LED twilight photometers and their data loggers in a cardboard box in the bed of my pickup, with two bricks to keep it steady.

What happened next was a real surprise. Lights were everywhere! Of course they were always there, but this experiment made their presence seem much more obvious. Along the road there were overhead streetlights, flashing warning lights, traffic lights and, worst of all, cars tailing me with bright headlights. There were also industrial and commercial sites with brilliant floodlights and security lights. The lights of San Marcos produced a distinct bubble of light over the city.

Assuming these lights would produce spikes in the data superimposed over a background signal from sky glow, I continued my transect, and you can see the results above. The spikes in the red trace from a 600nm orange LED are mainly from street and security lights and headlights. The underlying and more stable trace is the background sky glow scattered from the zenith sky. The blue squares indicate when I recorded brief comments about various locations. The numbers in the chart indicate:
1. Peak sky glow over Seguin
2. Darkest sky between Seguin and San Marcos
3. Peak sky glow over San Marcos
4. Sky glow spikes from two car dealerships outside San Marcos
5. Sky glow spikes while driving around San Marcos.

The sky glow over both cities in my simple survey swamped out the Milky Way, and only the brightest stars and planets could be seen. Amateur astronomers would have better viewing midway between these cities. Or they could head west for darker skies.

» For serious study, the Sky Quality Meter is designed specifically to measure sky glow.
» Globe at Night is “an international citizen-science campaign” to raise awareness of light pollution. Another excellent resource is the International Dark Skies Association. For a stunning dark sky global map, see Dark Site Finder.
» Have an iPhone? The Dark Sky Meter app uses the phone’s camera to measure sky glow at night.