Wednesday, November 4, 2015

Better Overhead Street Lights for Better Visibility

Overhead street lights provide many benefits.

  • Security for pedestrians
  • Help see around dark corners when driving
  • Help drivers on on/off ramps
  • Improves overall visibility to both pedestrians and drivers
There are also a few issues with overhead street lights.
  • Glare to drivers
  • Power usage
  • Light pollution
  • Color issues (cannot tell what color anything is with the orange lights)
We need overhead street lights and I have a few suggestions for fixing some of the issues. First of all there are many overhead lights that automatically dim when their motion sensors don't see anyone. This is a newer technology and will be slow to adopt. Also many of the newer lights are LED so the power consumption is much lower. Finally color issues are being resolved by installing white or close to white lights instead of the orange lights that are typically low pressure sodium vapor.

In the past light pollution was dealt with by getting rid of the white lights and installing the orange lights as the wavelength of these lights would not refract as much into the atmosphere. There are newer lights now that appear white but are missing some of the spectrum (filtered) that cause a lot of the light pollution. So you have a nicer light without all the light pollution.

The last issue I listed is glare. This is a hard one to deal with especially because glare has many causes.
  • Brightness of light
  • Color of light
  • Atmospheric conditions such as fog
My solution to all of these is a smart light that can:
  • Adjust its brightness based on objects and motion around it.
  • Adjust the color of the light based on the time of day and atmospheric conditions.
If it is a clear day with very little moisture in the atmosphere then a bright white light is best. As there is more moisture in the atmosphere adjusting the color output as well as applying filters to the light will cut down on the glare that would otherwise make driving more difficult as well as make it harder for pedestrians to see. Just like you don't drive in the fog with your high beams a bright white light from above will make it very hard to see anything in the fog.

Some basic light theory is that we perceive different colors based on their respective wavelength. Purple or bluer colors have a shorter wavelength and are more susceptible to bouncing around or "refract", especially when there are particulates in the air like fog, dust, and smog. You likely have seen some car headlights that are purplish and are annoying from the glare they cause. Think why the sky is blue.

At the other end there is red that has a longer wavelength. You will notice that the military uses red lights or filters on their lights so that they cannot be seen as far away giving their position away.

So many fog lights are yellow so they are still bright enough to cover some distance while not causing as much glare and refracting as much.

Color "temperature" is measured in Kelvin. The chart below shows some data points on the color temperature of light. Daylight is generally considered around 5000K.

The last piece I want to talk about is CRI or color rendering index, also sometimes referred to as color accuracy. Perfect daylight would be about 100 and some lights are actually negative. The reason I bring up CRI is that you want a street light or any light to have a good CRI as a higher quality of light means you don't need as much of it. So less energy and possibly less glare since it is not so bright.

Conclusion is that with a proper sensor to detect particulates in the air and then the ability for a light to change its color either by mixing different types of light sources (think different wavelengths of LEDs) or filters. We should be able to provide a overhead street light that is "smart" and will make the world a better place!