Design for Darkness: How Outdoor Lights Affect the Sky

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Seeking starlight from a city may seem futile, especially here in Chicago, home of the orange glow.  We are, after all, the literal poster city for light pollution,  since the 2008 National Geographic cover story “The End of Night” outed us as the worst offender.  Still, we can either be part of the problem or the solution.  

PROTECTING CITY BASED STAR GAZING

Our designs for the Wild Blossom Meadery / Brewery, are nearing completion.  As we specify details like light fixtures for various areas of the project we take all the program uses into consideration.  In this case, one of those purposes is using the side yard for a little glimpse of the night sky.

We try not to overlight the outsides of our buildings in any case – a city dwelling usually has a lot of ambient light to take advantage of.  Once directed downlights are placed near all the doors (to prevent key fumbling) we think twice before adding any more exterior light.

Since Wild Blossom is specifically asking for stargazing conditions (our clients are avid amateur astronomers), we did a little research to see if there was a recommended type of light for stargazing – or at least to find out what kind of lights contribute least to general light pollution.

DIRECT OUTDOOR LIGHTS DOWNWARDS

Unshielded lights are a waste of energy, a safety hazard and a nuisance to neighbors. They also fill the sky with unwanted light pollution. Any outdoor light should have a directional shield that aims light where its wanted – at the ground adjacent to the light. Light that travels directly from the bulb to an eye causes glare (scroll down to see why that’s bad) and light that passes eye level is simply wasted … and contributes to light pollution. The below sketch based on illinoislighting.org.

directed light

CHOOSE BULBS WITH CARE

Consider carefully what the minimum amount of light needed for any outdoor situation is … and design close to that amount.  Brightness in lights is measured in lumens.  Choosing lamps which generate less light also makes for more energy efficient outdoor lights.  Win, win!

Another aspect of choosing outdoor lights (or lights for any situation) is color spectrum. Different types of light bulbs throw (and let us see) different amounts of the visible spectrum.

The reason Chicago glows orange every night is that most of our streets are lit with High Low Pressure Sodium (EDIT: Its HPS, not LPS, as was brought to our attention by the Flagstaff Dark Skies Coalition) bulbs which put out light in the yellow orange wavelengths and everything they illuminate looks a uniform sepia toned orange. (This is starting to change.)  LED bulbs, by contrast, throw an almost complete spectrum of color and can render very much like daylight which can make it more difficult to view the night sky. These and more spectrum examples, at the Naval Observatory Flagstaff Station website here.

light spectrum

For Wild Blossom’s side yard, we are recommending a heavily shielded (light directed down) light fixture with a high pressure sodium bulb.  Since those emit a smaller segment of the visible spectrum, they throw less light into the surrounding area.

BUT WHAT ABOUT SAFETY?

In fact, the brightest lights don’t make the night time safer.  Lights that are too bright (in contrast to the shadows around them) or cast light directly into your eyes (glare) force your pupils to constrict which makes it harder to see the world around you – not easier.

For a creepy example of how this works, check out these two images (see them superimposed on each other at the Florida Atlantic University Physics Department Website).   On the left you can see the guy standing in the path.  On the right, he’s still there.  Look for his white sneakers and smudge of shadow where he stands just off the sidewalk in the deep shade.  Eeep.

glare and safety

LEARN MORE ABOUT DARK SKY INITIATIVES

There are a number of organizations devoted to advocating for reduced light pollution and providing advice you can use.  Check out the International Dark Sky Association at their website: darksky.org.  Or, for research closer to home, you can turn to the Illinois Coalition for Responsible Outdoor Lighting to find out why … and how … to make our Chicago sky a little darker.

  • We want
    to applaud you on being one of the few lighting design companies we’ve
    encountered that is concerned about the spectral issues of light at
    night. It is so refreshing to learn of a lighting design
    business that actually understands why!

    High
    pressure sodium lighting creates much less skyglow at night. The only
    lamp that is better than HPS for skyglow is LPS (or a narrow spectrum
    amber LED). Here are 2 links written by the author of the US Naval
    Observatory link that you posted on your blog that may be helpful:

    http://www.flagstaffdarkskies.org/for-wonks/lamp-spectrum-light-pollution/

    http://www.flagstaffdarkskies.org/low-pressure-sodium-lighting/

    Obviously,
    fully shielded lighting takes top priority when doing any outdoor
    lighting design. It’s more efficient and limits light directed up,
    which reduces glare and light pollution. The second priority is limiting
    the number of lumens (light output) on the site. The final priority is the lamp
    spectrum.

    Keep up the great work!

  • http://chicago.suntimes.com/politics/nearly-all-city-outdoor-lights-to-be-upgraded/ UPDATE: The City of Chicago is moving forward with an RFP to replace all of our outdoor lights to LED in an effort to reduce light pollution, save energy and reduce maintenance costs.

    However, there is more to light pollution than just swapping out the lamp type. Chicago suffers from light pollution because all of our standard street fixtures point straight up towards the sky with no shield. Further, and per our original post above, LED lights produce light the encompasses the entire light spectrum, making star gazing more difficult, not less.