There’s a lot of jargon that goes with the territory when working to protect our dark skies. It all boils down to a simple graphic for people who just want to know what to do in their own yards. If you want to dig deeper and learn what it all means, keep reading below the graphic.


CCT defines color, not the brightness, of a light source and is measured in degrees Kelvin. A low CCT is in the amber end of the color spectrum while a high CCT is in the blue-white end of the spectrum. For reference purposes, a standard incandescent light is about 3000K while the bright white xenon headlights of some newer cars are 4500K.

From the Town of Fountain Hills Outdoor Lighting Ordinance:

“In order to minimize the detrimental effects of blue light, the correlated color temperature (CCT) of any outdoor lighting fixture shall not exceed 3000K.”


For decades, watts were used to indicate the relative brightness of lights. This was fine when incandescent lights represented the majority of light sources. However, watts are a measure of power used, not brightness. Using watts to define brightness no longer works with new lighting technologies such as CFLs and LEDs.

For reference, a 100-watt incandescent bulb produces approximately 1600 lumens of emitted light. The latest LEDs can produce the same level of light output using less than 20 watts.

The following chart shows the lumens and power used for the three primary light sources:

From the Town of Fountain Hills Outdoor Lighting Ordinance:

“All outdoor light fixtures with light output greater than 2250 initial lumens shall be fully shielded. Fixtures with light output ranges from 1125 to 2250 initial lumens shall be at least partially shielded.”


A fully shielded light fixture has a solid barrier (cap) at the top of the fixture in which the lamp (bulb) is located. The fixture is angled so the lamp is not visible below the barrier with no light visible below the horizontal angle.

The goal of fully shielded light fixtures is to prevent the following:

  1. Light trespass, which adversely affects neighboring and distant properties. Unshielded light travels much farther than people realize.
  2. Glare, which can cause eye discomfort or temporary blindness.
  3. Sky glow, which keeps all of us from enjoying the splendor of the night sky while also harming animal and human health.

A variety of fully-shielded light fixtures are available for all lighting situations and complement buildings with various architectural styles. CLICK HERE to find Dark Sky-friendly lighting suggestions.

The following illustrates good and bad lighting as it relates to shielding.

All fully-shielded light fixtures provide more light on the ground while energy is no longer being wasted by lighting the sky.

Bad lighting without shielding

Good lighting with shielding

Landscape light fixtures that predominantly provide downlighting rather than uplighting should be chosen. In addition to shielding, the use of timers and sensors is encouraged to minimize unnecessary lighting rather than leaving outdoor lights on all night, which is a waste of energy and an annoyance to neighbors. It should also be noted that there is no clear scientific evidence that increased outdoor lighting deters crime.

Special care must be taken in selecting fixtures for buildings on hillsides, as even meeting the “horizontal angle” requirement is not sufficient to protect owners of properties lower than the hillside site. Non-clear glass and lamps with the lowest wattage or lumens possible help prevent light trespass.

From the Town of Fountain Hills Lighting Ordinance:

“All outdoor light fixtures with light output greater than 2250 initial lumens shall be fully shielded. Fixtures with light output ranges from 1125 to 2250 initial lumens shall be at least partially shielded.”


A nit is a measure of the luminous intensity per unit area. It’s used to indicate the brightness of flat-screen displays such as electronic message boards, computers, televisions, etc.


One nit is equivalent to one candela per square meter. A candela is a unit of luminous intensity approximately equal to the luminous intensity emitted by one common candle. If a surface area of one square meter were positioned one meter away from a candle, the luminous intensity of that square meter would be approximately one nit.


In 2016, the Fountain Hills lighting and sign ordinances were updated to address new technologies. One rapidly evolving technology is electronic message boards, which have become less expensive and capable of brighter light output. Arizona State University concluded from a research study that a brightness above 100 nits causes temporary blindness for drivers. For that reason, communities across the country are adopting the 100-nit maximum for electronic message boards and Fountain Hills incorporated this guideline in its sign ordinance.


From the Town of Fountain Hills Sign Ordinance: 

“LED signs shall not exceed the maximum lumination level of 100 nits.”