This image is a 20 minute exposure taken about 5 miles south of Stephenville, and looking east. To the right of the frame is normal night sky. To the left is tax money spent on light that does nothing useful. This is light sent off into the sky by poorly designed fixtures; up to 40% of the total light output of many of the streetlights and exterior building lighting in the city is wasted in this way.
This much shorter exposure looking north gives a much better idea of the total light waste. Remember that it's a clear night; that light isn't being reflected from clouds, just illuminating normal atmospheric haze. On a cloudy night, the glow can actually be enough to navigate by at this range.
Why does this happen? In order to define the problem, it is helpful to establish a few basic reference. As this image from Phillips/HADCO shows, there are multiple regions of illumination to consider with regard to the typical street lamp. The primary illumination area extends to a maximum of 80 degrees above the vertical line from the lamp to the ground. (It is worth noting at this point that most lamps do not emit enough light to provide useful illumination to this distance. In a typical streetlight installation, the angle is less than 70 degrees.) From 80 to 90 degrees, the light emitted is not useful for illumination at ground level, and contributes only to glare. Above 90 degrees, the light emitted becomes skyglow, as seen in the first two images.
Thus, a street lamp which emits light equally in all directions would waste 50% of its output as skyglow, and another 5% as glare. Fortunately, this would not be a typical streetlight design; the hood and overhead supports of most streetlights block most of the light from going directly upward. Unfortunately, this tends to be more of a design accident than an intentional and properly implemented approach to good lighting design.
This image from EVLuma illustrates the light output of a typical NEMA fixture, commonly used for street lighting in Stephenville, compared with a fixture retrofitted with their full-cutoff LED module. Note that in the normal fixture at left, there is a significant amount of light emitted both horizontally and well above the horizontal plane. This provides no useful illumination from an elevated fixture. The LED retrofit eliminates this waste light, but other methods, including reflective shields, are also available.
Taking, as an example, a typical 150 watt high pressure sodium bulb -the most common streetlight light source - and placing it in a reflector assembly to redirect the wasted light back into the useful area, we can then reduce the needed wattage by 40%, or 60 watts, without reducing the amount of useful light at all. Multiplying that by the 1,096 streetlights in Stephenville would give a savings of over 65 kilowatts of energy.
How well do full-cutoff area lights work? This photo of a local motel illustrates an excellent implementation of efficient, low-glare lighting. The parking lot is well illuminated, with very little light shining beyond the area of intended illumination, and no light going directly into the sky. Additionally, almost no light is directed into the rooms' windows, where it would serve no useful purpose, and would disturb the customers.
How can this problem be effectively addressed? Replacing all of the more than 1,000 streetlights in the city would be cost prohibitive. When New Mexico passed its lighting regulations, in 1999 all existing lights were grandfathered, but any replacements were required to be compliant with the new regulations. Texas Health and Safety Code chapter 425 requires that all new or replacement state-funded lighting comply with restrictions including that all light fixtures with output over 1800 lumens (roughly equivalent to a single 100 watt incandescent bulb) be of a full-cutoff design. As such, the city should adopt similar restrictions on at least its own outdoor lighting, and potentially on all outdoor lighting within the city.