LED Street Lighting: Hope or Hype?

Architects, lighting designers, and engineers nationwide are being asked to build with LEED certification, ENERGY STAR^® compliance, Dark Sky regulations, and California Title 24 restrictions in mind. Solid-state lighting products certainly serve as good alternatives, but are they the right solution? Among the many factors that need to be evaluated when considering LED luminaires are efficacy, effective life, energy savings, and potential expanded applications of LEDs.

Thomas Edison was once quoted as saying, “I don’t like to go into things connected with ancient history or the dead past. What I’m interested in is the future – what’s going to happen tomorrow.” And, while we can certainly agree with his disinterest in ancient history (unless we’re interested in studying the West Indian firefly box, which used real fireflies as a source of illumination), we must look to the recent past at a minimum to see what’s looming on the lighting horizon.  

Several municipalities are testing LED streetlights as replacements for metal-halide or high-pressure sodium (HPS) lighting fixtures. Their research and results provide some clear, practical direction, and some confusing comparisons. We can use these comparisons to see how LED units have changed in the past few years, how seriously they’re being considered (and should be considered) as a viable lighting option, and how they compare to the standard products that have been used for many years.

is a figure of merit for light sources: It’s the ratio of luminous flux (in lumens) to power (usually measured in watts). As most commonly used, it’s the ratio of luminous flux emitted from a light source to the electric power consumed by the source, and, thus, describes how well the source provides visible light from a given amount of electricity. This is also referred to as luminous efficacy of a source.
Luminous efficacy can also refer to luminous efficacy of radiation (LER), which is the ratio of emitted luminous flux to radiant flux. Luminous efficacy of radiation is a characteristic of a given spectrum that describes how sensitive the human eye is to the mix of wavelengths involved. Which sense of the term is intended must usually be inferred from the context, and is sometimes unclear. The luminous efficacy of a source is the LER of its emission spectrum times the conversion efficiency from electrical energy to electromagnetic radiation.

Wavelengths of light outside the visible spectrum are useless for illumination because they can’t be seen by the human eye. And, within the visible spectrum, the eye responds to some wavelengths more than others. This response of the eye is represented by the luminosity function. This is a standardized function that represents the response of a “typical” eye under bright conditions (photopic vision). One can also define a similar curve for dim conditions (scotopic vision). When neither is specified, photopic conditions are generally assumed.

Luminous efficacy of radiation measures the fraction of electromagnetic power, which is useful for lighting. It’s obtained by dividing the luminous flux by the radiant flux. Light with wavelengths outside the visible spectrum reduces LER because it contributes to the radiant flux while the luminous flux of such light is zero. Wavelengths near the peak of the eye’s response contribute more strongly than those near the edges.

In the Intl. System of Units (SI), luminous efficacy has units of lumens per watt (lm/W). Photopic luminous efficacy of radiation has a maximum possible value of 683 lm/W for the case of monochromatic light at a wavelength of 555 nm (green). Scotopic luminous efficacy of radiation reaches a maximum of 1,700 lm/W for narrowband light of wavelength 507 nm.¹

This is a very fancy way of saying that the human eye can only see certain wavelengths of color, and that the effect of ambient light or daylight impacts the way the eye catches and sees color. It also means that the luminous efficiency of most fixtures is figured in lumens per watt (not always so in the LED world). Interestingly, in dark environments – night, for instance – the eye is more in tune with red and green colors, and reads them as being brighter. The same source viewed by the human eye at daytime and nighttime will appear to be drastically different. This is especially true of LED fixtures.

Most high-brightness green and blue LEDs, and all of the usual high-brightness blue-green LEDs, have a spectrum that’s greatly more favorable for scotopic vision than the spectrum of incandescent lamps – especially lower-wattage/lower-current, longer-life incandescent lamps. A nightlight made with non-yellowish-green, blue-green, or turquoise-blue LEDs will appear to illuminate a room more brightly than an incandescent or neon nightlight with equal lumen output.

In fact, the usual white LEDs have a spectrum that’s somewhat more favorable to scotopic vision than the spectrum of typical incandescent lamps – especially 120-volt incandescent lamps that are 15 watts or less, and super-long-life lamps of up to 150 watts. In dim conditions, white LEDs can outperform most incandescents watt for watt. But, most fluorescent lamps are much more efficient than incandescents and most white LEDs (though this is in the early stages of changing) for photopic and scotopic vision.²

While this is good background information, the actual output of LED fixtures can be troubling to measure. Manufacturers use various methods to determine the information they publish regarding luminance and performance. As such, two manufacturers that offer similar products with a similar layout of LEDs can provide vastly different results. So, where can you find some impartial data to determine the effectiveness of LED streetlights? And, how do photopic and scotopic levels differ in measurement? The U.S. Department of Energy (DOE), in partnership with various municipalities, has been graciously publishing the results of its tests for the past few years.

The recent study by the DOE and the City of Oakland found that the LED streetlights used in their testing showed a lower photopic illuminance level and a higher scotopic illuminance level (see tables below³).

Table 1: Measured Photopic Illuminance Levels

Table 2: Scotopic Illuminance Levels

So, what do you learn from this? The clear lesson is that, while LED streetlights can be measurably lower in photopic illuminance levels, the scotopic illuminance level (as long as it maintains the minimum required footcandle output) may more than make up for any deficiencies. This is proven by the public comments received during the Oakland test and other municipal tests of LED streetlights in which average citizens thought the fixtures appeared to be brighter than the high-pressure sodium lamps they replaced. This appears to be attributable to improved visibility for drivers and pedestrians, and the overall positive effects of the new streetlights on several aspects of the neighborhood’s overall appearance and nighttime safety. This is consistent with the open-ended responses of those preferring the new streetlights – half of whom indicated that their expressed preferences were based on improved visibility.⁴

Effective Life
While we can measure the output of these luminaires, we’re still working on their useful, effective life. Concerns continue about the fact that, while most manufacturers claim a working life of 70,000 to 100,000 hours, the useful life of an LED begins to deteriorate at 50,000 hours. Given this, are LED streetlights really a viable alternative? The average annual number of hours estimated in the City of Oakland study for the existing HPS streetlights was 4,900. If we assume that the LED unit will begin to deteriorate at 50,000 hours, we can safely anticipate more than 10 years of service without a noticeable depreciation of light output. This gives us a more than satisfactory period to determine ROI. 

From a green standpoint, there’s legitimate concern about using a disposable product (although, hopefully, that means recyclable). Moreover, using LED luminaires means that there are no components to be changed (light bulbs), but that the entire unit is replaced. With the rapid growth of LED technology, this could mean that the replacement unit is brighter, or could even be an entirely different shape and style.

Still, the effective life of these units, and the lowered cost of maintenance, brings balance to the replacement issue.

Economic Savings
One of the most important reasons for using LED fixtures of any sort is their energy efficiency. In the City of Oakland study, a side-by-side analysis of maintenance and energy costs showed that the LED units were roughly one-half the cost of the high-pressure-sodium units they were replacing. This study assumed that the HPS fixture maintenance included lamp, photodiode, starter, ballast, and fixture material costs, as well as estimated labor and vehicular costs for the work performed. The LED units were assumed to have no maintenance costs.

Table 3: Annual Luminaire Costs⁵

While this appears to be a clear mandate for the use of LED streetlights, the study went on to compare new construction and retrofit ROIs.

Table 4: New Construction Economics⁶

Table 5: Retrofit Economics⁷

Clearly, the concept of a 25.5-year ROI makes little sense with a fixture that has a useful life of 10 to 20 year. But, it’s hard to anticipate the cost of energy; the increased manufacturing efficiency of LED products, which can lead to lower prices; and many other outlying factors.

Expanded Uses
In the meantime, there are many LED units that can replace metal-halide and HPS exterior lights. The DOE recently released a study conducted in Sacramento, CA, of parking-lot lights. The fixtures were a combination of dim and high output based on motion sensors. The findings were surprising in a number of ways.

LED lighting for outdoor parking lots shows great potential for energy savings. This demonstration provides evidence of increased potential savings using bi-level operation luminaires, and further evidence of the improvements in performance of LED luminaires. The costs and savings for this host customer provided favorable payback scenarios for both new construction and retrofit (less than 5 years) due to significant maintenance and energy cost savings. Utility or government incentive programs could also encourage greater adoption of LED luminaires for outdoor parking-lot applications by reducing the initial investment. These utility incentive programs should require minimum performance standards for qualifying products to ensure long-term energy savings.⁸

And, if LED streetlights don’t represent the panacea we think they could be, remember what Edison said: “Look to the future.” Not more than 10 years ago, LED streetlights were a concept in someone’s mind. Today, they’re actively being tested by numerous municipalities with mixed results. Two years from now, the efficiency and economics of LEDs will likely make more sense. And, with more mandates to go green, they will likely become standard fare.

John Mitchell is the director of sales for ELS in Sun Valley, CA. Mitchell has been in the entertainment and architectural lighting industries for more than 17 years. He may be reached at [email protected].  

About Entertainment Lighting Services, Inc. (ELS)
Established in 1991, ELS is one of the country’s leading suppliers of lighting staging products, providing the most technologically advanced lighting equipment for rental and sale, as well as a full range of production services. ELS serves the entertainment industry, corporate customers, and commercial developers. ELS prides itself on the craftsmanship of its installations. Director of Sales John Mitchell says, “ELS can provide design services, installation services, equipment sales, and service. We are truly a turnkey operation for lighting systems on the West Coast. Our continued expansion into new markets shows the value of the services we provide.”