Traffic Signal and Street Light Information Sheets
Willis Lamm
  Preserving Historic
Street Lights

Concepts for improving the
efficiency of historic and
vintage street light systems

Part Two

Solution Options

Please note: This feature is a continuation of Part One.

Series Circuits

In their day, series circuits were the most efficient means of lighting in terms of lumens produced per watt. However the voltage across a typical series 6.6 amp 4000 lumen incandescent lamp was under 35 volts. These lamps, as well as the entire circuit, required a constant current in order to operate properly. Such a circuit would not be conducive to present day high efficiency lighting. So the issue at hand involves getting reliable 110 volt output from a 2+kv 6.6 amp series circuit.

As far back as the 1890s manufacturers produced isolation transformers for series circuits that could power subordinate multiple circuits of "line voltage" lamps, generally from 110 to 240 volts. That technology certainly worked, although it was not highly efficient and some transformers were found to produce radio frequency interference (RFI,) particularly when lamps failed and there was no load on the transformers.

A modern version of the isolation transformer exists that is more efficient and does not produce RFI issues. An isolation transformer mounted in the base compartment of a luminaire (or pole mounted, where necessary) can reliably deliver 120 volts from a 6.6 amp constant current series circuit.

The STM-140 series isolation transformers made by OV20 Systems operate on 6.6 amp series circuits. Regardless of voltage supplied by the constant current regulator, they deliver clean 120 or 240 volt power through secondary windings, sufficient for powering LED luminaires or any of a number of retrofits for classic luminaires.

Any isolation transformer selected should be designed to withstand both an open circuit and a short circuit on its secondary windings - just in case old wiring does short circuit or someone removes a "Jones" socket from its receptacle while the circuit is energized.

"Stab-in" and head mounted autotransformers.

Sequoia Lighting Corporation manufactures a whole line of "in head" autotransformers and kits, including "stab-in" retrofits that fit into Jones sockets. These kits directly supply HPS, CMH and similar improved efficiency lamps from existing series incandescent or mercury vapor circuits. For historic districts and other locations where aesthetic concerns apply, CMH retrofits can provide "incandescent-like" illumination on these old series circuits at a fraction of the cost of incandescent lighting.

An illustration of some of these kits appears below.

Sequoia also makes complete replacement luminaires designed to be served by traditional series circuits. A more complete copy of Sequoia's cut sheets can be found here. (Large file, may take time to download.)

With 120 or 240 volts provided at the luminaire, or direct conversion for lower cost light sources without involving costly circuit replacement, the range of upgrade options expands exponentially.

Retrofit or Replace the Luminaire?

With the relatively low cost ability to provide 120 or 240 volts or direct supply to lower energy lamps from a series circuit, the next logical decision point involves whether to retrofit a luminaire, replace the luminaire head or replace the entire post and luminaire assembly.

There are a number of high efficiency luminaires that can be mounted on existing bracket arms and column posts that will preserve an historic look. Given that catalogs abound with such options, this section will focus on making existing luminaires more efficient rather than discuss replacements.

With NEMA style luminaires or post top luminaires having standard socket mounting hardware, a ceramic metal-halide (CMH) retrofit package or "stab-in" kit could be a relatively inexpensive and reliable option to replace a series incandescent lamp. Care should be taken to use a CMH replacement that is similar in light output as "trespassing" light can become an issue with higher intensity light sources. CMH produces a "natural" light that is generally not offensive and provides a high degree of color rendering. (A number of manufacturers such as Sequoia Lighting and Ray-Lite can also provide more modern globes for decorative post-top lamps that help reduce trespassing light.)

Where changing out series socket hardware is problematic, the series lamp can be adapted to accept an HPS, CMA, CFL or LED replacement. When providing 110 volt service to series luminaires that have removable Jones sockets, it is important to replace the film cutout disk in the socket bayonet with appropriate insulating material, or if powered by an isolation transformer, use a "red dot" cutout that has a 250 volt breakover voltage. Failure to properly insulate the bayonet contacts from each other will create a direct short circuit and the lamp won't light.

If you can reliably deliver 120 volts to the lamp socket, whether through use of an autotransformer or in a luminaire now served bya 120 volt circuit, high output CFL or LED screw-in lamps can be very cost-effective lighting options in addition to the CMH kits.


Various studies over the years, particularly focusing on low pressure sodium and early non-color corrected mercury vapor lamps, showed improved driver perception and reaction as well as more "useful" light per lumen when light sources were less monochromatic and closer to the range of 3000 to 5000 Kelvins. As an example, drivers' reaction times for braking upon perceiving danger improved by 25% under incandescent lighting as compared with monochromatic low pressure sodium lighting. See footnote below.

Most incandescent lamps, including household lamps, produce light in the 2800 Kelvin range. Therefore 2800 is a portion of the spectrum that most of us are "tuned" to for seeing and viewing objects. How well a given source of light allows us to perceive objects is perhaps more important than raw intensity. Our ability to distinguish colors under artificial light is a key factor in our ability to perceive objects in our environment.

For security and workspace lighting the most desirable outcome is to provide light that produces the best Color Rendering Index (CRI.) The CRI scale ranges from 1 (the worst) to 100 (the best.) Most incandescent lamps, for example, give light at around 2800K have a CRI of 100. Therefore if we want to provide the most usable light per candlepower, we want to get as close to 2800K and a CRI of 100 as possible.

One advantage of CFL illumination is that many high output CFL lamps burn near 2800K, providing a CRI of nearly 100. Thus a softer, lower intensity can provide more usable light on the street. Lower operating costs, less glare and more usable light are desirable attributes.

For higher output CFL field tests we used lamps from TCP

Lighting for traffic purposes is more effective when in the "cooler" color range, generally above 9,000K. Brake lights, turn signals and traffic signals are more distinctive and more clearly recognized under cooler street lighting such as induction or LED as compared with sodium vapor, particularly when these indications appear in a driver's peripheral vision. This improved traffic safety is due in part to the contrast between red and amber lamp colors and the cooler ambient light provided by street lighting.

Useful links regarding color temperatures and the Color Rendering Index.

Damp Location Rated LEDs

LEDs are being produced that are certified for damp locations such as open reflector luminaires. The more powerful ones tend to have color temperatures of 3000k, just slightly whiter than typical incandescent lamps. We field tested Feit's 33-watt 300-watt equivalent and the results were extremely encouraging.

Glare and Trespassing Light

We are in an age where glare and trespassing light are growing issues with which lighting engineers must contend. Higher intensity lighting, particularly lighting that tends to be more monochromatic, can be annoying when it strays beyond its intended area of illumination.

"Softer" light producing a more conventional color temperature, of modest intensity and that is appropriately diffused, tends to draw fewer complaints. More natural light doesn't cast sharp shadows contrasted by strange colored light, isn't harsh to look at and appears more "natural" to the eye. This statement is, of course, subjective but it can be compared with indoor lighting that can be pleasing or displeasing as a result of color, intensity and diffusion.

Practical Options

Lighting technology is constantly evolving and undoubtedly additional practical lighting options having lower operating costs will be forthcoming. Meanwhile some reasonable and affordable options presently exist that can improve the efficiency of existing lighting systems.

Illustrated below is a table of common low to medium output street light lamps to help compare the various types and their performance.

As the chart illustrates, Ceramic Metal-Halide (CMH) lamps provide the best CRI for lower power consumption lamps, ranging from a CRI of 85 to 92. Next best are the CFLs, producing a CRI of about 82. The most lumens per watt are provided by LEDs with CRIs in the upper 70s and above.

Street Views of CFL Retrofits

Practical Considerations

Three viable options exist for retrofitting existing luminaires. Each has its advantages and disadvantages.

CMH lamps provide strong light at reasonable operating costs with a CRI that makes the light more pleasing, makes the light more useful and increases safety when compared with sodium vapor illumination. However a CMH lamp will require a retrofit kit that includes a magnetic ballast or electronics package and associated components. Some luminaires might not accommodate this equipment. However where these components can be installed, CMH provides a very practical and low maintenance source of light.

CFL retrofit lamps provide a very "viewer friendly" soft light that can be simply screwed into mogul lamp sockets. Most models operate off 110 or 220 volts that can be supplied via isolation transformers installed on series circuits or by bypassing the ballasts in 110 or 220 volt HID luminaires. There are also direct retrofits available for 175 watt mercury vapor luminaires that take power from the MV ballast at just over one-third the operating cost.

As of this writing, the intensity of CFL lamps is limited to around 10,000 lumens and they can experience a slight loss in intensity during extremely cold weather. However the usefulness of the light produced is much greater than equivalent MV or HPS.

LED retrofit lamps provide the most light output per watt consumed and their CRI ranges are practical. However at the present time LED retrofits are expensive and most of the roadway lighting models being produced are for post top luminaires. However as the technology evolves the prices of LED retrofits will likely decline substantially and they will likely be designed for additional applications.

Please note: There are a number of issues to consider when preserving and/or repurposing vintage lamps for parking lot and roadway use including economy and roadway safety. There have been a number of relevant studies published regarding driver response to intensity and colors. Links to several of these reports can be found in the Street Light History Section.

Continue to
Field Trials

Return to Part One

If you have comments, see an error or think of something that should be added to this section, please
Email me.

For more information on how series circuits work, please see
Understanding Series Street Light Systems

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