LED lighting: Do you know what you—€™re getting?

June 2012 » Web Exclusive » ENGINEERED SOLUTIONS
Understand fixture considerations for development of concise specifications.
Ray Schork, P.E.

Like many CE News readers, I have spent most of my career designing. My core competency is structural design, but with the economy the way it is, we can’t always be picky when it comes to design projects. This means learning new design skills to remain billable.

Learning new design skills comes with being in the right place at the right time. I had an opportunity to be the lead designer for a significant lighting project so I did some on-the-job training, used my “phone a friend” calls, and leaned heavily on my lighting supplier, Holophane, for help. I also have to give credit to my stepfather who just happens to be an electrical engineer. In the end, I passed my lighting certification with the Ohio Department of Transportation (ODOT).

With a few lighting design projects under my belt, I thought I had the hang of it until one of my clients, the City of Blue Ash, Ohio, requested light emitting diode (LED) light fixtures on a particular project. I looked for that all-encompassing specification for LED lights but couldn’t find anything on the subject. ODOT doesn’t use LED lights, so using its “Construction and Materials Specifications” book was not an option. As I dug deeper, looking into what would be a good specification, I began to get buried as more information became available.

At this point, I took a step back and assessed my situation. Just like I had to learn the basics of conventional lighting design, I now had to learn the basics of LED lighting design. Not all products are equal; this is why we have specifications in the first place. We could put that catch-all phrase “or equal as approved by the engineer,” but what this creates is a lot more work for the engineer if the contractor wants to use some cheaper product that we really don’t want in the first place. We are now in a position to defend the product we have called out on the plans. As designers, we try to keep the plans proprietary, so this sends us right back to writing a clear and concise specification for the quality of the product you really want to be used on your job. With all that said, there are many considerations that a designer must look at for LED light fixtures.

In my search for the Holy Grail of LED lighting specifications, I found numerous manufacturers that would say their product is the best. Of course they are going to say that, but what really makes them the best? There is a laundry list of items to consider when selecting an LED light fixture. It’s best to weed out the ones you don’t want and then present the remaining options to your client.

LED roadway lighting fixture considerations include style, color, surge protection, dual drivers, driver life, bracket mounts, distribution, light color, optics material, L70 useful life, limited warranty, dimmable, IP66 rating, and cost.

Fixture considerations include style, color, surge protection, dual drivers, driver life, bracket mounts, distribution, light color, optics material, L70 useful life, limited warranty, dimmable, IP66 rating, and cost.

Style and color
I can barely match my clothes in the morning let alone try to guess what type of light fixture a client wants on a particular project. There is no point in diving into the design until you get this worked out.

Typical LED street light fixtures can be an ice cube tray style — a rectangular box with LEDs — or shaped to look like traditional lighting fixtures, which allow quick replacement using existing poles.

Typical LED fixtures are an ice cube tray style — a rectangular box with the LEDs. This lends itself well to the construction of bird nests. Some manufacturers have taken their conventional fixtures and designed them for LEDs. This allows a quick replacement of the fixture itself while the pole can remain in place. Some fixtures are shaped to look like traditional lighting fixtures, for example like cobra heads.

Generally, the style and color of the light fixture is controlled by the architect, reviewing agency, downtown committee, city council, etc.

Surge protection
LED fixtures are electronic devices and consideration for surge protection is critical. If the fixture is not designed appropriately, your client will be replacing it sooner than expected if a surge exceeds the fixture’s rating. To be specific, specify Class C High 2002 ANSI 10kv & 10kva. Avoid the fixtures rated just for Class C 10kv.

Dual drivers
A driver is to LED fixtures as ballast is to fluorescent fixtures. One appeal of LED lights is that maintenance of the light is minimal compared with conventional lights. To minimize maintenance and keep the light in operation, dual drivers can be used. This is a backup system; if the first driver fails, the second driver kicks in. A particular light fixture may have more than one driver but they may be needed just to run all the LEDs. Normally, dual drivers are specified as an upgrade to an LED light fixture, if it is even available. It will cost more, but with a backup driver, your client will be able to replace a blown driver when their schedule allows and the service life of the light will be double.

Driver life
It is important to specify the life of the LED light driver. A typical driver life for LEDs is 50,000 hours. If the driver life is more than this you will be ahead of the game.

Bracket mounts
This refers to the number of mounts the fixture has to the support. The more mounts, the more secure the connection is. Make sure a fixture that mounts to a bracket arm has two mounts.

Distribution
Light distribution for a fixture is critical for design. In general, there are four types of light distributions. In some cases, you will want a long narrow distribution for long lighting runs or you will want to push the light further way from the fixture, for example at an intersection. If the light you want is limited in its distribution, this will limit what you can do when you start to design the system.

Light color
The color of the light is measured in Kelvins. Typical light color ranges are 4,000 degrees K (yellowish), 5,000 degrees K (pure white), and 6,000 degrees K (bluish white). The color of the light will be critical for the application of the fixture. For example, you would not want to have a yellowish light in a shopping area or a bluish white light for a street design. Again, if only one color range for a particular fixture is available, this will limit what you can do or will just not be acceptable to your client.

Optics material
Choices for optics material are prismatic glass or molded acrylic. The prismatic glass is generally more expensive but the direction of the light can be better controlled. Keep in mind that years from now the glass will still be glass — clear and durable. Molded acrylic will yellow over time. This will result in aesthetic as well as performance issues. Prismatic glass will save you headaches in the long run.

L70 useful life
LEDs can stay lit almost indefinitely, so to define the lumen (light) output, the L70 useful life term is used. You can read volumes on this subject, but to keep it simple, the useful light for LEDs is defined for a specific temperature for 70 percent of its useful life in hours. Temperature plays a key factor — ambient temperature as well as the fixture temperature. If the fixture is located in Texas, it will not have as much light output as the same fixture in Alaska. Also, LED fixtures have cooling fins, and if it isn’t designed properly or the fins get full of debris it will run hot and put out less light. A typical specification for the L70 useful life of LED’s is 80,000 hours at 40 degrees C and 105,000 hours at 25 degrees C.

Limited warranty
How long will the company stand behind its product in case something goes wrong? A typical limited warranty for LEDs is five years; anything more than this is great.

Dimmable LED lights can be used with Remote Operations Asset Management — ROAM — systems to conserve energy.

Dimmable
If you want to dim the LED lights for any reason, you need to specify that they can be dimmed. This works well with lighting management systems such as Remote Operations Asset Management (ROAM) to dim lighting zones to conserve energy.

IP66 rating
IP66 refers to the Ingress Protection Rating. The first six in “66” is for dust protection, and the second six is for water jet protection. Either the fixture has it or it doesn’t. You will have problems if dust or water gets into the fixture, so specify that the fixture is designed for IP66 and you will be fine.

Cost
How much is this fixture going to cost the client? Keep in mind it is not just the fixture cost; it is the installation and energy usage also. Generally, the installation should be the same for any fixture. A fixture meeting all of the criteria above will probably be more expensive than one that doesn’t, but when there are problems down the road it will be your problem not your client’s problem.

Photometric design
Not all LEDs are created equal, so it is important to educate yourself (and your client) about what they are actually getting. Assuming you have worked out the fixture selection, you are ready to start your design. The most important point is that you have an idea what fixture you are going to use beforehand. Without this as a starting point, there will be endless possibilities of fixtures, pole spacing, etc.

The most important design aspect is the photometric design. Without a proper photometric design you will have no idea how your light fixture will perform for its intended purpose. The photometric design is prepared with a software program such as Acuity Brands Lighting’s Visual. Key outputs are lumen (light) maximum/minimum ratio, average/minimum ratio, and minimum foot-candles. Pole spacing, mounting heights, fixture distribution, and fixture intensity all factor into the photometric design. Your lighting manufacturer should have Illuminating Engineering Society (.IES) files for whatever fixture and distribution you choose to input into the program.

Pole spacing, mounting heights, fixture distribution, and fixture intensity all factor into the photometric design.

It is important to note that if you want to keep the existing pole spacing and simply want to switch out the conventional fixtures for LEDs you still need to analyze the photometrics to see if it will work.

My first LED lighting project for the City of Blue Ash, Ohio was a success for me as well as for my client. Hopefully, the information in this article will help you attain the same success on your next LED lighting project.

Ray Schork is currently a quality assurance manager for planning and design projects working in the Cincinnati-metro region.


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