1. How do LED based products actually save money? An analogy that may help would be something like buying a new expensive fuel efficient car that doesn't need service and lasts 10-50 times longer than it's replacement. At some point in the calculation, you will reach a break even point, and beyond that, the rest is pure savings. We typically don't think about the extra costs when replacing a light bulb, but these costs really do add up as you can see from the energy savings calculator.
Having a high lm/W (think MPG) rating combined with a long service free life will add up to huge savings with a rapid pay back. The most important factor is the reliability of the product and the true efficiency.
2. Can LED lamps and fixtures be used in permanent installations? DMS Lighting products are designed for very long service life which includes the body housing as well as the LED light system. Providing for a practical period of time that the product may be installed and not require service vs. the alternatives available at the moment, it would be a good choice for a permanent installation. Again, a trusted source is important in terms of quality and ratings.
DMS products are also useful in hard to access areas where service costs and or replacement becomes a problem.
3. What are the advantages of using LED lamps instead of Compact Fluorescent lamps (CFL)? One difference is environmental in that CFLs generally contain some mercury in the tubes whereas LED lamps don't, making them more environmentally friendly. This is considering that both products are lead free.
Properly designed LED lamps have a useful lifetime of 50-100 thousand hours and generally grow less bright as they age. CFLs on the other hand, have a shorter life span of <10 thousand hours and tend to change color and then flicker as they reach the end of useful life.
LED lamps come on instantly and at full brightness. CFLs generally come on slowly and warm up to their operating brightness and color.
LED lamps are solid state and therefore more rugged than the glass based CFL.
LED lamps are better suited for spotlighting applications compared to CFL due to their point source nature and optics.
LED lamps can emit light of different colors very efficiently, but a painted CFL has a very low efficiency rating.
LED lamp with white light output produce less UV radiation than CFL lamps. UV can be degrade plastics and fade colors on paintings and prints.
4. What are some examples of where I would use LED lamps or fixtures?
LED lamps are well suited to be used as replacement spot lamps such as halogen or standard type MR11, MR16, PAR lamps and other types of in ceiling, in-wall, or inground fixtures. For instance, lighting up your palm tress with MR16 halogen lamps is quite common. Switching to LED lamps in this application will allow you to run a much greater nuimber of lamps and provide a consistant quality of light without rewiring existing systems.
Deck fixtures are another really good application. When using inground halogens for deck lighting, the temperature on the surface of the fixture can be too hot to step on with bare feet. With DMS LED lamps for fixtures, that is not a problem. Also colors and color mixing are possible when needed but would require rewiring and some additional control equipment.
Another cool application is spotlighting paintings or objects in the house. A 1 watt LED spotlamp can do this job quite well and can act as an abient background light as well since it draws almost no power and produces very little heat. This is not possible when using halogen lamps.
White LED lamps have been adopted early on by museums, art galleries and hotels. In many cases, white color derived from RGB was found to offer superior color enhancement for paintings and colored materials. Another large segment using LED based colored lamps early on were entertainment venues and amusement parks.
General lighting uses include spotlighting, wall washing, landscape, cabinet lighting, and low/medium intensity flood lighting.
5. What are the key considerations for selecting LED based solid state lamps?
Assuming that the reasons for switching to LED SSLs are to lower the lighting maintanance cost, reduce energy usage, provide specialty lighting, enhance lighted objects, lower the thermal A/C loading, and perhaps produce special effects, the next considerations which are important will be explained here:
1. Lumen maintenance life. Most if not all companies seem to claim to provide 50,000 to 100,000 hours of lifetime from their LED based products. In order to accomplish this amazing figure, it is critically important that the LED emitter source is known to be a varifiable quality maker and that maker's thermal and power derating curves are adhered to in the design of the product. Only buy from reputable sources that have the knowhow and experience in the design of these products, otherwise, you most likely will experience very early failure of the devices. Many companies quote the LED specifications of the best devices known at the time and not always the ones that are used in the product and do not have the experience or knowhow to execute a design that can meet the requirements to acheive the lumen maintenance figures expected. This figure of merit is a key basis for the return on investment calculation and is also related to expected operation temperature range, which if exceeded will reduce the expected life of the product.
2. Efficacy. Efficacy is expressed in lumens per watt or lm/W on the data sheets. This figure of merit is monumentally important in that it expresses not only the electrical efficiency of the light source but also its rank amoung other competing light sources such as Halogen and CFL class products. Again, some awareness of how this figure is misused is needed to make sure you are getting really what your are paying for. Many companies again follow the specification sheet or quote "marketing numbers" they need to sell their lamps. Part of the confusion also comes from the fact that there can be an apple and oranges comparison going on which sometimes cannot be avoided due to connection configurations. This number is most meaningful when the test conditions are spelled out and the data from a metrology lab is available for correlation.
It is important to note that the lumen maintenance and efficacy are related when an LED may be overdriven to produce a high lumen output at the expense of the temperature rise beyond the Safe Operating Area (SOA) or out of the derating curve. This is common in flashlights where 1,000 hours of operation are acceptable and brightness is paramount.
It is recommended to purchase from companies that have the measurement and testing data on hand or can supply it when required.
3. Energy Star Program. Although the proposal was just launched by the DOE's Energy Star program in late 2008, it gives you some good guidelines to follow for the future selection of products. This program has taken into consideration many of the design targets LED SSL products should be heading for and will make this part of the Energy Star rating system. Products with the mark, should be fully qualified which could happen in the near future. This program will most likely weed out the under engineered and overstated products that have caused some confusion and dissapointments in this exciting product area.
6. What can I expect at this time as a realistic performance figure for a properly designed LED SSL product?
As an example, let us start at the heart, which would be the LED emitter. For this example, we have chosen the Cree XR-E-Q5 in cool white which just happens to be the best performing LED emitter available. Let us do some calculations from here so you can have a clearer picture of what is involved.
First we need to decide on our target driving current which we have decided will be 750mA. Looking at the electrical characteristics of current vs. forward voltage, we can see this driving current will develop 3.5V across the device with a power draw at the device of 0.750 x 3.5 = 2.625 W. Now we design a heatsink structure that allows a certain temperature rise above our maximum ambient operating temperature that is within the thermal design derating curves provided by Cree for this device. In fact, this is how we have chosen our input current initially. If the die junction temperature exceeds 80 degrees C in the design, the 50,000 hour lumen maintenance projection will have to be reduced. This ties into what was mentioned before regarding the ambient operating temperature being related to the life rating.
Considering we did a good job designing a proper thermal management system and specifed the correct maximum ambient temperature, we can be assured with proper usage the device will meet its life expectancy of 50,000 hours.
At this point we can calculate the Lumen output of the raw device. Again, refering to the device data sheet from Cree, we see that at 750mA we can expect around 175 lm at 25 degrees C. This figure should be degraded by 10% at a die temperature of 80 degrees C, so we should use 158 lm worst case. As this device has a beam pattern that is 90 degress Full Width Half Max (FWHM) we have chosen to place a 25 degree optic on the device. This optic, if well designed, has an efficiency of 80% so our lumen output is now at 126 lm. If we decide not to include the constant current supply into our final calculation, which in some instances isn't required, we would be producing 47.5 lumens per watt. This would represent a good indication of a typical LED lamp design using the State of the Art Cree Q5 device. Throwing a constant current power supply into the equations would net around 40.3 lumen per watt.
Interesting stuff to say the least. Don't be fooled faulty claims or improper designs when choosing a long term product such as LED based solid state lighting as anything above the figure is doubtful at present and even less likely when using Warm White, which cannot be driven as hard and as Luminous flux is weighted to human vision, the output will also register a lower output.
