Wednesday, September 10, 2008
Illumination coverage area from leds
Please comment on your experience with coverage area you have observed with various LED lighting products.
Thursday, September 4, 2008
Thermal Issues with Outdoor SSL
Most of us know that solid state lighting design requires effective thermal management. Thermal discussion was the #1 hold up in the early days of LED traffic signals. In fact, for the first 2 years after ITE and NEMA became involved with LED Traffic Signal Standards, they conducted studies of solar heating effects in various geographical locations. But that's history, and obviously LED traffic signals are a huge success.
Traffic signals must be their brightest at the hottest time of day. Since outdoor lighting with LEDs takes place at night, do we assume the Sun is not an issue? This would definitely be a bad assumption. Referring back to the traffic signal studies, it was demonstrated that non-energized traffic signal heads of any exterior color, exposed to the Sun, had internal tempratures exceeding 85°C. This fact must not be overlooked in the design of outdoor solid-state luminaires; after all, they will be in the Sun all day long.
Why? The LED luminaires will not be on during the day, so why be concerned with the solar heat?
It's true that LEDs degrade with excessive junction temperature while driven with current; the LED junction typically will not be degraded without electrical bias, even at storage temperatures.
However, white LEDs of the phosphor conversion type can be degraded when the phosphor is subjected to excessive heat, with or without electrical bias. That would be the case in luminaires that behave as solar collectors.
(Note that previous LED technologies by some manufacturers had epoxy encapsulant that was sensitive to the blue light or Ultra-Violet, and it yellowed over time. Obviously for outdoor applications, no element should yellow from UV exposure because of the required daily exposure to UV from the Sun.)
LEDs rated for 50,000 hours that are stated to have a 12-year service life may give a surprising reduction in lumen maintenance when subjected to 85°C on a daily basis for several years. A nighttime Luminaire is often characterized at 50% duty cycle, i.e. 50,000 hours is stated as a 12-year lifetime of lumen maintenance. However, if that Luminaire is powered at night and collecting solar heat by day, it may actually be a 6-year product.
Phosphor manufacturers have thermal degradation graphs for their product, which may show a permanent shift in CCT as well as conversion efficiency degradation. Some LED manufacturers have taken steps to distance the phosphor coating from the heat-producing junction.
There are SSL Luminaires with heat exchangers designed to address the solar issues, and some Luminaires really are efficient solar collectors. Can you spot the difference?
Traffic signals must be their brightest at the hottest time of day. Since outdoor lighting with LEDs takes place at night, do we assume the Sun is not an issue? This would definitely be a bad assumption. Referring back to the traffic signal studies, it was demonstrated that non-energized traffic signal heads of any exterior color, exposed to the Sun, had internal tempratures exceeding 85°C. This fact must not be overlooked in the design of outdoor solid-state luminaires; after all, they will be in the Sun all day long.
Why? The LED luminaires will not be on during the day, so why be concerned with the solar heat?
It's true that LEDs degrade with excessive junction temperature while driven with current; the LED junction typically will not be degraded without electrical bias, even at storage temperatures.
However, white LEDs of the phosphor conversion type can be degraded when the phosphor is subjected to excessive heat, with or without electrical bias. That would be the case in luminaires that behave as solar collectors.
(Note that previous LED technologies by some manufacturers had epoxy encapsulant that was sensitive to the blue light or Ultra-Violet, and it yellowed over time. Obviously for outdoor applications, no element should yellow from UV exposure because of the required daily exposure to UV from the Sun.)
LEDs rated for 50,000 hours that are stated to have a 12-year service life may give a surprising reduction in lumen maintenance when subjected to 85°C on a daily basis for several years. A nighttime Luminaire is often characterized at 50% duty cycle, i.e. 50,000 hours is stated as a 12-year lifetime of lumen maintenance. However, if that Luminaire is powered at night and collecting solar heat by day, it may actually be a 6-year product.
Phosphor manufacturers have thermal degradation graphs for their product, which may show a permanent shift in CCT as well as conversion efficiency degradation. Some LED manufacturers have taken steps to distance the phosphor coating from the heat-producing junction.
There are SSL Luminaires with heat exchangers designed to address the solar issues, and some Luminaires really are efficient solar collectors. Can you spot the difference?
Thursday, August 7, 2008
Scotopic verses Photopic for Best Night Vision
An irony - for as long as we've been able to measure outdoor lighting to light the streets, nighttime luminance criteria has been based on the wrong color, and has given a false impression of safety.
When luminance is greater than 3 cd/m2, our "daylight vision" peak sensitivity is 560nm, this is the Photopic scale. For less than 0.001 cd/m2, our color sensitivity peaks at 500nm, the Scotopic scale. The scale between Scotopic and Photopic is Mesopic. Most outdoor lighting aligns with the center of this scale at 0.1 cd/m2.
More to the Scotopic-Photopic debate: when we must focus on an object, the cones of our retinas come into play regardless of luminance or color. The color of HPS lighting, at any luminance, is misaligned with our photopic peak. See an interesting article with Mesopic scale here. Specifically, we can be far more efficient with less light of the proper color.
Also see Graphic Comparisons and Explanation
When luminance is greater than 3 cd/m2, our "daylight vision" peak sensitivity is 560nm, this is the Photopic scale. For less than 0.001 cd/m2, our color sensitivity peaks at 500nm, the Scotopic scale. The scale between Scotopic and Photopic is Mesopic. Most outdoor lighting aligns with the center of this scale at 0.1 cd/m2.
More to the Scotopic-Photopic debate: when we must focus on an object, the cones of our retinas come into play regardless of luminance or color. The color of HPS lighting, at any luminance, is misaligned with our photopic peak. See an interesting article with Mesopic scale here. Specifically, we can be far more efficient with less light of the proper color.
Also see Graphic Comparisons and Explanation
Sunday, April 13, 2008
CRI is not the best metric with LEDs
Color Rendering Index (CRI) indicates how well a test source renders eight standard colors of intermediate saturation, when compared to a reference lamp of the same color temperature. Lab measured CRI is a comparison against a spectrally continuous red-weighted reference. Field conducted CRI tests are subjective with Human observers when luminance levels are below 3 cd/m2.
The industry is discovering that CRI is not the best metric for comparing LED light sources, especially at Mesopic levels. Originally developed in 1964, this index is based on outdated color models and assumes illumination sources with broad spectral distributions, whereas LEDs are narrow-band sources. And, nighttime lighting requirements fall primarily in the Mesopic range where our color sensitivity shifts with luminance, and there is no defined index.
Several standards bodies are addressing this deficiency, and in the interim, Color Temperature may be the most suitable tool for comparison because it is independent of observer subjectivity.
Here are some related links:
Metrics for solid-state lighting
Lumileds LED Glossary
www.netl.doe.gov PDF
The industry is discovering that CRI is not the best metric for comparing LED light sources, especially at Mesopic levels. Originally developed in 1964, this index is based on outdated color models and assumes illumination sources with broad spectral distributions, whereas LEDs are narrow-band sources. And, nighttime lighting requirements fall primarily in the Mesopic range where our color sensitivity shifts with luminance, and there is no defined index.
Several standards bodies are addressing this deficiency, and in the interim, Color Temperature may be the most suitable tool for comparison because it is independent of observer subjectivity.
Here are some related links:
Metrics for solid-state lighting
Lumileds LED Glossary
www.netl.doe.gov PDF
Saturday, April 12, 2008
Parking Garage LED Lights
How many LED parking garage lights have we found that were actually designed for the IESNA RP20 standard, and the NPA criteria? This means achieving the vertical footcandles without making a glare bomb.
A parking garage luminaire is a special critter, it has entirely different photometrics than typical outdoor luminaires. So this may be considered off-topic.
Here is a Blog on Raleigh for their selection of parking garage light. Clearly this luminaire puts all its light on the floor beneath, and up-light or lighting the walls at 5-ft is by chance. Comments found on that Blog pretty much make that point.
Another site also has illustrations in application which is more than a lot of sites I've found.
Let us hear about better garage luminaires you've found.
A parking garage luminaire is a special critter, it has entirely different photometrics than typical outdoor luminaires. So this may be considered off-topic.
Here is a Blog on Raleigh for their selection of parking garage light. Clearly this luminaire puts all its light on the floor beneath, and up-light or lighting the walls at 5-ft is by chance. Comments found on that Blog pretty much make that point.
Another site also has illustrations in application which is more than a lot of sites I've found.
Let us hear about better garage luminaires you've found.
Sunday, November 4, 2007
Utility Co. Rebates, Grants and other Incentives
There have been requests to add this category to assist users in getting rebates from their utility companies, or in finding grant money for deployment of this new technology.
In this section you could discuss "Performance Contracts" or Energy Services Contracts, etc.
If you have information you can discuss in this forum, please post here, or post an email address where you can be contacted.
Remember that new technology brings controversy. Some refuse to accept new things, so don't be alarmed by comments.
In this section you could discuss "Performance Contracts" or Energy Services Contracts, etc.
If you have information you can discuss in this forum, please post here, or post an email address where you can be contacted.
Remember that new technology brings controversy. Some refuse to accept new things, so don't be alarmed by comments.
Saturday, August 4, 2007
Pay Back or Return on Investment (ROI)
Here's where we determine justification. Don't forget warranty provisions, and whether the company can actually afford to replace defective units.
How much does a LED light really save on your electric bill? If the LED is expected to light as much area as a conventional light, and it's efficacy is not as high as High Pressure Sodium or Metal Halide, then it will consume more electricity, which is no bargain at all.
We know the LED can use less electricity if it reduces the excessive light beneath the fixture which is wasted light anyway, but that may be only 20%, correct? Then a LED light installation must pay back the purchaser with reduced maintenance.
If a light is a security or safety issue, a burned-out light must be replaced as soon as possible, so there is a call-out factor which may be overtime pay with a bucket truck. This may pay back the cost of a LED fixture by eliminating only a single repair incident. For a light that is costly to replace, such as on a bridge or in a tunnel, LEDs can offer a huge advantage. These cases are ideal to justify the higher initial cost for a longer life, robust product.
Calculating the payback isn't difficult, and some companies offer a spreadsheet to do that for you.
For example, a 100 watt HID lamp will have a ballast that pulls an additional 20 watts, and replacing that with a 75 watt LED at 8-cents/kWh will save you less than $3 per month. How many years will it require to pay back on energy savings?
If routine maintenance requires re-lamping or a new ballast every 2 years, and if that costs $50 then a $400 LED light could pay for itself in 8-years. Many agencies use a streetlight budget of $50 per year, and probably visit the light only when it's not working. So, the years they don't visit the light amounts to money used for something else within the department.
By switching to LED lights for maintenance savings, the department budget gets slashed by that amount.
Given those figures, payback from energy and routine maintenance savings only, would require cost of a LED streetlight to be less than $200 for a payback in 3-years.
Here is a Calculator "DEUS"
How much does a LED light really save on your electric bill? If the LED is expected to light as much area as a conventional light, and it's efficacy is not as high as High Pressure Sodium or Metal Halide, then it will consume more electricity, which is no bargain at all.
We know the LED can use less electricity if it reduces the excessive light beneath the fixture which is wasted light anyway, but that may be only 20%, correct? Then a LED light installation must pay back the purchaser with reduced maintenance.
If a light is a security or safety issue, a burned-out light must be replaced as soon as possible, so there is a call-out factor which may be overtime pay with a bucket truck. This may pay back the cost of a LED fixture by eliminating only a single repair incident. For a light that is costly to replace, such as on a bridge or in a tunnel, LEDs can offer a huge advantage. These cases are ideal to justify the higher initial cost for a longer life, robust product.
Calculating the payback isn't difficult, and some companies offer a spreadsheet to do that for you.
For example, a 100 watt HID lamp will have a ballast that pulls an additional 20 watts, and replacing that with a 75 watt LED at 8-cents/kWh will save you less than $3 per month. How many years will it require to pay back on energy savings?
If routine maintenance requires re-lamping or a new ballast every 2 years, and if that costs $50 then a $400 LED light could pay for itself in 8-years. Many agencies use a streetlight budget of $50 per year, and probably visit the light only when it's not working. So, the years they don't visit the light amounts to money used for something else within the department.
By switching to LED lights for maintenance savings, the department budget gets slashed by that amount.
Given those figures, payback from energy and routine maintenance savings only, would require cost of a LED streetlight to be less than $200 for a payback in 3-years.
Here is a Calculator "DEUS"
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