PDA

View Full Version : LED luminance considerations


steveparrott
11-08-2008, 08:30 AM
As I've brought up in many previous posts, aside from considerations of color and beam characteristics, maintenance of LED luminance is really the key concern for landscape lighting designers.

Standards for LED lamp life have been proposed by the LRC and are being integrated into IESNA guidelines. The primary standard (life definition), designated by L(70%), is the number of operational hours when total lumens have depreciated to 70% of their original value.

70% was chosen because research has shown that a 30% reduction in light level is at about the threshold of detectable difference and judged to be acceptable by occupants of the space.

A couple points about this:

1. This research was conducted in well-lit interior spaces where light levels are fairly high. Light levels in landscape lighting are very low (in the Mesopic region - the region where retinal cones that detect color and fine detail are losing sensitivity and rods that detect contrast and movement are becoming active). If the design starts with brightness in the Mesopic region, then as brightness decreases, vision moves into the Scotopic region (night vision) where color perception disappears. To my knowledge, there's no research looking into the detectable threshold of brightness difference when you're starting with such low levels. It's possible that, after two or three years an LED landscape lighting job will look drab because brightness levels have diminished to the point where colors are poorly perceived.

2. The research also indicates that when light sources are in critical applications (such as wall washes with side-by-side beams) the standard should be set at 80%. An LED with a L(70%) of 50,000 hrs, has an L(80%) of about 30,000 hrs. Given that these standards are set in laboratory conditions, actual life may be more in the range of 20,000 hrs. We just don't know.

irrig8r
11-08-2008, 09:58 AM
Also, in a landscape setting, other factors such as hard water or soil deposits on lenses and plant overgrowth can also diminish light output, so that even if the lamp needs replacing less frequently, maintenance labor is still a cost to be factored into the total picture.

Mike M
11-08-2008, 01:41 PM
Gregg, I don't think that is what Steve is getting at. I think the main concern is that the LED may still be going 7 or 8 or 9 years after install, but it may need to be replaced because of noticeable loss of output. I guess we just won't know until we see what happens.

Steve's point reminds me of how a lot of MR bulbs should be replaced because of the mirror finish wearing off. The bulb is still going, but it loses its focus and becomes a mini-wash.

JoeyD
11-11-2008, 10:42 AM
What Steve is pointing out is something we are clearly aware of and fear. It is this that has kept us from developing LED fixtures and making claims as to them having equal lumen output when these manufactuers making them know darn well that the lumens will deteriorate. Now maybe this LUXXO MR16 LED has something that no one else does but even though we are supplying it we are still skeptical. Only time will tell!!

irrig8r
11-11-2008, 11:48 AM
Mike, what I was saying was that even if lamps don't need replacing there is often other maintenance work to be done that might even involve more labor than changing a lamp, and so before making any claims as to how using LEDs might effect the bottom line when considering the overall cost over the life of the system, that should be included.

(Do I get a run-on sentence award for that one?)

JoeyD
11-11-2008, 12:44 PM
forget about lamps....SYSTEMS need maintenance and monitoring. You need to adjust timers, adjust fixture alignement, clean lenses, add fixtures, etc....

NightScenes
11-19-2008, 02:58 PM
Don't forget that all lamps' output gradually diminishes over time. Lamps don't just go out, they gradually decay until the finally go out. A one year old MR16 will not be as bright as a new one.