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.