We've been Misled about Light Levels - especially L70
If you're like me, you believed the IESNA when they said that our vision can only detect differences in light levels of about 30% (I must admit I repeated this claim many times). That means you can't tell the difference in perceived brightness between a lamp at 100% and one at 71%; Only when a lamp diminishes in luminance to 70% of its original value can you tell the difference - hence L70.
This is patently false for landscape lighting applications.
The IESNA L70 standards were based primarily on typical interior lighting - with a focus on office lighting. Their research indicated that a drop in less than 30% of typical light levels in interior lighting was not noticeable.
Office lighting is very different from landscape lighting. Office lighting with typical footcandle levels of 10 to 50, compare to landscape lighting with typical levels of one tenth of those values.
Now, it's important to note (in the IESNA's defense) that they said it may be appropriate to use L80 or a higher number for other (more critical) applications. But the landscape lighting industry has chosen not to embrace the higher standards - probably because the testing has not been done to identify the appropriate levels.
And, it's probably true that lights projecting on plant material can diminish significantly in light levels before the client notices - the L70 standard is probably appropriate for plants.
Architectural lighting, on the other hand, is far less forgiving. The other day I held a dimmer in my hand and adjusted the light level of a directional fixture projected on the side of a house. I increased and decreased the light level in 5% increments. I was shocked to find that each 5% change in light level was obvious to see. At 65% the light was overly bright. At 55% it was too dim. At 60% it was just right.
Every lighting designer should do the above test. I predict you will be as shocked as myself that a 5% difference in light level can make the difference between a design that works and one that doesn't.
To conclude, I wanted to make two points:
1. L70 is not an appropriate standard for exterior architectural lighting - so when evaluating specs, look for fixtures with robust thermal management likely to lead to extended LED life without appreciable lumen depreciation. Otherwise, a few years down the road your LED source may fail to satisfy the needs of the design.
2. Select LED fixtures that allow you to fine-tune lumen level - LED's give us this capability - we should fully leverage the technology to become better designers.