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We've been Misled about Light Levels - especially L70

Discussion in 'Landscape Lighting' started by steveparrott, Aug 19, 2013.

  1. steveparrott

    steveparrott Sponsor
    Posts: 1,196

    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.
  2. LLC RI

    LLC RI LawnSite Member
    Posts: 149

    Steve... I saw the demo of the new Impressionist series... looks pretty cool.. it will have it's niche.. for sure.

    Also.. on the L70... I think that another factor that presents itself in that whole discussion with regards to office lighting, is that most office lighting will be effected by ambient light that comes in through the windows. As such, that ambient light will fill in the void created by dimming down to 70 %. Just a thought that popped in my head when I read your post.


  3. Viewpoint

    Viewpoint LawnSite Member
    Posts: 75

    One thing to consider Steve- When using a dimmer to control lighting, the dimmer controls power, not light output. When you're making those 5% incremental changes, are you measuring the CBCP output of the light? I'm pretty sure that a 5% change at the dimmer will not be an exact correlation to the measured light output and certainly not exact in percieved light output. This will be different from each light manufacturer and each dimmer manufacturer.

    Our eyes change and adapt instantaneosly to light input, so percieved light is like beuaty, as always, in the eye of the beholder. We can get as scientific as we want making all these measurements, but the only thing that really matters is what we percieve.

    L70 is noticeable if we replace one lamp and stick it next to one that is at 100%. But, if all the lamps are deteriorating at the same rate, a light putting out 70% next to a whole bunch of other lights at 70% is no big deal. Our eyes (and brain) will make up the difference.
  4. steveparrott

    steveparrott Sponsor
    Posts: 1,196

    Andy, you're absolutely right. The 5% dimming increments are power measurements - which correlate to light level (e.g. CBCP) changes but not in a linear way. Still, when doing the design and talking about the levels it is useful to have a number to reference.

    In practice, when I dim our new lights, I perceive each 5% change to be roughly equivalent - meaning a change from 95% to 90% seems like a similar change as from 20% to 15% - though changes at higher levels are slightly more noticeable.

    Regarding the rate at which lights diminish in light output (re: L70), this can vary siginificantly depending on the thermal transfer for each fixture at the site. Even relatively minor differences in patterns of air flow around the fixture can significantly alter heat dissipation. Also, if you look at LED lumen depreciation curves you usually see sharp declines in lumen levels once the levels dip below the 80-70% level. So, fixtures nearing their end of life may show drastically different lumen levels.

    It's still early to see these end-of-life scenarios in LED jobs installed over the past few years. The next two or three years should be interesting.

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