Originally Posted by Viewpoint
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.
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.