[Bldg-sim] question regarding lighting efficiency/gains units of measurement

[Guglielmetti, Robert]

"Anyway, for the sake of a dimensional analysis:
1 lux = 1 lumen/W. "

Uhm, no it doesn't.

Refreshing as it is to see this discussion trending toward the true meaning of lighting efficiency, this is a wiggly target. Too often here in the US we see "payback analysis" on lighting efficiency upgrades based solely on luminous efficacy, or lumens per watt, or the amount of light (luminous flux) that comes outta the lamp. Problem is, we're trying to illuminate a space and a task, and the metric here is luminous flux density, or lux, which is *lumens per meter squared*.

Lamps (light bulbs) get shoved into light fixtures (or fittings, or luminaries, whatever), and they spew light, and the light fixture (reflector, lens, whathaveyou) sends some of that light on its way into the room, which in turn does more stuff to the light -- it absorbs some, it reflects some, and some literally goes out the window. The final answer of how much light makes it to the task is expressed in lux, yes. But this result is derived by calculating the effect of the *space* on the light that made it out of the fixture. This will depend on the shape of the room (specifically, its room cavity ratio) and the reflectance of its surfaces, primarily. The answer is different for every unique space.

The point is you need to do at least a rudimentary illuminance calculation with the new lighting to confirm the ultimate goal (task illuminance) is still being met with the new tech. ASSuming it does, then you are able to simply relate the new tech to the old in terms of lux/watt, i.e. How much "seeing power" you end up with on the desk for a given amount of electrical power consumed.

This of course tells you nothing about the overall quality of the visual environment, but it's still better than just looking at lamp efficacy.



Rob Guglielmetti
NREL Commercial Buildings Research Group
Golden, CO 80401
robert.guglielmetti at nrel.gov

-----Original Message-----
From: Julien Marrec [julien.marrec at gmail.com<mailto:julien.marrec at gmail.com>]
Sent: Wednesday, May 28, 2014 08:24 AM Mountain Standard Time
To: Nick Caton
Cc: bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] question regarding lighting efficiency/gains units of measurement

Hi,

I agree with Nick, usually the metrics used in the US concerns the Lighting Power Density (LPD), in W/sqft, but this is also the case in most of Europe, of course in W/m² (at least in France and I'm guessing the UK too considering the BREEAM Standard).
Usually standards adapt to different required lighting levels by giving max LPD per space type (to adjust for required lux levels)


Anyway, for the sake of a dimensional analysis:

1 lux = 1 lumen/W.

<=> W/(m².100.lux) = (1/100) / (W/lm)

So it is the inverse of the luminous efficacy (expressed as a percentage).

Now if you want to throw in foot-candles (1 fc = 1 lm/ft²) just for fun... be my guest. that'd be W/(m².100.lux) = 1/100 * W / (ft².fc) but I have never seen that being used...


(I can't resist the opportunity to say it'd be about time to start using the SI units in the US too :))

Best,
Julien

--
Julien Marrec, EBCP, BPI MFBA
Ingénieur en efficacité énergétique du bâtiment
T: +33 6 95 14 42 13
www.julienmarrec.com<http://www.julienmarrec.com>

DoYouBuzz : www.doyoubuzz.com/julien-marrec_1<http://www.doyoubuzz.com/julien-marrec_1>
LinkedIn (fr) : www.linkedin.com/in/julienmarrec/fr<http://www.linkedin.com/in/julienmarrec/fr>
LinkedIn (en) : www.linkedin.com/in/julienmarrec<http://www.linkedin.com/in/julienmarrec>


2014-05-28 15:51 GMT+02:00 Nick Caton <ncaton at smithboucher.com<mailto:ncaton at smithboucher.com>>:
I’ll take a stab:

First, watts per square foot is the typical unit of measure for installed lighting power in the states.  Installed lighting power is what comes up most often in discussions of lighting system efficiencies/gains (in my experience).

Luminous efficacy (Lumens per Watt ) pops up when comparing sources (i.e. LED vs. Fluorescent).  When comparing lighting fixtures as a delivery method, “delivered” lumens per watt is the preferred metric, which additionally takes into account the efficiency of the fixture/housing involved.

Tying these all together:  Watts = 0.09290304 x lux x (square feet) / (lumens per watt)

(yep, I had to google that)

Ignoring the constants for a moment, W / (m2*lx) looks something like the inverse of what I’m used to calling luminous efficacy, so where you might describe a lowering of W / (m2 * lx), that would equate to a gain in luminous efficacy (lm/W).

Is that messy enough?

[cid:image001.jpg at 01CF7A50.53AE06C0]

NICK CATON, P.E.
SENIOR ENGINEER

Smith & Boucher Engineers
25501 west valley parkway, suite 200
olathe, ks 66061
direct 913.344.0036
fax 913.345.0617
www.smithboucher.com<http://www.smithboucher.com>

From: Bldg-sim [mailto:bldg-sim-bounces at lists.onebuilding.org<mailto:bldg-sim-bounces at lists.onebuilding.org>] On Behalf Of Christian Stalberg
Sent: Wednesday, May 28, 2014 5:22 AM
To: bldg-sim at lists.onebuilding.org<mailto:bldg-sim at lists.onebuilding.org>
Subject: [Bldg-sim] question regarding lighting efficiency/gains units of measurement

In Europe one sees W/m2-100 lux as a measure of lighting related gains or lighting efficiency. What is the equivalent here in the states? I suspect it is a messy conversion.

Thank you in advance.

Regards,
_
Christian Stalberg
Natural Intelligence, LLC
http://naturalintelligence.us
Tel. 415.531.4610


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