Re: [EnergyPlus_Support] Re: lux and watts

No, Lumen and Lux are not the same thing.

Lumens measure the amount of light generated by the lamp.Â The distribution of the light will depend on the shape of any diffuser or reflector etc.

Lux is the light density i.e. lumens per square meter.

The LED lamp giving 50 lumens for 1W may have a narrow beam reflector and give a high Lux level, or a wide beam reflector and a low Lux level.Â Also the lux level will be higher close to the lamp and drop further away as the light spreads out.

Lighting design must take into account how the light is distributed by lights and also reflected from walls and ceiling before it reaches the reference point.Â E.g. usually we have a target of 500 Lux at desk level in an office. The number and spacing of lights required to give this has to be calculated.

EnergyPlus does not do this, it assumes is has been done and you then tell it what the desired level is and the number of watts required to achieve this.Â EP then calculates how much the watts can be reduced as a result of daylight available.

Donald

Donald MacSween

Energy Consultant

GreenspaceLive

_________________________________________Â Â Â
Â T. +44(0)1851 707876Â Â M. +44 (0) 7708 339647

W. www.greenspacelive.com

On Mon, Jun 23, 2014 at 6:30 AM, 'jeannieboef@xxxxxxxxx' jeannieboef@xxxxxxxxx [EnergyPlus_Support] <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:

Â

Lumen and lux are not the same thing. Just checking.

Mit freundlichen GrÃ¼Ã?en- Sent from my iPhoneÂ (excuse the brevity)

i. A.
Jean Marais

b.i.g. bechtold
Tel. Â +49 30 6706662-23

On 22.06.2014, at 19:49, "vincent lacour vlacour@xxxxxxxxx [EnergyPlus_Support]" <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:

Â

Thanks Ery,Â
but we need to go one step further. Â I want to use the daylighting:controls. Â To do this I need to to fill in the Illuminance SetPoint at First Reference Point field. Â This field uses lux as a unit. Â Quoting from the IinputOutputReference:"The desired lighting level (in lux) at the First Reference Point. This is the lighting level that would be produced at this reference point at night if the overhead electric lighting were operating at full input power."

If I understand it correctly, starting from the desired lighting level, I have to design my lighting system in lumen and define the power demand in watts.
It makes sense.
thanks again.

Vincent

On Sun, Jun 22, 2014 at 7:19 PM, Ery Djunaedy ery.mailinglist@xxxxxxxxx [EnergyPlus_Support] <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:

Â

Vincent

The efficacy of the lights is in your input, EnergyPlus will take
whatever you define as the efficacy.

If you define 10W/m2 for a zone, and your daylight sensor is set at
500 lux, then YOU are telling EnergyPlus that your light is at 50
lumen per watt.

You can change the LPD to 1 W/m2, with the same 500 lux setpoint, then
YOU are telling EnergyPlus that your light is at 500 lumen per watt.

EnergyPlus always assumes that in the absence of daylighting, your
electric lights will be able to meet the daylight sensor setpoint.

It's not a mystery under the hood, but it is simply your input.

Makes sense?

Ery

On Thu, 19 Jun 2014, vincent lacour vlacour@xxxxxxxxx [EnergyPlus_Support] wrote:

> Thanks Rahul, but this does not address the issue. Everything in
> Daylighting:controls is inLUX whereas everything in Lights is in Watts.
> What is missing is the efficiency of the lights, i.e. how much LUMEN you
> can get with 1W. With a led you get 50 lumen; with an incandescent light
> bulb you get only 10 lumen. There must be a lumen/watt ratio under the
> energyplus hood, but where????
>
>
> On Thu, Jun 19, 2014 at 6:09 PM, rahula7@xxxxxxxxx [EnergyPlus_Support] <
> EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:
>
> >
> >
> > If you look at the Daylighting:Controls object, there are a number of
> > fields:
> > - Lighting control type
> > - Minimum input power fraction for continuous dimming control
> > - Minimum input power fraction for continuous dimming control
> > - Number of control steps
> >
> > These fields combine to determine how much lighting power will be supplied
> > after subtracting the daylight contribution. It's all there in the I/O
> > Reference.
> >
> > Rahul
> >
> >
> >

--
Dr. Ery Djunaedy
Research Scientist - Building Simulation Specialist
Integrated Design Lab University of Idaho
306 S 6th ST, Boise, ID 83702
Phone: 208-429-0220