Folks,

This may not be a relevant question for Radiance list, but I ask it anyway.

If I have the solar incident (irradiance, direct and diffuse) on the external facade, is there a simple way to calculate/estimate the illuminance on the facade? The magic number 179?

Thanks,

Ery

Dear Ery,

the 179 is the magic number just for Radiance... is the Luminous Efficacy.
My understanding is that the Luminous Efficacy of the clear sky is around
100; but it vary.

My answer would be NO. But wait for others, there might be some hope yet.

Regards,

German

···

2013/3/20 Ery Djunaedy <[email protected]>

Folks,

This may not be a relevant question for Radiance list, but I ask it anyway.

If I have the solar incident (irradiance, direct and diffuse) on the
external facade, is there a simple way to calculate/estimate the
illuminance on the facade? The magic number 179?

Thanks,

Ery

______________________________**_________________

Ery,

German is correct. 179 lumens/watt is the efficacy for uniform
distribution of visible light. And only useful after you've converted to
visible energy.

The efficacy for the solar spectrum varies based on properties of the
atmosphere (which is constantly changing). The efficacy for direct sun is
generally 80-120 lumens/watt. The efficacy for sky is typically 100-140
lm/watt. The efficacy of light on a facade is further complicated since it
is a constantly changing mix of direct and sky.

Sorry I can't give you an easy number!

Andy

···

On Wed, Mar 20, 2013 at 3:36 PM, Germán Molina Larrain <[email protected]>wrote:

Dear Ery,

the 179 is the magic number just for Radiance... is the Luminous Efficacy.
My understanding is that the Luminous Efficacy of the clear sky is around
100; but it vary.

My answer would be NO. But wait for others, there might be some hope yet.

Regards,

German

2013/3/20 Ery Djunaedy <[email protected]>

Folks,

This may not be a relevant question for Radiance list, but I ask it
anyway.

If I have the solar incident (irradiance, direct and diffuse) on the
external facade, is there a simple way to calculate/estimate the
illuminance on the facade? The magic number 179?

Thanks,

Ery

______________________________**_________________

_______________________________________________
[email protected]

Hi Ery,

it might be worth looking at the gendaylit source. There should be some models implemented for estimation of the luminous efficacy, and thus conversion between radiometric and photometric units. Still, it is certainly a good idea to measure irradiance instead of illuminance if this is what you need.

Cheers, Lars.

All,

A bit more background information. We are doing a coupled EnergyPlus-Radiance simulation on blinds control. The blinds control is in EnergyPlus, and we use Radiance to calculate the illuminance in the space.

One of the blinds control algorithm that we tested uses the vertical illuminance at the exterior facade. As you said, there is no easy way to calculate this in EnergyPlus. So we need to have another Radiance run just to calculate the vertical illuminance at the external facade. I am not happy with having to call Radiance twice, because it adds a bit of complexity and a bit of time to the whole scheme. So that is why I was wondering about simple calculation of vertical illuminance if we know the total solar irradiance on the surface.

So I dig further. The file below contains some graphs with simple regression on irradiance and illuminance on external facade:
https://www.dropbox.com/s/mqs4vycp1vc01w7/vertical_illum_solar.zip

The irradiance is the total solar incidence (direct, diffuse plus ground reflection) calculated by EnergyPlus and the vertical illuminance is calculated by Radiance (gendaylit and rtrace). They both came from the same weather file (Boise TMY3 EPW).

Apart from anomalies on the some points on West and North facade, the slope is about 100-110 lm/W. I will dig deeper to look at some outliers in the data.

Any thoughts?

Ery

···

On 03/20/2013 05:54 PM, Andrew McNeil wrote:

Ery,

German is correct. 179 lumens/watt is the efficacy for uniform distribution of visible light. And only useful after you've converted to visible energy.

The efficacy for the solar spectrum varies based on properties of the atmosphere (which is constantly changing). The efficacy for direct sun is generally 80-120 lumens/watt. The efficacy for sky is typically 100-140 lm/watt. The efficacy of light on a facade is further complicated since it is a constantly changing mix of direct and sky.

Sorry I can't give you an easy number!

Andy

On Wed, Mar 20, 2013 at 3:36 PM, Germán Molina Larrain <[email protected] > <mailto:[email protected]>> wrote:

Dear Ery,

the 179 is the magic number just for Radiance... is the Luminous
Efficacy. My understanding is that the Luminous Efficacy of the
clear sky is around 100; but it vary.

My answer would be NO. But wait for others, there might be some
hope yet.

Regards,

German

2013/3/20 Ery Djunaedy <[email protected]
<mailto:[email protected]>>

Folks,

This may not be a relevant question for Radiance list, but I

If I have the solar incident (irradiance, direct and diffuse)
on the external facade, is there a simple way to
calculate/estimate the illuminance on the facade? The magic
number 179?

Thanks,

Ery

Hi Ery,

Tinkering with the Boise TMY3 data I was a little surprised to see how some of the efficacy values turned out. For those unfamiliar, the Boise TMY3 climate file (much like many of the others from the Energy+ website) contains both illuminance and irradiance data for these quantities:

- Global horizontal (Gh)
- Diffuse horizontal (Dh)
- Direct normal (Dn)

Sometimes illuminance data is derived from irradiance measurements, though nowadays it is more likely to be measured separately. Which it is could be identified in the climate file (I haven't checked).

The following table shows how the mean luminous efficacy (within 5klux bands of global horizontal illuminance) varies for: the global horizontal; the diffuse horizontal; and, the direct normal quantities:

gh_lo gh_hi Eff_gh Eff_dh Eff_dn

5000.00 10000.0 106.145 135.606 60.1583
10000.0 15000.0 106.610 147.202 73.0784
15000.0 20000.0 107.339 258.167 81.6775
20000.0 25000.0 108.312 140.848 88.2379
25000.0 30000.0 107.261 156.417 92.4818
30000.0 35000.0 106.713 184.455 94.1426
35000.0 40000.0 106.456 197.996 96.1488
40000.0 45000.0 106.142 213.014 97.9623
45000.0 50000.0 105.894 177.705 99.2376
50000.0 55000.0 105.992 173.584 100.170
55000.0 60000.0 105.812 179.285 100.867
60000.0 65000.0 106.057 300.989 101.701
65000.0 70000.0 105.593 129.234 101.205
70000.0 75000.0 105.347 145.367 101.420
75000.0 80000.0 105.333 193.799 101.418
80000.0 85000.0 105.044 135.980 101.445
85000.0 90000.0 104.697 134.219 100.965
90000.0 95000.0 103.924 123.413 100.588

The global efficacy value (Eff_gh), as you noted, is between 100 and 100 lm/W -- I'd expect global vertical to be very similar to global horizontal, at least for sun illuminated orientations. Direct normal efficacy across the ranges is also pretty much how I expected it to vary. However, diffuse horizontal efficacy seems to flap-around quite a bit -- four maxima, two of them quite conspicuous. I wouldn't have been too surprised to see general trends, but this was unexpected. Can't say that I've noticed or given much thought to it before, but I am a little intrigued by those high values, i.e. > 200 lm/W.

Best
John

John Mardaljevic
Professor of Building Daylight Modelling
School of Civil & Building Engineering
Loughborough University
Loughborough
Leicestershire
LE11 3TU, UK

Tel: +44 1509 222630 (Direct)
Tel: +44 1509 228529 (Pam Allen, secretary)

Personal daylighting website:
http://climate-based-daylighting.com

Hi John,

Thanks for looking at this. This is indeed interesting. When I get back in the office I can at least ask around if the measured versus derived illuminance question is answered somewhere in the weather file. As to the reason(s) why the efficacy for diffuse horizontal flaps around, I'm stumped, and unclear if its cause for alarm. Anyone?

Rob

Rob Guglielmetti
NREL Commercial Buildings Research Group
Golden, CO 80401
[email protected]

···

-----Original Message-----
From: John Mardaljevic [[email protected]<mailto:[email protected]>]
Sent: Monday, March 25, 2013 03:44 PM Mountain Standard Time
To: [email protected]

Hi Ery,

Tinkering with the Boise TMY3 data I was a little surprised to see how some of the efficacy values turned out. For those unfamiliar, the Boise TMY3 climate file (much like many of the others from the Energy+ website) contains both illuminance and irradiance data for these quantities:

- Global horizontal (Gh)
- Diffuse horizontal (Dh)
- Direct normal (Dn)

Sometimes illuminance data is derived from irradiance measurements, though nowadays it is more likely to be measured separately. Which it is could be identified in the climate file (I haven't checked).

The following table shows how the mean luminous efficacy (within 5klux bands of global horizontal illuminance) varies for: the global horizontal; the diffuse horizontal; and, the direct normal quantities:

gh_lo gh_hi Eff_gh Eff_dh Eff_dn

5000.00 10000.0 106.145 135.606 60.1583
10000.0 15000.0 106.610 147.202 73.0784
15000.0 20000.0 107.339 258.167 81.6775
20000.0 25000.0 108.312 140.848 88.2379
25000.0 30000.0 107.261 156.417 92.4818
30000.0 35000.0 106.713 184.455 94.1426
35000.0 40000.0 106.456 197.996 96.1488
40000.0 45000.0 106.142 213.014 97.9623
45000.0 50000.0 105.894 177.705 99.2376
50000.0 55000.0 105.992 173.584 100.170
55000.0 60000.0 105.812 179.285 100.867
60000.0 65000.0 106.057 300.989 101.701
65000.0 70000.0 105.593 129.234 101.205
70000.0 75000.0 105.347 145.367 101.420
75000.0 80000.0 105.333 193.799 101.418
80000.0 85000.0 105.044 135.980 101.445
85000.0 90000.0 104.697 134.219 100.965
90000.0 95000.0 103.924 123.413 100.588

The global efficacy value (Eff_gh), as you noted, is between 100 and 100 lm/W -- I'd expect global vertical to be very similar to global horizontal, at least for sun illuminated orientations. Direct normal efficacy across the ranges is also pretty much how I expected it to vary. However, diffuse horizontal efficacy seems to flap-around quite a bit -- four maxima, two of them quite conspicuous. I wouldn't have been too surprised to see general trends, but this was unexpected. Can't say that I've noticed or given much thought to it before, but I am a little intrigued by those high values, i.e. > 200 lm/W.

Best
John

John Mardaljevic
Professor of Building Daylight Modelling
School of Civil & Building Engineering
Loughborough University
Loughborough
Leicestershire
LE11 3TU, UK

Tel: +44 1509 222630 (Direct)
Tel: +44 1509 228529 (Pam Allen, secretary)

Personal daylighting website:
http://climate-based-daylighting.com<http://climate-based-daylighting.com/>

Rob and others
The very high numbers (>300) are not physically plausible (at least on this
planet with its atmosphere and sun). You can get numbers in the 200l/w
range if you filter sunlight through spectrally selective glass. If I had
to guess, without doing any homework, its an artifact of the derivation as
follows: I assume diffuse radiation is calculated by processing measured
global irradiance and measured or assumed beam, and that diffuse
illuminance is calculated by processing measured global illuminance and
measured or assumed beam, so you are comparing two "small" numbers, each
the result of subtraction of two much larger numbers, so that relatively
small errors in the original calcs are magnified by that derivation. If
someone has time to dig into the source data you could verify or expose
another source. Of course it could be direct measurement and just a batch
of poorly measured/calibrated/reported data.

Steve

···

**********************************************************************

Stephen Selkowitz

Building Technology and Urban Systems Department (510) 486-5064

Bldg.90-3111
fax (510) 486-4089

Lawrence Berkeley National Laboratory
[email protected]

Berkeley, CA 94720
http://buildings.lbl.gov/ <http://BTECH.lbl.gov/&gt;

**********************************************************************

On Wed, Mar 27, 2013 at 8:51 AM, Guglielmetti, Robert < [email protected]> wrote:

**
Hi John,

Thanks for looking at this. This is indeed interesting. When I get back in
the office I can at least ask around if the measured versus derived
illuminance question is answered somewhere in the weather file. As to the
reason(s) why the efficacy for diffuse horizontal flaps around, I'm
stumped, and unclear if its cause for alarm. Anyone?

Rob

Rob Guglielmetti
NREL Commercial Buildings Research Group
Golden, CO 80401
[email protected]

-----Original Message-----
*From: *John Mardaljevic [[email protected]]
*Sent: *Monday, March 25, 2013 03:44 PM Mountain Standard Time
*To: *[email protected]

Hi Ery,

Tinkering with the Boise TMY3 data I was a little surprised to see how
some of the efficacy values turned out. For those unfamiliar, the Boise
TMY3 climate file (much like many of the others from the Energy+ website)
contains both illuminance and irradiance data for these quantities:

- Global horizontal (Gh)
- Diffuse horizontal (Dh)
- Direct normal (Dn)

Sometimes illuminance data is derived from irradiance measurements, though
nowadays it is more likely to be measured separately. Which it is could be
identified in the climate file (I haven't checked).

The following table shows how the mean luminous efficacy (within 5klux
bands of global horizontal illuminance) varies for: the global horizontal;
the diffuse horizontal; and, the direct normal quantities:

gh_lo gh_hi Eff_gh Eff_dh Eff_dn

5000.00 10000.0 106.145 135.606 60.1583
10000.0 15000.0 106.610 147.202 73.0784
15000.0 20000.0 107.339 258.167 81.6775
20000.0 25000.0 108.312 140.848 88.2379
25000.0 30000.0 107.261 156.417 92.4818
30000.0 35000.0 106.713 184.455 94.1426
35000.0 40000.0 106.456 197.996 96.1488
40000.0 45000.0 106.142 213.014 97.9623
45000.0 50000.0 105.894 177.705 99.2376
50000.0 55000.0 105.992 173.584 100.170
55000.0 60000.0 105.812 179.285 100.867
60000.0 65000.0 106.057 300.989 101.701
65000.0 70000.0 105.593 129.234 101.205
70000.0 75000.0 105.347 145.367 101.420
75000.0 80000.0 105.333 193.799 101.418
80000.0 85000.0 105.044 135.980 101.445
85000.0 90000.0 104.697 134.219 100.965
90000.0 95000.0 103.924 123.413 100.588

The global efficacy value (Eff_gh), as you noted, is between 100 and 100
lm/W -- I'd expect global vertical to be very similar to global horizontal,
at least for sun illuminated orientations. Direct normal efficacy across
the ranges is also pretty much how I expected it to vary. However, diffuse
horizontal efficacy seems to flap-around quite a bit -- four maxima, two of
them quite conspicuous. I wouldn't have been too surprised to see general
trends, but this was unexpected. Can't say that I've noticed or given much
thought to it before, but I am a little intrigued by those high values,
i.e. > 200 lm/W.

Best
John

John Mardaljevic
Professor of Building Daylight Modelling
School of Civil & Building Engineering
Loughborough University
Loughborough
Leicestershire
LE11 3TU, UK

Tel: +44 1509 222630 (Direct)
Tel: +44 1509 228529 (Pam Allen, secretary)

Personal daylighting website:
http://climate-based-daylighting.com

_______________________________________________
[email protected]

Just to muddy the question a little more, why do the TMY3 files report illuminance in lux but efficacy in lms/Watt?

···

On 3/28/2013 8:24 AM, [email protected] wrote:

-----Original Message-----
>*From: *John Mardaljevic [[email protected]]
>*Sent: *Monday, March 25, 2013 03:44 PM Mountain Standard Time
>*To: *[email protected]
>
>Hi Ery,
>
>Tinkering with the Boise TMY3 data I was a little surprised to see how
>some of the efficacy values turned out. For those unfamiliar, the Boise
>TMY3 climate file (much like many of the others from the Energy+ website)
>contains both illuminance and irradiance data for these quantities:
>
>- Global horizontal (Gh)
>- Diffuse horizontal (Dh)
>- Direct normal (Dn)
>
>Sometimes illuminance data is derived from irradiance measurements, though
>nowadays it is more likely to be measured separately. Which it is could be
>identified in the climate file (I haven't checked).
>
>The following table shows how the mean luminous efficacy (within 5klux
>bands of global horizontal illuminance) varies for: the global horizontal;
>the diffuse horizontal; and, the direct normal quantities:
>
>gh_lo gh_hi Eff_gh Eff_dh Eff_dn
>
> 5000.00 10000.0 106.145 135.606 60.1583
> 10000.0 15000.0 106.610 147.202 73.0784
> 15000.0 20000.0 107.339 258.167 81.6775
> 20000.0 25000.0 108.312 140.848 88.2379
> 25000.0 30000.0 107.261 156.417 92.4818
> 30000.0 35000.0 106.713 184.455 94.1426
> 35000.0 40000.0 106.456 197.996 96.1488
> 40000.0 45000.0 106.142 213.014 97.9623
> 45000.0 50000.0 105.894 177.705 99.2376
> 50000.0 55000.0 105.992 173.584 100.170
> 55000.0 60000.0 105.812 179.285 100.867
> 60000.0 65000.0 106.057 300.989 101.701
> 65000.0 70000.0 105.593 129.234 101.205
> 70000.0 75000.0 105.347 145.367 101.420
> 75000.0 80000.0 105.333 193.799 101.418
> 80000.0 85000.0 105.044 135.980 101.445
> 85000.0 90000.0 104.697 134.219 100.965
> 90000.0 95000.0 103.924 123.413 100.588
>
>The global efficacy value (Eff_gh), as you noted, is between 100 and 100
>lm/W -- I'd expect global vertical to be very similar to global horizontal,
>at least for sun illuminated orientations. Direct normal efficacy across
>the ranges is also pretty much how I expected it to vary. However, diffuse
>horizontal efficacy seems to flap-around quite a bit -- four maxima, two of
>them quite conspicuous. I wouldn't have been too surprised to see general
>trends, but this was unexpected. Can't say that I've noticed or given much
>thought to it before, but I am a little intrigued by those high values,
>i.e. > 200 lm/W.
>
>Best
>John
>
> John Mardaljevic
>Professor of Building Daylight Modelling
>School of Civil & Building Engineering
>Loughborough University
>Loughborough
>Leicestershire
>LE11 3TU, UK
>
>Tel: +44 1509 222630 (Direct)
>Tel: +44 1509 228529 (Pam Allen, secretary)
>
>[email protected]
>http://www.lboro.ac.uk/departments/cv/staff/profile/367.html&lt;http://www.lboro.ac.uk/departments/cv/&gt;
>
>Personal daylighting website:
>http://climate-based-daylighting.com
>
>_______________________________________________
>[email protected]
>

--

*Jill Dalglish, LEED AP BD+C*

www.dalglishdaylighting.com <http://www.dalglishdaylighting.com/&gt;
303-955-4945

Illuminance in lux is really lumens/m² and irradiance is Watts/m². Divide the two and the unit of area cancels out.

···

From: Jill Dalglish [mailto:[email protected]]
Sent: Thursday, March 28, 2013 10:49 AM
To: [email protected]