# why "global horizontal illuminance" is smaller than "global horizontal radiation" multiplied by 179 in epw file?

Dear list, Happy New Year!

I have a simple question related to conversion from irradiance value to
illuminance value, and pls correct me if I'm wrong:

Usually we can estimate the *illuminance (lux)* for a given point by
by *179 (lm/w)* which is the luminous efficacy used in Radiance, or more
strictly (R*0.265+G*0.670+B*0.065)*179.

However, it seems that in a epw weather file the* "global horizontal
illuminance"* value is not equal to but smaller than the *"global
horizontal radiation" value multiplied by 179*.

1. why there's such a large discrapency?
2. Will this lead to over-estimation of illuminance when using cumulative
sky derived from "global horizontal radiation" ?

- Ji

Dear list, any comments as a celebration for avoiding the avoidable "fiscal
cliff" ?

- Thanks! Ji

···

On Wed, Jan 2, 2013 at 12:36 AM, Ji Zhang <[email protected]> wrote:

Dear list, Happy New Year!

I have a simple question related to conversion from irradiance value to
illuminance value, and pls correct me if I'm wrong:

Usually we can estimate the *illuminance (lux)* for a given point by
multiplying the *irradiance (w/m2)* for the point as simulated via
Radiacne by *179 (lm/w)* which is the luminous efficacy used in Radiance,
or more strictly (R*0.265+G*0.670+B*0.065)*179.

However, it seems that in a epw weather file the* "global horizontal
illuminance"* value is not equal to but smaller than the *"global
horizontal radiation" value multiplied by 179*.

1. why there's such a large discrapency?
2. Will this lead to over-estimation of illuminance when using cumulative
sky derived from "global horizontal radiation" ?

- Ji

Hi Ji,

179 is the efficacy of white (equal energy) light over the visible
spectrum.

Daylight is composed of a broader spectrum, so the efficacy (visible light
per watt of energy) is lower. Usually around 90 for the sun and 110 for
the sky, but changes based on various factors.

179 is used in Radiance as a convention since we are simulating visible
light. So when you're defining you sky using gensky with weather data you
need to either use the measured illuminance values and divide by 179 to get
values (for solar spectrum), multiply by an approximate efficacy, then
divide 179 to get radiometric units for the visible spectrum only.

If you use gendaylit all the conversions are done for you.

Best,
Andy

···

On Tue, Jan 1, 2013 at 8:36 AM, Ji Zhang <[email protected]> wrote:

Dear list, Happy New Year!

I have a simple question related to conversion from irradiance value to
illuminance value, and pls correct me if I'm wrong:

Usually we can estimate the *illuminance (lux)* for a given point by
multiplying the *irradiance (w/m2)* for the point as simulated via
Radiacne by *179 (lm/w)* which is the luminous efficacy used in Radiance,
or more strictly (R*0.265+G*0.670+B*0.065)*179.

However, it seems that in a epw weather file the* "global horizontal
illuminance"* value is not equal to but smaller than the *"global
horizontal radiation" value multiplied by 179*.

1. why there's such a large discrapency?
2. Will this lead to over-estimation of illuminance when using cumulative
sky derived from "global horizontal radiation" ?

- Ji

_______________________________________________
[email protected]

Can we have this reply laminated onto cards? =8-)

Thanks Andy!

Rob Guglielmetti
National Renewable Energy Laboratory (NREL)
Commercial Buildings Research Group
15013 Denver West Parkway MS:RSF202
Golden, CO 80401
303.275.4319
[email protected]

···

On 1/2/13 12:23 PM, "Andrew McNeil" <[email protected]<mailto:[email protected]>> wrote:

Hi Ji,

179 is the efficacy of white (equal energy) light over the visible spectrum.

Daylight is composed of a broader spectrum, so the efficacy (visible light per watt of energy) is lower. Usually around 90 for the sun and 110 for the sky, but changes based on various factors.

179 is used in Radiance as a convention since we are simulating visible light. So when you're defining you sky using gensky with weather data you need to either use the measured illuminance values and divide by 179 to get radiometric units for the visible spectrum, or use the measured radiance values (for solar spectrum), multiply by an approximate efficacy, then divide 179 to get radiometric units for the visible spectrum only.

If you use gendaylit all the conversions are done for you.

Best,
Andy

On Tue, Jan 1, 2013 at 8:36 AM, Ji Zhang <[email protected]<mailto:[email protected]>> wrote:
Dear list, Happy New Year!

I have a simple question related to conversion from irradiance value to illuminance value, and pls correct me if I'm wrong:

Usually we can estimate the illuminance (lux) for a given point by multiplying the irradiance (w/m2) for the point as simulated via Radiacne by 179 (lm/w) which is the luminous efficacy used in Radiance, or more strictly (R*0.265+G*0.670+B*0.065)*179.

However, it seems that in a epw weather file the "global horizontal illuminance" value is not equal to but smaller than the "global horizontal radiation" value multiplied by 179.

1. why there's such a large discrapency?
2. Will this lead to over-estimation of illuminance when using cumulative sky derived from "global horizontal radiation" ?

- Ji

_______________________________________________
[email protected]<mailto:[email protected]>

I see, thank you very much, Andy!

So, can we put it as:
The *"global horizontal radiation" *in the epw file is a total solar
radiation value* NOT yet *integrated over the visible spectral range
(380-780 nm) (from gendaylit man page), so we can't simply multiply it by
179 to get the illuminance value. We need to "break" the *"global
horizontal radiation" *into its RGB components and then use
(R*0.265+G*0.670+B*0.065)*179 to convert it into a illuminance value.

- Ji

···

On Thu, Jan 3, 2013 at 3:23 AM, Andrew McNeil <[email protected]> wrote:

Hi Ji,

179 is the efficacy of white (equal energy) light over the visible
spectrum.

Daylight is composed of a broader spectrum, so the efficacy (visible light
per watt of energy) is lower. Usually around 90 for the sun and 110 for
the sky, but changes based on various factors.

179 is used in Radiance as a convention since we are simulating visible
light. So when you're defining you sky using gensky with weather data you
need to either use the measured illuminance values and divide by 179 to get
values (for solar spectrum), multiply by an approximate efficacy, then
divide 179 to get radiometric units for the visible spectrum only.

If you use gendaylit all the conversions are done for you.

Best,
Andy

On Tue, Jan 1, 2013 at 8:36 AM, Ji Zhang <[email protected]> wrote:

Dear list, Happy New Year!

I have a simple question related to conversion from irradiance value to
illuminance value, and pls correct me if I'm wrong:

Usually we can estimate the *illuminance (lux)* for a given point by
multiplying the *irradiance (w/m2)* for the point as simulated via
Radiacne by *179 (lm/w)* which is the luminous efficacy used in

However, it seems that in a epw weather file the* "global horizontal
illuminance"* value is not equal to but smaller than the *"global
horizontal radiation" value multiplied by 179*.

1. why there's such a large discrapency?
2. Will this lead to over-estimation of illuminance when using cumulative
sky derived from "global horizontal radiation" ?

- Ji

_______________________________________________
[email protected]

_______________________________________________
[email protected]

···

On Thu, Jan 3, 2013 at 3:55 AM, Guglielmetti, Robert < [email protected]> wrote:

Can we have this reply laminated onto cards? =8-)

Thanks Andy!

Rob Guglielmetti
National Renewable Energy Laboratory (NREL)
Commercial Buildings Research Group
15013 Denver West Parkway MS:RSF202
Golden, CO 80401
303.275.4319
[email protected]

On 1/2/13 12:23 PM, "Andrew McNeil" <[email protected]<mailto: > [email protected]>> wrote:

Hi Ji,

179 is the efficacy of white (equal energy) light over the visible
spectrum.

Daylight is composed of a broader spectrum, so the efficacy (visible light
per watt of energy) is lower. Usually around 90 for the sun and 110 for
the sky, but changes based on various factors.

179 is used in Radiance as a convention since we are simulating visible
light. So when you're defining you sky using gensky with weather data you
need to either use the measured illuminance values and divide by 179 to get
values (for solar spectrum), multiply by an approximate efficacy, then
divide 179 to get radiometric units for the visible spectrum only.

If you use gendaylit all the conversions are done for you.

Best,
Andy

On Tue, Jan 1, 2013 at 8:36 AM, Ji Zhang <[email protected]<mailto: > [email protected]>> wrote:
Dear list, Happy New Year!

I have a simple question related to conversion from irradiance value to
illuminance value, and pls correct me if I'm wrong:

Usually we can estimate the illuminance (lux) for a given point by
by 179 (lm/w) which is the luminous efficacy used in Radiance, or more
strictly (R*0.265+G*0.670+B*0.065)*179.

However, it seems that in a epw weather file the "global horizontal
illuminance" value is not equal to but smaller than the "global horizontal

1. why there's such a large discrapency?
2. Will this lead to over-estimation of illuminance when using cumulative
sky derived from "global horizontal radiation" ?

- Ji

_______________________________________________
[email protected]<mailto:
[email protected]>

_______________________________________________
[email protected]

Haha, so you have a bible going? Good for you. I think many of us have something similar, in some form or fashion. I managed to document a couple of my notes online, here:

Francesco Anselmo's site is another repository:

Axel's stuff is awesome:

…and of course there is an effort underway to buff up the content right on the radiance-online.org site.

As Greg Ward has said: "everything about Radiance is documented, somewhere". Trick is when you find a nugget, save it someplace!

Rob Guglielmetti
National Renewable Energy Laboratory (NREL)
Commercial Buildings Research Group
15013 Denver West Parkway MS:RSF202
Golden, CO 80401
303.275.4319
[email protected]

···

On 1/2/13 8:36 PM, "Ji Zhang" <[email protected]<mailto:[email protected]>> wrote:

On Thu, Jan 3, 2013 at 3:55 AM, Guglielmetti, Robert <[email protected]<mailto:[email protected]>> wrote:
Can we have this reply laminated onto cards? =8-)

Thanks Andy!

Rob Guglielmetti
National Renewable Energy Laboratory (NREL)
Commercial Buildings Research Group
15013 Denver West Parkway MS:RSF202
Golden, CO 80401
303.275.4319<tel:303.275.4319>
[email protected]<mailto:[email protected]>

On 1/2/13 12:23 PM, "Andrew McNeil" <[email protected]<mailto:[email protected]><mailto:[email protected]<mailto:[email protected]>>> wrote:

Hi Ji,

179 is the efficacy of white (equal energy) light over the visible spectrum.

Daylight is composed of a broader spectrum, so the efficacy (visible light per watt of energy) is lower. Usually around 90 for the sun and 110 for the sky, but changes based on various factors.

179 is used in Radiance as a convention since we are simulating visible light. So when you're defining you sky using gensky with weather data you need to either use the measured illuminance values and divide by 179 to get radiometric units for the visible spectrum, or use the measured radiance values (for solar spectrum), multiply by an approximate efficacy, then divide 179 to get radiometric units for the visible spectrum only.

If you use gendaylit all the conversions are done for you.

Best,
Andy

On Tue, Jan 1, 2013 at 8:36 AM, Ji Zhang <[email protected]<mailto:[email protected]><mailto:[email protected]<mailto:[email protected]>>> wrote:
Dear list, Happy New Year!

I have a simple question related to conversion from irradiance value to illuminance value, and pls correct me if I'm wrong:

Usually we can estimate the illuminance (lux) for a given point by multiplying the irradiance (w/m2) for the point as simulated via Radiacne by 179 (lm/w) which is the luminous efficacy used in Radiance, or more strictly (R*0.265+G*0.670+B*0.065)*179.

However, it seems that in a epw weather file the "global horizontal illuminance" value is not equal to but smaller than the "global horizontal radiation" value multiplied by 179.

1. why there's such a large discrapency?
2. Will this lead to over-estimation of illuminance when using cumulative sky derived from "global horizontal radiation" ?

- Ji

_______________________________________________
[email protected]<mailto:[email protected]><mailto:[email protected]<mailto:[email protected]>>

_______________________________________________
[email protected]<mailto:[email protected]>

Great! Thanks, Rob! - Ji

···

On Thu, Jan 3, 2013 at 11:53 PM, Guglielmetti, Robert < [email protected]> wrote:

Haha, so you have a bible going? Good for you. I think many of us have
something similar, in some form or fashion. I managed to document a couple
of my notes online, here:

http://www.rumblestrip.org/interests/light/rtrace-multiprocessing-option-initial-test-results/

Francesco Anselmo's site is another repository:

Axel's stuff is awesome:

I got a server error!

…and of course there is an effort underway to buff up the content right on

somewhere". Trick is when you find a nugget, save it someplace!

Rob Guglielmetti
National Renewable Energy Laboratory (NREL)
Commercial Buildings Research Group
15013 Denver West Parkway MS:RSF202
Golden, CO 80401
303.275.4319
[email protected]

On 1/2/13 8:36 PM, "Ji Zhang" <[email protected]<mailto:[email protected]>> > wrote:

On Thu, Jan 3, 2013 at 3:55 AM, Guglielmetti, Robert < > [email protected]<mailto:[email protected]>> wrote:
Can we have this reply laminated onto cards? =8-)

Thanks Andy!

Rob Guglielmetti
National Renewable Energy Laboratory (NREL)
Commercial Buildings Research Group
15013 Denver West Parkway MS:RSF202
Golden, CO 80401
303.275.4319<tel:303.275.4319>
[email protected]<mailto:[email protected]>

On 1/2/13 12:23 PM, "Andrew McNeil" <[email protected]<mailto: > [email protected]><mailto:[email protected]<mailto:[email protected]>>> wrote:

Hi Ji,

179 is the efficacy of white (equal energy) light over the visible
spectrum.

Daylight is composed of a broader spectrum, so the efficacy (visible light
per watt of energy) is lower. Usually around 90 for the sun and 110 for
the sky, but changes based on various factors.

179 is used in Radiance as a convention since we are simulating visible
light. So when you're defining you sky using gensky with weather data you
need to either use the measured illuminance values and divide by 179 to get
values (for solar spectrum), multiply by an approximate efficacy, then
divide 179 to get radiometric units for the visible spectrum only.

If you use gendaylit all the conversions are done for you.

Best,
Andy

On Tue, Jan 1, 2013 at 8:36 AM, Ji Zhang <[email protected]<mailto: > [email protected]><mailto:[email protected]<mailto:[email protected]>>> > wrote:
Dear list, Happy New Year!

I have a simple question related to conversion from irradiance value to
illuminance value, and pls correct me if I'm wrong:

Usually we can estimate the illuminance (lux) for a given point by
by 179 (lm/w) which is the luminous efficacy used in Radiance, or more
strictly (R*0.265+G*0.670+B*0.065)*179.

However, it seems that in a epw weather file the "global horizontal
illuminance" value is not equal to but smaller than the "global horizontal

1. why there's such a large discrapency?
2. Will this lead to over-estimation of illuminance when using cumulative
sky derived from "global horizontal radiation" ?

- Ji

_______________________________________________
[email protected]<mailto:
[email protected]><mailto:
[email protected]<mailto:
[email protected]>>

_______________________________________________
[email protected]<mailto:
[email protected]>

_______________________________________________
[email protected]

Hi Ji Zhang!

So, can we put it as:
The *"global horizontal radiation" *in the epw file is a total solar radiation value*NOT yet *integrated over the visible spectral range (380-780 nm) (from gendaylit man page), so we can't simply multiply it by 179 to get the illuminance value. We need to "break" the *"global horizontal radiation" *into its RGB components and then use (R*0.265+G*0.670+B*0.065)*179 to convert it into a illuminance value.

I would not put too much effort into understanding why it is 179 lm/W in Radiance. It usually really does not matter for you. You can use it as a constant as long as you are interested in photometric quantities, as in such case, you input and your output will be in lumens, not watt. So feed in by dividing by 179, read out by multiplying with the same. That´s it. This is completely unrelated to the "real" luminous efficacy of lamps, skies, stars (aka the sun...), which is not that easy to predict. So, whenever possible, I'd use the photometric units for input when I need photometric results. This means you are working with well-defined conversions.

Cheers, Lars.

Noted with many thanks, Lars! and Happy New Year!
- Ji

···

On Fri, Jan 4, 2013 at 6:50 PM, Lars O. Grobe <[email protected]> wrote:

Hi Ji Zhang!

So, can we put it as:
The *"global horizontal radiation" *in the epw file is a total solar
radiation value* NOT yet *integrated over the visible spectral range
(380-780 nm) (from gendaylit man page), so we can't simply multiply it by
179 to get the illuminance value. We need to "break" the *"global
horizontal radiation" *into its RGB components and then use
(R*0.265+G*0.670+B*0.065)*179 to convert it into a illuminance value.

I would not put too much effort into understanding why it is 179 lm/W in
Radiance. It usually really does not matter for you. You can use it as a
constant as long as you are interested in photometric quantities, as in
such case, you input and your output will be in lumens, not watt. So feed
in by dividing by 179, read out by multiplying with the same. That´s it.
This is completely unrelated to the "real" luminous efficacy of lamps,
skies, stars (aka the sun...), which is not that easy to predict. So,
whenever possible, I'd use the photometric units for input when I need
photometric results. This means you are working with well-defined
conversions.

Cheers, Lars.

_______________________________________________
[email protected]

Hi Aksel,
Unfortunately I don't have sources for this other than Greg's emails in the
vintage radiance digests (1990's). Search these pages for "179", there are
several emails on this topic):
Best,
Andy

···

On Fri, Jan 4, 2013 at 10:34 AM, Aksel Groß <[email protected]> wrote:

Hi Andy,

Interesting rule. Could you recommend further reading for my interest on
how to get to the number 179? I would like to get a tiny grasp on the
underlying principle to better understand.

Thanks a lot,
Aksel

Am 02.01.2013 um 20:23 schrieb Andrew McNeil <[email protected]>:

> Hi Ji,
>
> 179 is the efficacy of white (equal energy) light over the visible
spectrum.
>
> Daylight is composed of a broader spectrum, so the efficacy (visible
light per watt of energy) is lower. Usually around 90 for the sun and 110
for the sky, but changes based on various factors.
>
> 179 is used in Radiance as a convention since we are simulating visible
light. So when you're defining you sky using gensky with weather data you
need to either use the measured illuminance values and divide by 179 to get
values (for solar spectrum), multiply by an approximate efficacy, then
divide 179 to get radiometric units for the visible spectrum only.
>
> If you use gendaylit all the conversions are done for you.
>
> Best,
> Andy
>
>
> On Tue, Jan 1, 2013 at 8:36 AM, Ji Zhang <[email protected]> wrote:
> Dear list, Happy New Year!
>
> I have a simple question related to conversion from irradiance value to
illuminance value, and pls correct me if I'm wrong:
>
> Usually we can estimate the illuminance (lux) for a given point by
by 179 (lm/w) which is the luminous efficacy used in Radiance, or more
strictly (R*0.265+G*0.670+B*0.065)*179.
>
> However, it seems that in a epw weather file the "global horizontal
illuminance" value is not equal to but smaller than the "global horizontal
>
> 1. why there's such a large discrapency?
> 2. Will this lead to over-estimation of illuminance when using
cumulative sky derived from "global horizontal radiation" ?
>
>
> - Ji
>
> _______________________________________________
> [email protected]
>
>
> _______________________________________________
> [email protected]

--

Aksel Groß
Dipl.Ing.Arch., Dipl.Szeno.

Electric Gobo
Schönhauser Allee 182
10119 Berlin, Germany

T +49 30 559 531 75
M +49 179 394 30 92
[email protected]

http://gobo.io

_______________________________________________