Ground Light in Daylight factor

Hi all,

I have a question concerning the light source (.sky file) for daylight factor simulation.
While comparing the results with Dial+ and radiance/sketchup, I found out that honeybee isn’t considering ground glow (usually representing a 20% ground reflection) while other software do consider it.
This leads to lower results of daylight factor all over the zone, but particularly on the back on the zone as it is mainly lit by groung glow in other softwares.

So my questions are :
1 - Is this difference normal / wanted ?
2 - Is there a way to consider ground glow easily (I tried modifying the sky file before launching the simulation but it is overwritten each time)
3 - Is HB[+] doing the same thing or not ?

Kind regards

The good practice is to add a ground plane as a geometry to your scene. In your case you can add a ground plane with 20% reflectance.

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Dear Mostapha,

Thank you for this remark. I agree with you on this but am quite surprised as other software do consider a “luminous ground” anyway. It seems HB+ and other recipes include a luminous ground while the DF recipe in HB does not.

In this pdf document from Mardaljevic, it is explained (section 6.3.4) :

Although it might seem too self-evident to point out, we should remind ourselves
that at the horizon the sky “meets” the ground. An actual ground plane of finite
extent, say, a disc of radius r, will always fall short of an “infinite” horizon. For any
given view toward the horizon, we can make the gap (a black void) between the
edge of the ground and the sky appear smaller by using a larger r. However, we can
never make them meet. Furthermore, there are good reasons not to introduce an
actual ground plane of inordinately large size: the resolution of an ambient calculation
will be dependent on the maximum dimension of the scene.
To get around this problem, we use an upside-down sky to represent a luminous
ground. To do this, we apply the skyfunc modifier to a 180-degree glow source,
where the direction vector is pointing downward.

It seems indeed that without this luminous sky, daylight factor results are quite pessimistic, even with a local ground plane with 20% reflectance.

I will continue my research on the subject but would be very happy to have your insight on the subject.


Here some example of “typical” Reflectance for outdoor surfaces.

Outdoor surface type Typical Reflectance
Water surfaces 7%
Soil (clay, marl) 14%
Dirt roads 4%
Coniferous forest in winter 7%
Forest in the fall 26%
Fields with ripe branches and plants 26%
Aged asphalt 10%
Aged concrete 22%
Dead leaves 30%
Dry grass 20%
Green grass 26%
Roofs or terraces in bitumen 13%
rubble/gravel 20%
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Nice share @giorgio_butturini,
Can you please share the source of these reflectances?

@devang: Italian code UNI 8477-2:1985 Title: Solar energy. Calculation of energy gains for building applications.

Turns out that I have removed the ground at some point intentionally to check the LEED standard for daylight factor sky.

I added it back as I see your point and what John has mentioned in the pdf file.

Is it solar reflectance or visible transmittance? @giorgio_butturini

They are Solar reflectances

oups I meant solar reflectance or visible reflectance.
@devang thanks, would you know any other source than (which is down now)?

IESNA handbook would be a resource to check. Also, please check this