Is this a bug in the source code for Calculating DHI?

cdwu · September 4, 2026, 3:36pm

Hi Mostapha and Chris,

While I was working on a research project using the “direct solar irradiance” component in Ladybug, I realized that the equation used to calculate diffuse irradiance for anisotropic sky is (0.313 * math.cos(vec_angle) * 0.313 * math.cos(vec_angle), in https://www.ladybug.tools/ladybug/docs/_modules/ladybug/wea.html , while in an ASHRAE document (page 7) the equation is 0.313 x cos^2(theta). I wonder if this is a bug or if there was a reason to use 0.313^2 instead of just 0.313?

ASHRAE document:

Alax_Morgan · September 15, 2026, 8:41pm

Good catch. The difference between 0.313 × cos²(theta) and 0.313² × cos²(theta) seems worth checking, especially since it can affect the calculated diffuse irradiance. Hopefully someone familiar with the implementation can clarify whether this was intentional or just a typo in the source code.

chris · September 15, 2026, 8:49pm

Thanks, @Alax_Morgan and @cdwu ,

Looks like it’s a bug that goes all of the way back to Ladybug Legacy.

github.com/ladybug-tools/ladybug-legacy

src/Ladybug_Surface%20Hourly%20Solar.py

d90db02f2


      
              srfDirect = createHeader('Direct ' + dataType + ' for Surface - Az' + str(int(srfAzimuth)) + ', Alt' + str(int(srfAltitude)), header)
              srfDiffuse = createHeader('Diffuse ' + dataType + ' for Surface - Az' + str(int(srfAzimuth)) + ', Alt' + str(int(srfAltitude)), header)
              srfGlobal = createHeader('Total ' + dataType + ' for Surface - Az' + str(int(srfAzimuth)) + ', Alt' + str(int(srfAltitude)), header)
          
          # Calculate solar on the surface.
          anisotropicViews = []
          for i, vec in enumerate(sunVectors):
              srfDir = 0
              if vec != None:
                  angle = rc.Geometry.Vector3d.VectorAngle(vec, normalVec)
                  y = max(0.45, 0.55 + (0.437*math.cos(angle)) + 0.313 * math.cos(angle) * 0.313 * math.cos(angle))
                  anisotropicViews.append(y)
                  if angle < math.pi/2:
                      srfDir = directNormal[i]*math.cos(angle)
                  srfDirect.append(srfDir)
              else:
                  anisotropicViews.append(0)
                  srfDirect.append(0)
              
              if skyModel == False:
                  srfDif = diffuseHorizontal[i] * (anisotropicViews[i]*(math.sin(math.radians(abs(90-srfAltitude)))) + math.cos(math.radians(abs(90-srfAltitude))))

I must have made the mistake 9 years ago when I added the anisotrophic sky model there:

… and then I must have copied into the new ladybug source code without double-checking the primary source. I’ll push a fix to the latest ladybug source code shortly.

chris · September 15, 2026, 10:56pm

The bug was fixed here:

.. .and the fix will be available via the LB Versioner component in an hour or so. Thanks again for reporting, @cdwu !

cdwu · September 16, 2026, 2:19pm

Thank you Chris for fixing the bug! Your contribution to the community is invaluable. Cheers! - Chengde.