[Radiance-general] Radiance and Ecotect

[Nick Devlin]

Nick, 

John's recommendations are on the nose. It took me a while to come to the
same conclusions, but using Optic to create the .rad description for the
glazing is the simplest way to go. You just need to be certain that Optic is
providing the description for your complete window build-up. I vaguely
remember recent posts regarding Optic and this topic. 

Given the calculation speed, and greater transparency of the settings using
radiance, I gave up on using Ecotects internal calcs a long time ago. With a
minimum of tweaks, you can also get Radiance to do VSC or Sky component
calcs too. 

Hope that is useful

Nickd

Message: 2
Date: Tue, 09 May 2006 23:51:53 -0400
From: John An <ja at a10nyc.com>
Subject: [Radiance-general] Re: Radiance and Ecotect
To: <radiance-general at radiance-online.org>
Message-ID: <C086DB99.186D%ja at a10nyc.com>
Content-Type: text/plain;	charset="US-ASCII"

Nick,
Ecotect uses the split-flux method for daylight factor calculation. Radiance
calculates the daylight factor using backward raytracing. Thus, if the
surface reflectance values is accurate, the Radiance method should be more
accurate. There is the argument that if the surface reflectance value is
slightly off, then the raytracing method magnifies the error with each
bounce of light, where with the split-flux method, the error is not
magnified.

With regard to glass definition, Ecotect has been known not to transfer
glazing properties correctly into Radiance definition. If the discussion
forum was still online, then you would be able to follow a forum discussion
regarding the difficulties in translating Ecotect glass to Radiance
definition. The way I get around this problem is by using Optics (not
Window, as I had previously mentioned) from LBNL to create a Radiance
definition of a commercially available glass, and use the Optics definition
to replace the Ecotect glass. If you want a step-by-step method, email me
off the forum.

So, to answer your ultimate question, I generally trust Radiance results
over Ecotect results. But then again, this is just my opinion. The
difference between Ecotect and Radiance results are simply because of the
algorithm used. This does not necessarily mean that one is wrong and the
other is right.

Hope this helps

John


On 5/9/06 6:00 AM, "radiance-general-request at radiance-online.org"
<radiance-general-request at radiance-online.org> wrote:
Can anyone help here?

I'm trying to model daylight factors on the vertical surface of a 10m high
warehouse rack.  The reflectances are fairly low, 0.3 for the rack and 0.6
for the other surfaces.  I've compared daylight factor results from Ecotect
and Radiance and found big differences in the magnitude and range of values
across the surface.  In one case, Ecotect is giving values varying between
about 7.5% and 1%, the corresponding values from Radiance are between about
2.5% and 0.25%.

Ecotect's export to Radiance facility appears to incorrectly set the
transmissivity of the glazing material in the .rad output.  Setting the
glazing transparency (I guess this is the same as tranmsmittance) in Ecotect
to 0.88 and using the 'clean' glass preset I would expect to see
transmissivity values of 0.96 in the .rad file output.  I'm actually getting
values of 0.66.  Even after correcting the .rad file I still find the output
values to be considerably smaller than those from Ecotect.

I'm using the high quality preset for Ecotect's calculation (further
increasing the quality appears to give slightly larger daylight values), and
an 'indirect reflections' value of 5 for Radiance, all other parameters are
the defaults for Ecotect's Radiance interface.

I suppose my real question here is which values should I trust?  Perhaps
neither.

Nick




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