[Radiance-general] HDR Sky Camera @ LBNL's Flexlab

[Greg Ward]

I think that a reasonable approach is to use the global illuminance reading to determine the direct + indirect integrated total, then compare this to the integration of the HDR sky dome recording.  You can then add in a solar source whose solid angle is enlarged to cover the "clipped" pixels in the HDR image, adjusting its radiance such that the new total:

	integrated_masked_HDR + solar_source*adj_solid_angle*cos_theta = measured_illuminance

The larger sun that includes circumsolar will lead to slightly enlarged penumbras in some cases, but it's a reasonable compromise.  I'm sure a script could be made to automate the above process without too much effort.

Cheers,
-Greg

> From: Andrew McNeil <amcneil at lbl.gov>
> Subject: Re: [Radiance-general] HDR Sky Camera @ LBNL's Flexlab
> Date: June 28, 2014 1:28:09 AM GMT+02:00
> 
> Hi All,
> 
> I haven't done any testing myself, I wanted to make the data available right away so that others could tinker too (and maybe make it easier for me).
> 
> Joe - You're correct that the HDR sky image essentially replaces the skyfunc modifier, here's a thread where Kyle was doing the same thing: http://www.radiance-online.org/pipermail/radiance-general/2012-October/008962.html 
> 
> Rob - In a clear sky condition our HDR images won't capture the full luminance of the sun. Mksource would be helpful to zero out the pixels, but the source that it makes won't be useful without adjusting the radiance of the source to match that of the sun.
> 
> Best,
> Andy
> 
> 
> On Fri, Jun 27, 2014 at 2:58 PM, Rob Guglielmetti <rob.guglielmetti at gmail.com> wrote:
> Hi German, and everyone else. Certainly one could use these HDR images to generate sky vectors and apply them to daylight coefficients for a given model(s). Greg Ward has created a cool tool called mksource to facilitate this process in Radiance, identifying small, intense pixels in the image; creating and placing Radiance light sources in their stead, and zeroing the pixels to avoid double counting. 
> 
> Considerations: 
> - Capturing the true (full) dynamic range of an exterior scene with direct sun is difficult. 
> - Using locally-captured HDR images for daylight availability analysis is statistically dubious. Granted, so is using TMY data, for different reasons. This is why I changed the title of this list to "considerations", from "problems". =)
> 
> 
> 
> On Fri, Jun 27, 2014 at 3:38 PM, CHI-German Molina <gmolina at hdlao.com> wrote:
> Wow, I have been thinking on doing this for a while... although I have no idea where to start from.
> 
> Is it possible to calculate the Daylight Coefficients of the building; and use the HDR image to generate sky vectors and calculate different options for optimizing daylighting? 
> 
> I am picturing a computer that, every 5 minutes, calculate the sky vector, computes the interior lighting conditions, and simulates the different lighting options performing a whole-building lighting control with no photo sensors. Even more, maybe a whole neighborhood could use the same camera. Nonsense?
> 
> Thanks for sharing!
> 
> 
> 2014-06-27 0:59 GMT-04:00 Joe Smith <the.oat.cracker at gmail.com>:
> Hi, Andy, thanks for sharing LBNL's sky mapping experiment! 
> 
> Can you kindly advice on resources that elaborate on how to use HDR sky image for daylight simulation? Does it involve specifying the HDR image, rather than a "skyfunc", as the material identifier for the sky geometry? How is the pixel value of a given point on the HDR image converted to luminance value of the corresponding position on the sky?  
> 
> Thanks! 
> - Joe
> 
> Andrew McNeil <amcneil at lbl.gov>于2014年6月27日星期五写道：
> 
> Hi Everyone,
> 
> LBNL has installed an HDR sky camera at our new FLEXLAB site: http://flexskycam.lbl.gov. I've uploaded sample data, including hdr images and csv datafiles, recorded by the camera for three days over the past week( clear, partly cloudy and overcast). We're happy to share more data with other researchers and daylight practitioners (but we don't have much to offer yet).
> 
> The images can be mapped to a Radiance sky for simulation under real sky conditions. I have not used the sky HDR images yet, myself, so if anybody uses them successfully please report back and share what you've done!
> 
> Questions about the camera hardware and capabilities should be directed to Chris Humann at Terrestrial Light.
> 
> Best,
> Andy
> 
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