"perfectly diffuse" light source

Hi-

I'm working on a lighting study for an architectural project where the
designers wish to create a large, highly reflective, light shaft that
runs the length of the building. On each floor, there will be openings
that allow the light to diffuse from the shaft to the occupants. At
this stage, I don't want to go through a complex exercise to model the
light shaft and simulate the large number of reflections that would
result from the highly reflective surface. We do not have the time or
budget to do this. Instead, I'd just like to model a perfectly diffuse
light source that outputs a light power in line with theoretical
estimates for losses through a light shaft. Essentially, I'll just
model a light source with a certain lumen output.

I don't have any type of perfectly diffusing light source in my
luminaire library right now, but I can construct one. I'm wondering if
anyone has any thoughts or experience with this type of study, and if
any words of wisdom or past experience that might be useful.

Thanks!

Chien Si Harriman LEED AP

Senior Building Performance Engineer

(P) 415.655.4005

GUTTMANN & BLAEVOET Consulting Engineers

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I believe a surface modeled with the material type "light" behaves as a diffuse source. You can use the lampcolor table to calculate the RGB definition based on lumens and surface area.

How will you calculate the brightness of this diffuse surface at various points along the light shaft?

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That is an excellent question. Actually, at this stage, I am not sure.
I'm working with an architect who is still in the phase of feasibility
studies. Right now, a research group at the local university has given
me expected lumens under various sky conditions based on their own
research and calculations (where they conservatively have assumed 40% of
the light available from the sky vault will be lost as the light travels
down the shaft). At this stage, this is the best information that I've
been given.

I have been raising the issue that this system would have to be highly
engineered, in order to take light traveling vertically and project it
horizontally on three different levels of the building. There would
obviously be light losses as light passes from each successive floor
from the roof. But all of the calculations now are based on a very
rough idea of losses, as mentioned above.

Do you have any ideas for how to calculate the losses down the shaft?

Chien Si Harriman LEED AP

Senior Building Performance Engineer

(P) 415.655.4005

GUTTMANN & BLAEVOET Consulting Engineers

San Francisco | Sacramento | Santa Rosa <http://gb-eng.com/page/Contact>

Website <http://www.gb-eng.com/>

Certified Green Business <http://www.greenbiz.ca.gov/AboutUs.html>
committed to the 2030 Challenge <http://www.2030challenge.com/>

HVAC, Plumbing, Fire Protection, Electrical, Lighting,
Telecom/Technology <http://gb-eng.com/page/services>

Commissioning, Retro Commissioning, Energy & Building Performance
Modeling <http://gb-eng.com/page/services>

In a recent project I used mkillum to calculate the light available to a
disk at the top of the building, then moved the disk down to the level where
I wanted the light pipe to terminate. The output was reduced by 6% per meter
which is a value the manufacturer mentioned in one of their brochures
(although on the phone they were much more pessimistic about the performance
of a very long light pipe).

I don't know if this is an accurate way of calculating the performance. If
you think a light pipe is a realistic option in your situation I would
recommend a mock up.

Regards,
Thomas

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On Thu, May 27, 2010 at 8:08 PM, Chien Si Harriman <[email protected]>wrote:

Do you have any ideas for how to calculate the losses down the shaft?

Thomas-thanks a lot for the reply. I am liking the radiance community.
I don't know if we are going to have money at this point for a physical
mockup. At this point, they are just wanting to prove the concept could
realistically work. I am not sure how we will be able to physically
simulate design options of the light pipe should the architects like
what they see and want to move forward though, on what is essentially a
municipal project budget. I'm hoping that the University has something
powerful, but not sure. I am not sure that my workstation has the
horsepower to crank out iterations of engineering designs that will
transfer the light from a horizontal to vertical direction. Readings
I've done with Radiance suggest that Radiance may not be well-suited to
this type of study, either. I do believe I will run into problems with
the simulation, but am working hard to tackle it!

Anyone with words of wisdom, again, is greatly appreciated. This is
just my thinking based on my education in the subject.

Chien Si Harriman LEED AP

Senior Building Performance Engineer

(P) 415.655.4005

GUTTMANN & BLAEVOET Consulting Engineers

San Francisco | Sacramento | Santa Rosa <http://gb-eng.com/page/Contact>

Website <http://www.gb-eng.com/>

Certified Green Business <http://www.greenbiz.ca.gov/AboutUs.html>
committed to the 2030 Challenge <http://www.2030challenge.com/>

HVAC, Plumbing, Fire Protection, Electrical, Lighting,
Telecom/Technology <http://gb-eng.com/page/services>

Commissioning, Retro Commissioning, Energy & Building Performance
Modeling <http://gb-eng.com/page/services>

Hi!

There are some papers out there trying to give algorithms for calculation of flux through mirror pipes. Two typically unsolved problems are that they either assume purely diffuse or purely specular reflection, and are difficult to apply to different sky conditions - cloudy, sunny, all the distributions are hard to get in there. It is possible to do simulations in radiance, which may require extensions. However modelling a complete system is challenging.

By the way, depending on what you are exactly modelling, glow may be a useful option. I do not know how large these luminous areas are though...

Cheers, Lars.

Just in case anyone missed this nifty little method to measure the flux from light pipes:

http://www.radiance-online.org/radiance-workshop5/2006_Radiance_Workshop/Presentations/MardaljevicKrausseAndersen.pdf

[8Mb]

Paper: Mardaljevic, J., Krausse, B. and Andersen, M. Transmission illuminance proxy HDR imaging: A new technique to quantify luminous flux.Lighting Res. Technol. Vol. 41, No. 1, pp. 27-49, 2009

-John

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-----------------------------------------------
Dr. John Mardaljevic
Reader in Daylight Modelling
Institute of Energy and Sustainable Development
De Montfort University
The Gateway
Leicester
LE1 9BH, UK
+44 (0) 116 257 7972
+44 (0) 116 257 7981 (fax)

[email protected]
http://www.iesd.dmu.ac.uk/~jm