effectiveness of light shelves

Hello all,

I have a question that is not very specific to radiance usage but felt that
most of the online forum users might have a good idea about it:

Though light shelves are widely perceived to bounce daylight into interiors
off of the ceiling and also cut down glare, they are more effective in
cutting down the glare and do not actually increase the daylight levels at
the back of the room.

True or false? Why?

Thanks in advance,

Ramana Koti, Sustainable Building Analyst
LEED AP

ELEMENTS | a division of berkebile nelson immenschuh mcdowell architects
106 W. 14th Street Suite 200
Kansas City, Missouri 64105

p 816.783.1635 f 816.783.1501
www.bnim.com| elements.bnim.com

Mobile Phone: 480.326.9275

Both. Because light is a wave (and complicated).

I've not done a proper study on the effectiveness of light
shelves but my general understanding is as follows:

1) Light shelves are useful when there is some sunlight to
    redirect into the room. In this case they should increase
    the daylight factor further into the room. If the room
    is too long even a light shelve will not help. The room
    geometry (and of course surface colours) have to fit to
    benefit of a light shelve treatment.

2) Because they are effectively an obstruction in the window
    they will reduce the DF for the front and the middle of
    the room. At the front we usually have plenty of daylight
    and don't mind; in the middle the loss in DF might in fact
    be bigger than the gain at the back which would reduce the
    overall DF.

3) Daylight studies (at least DF calculations) are usually done
    with a standard CIE overcast sky. This sky model has no sunlight
    component and is therefore most inappropriate to estimate the
    efficiency of light shelves. A light shelve will only reduce the
    available window opening and the light coming in.
    However it's the only sky model mentioned in the British Standard
    - and I suppose everywhere else - so it's used to get the one or
    two numbers the architects need to get their BREEAM or LEED browny
    points. [1]

Thomas

[1] I'm open for any suggestion to improve this sad status of
     daylight design in the current practitioners work - but please
     start a new thread.

···

On 28 Jul 2006, at 21:56, Ramana Koti wrote:

Though light shelves are widely perceived to bounce daylight into
interiors off of the ceiling and also cut down glare, they are
more effective in cutting down the glare and do not actually
increase the daylight levels at the back of the room.

True or false? Why?

Hi Ramana,

I think it will be imprecise to answer your question in terms of black or
white. In fact it depends on several factor such lightshelf type, geometry,
size, location, materials, window orientation and size, room dimensions, sky
conditions ("luminous climate"), building maintenance among others.

Sometimes, controlling the amount of radiation entering a room and therefore
reducing very high light levels close to the window wall helps creating a
comfortable lighting environment, by reducing the contrast between levels at
the view window and those at the back of the room, yet sacrificing on
illuminance levels.
Best regards,

Guedi Capeluto

Arch. Guedi Capeluto, D.Sc.
Faculty of Architecture and Town Planning
Technion - Israel Institute of Technology
Technion City, Haifa 32000. Israel
Tel:+972.4.8294012 Fax:+972.4.8294617
email: [email protected]
web : http://tx.technion.ac.il/~arrguedi

···

----------------------------------------------------------------------

Message: 1
Date: Fri, 28 Jul 2006 14:56:13 -0500
From: "Ramana Koti" <[email protected]>
Subject: [Radiance-general] effectiveness of light shelves
To: "Radiance general discussion"
  <[email protected]>
Message-ID:
  <[email protected]>
Content-Type: text/plain; charset="iso-8859-1"

Hello all,

I have a question that is not very specific to radiance usage but felt that
most of the online forum users might have a good idea about it:

Though light shelves are widely perceived to bounce daylight into interiors
off of the ceiling and also cut down glare, they are more effective in
cutting down the glare and do not actually increase the daylight levels at
the back of the room.

True or false? Why?

Thanks in advance,

Ramana Koti, Sustainable Building Analyst
LEED AP

ELEMENTS | a division of berkebile nelson immenschuh mcdowell architects
106 W. 14th Street Suite 200
Kansas City, Missouri 64105

p 816.783.1635 f 816.783.1501
www.bnim.com| elements.bnim.com

Mobile Phone: 480.326.9275
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Hi Ramana,

Light shelves do redirect light deeper into a space under both overcast and clear sky conditions. Whether they do this effectively and for all overcast and clear sky conditions depends on building and light shelf design. They do not necessarily block nor reduce glare. That depends on design particulars as well.

Paul

···

On Jul 28, 2006, at 12:56 PM, Ramana Koti wrote:

Though light shelves are widely perceived to bounce daylight into interiors off of the ceiling and also cut down glare, they are more effective in cutting down the glare and do not actually increase the daylight levels at the back of the room.
True or false? Why?