[Radiance-general] effectiveness of light shelves

[Thomas Bleicher]

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?

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.