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A daylight factor is the ratio of internal light level to external light level and is defined as follows:

DF = (Ei / Eo) x 100%

where, Ei = illuminance due to daylight at a point on the indoors working plane, Eo = simultaneous outdoor illuminance on a horizontal plane from an unobstructed hemisphere of overcast sky.

In order to calculate Ei, one must establish the amount of light received from the outside to the inside of a building. There are three paths along which light can reach a point inside a room through a glazed window, rooflight, or aperture, as follows:

  • Direct light from a patch of sky visible at the point considered, known as the sky component (SC),
  • Light reflected from an exterior surface and then reaching the point considered, known as the externally reflected component (ERC),
  • Light entering through the window but reaching the point only after reflection from an internal surface, known as the internally reflected component (IRC).

The sum of the three components gives the illuminance level (lux) at the point considered:

Lux = SC + ERC + IRC

The daylight factor can be improved by increasing SC (for example placing a window so it "sees" more of the sky rather than adjacent buildings), increasing ERC (for example by painting surrounding buildings white), increasing IRC (for example by using light colours for room surfaces). In most rooms the ceiling and floor will be of fixed colour, and much of the walls covered by furnishings. This gives less flexibility in changing the daylight factor by using different wall colours than might be expected[1] meaning changing SC is often the key to good daylight design.

A study of daylight factors within a single storey building resulting from different perimeter glazing and rooflight designs and glass types. Undertaken using the IES Raidance software Module.

Daylight factors are used in architecture and building design in order to assess the internal natural lighting levels as perceived on the working plane or surface in question, in order to determine if they will be sufficient for the occupants of the space to carry out their normal duties. The design day used for daylight factor calculations is based upon the Standard CIE overcast Sky for 21 September at 12:00pm, and where the Ground Ambient light level is 11921 Lux. CIE being the Commission Internationale de l´Eclairage, or International Commission on Illumination.

Calculating daylight factors requires complex repetition of calculations and thus is generally undertaken using a complex software product such as Radiance. This is a suite of tools for performing lighting simulation which includes a renderer as well as many other tools for measuring simulated light levels. It uses ray tracing to perform all lighting calculations. One failing in many of these calculations is that they are often completed without wall hangings of furniture against the walls. This can lead to high predictions of the daylight factor than is correct.[2]

In order to assess the effect of a poor or good daylight factor, one might choose to compare the results for a given calculation against published design guidance. In the UK this is likely to be CIBSE Lighting Guide 10 (LG10-1999) which broadly bands average daylight factors into the following categories:[3]

  • Under 2 – Not adequately lit – artificial lighting will be required.
  • Between 2 and 5 – Adequately lit but artificial lighting may be in use for part of the time.
  • Over 5 – Well lit – artificial lighting generally not required except at dawn and dusk – but glare and solar gain may cause problems.

See also[edit]

Notes[edit]

  1. ^ Simm, S. and Coley, D., 2011. The relationship between wall reflectance and daylight factor in real rooms. Architectural Science Review, 54 (4), pp. 329-334.
  2. ^ Simm, S. and Coley, D., 2011. The relationship between wall reflectance and daylight factor in real rooms. Architectural Science Review, 54 (4), pp. 329-334.
  3. ^ CIBSE Lighting Guide 10: Daylighting and window design, Year: 1999, ISBN 0-900953-98-5, Publisher: CIBSE

External links[edit]


Original courtesy of Wikipedia: http://en.wikipedia.org/wiki/Daylight_factor — Please support Wikipedia.
This page uses Creative Commons Licensed content from Wikipedia. A portion of the proceeds from advertising on Digplanet goes to supporting Wikipedia.
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26 news items

 
Glass on Web
Sun, 16 Nov 2014 23:18:45 -0800

Today, some European countries have already introduced a daylight factor and minimal glazing requirements within their national construction regulations. Glass for Europe welcomes the pioneering initiatives of countries such as France which requires a ...
 
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Sat, 01 Nov 2014 21:41:15 -0700

At the new Leeds East Academy in West Yorkshire, Mardome dome rooflights from Brett Martin Daylight Systems have helped to provide a lighter, brighter and healthier, more functional internal teaching environment – whilst reducing demand for electric ...
 
ArchDaily
Tue, 19 Aug 2014 00:52:30 -0700

... widely trusted daylighting analysis engines Radiance and DAYSIM to provide key metrics including daylight levels at specific times, annual availability of quality daylight (also known as Daylight Autonomy), annual risk of glare, as well as Daylight ...

Green Building Elements

Green Building Elements
Mon, 28 Apr 2014 02:02:21 -0700

It found that by providing daylight via skylights, the total glazing area could be reduced, on average, from a maximum 20 percent of floor area to 12 percent of floor area while achieving the same baseline average of daylight factor target of five percent.

The Inquisitr

The Inquisitr
Mon, 04 Aug 2014 07:28:55 -0700

Regardless, the appeal of a WrestleMania will be hindered by the lower budget and daylight factor. The shocking news comes from the combination of Raw and Smackdown to re-create the “Supershow” format. The combined shows began in 2011 and ended ...
 
PR Newswire (press release)
Tue, 22 Apr 2014 02:20:56 -0700

It found that by providing daylight via skylights, the total glazing area could be reduced, on average, from a maximum 20 percent of floor area to 12 percent of floor area while achieving the same baseline average of daylight factor target of five ...
 
Marketwired (press release)
Tue, 12 Aug 2014 05:56:15 -0700

... widely trusted daylighting analysis engines Radiance and DAYSIM to provide key metrics including daylight levels at specific times, annual availability of quality daylight (also known as Daylight Autonomy), annual risk of glare, as well as Daylight ...

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Mon, 07 Apr 2014 09:18:45 -0700

Ignoring [their] own policies, they have approved a development with 70 per cent of the four-bedroom units undersized; 42 per cent of all the flats have poor light levels; at least 15 per cent of the units do even not meet required the Average Daylight ...
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