# Daylight factor

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 Radiance 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.