Daylight should become the primary light source in building for health, productivity and sustainability. Architects should design buildings to maximise the use of daylighting and in many countries are rewarded for doing so by gaining points, e.g. under LEED.

In the healthcare sector, for example, the use of daylighting has proved that it obtains the following benefits[i]:

• sunlight has disinfectant qualities;
• a reduction in the average length of hospital stay
• quicker post-operative recovery;
• reduced requirements for pain relief;
• quicker recover from depressive illness;
• benefits with obesity and heart disease.

There are legislative requirements for ensuring adequate daylight provision in new buildings in many countries. But how should we go about encouraging and maximising its use?

Light

Light is measured in lumens, and illumination in lux. Lux reveal how many lumens you need to light a given area. A lux (symbol: lx) is equal to an illumination level of one lumen per square metre. (In non-SI units, one footcandle is equal to approximately 10 lux.)  

Table: How many lux are needed for different applications:

Source: Carbon Trust and lighting manufacturer Veelite.

Daylight factor

The daylight factor (DF) is commonly used to determine the ratio of internal light level to external light level and is defined as follows:

DF = (E_i / E_o) x 100%

where: E_i = illuminance due to daylight at a point on the indoors working plane

          E_o = simultaneous outdoor illuminance on a horizontal plane from an unobstructed hemisphere of overcast sky.

Factors affecting E_i are:

• the sky component (SC): direct light from a patch of sky visible at the point considered;
• the externally reflected component (ERC): the light reflected from an exterior surface and then reaching the point considered;
• the internally reflected component (IRC): the light entering through glazing and reflected from an internal surface.

The illuminance level (lux) at any point being considered is the sum of these:

Lux = SC + ERC + IRC

Each of these components may be adjusted by the designer (e.g. the reflectivity of internal surfaces) to achieve the required level of lighting.

A DF of the following level has these properties[ii]:

• <2: not adequately lit – artificial lighting will be required.
• 2 – 5: adequately lit but artificial lighting may be in use for part of the time.
• >5: well lit – artificial lighting not required except at dawn and dusk. However glare and solar gain may cause problems.

Note that light intensity decreases by the square of the distance from the point source. Therefore, 500 lux directed over ten square metres will be dimmer than the same amount spread over one square metre.

Right: Illumination decreases by the inverse square law with distance from the light source. As an example, if a bulb gives off 400 lux at 1m, at a distance of 4m the irradiation will be one sixteenth of this, or 25 lux (reading from the graph). If 300 lux is required at 4m distance, then 12 lamps each giving off 400 lux would be required (300/25). This illustrates the importance of positioning in lighting. Source: Author & Wiki Commons, author: Borb. 

Once the lux level has been decided upon and the daylight factor calculated for a particular point, then the daylight autonomy (DA) can be calculated. DA is the percentage of the occupied times of the year (in hours) when the illuminance requirement is met by daylight alone.

But this approach has limitations: DF and DA do not say anything about the quality of the daylight, which is subjective. For example, the degree of shading or contrast can affect what may be visually discerned. This may depend upon whether the sky outside is cloudy or clear.

A refinement to using DF and DA is climate-based daylight modelling which is a new art, but software tools are being developed. Readers are directed to the European Committee for Standardisation, the Illuminating Engineering Society Daylight Metrics Committee and the International Commission an Illumination D3/D6 Technical Committee for further information.

Techniques for using natural light and glazing

Passive daylighting is a system of collecting sunlight to maximise its benefits for lighting, in a controlled manner to avoid unwanted glare. The following tactics may be deployed:

• window size, shape, position and orientation;
• glazing coatings;
• reveal angles;
• shading devices (interior and exterior);
• light shelves;
• skylights and rooflights;
• atrium spaces;
• light wells;
• fibre optic cable networks connected to rooftop light traps;
• tubular daylight devices (sun pipes);
• reflective or pale painted surfaces and interior decor;
• daylight responsive electric lighting controls.

Where possible, ceilings might be sloped to direct more light inward.

It is vital to prevent direct daylight reaching critical visual task areas, and so it needs to be filtered. Artificial light should be brought in gradually further within spaces, so that there is not a sudden contrast between natural and artificially lit areas. The intention is to direct low daylight high into a space (to reduce the likelihood of excessive brightness). 

Window size, position and latitude

To capture the requisite amount of solar light, in progressively high latitudes windows will tend to become larger and equator-facing, while in progressively lower latitudes they will tend to become smaller, especially on the east and west sides, where the sun is lower in the morning and evening.

Separation from solar gain

The management of daylighting will help to separate it from that of heat gain, which may not always be required. This can be achieved by choosing the light-to-solar gain (LSG) of glazing.

This is the ratio between the solar heat gain coefficient (SHGC) and Visible Transmittance (VT). It provides a gauge of the relative efficiency of different glass or glazing types in transmitting daylight while blocking heat gains. The higher the number, the more light is transmitted without adding excessive amounts of heat.

• Visible Transmittance is a fraction of the visible spectrum of sunlight (380 to 720 nanometers), weighted by the sensitivity of the human eye that is transmitted through the glazing of a window, door, or skylight. VT is expressed as a number between 0 and 1, with 0 signifying completely opaque and 1 completely transparent.
• The solar heat gain coefficient (SHGC) of glazing (called the called the 'G-value' in Europe) is the proportion of total solar energy that enters via the window.

Satisfying the remaining lighting requirements artificially

The efficacy of a lamp is measured in the number of lumens it produces per watt input. The strategy to follow is to maximize the number of lumens obtainable for the least number of watts. If an office, which requires 500 lux, has an area of 100 square metres, and the lamps are two metres above the desk level, then this will need 500 x 100 x 4 = 200,000 lumens.

This office could therefore be lit (at night) by (using the figures in the table below): 200,000/70 = 2156W of CFLs, 200,000/90 = 2,224W of LEDs, or 2,000W of high-frequency fluorescent tubes.

Table: The performance of typical 12V lamps.

Source:  Manufacturers' data

NB The performance of LEDs varies considerably according to the manufacturer. Choosing the right LED products is very important.