Spatial distribution from luminaires

Generally, light shining on task surfaces is "direct" light, while light shining away from such surfaces is "indirect" light. Sometimes designers instead describe fixtures based on the direction of the light: "uplights", "downlights", or "sidelights".

The output distribution of a luminaire can be measured over a range of angles and plotted on a polar diagram. In it, the lines represent light levels actually measured at a fixed distance from the luminaire (usually 1 m) as it was rotated around to different angles. In most cases, the units of this graph will be lumens. However, these levels can be normalized from 0-1 to represent a multiplication factor instead; this provides for the use of different lamp powers in the fixture.

Types of Luminaire
(click to enlarge, see more detail in Building Science Fundamentals)

A luminaire refers to a complete lighting unit, including one or more lamps, reflective surfaces, protective housings, electrical connections, and circuitry. All these components affect the overall output of light from the unit. Even with a very bright lamp, translucent or semitransparent materials may reduce the flow of light, and reflective surfaces can quickly become dusty and dirty in some environments, also reducing output.

Lighting Layouts

Given the wide choice of different lamps and luminaires available, there is an almost infinite set of different arrangements of electric lights within a room that will provide a certain illumination level.

The primary concern in lighting layout is to avoid glare on activity surfaces. Such glare is a result of light bouncing directly into user's eyes, rather than diffusely.

Good and bad lighting locations, causing or avoiding glare

The ultimate choice of lighting layout will invariably be an aesthetic one— achieving the right feel or ambiance within the space. Doing this well comes from experience as a lighting designer and from simply applying a bit of thought to the space and how it is likely to be used.

The images below demonstrate just two of the many different ways of lighting a simple room to give exactly the same lighting levels on each work surface. Each layout has advantages and disadvantages. For instance, the first layout might use less energy, but the second layout will cause less glare and will have fewer shadows cast on work surfaces by occupants.

Different fixtures and layouts providing the same lighting levels with very different distribution

The Lumen Method of Lighting Design

The intuition of the architect alone is not sufficient to fully comprehend the interplay of light and predict with certainty the amounts of illumination in all parts of the room. On the other hand, computational tools available are also often insufficient in themselves to achieve "beautifully" lit buildings.

The simplest method of calculating the overall illumination level for evenly lit spaces is the Lumen Method. It uses both computation and intuition and is the calculation most used by lighting engineers when determining the number of luminaires for a given lighting level. The simple formula is as follows:

E = F / A

Where E is the average (or minimum) illumination level at the work plane (in lux), F is the useful lumen output of all sources (lumens) and A is the total surface area of the working plane (in m²). In terms of architectural design, solving for F allows the architect or engineer to determine the total amount of light required in the room. This is given by rearranging the formula as follows:

F = AE

Be aware that the resulting value is not the lamp lumens, because not all of the light produced by each lamp actually reaches the work plane. Many factors affect the amount of light reaching the work plane:

• The size and proportion of the room.
• The height of the light fixtures above the work plane.
• The reflectance of wall and ceiling surfaces.
• The nature of the light fixture and its distribution of light.
• Light loss due to ageing, dust collection and yellowing.
• Atmospheric particles such as smoke or dust.

Some of the most efficient fixtures in the most effective layout can result in up to 80% of the installed lamp lumens reaching the work plane, while ineffective fixtures in a dark-colored room can result in only 2%.

Steps of the lumen method:

1. Select Required Illumination

2. Determine Received Flux

This is simply a matter of calculating the total surface area over which the required illuminance is to be distributed and multiplying this by the required illumination level using the formula F = AE. This gives the amount of "useful" light required. From this, the total installed flux can be determined.

3. Select a Light Fixture

A preliminary assessment must be made of the type of lighting required, a decision most often made as a function of both aesthetics and economics. This fixture may prove unsuitable for the lighting task, however. The next few steps are used to determine this.

4. Determine Mounting Height

The distance from the source to the working plane is very important, as it is a major determinant of the final illumination level. This is a function of the inverse square law.

5. Determine Room Index

The room index is a ratio, describing how the room's height compares to its length and width. It is given by:

Where L is the length of the room, W is its width, and Hm is the mounting height above the work plane.

6. Determine Utilization Factor (UF)

The Utilization Factor, or Coefficient of Utilization, brings together all of the variables above (reflectance of both the walls and ceiling, the room index, and the type of luminaire) into a single value. Use tables available from manufacturers (all architects' offices will have several) to determine the coefficient of utilization for different light fixtures.