To achieve the best overall outcome in a lighting installation, it is important to avoid the tendency of rushing straight into luminaire selection before determining more broadly what is required from the system. The use of a structured design process helps to avoid this.
The key steps in the design process are:
- Identify the requirements
- Determine the method of lighting
- Select the lighting equipment
- Calculate the lighting parameters and adjust the design as required
- Determine the control system
- Choice of luminaire
- Inspect the installation upon completion
(and if possible, a few months after occupation, to determine what worked and what didn’t. This is the only way to build up experience to apply to future designs)
The five initial stages are considered in more detail in the following lines.
This involves gaining a full understanding of what the lighting installation is intended to achieve. This includes the following:
- Task Requirements ?
- Mood of the space
- Relation to shape of space
- Things to be emphasised
- Things to hide
- Direction of light
- Interaction of daylight
At this stage, consideration is given to how the light is to be delivered, e.g. will it be recessed, surface mounted, direct or indirect, or will up-lighting be used, and its primary characteristics, e.g. will it be prismatic, low brightness or mellow light.
Consideration should be given at this stage to the use of daylight to minimise the need for artificial light.
Once the method of lighting has been selected, the most appropriate light source can then be chosen followed by the luminaire.
The following attributes should be studied when choosing the light source:
- Light output (lumens)
- Total input wattage
- Efficacy (lumens per Watt)
- Physical size
- Surface brightness / glare
- Colour characteristics
- Electrical characteristics
- Requirement for control gear
- Compatibility with existing electrical system
- Suitability for the operating environment
A number of factors also affect luminaire choice:
- Characteristics of the light source and control gear
- Luminaire efficiency (% lamp light output transmitted out of the fixture)
- Light distribution
- Glare control
- Finish and appearance
- Accessibility of components for maintenance
- Ability to handle adverse operating conditions
- Thermal management
Lighting calculation methods fall into three broad categories:
Photometric data for light sources and luminaires is commercially available to contribute to these calculations.
There are a wide range of manual computation methods for the calculation of different lighting aspects. These include complex methods for calculating the illuminance from a wide variety of shapes of luminous objects. The majority of these have now been superseded by computer programs (check our free software).
The Lumen Method was the mainstay for interior lighting and has remained in use as a quick and relatively accurate method of calculating interior illuminance.
Inadequate attention to the assumptions will produce poor results.
The basic assumptions are:
- All the luminaires in the room are the same and have the same orientation
- The luminaires do not have a directional distribution and are aimed directly to the floor
- The luminaires are arranged in a uniform array on the ceiling and have the same mounting height
- The luminaires are spaced less than the maximum spacing to mounting height ratio nominated in the coefficient of utilisation tables
The average illuminance produced by a lighting installation, or the number of luminaires required to achieve a specific average illuminance, can be calculated by means of utilization factors (UF), a UF being the ratio of the total flux received by a particular surface to the total lamp flux of the installation.
Lumen method formula //
The average illuminance E(h) over a reference surface s can be calculated from the “lumen method” formula.
- F – the initial bare lamp flux (lumens)
- n – the number of lamps per luminaire
- N – the number of luminaires
- LLF – the total light loss factor
- UF(s) – the utilization factor for the reference surface s of the chosen luminaire
Utilization factors can be determined for any surface or layout of luminaires. The “UF” symbol is normally shown followed by an extra letter in brackets, to denote the surface, for example, UF(F) is the utilisation factor for the floor cavity and UF(W) is the utilisation factor for the walls.
The lowest surface, the F surface (for floor Cavity), is a horizontal plane at normal working height (i.e. table height), which is often assumed to be 0.85 m above the floor.
The middle surface, the W surface (for walls), consists of all the walls between the C and F planes.
Although the lighting designer can calculate utilization factors, lighting companies publish utilization factors for standard conditions for their luminaires. The standard method of presentation is shown below. To use this table, it is only necessary to know the Room Index and the effective reflectance of the three standard surfaces (floor cavity, walls and ceiling cavity).
Room Index //
The Room Index is a measure of the angular size of the room, and is the ratio of the sum of the plan areas of the F and C surfaces to the area of the W surface. For rectangular rooms the room index is given by:
- L – the length of the room
- W – the width of the room
- Hm – the height of the luminaire plane above the horizontal reference plane.
If the room is re-entrant in shape, for example L shaped, then it must be divided into two or more non-re- entrant sections, which can be treated separately.
Spacing to Mounting Height Ratio (SHR)
The Spacing to Mounting Height Ratio (SHR) is the spacing between luminaires divided by their height above the horizontal reference plane.
Although it was possible to calculate the luminance of all the surfaces in a room, the calculations were extremely laborious and could only be justified in the most special cases. However, the advent of computer modelling enabled a more flexible approach to lighting design and significantly increased the information available to the designer.
- A mixture of luminaires can be used
- The luminaires no longer have to be arranged in a regular array
- Directional luminaires can be modelled
- A large number of calculation points can be considered to give a meaningful uniformity calculation
- The illuminance and luminance of all surfaces can be calculate
This gives the lighting designer a much greater understanding of what is happening in the room.
However there has been considerable research, experience and documentation over the past 80 years that has developed the current thinking in the adequacy of various illuminance levels for various tasks and functions.
Although there is some general understanding of the need for appropriate luminance distribution in the vertical plane, there is little information, experience or understanding for many designers to determine:
- What the luminance of surfaces should be in varying situations
- What is an acceptable luminance uniformity
- Whether there should there be a maximum luminance uniformity
- What is the desired graduation in luminance
- At what point is the luminance distribution of the wall unacceptable
It is important in using a lighting calculation program that the output records the type of luminaire used, the location of the luminaires, the assumed lumen output of the lamp, the light loss factor and the aiming points. If this is not recorded you have a pretty picture of the installation and no way of making it a reality.
These are programs that create a perspective rendering of the space in levels of detail that vary from a block representation of the space, to photographic quality renderings, depending on the sophistication of the program and the level of detail of the interior to be entered.
The programs fall into two basic types:
- Flux transfer or radiosity calculations
- Ray tracing calculations
A Lambertian surface is a perfect diffuser, where light is reflected in all directions, irrespective of the angle of incidence of the light such that irrespective of the viewing angle the surface has the same luminance. A specular surface is a mirror like surface, where the angle of reflection of the light is the same as the angle of incidence.
A real life surface is a combination of both surfaces (semi-specular) and has both specular and diffuse characteristics. Some materials are more specular while others are more diffuse.
A flux transfer or radiosity program treats all surfaces as diffuse or Lambertian surfaces, as a result their rendering tends to appear flat with soft shadow details. It will tend to overestimate the uniformity. Ray tracing traces the individual rays of light from the source to the eye as it reflects from surface to surface around the room. As a result ray tracing can allow for the specular component of the surfaces.
Some programs calculate the entire lighting by ray tracing while others calculate the space on a flux transfer basis and have an overlay of ray tracing of specific areas to improve the quality of the rendering. When ray tracing is added, reflections are added in polished surfaces and shadows become sharper.
The programs can show the designer how a specific design will perform but that they cannot reliably be used to assess the acceptability of a design.
Irrespective of the form of the visualisation output, it is important that the program provides adequate information to enable the construction and verification of the lighting design.
The output should include:
- Installation information – the type and location of all luminaires and the aiming information. The lamp details should be included as well as the specific catalogue number of photometric file that has been used.
- Light technical parameters – the illuminance, uniformity and other parameters that have been calculated to achieve the design.
- Verification information – adequate details to enable the lighting calculation to be verified. This should include the luminaire type, the photometric file, surface reflectances that were assumed, light loss factors, lumen output of lamps and mounting and aiming locations.
The effectiveness and efficiency of any lighting installation is affected as much by the control system as by the light sources and fixtures chosen.
Give consideration to:
- Providing multiple switches to control the number of lights that come on at any one time. Using one switch to turn on all the lights in a large room is very inefficient.
- Placing switches at the exits from rooms and using two-way switching to encourage lights to be turned off when leaving the room.
- Using ‘smart’ light switches and fittings which use movement sensors to turn lights on and off automatically. These are useful in rooms used infrequently where lights may be left on by mistake, or for the elderly and disabled. Make sure they have a built-in daylight sensor so that the light doesn’t turn on unnecessarily. Models which must be turned on manually and turn off automatically, but with a manual over-ride, are preferable in most situations. Be aware that the sensors use some power continuously, up to 5W or even 10W in some cases.
- Using timers, daylight controls and motion sensors to switch outdoor security lights on and off automatically. controls are particularly useful for common areas, such as hallways, corridors and stairwells, in multi-unit housing.
- Using solar powered lighting for garden and security lights.
- Using dimmer controls for incandescent lights (including halogens). This can save energy and also increase bulb life. Most standard fluorescent lamps cannot be dimmed, but special dimmers and lamps are available. If lamps are to be dimmed it is important to ensure that the correct equipment is used, especially when retrofitting more energy efficient lamps.
The performance of a luminaire should be considered just as carefully as its cost. In the long term a well designed, well constructed luminaire will be cheaper than a poor quality unit; and the salient features of a good quality luminaire are:
- Sound mechanical and electrical construction and a durable finish
- Adequate screening of high luminance lamps to minimise discomfort and glare
- Adequate heat dissipation to prevent over-heating of the lamp, wiring and ancillary equipment
- High light output ratio with the appropriate light distribution
- Ease of installation, cleaning and maintenance
Reference // The Basics of Efficient Lighting – A Reference Manual for Training in Efficient Lighting Principles – National Framework For Energy Efficiency