The route to good lighting

For anyone who is involved in lighting design on a daily basis, coming up with a scheme that meets the needs of the end-user and complies with best practice is almost second nature. For those who design lighting occasionally as part of a wider ranging electrical design, taking all factors into account can seem daunting.

In an office installation, for example, the final scheme must comply with LG3:2001 if it is to meet best practice. This is the latest version of Lighting Guide 3, produced as a guide to good commercial lighting by the Society of Light and Lighting. Since LG3 was upgraded, designing an office lighting scheme is not as simple as it once was.

Fortunately, there is now a set of seven criteria that can be used to assess a design, providing a simple way of verifying that the lighting will do the required job.

Key design criteria

When a car is designed, the designer considers the criteria that will be important to the customer including comfort, ergonomics, style, speed, safety and price. Generally the customer will choose a car that offers the best combination of these to meet their needs.

This same principle can be applied to a commercial office lighting design, whether it’s the luminaire or the installation that is being assessed. The following seven key criteria cover the requirements of LG3:2001 and those relating to energy efficiency as dictated by Approved Document L2 of the Building Regulations:

• luminance limit;
• contrast control;
• room surface illuminance;
• aesthetics;
• energy consumption;
• maintenance costs;
• installed cost.

Ensuring the right luminance limit will help to minimise screen reflections and discomfort glare and should be calculated at the precise limiting angle. If the luminance limit is too high, it will cause reflections on screens; if it is too low there will be dark areas on the ceiling.

A luminance of 1000 cd/m2 is suitable for most modern display screens, though where positive polarity software is being used, for example dark objects on a light background, a luminance of 1500 cd/m2 is acceptable. Levels above this should only be used under exceptional circumstances.

Controlling contrast will minimise discomfort glare within the space. There is no precise measure of contrast but research has shown that maintaining a 10:1 ratio of luminance levels between adjacent areas will provide a comfortable level. To that end, it is necessary to avoid creating hot spots on the walls or ceiling from indirect lighting, using downlighters with sharp cut-off angles and very bright ‘gull’ reflectors on architectural recessed up/downlighters.

Achieving the right levels of brightness on the surfaces around the room is the key to creating a bright and interesting environment. As a guide, LG3 states that illuminance on the walls should be 50% of that on the working plane and the ceiling illuminance should be 30%. LG3 recognises that it can be difficult to achieve this in spaces with low ceilings, but still requires that surface illuminances are considered. A useful rule of thumb when considering surface illuminances is that in an open plan office the ceiling plane is dominant, whereas in a smaller area (under 60 m2) the walls are dominant.

Aesthetics are very subjective and the best approach is to score alternative schemes against each other. In general, minimal protrusion below the ceiling by the luminaires is the preferred option but, as this is not generally a technical issue, the final choice is often determined by personal preferences of designers and end-users.

Considering life cycle costs

Optimising energy consumption while delivering an effective lighting solution is now a requirement of the Building Regulations, which provide guidance on how this can be achieved. Energy consumption is measured in terms of W/m2 and will be determined by the choice of lamps and control gear; the efficiency of the luminaires; the average target illuminance; and the lighting control strategy.

It is also necessary to minimise planned maintenance and cleaning cycles to provide a low cost of ownership. Considerations such as lamp life and whether the design of the luminaire allows easy access for maintenance all need to be considered.

Delivering an installed cost that is within budget, is perceived as good value and provides an effective lighting solution is a challenge for any project. To achieve this it is important to consider the lighting needs early in the project and set realistic budgets. Choosing luminaires that provide good light distribution, so that the number can be minimised, will also help to reduce installed costs.

Putting theory into practice

There is a wide choice of luminaires that can be used to meet the key criteria, often in combinations to ensure lighting to all surfaces and reduce contrast. For example, suspended direct/indirect luminaires provide downward and upward lighting and, in some cases, wall-washing as well. However, they do require the luminaire to be positioned far enough below the ceiling to deliver a smooth spread of light and avoid abrupt luminance change across the ceiling plane. Depending on the configuration of the space and positioning of luminaires, suspended solutions may require supplementary wall-washing to meet the >50% average wall illuminance need relative to the working plane average.

Ceiling-mounted fittings can provide a wash of light across the ceiling as well as downlighting; asymmetrical versions can also provide wall-washing. However, because they are highly visible, they will often fail to meet aesthetic needs.

Fully recessed fittings provide a neat and discreet solution with the option for direct downlight and wall-washing, but they cannot provide a wash of light across the ceiling. Supplementing them with wall-mounted or free-standing uplighters is a common solution but this does have implications for capital, installation and ongoing energy and maintenance costs.

A good solution is a luminaire that is recessed with a modest projection into the space to provide lighting to the ceiling. Such a luminaire needs to have a style that minimises visual intrusion and a profile that will not disrupt the flow of air across the ceiling from ventilation or air conditioning systems. When used in a standard office these luminaires can meet all of the requirements of LG3:2001.

The final choice of luminaires will vary between projects, but by applying the key criteria it is possible to evaluate different schemes to arrive at the optimum solution. For those responsible for the entire electrical design there is a compelling argument to work with a supplier with strong in-house design expertise and an extensive product portfolio that will meet the varied lighting needs of a modern, commercial space.