Wherever the office and whatever its size,it deserves to be lit well.

Whether a lawyer’s office is on the 20th floor of Canary Wharf, central London,or above a parade of shops,it needs to have the best lighting for the tasks that the lawyer needs to perform.

The other misconception is that office lighting is all about creating a uniform lighting level across the whole space.What is needed is uniform lighting across each task area,which normally consists of relatively small areas on each desk.The lighting in the wider office space can,and indeed should,vary somewhat to create visual interest.Even the most dedicated office worker looks up from his or her work from time to time,and when they do they need to see an interestingly lit office space and,ideally,a more distant view out of a window.

The new edition of Lighting Guide LG7:Office lighting published by the Society of Light and Lighting provides a valuable guide to this subject and is aimed at building designers and estates and facilities managers.

Design criteria for office lighting

There are many ways to light an office space: with direct light down from above,from indirect light bounced from the ceiling,or from a combination of both.Many factors will dictate or influence the choice of which technique to use.Low ceiling heights or exposed building structure may rule out certain methods or dictate certain layouts.Other building services,such as chilled beams or exposed ductwork,may prevent indirect lighting or provide ideal mounting locations for certain types of lighting.The client,interior designer or architect may have strong views on the style of lighting or lit effect that needs to be created.

For most sizes of office building,the design may be for a known user or a speculative developer and may be a refurbishment or new-build.

Where appropriate,the guide makes reference to the relevant regulations and standards that apply to lighting in offices.

Design brief and information

New build can broadly be divided into:

A.Developments where the occupier and use of the new building is known beforehand.

B.Speculative commercial developments where the final fit-out is left until the new tenant is secured.

Where designers are involved with new office space for a specific client,then it is important that they liaise with the client to determine the types of tasks that will be carried out in each area of

the building.The needs of a publisher,where many people have to carry out varied and sustained reading tasks,are very different from those of a recruitment agency where there may be a lot of face-to- ace interviewing and little paperwork.It is also important to establish what display screen equipment is to be used,where it is to be installed and the way the users intend working with the equipment.

For designers of speculative office space where the working spaces are usually flexible and not designed for specific users or tasks,a more general lighting approach is needed.The end-users could be carrying out almost any type of task or bring in any old type of display screen.For this reason,the exact nature of the lighting and decor to be provided must be established with the building’s owner or developer.

Design criteria

Once the designer understands what limitations the building puts on the design,he/she will need to consider what recommended task illuminances to use for each task area.The effects of the décor and surface reflectance in each area will then need to be taken into account.Unless these are defined in a brief,the designer should confirm to the client the lighting levels being used for the design and any assumptions being made about surface reflectances or maintenance cycles.

Selection of illuminance

The recommended design maintained illuminance over the task area in any room where office work is carried out is generally in the range 300 to 500 lux.This range allows some scope for tailoring for the exact type of work being carried out in the space.Where the tasks are mainly screen based,such as data retrieval or telephone sales, then illuminances at the lower end of this range should be used. Where the tasks are mainly document based,such as writing or copy typing,then 500 lux will be required.Where there are visually more onerous tasks,such as proof reading or technical drawing,even higher levels should be considered.Where the task is of short duration or is unusually large or of high contrast,a reduction in illumination level may be possible.The minimum level set by the Health and Safety Executive for any permanently occupied area is 200 lux.Areas that are not continuously occupied,such as circulation spaces,do not need to have this level.

The use of area or individual dimming of the luminaires should be considered.This will allow users to reduce the lighting level if they prefer to do so.It can also allow for reductions in electric lighting at times and in areas where there is sufficient natural light.

It should be checked that one user’s reduction in his/her lighting level does not adversely affect the lighting level in adjacent workspaces where users may prefer a higher lighting level.

Energy usage

As with all building types,energy usage is a combination of the energy consumption of the individual luminaires and the time that they are in use.The time for which any particular group of luminaires is on depends on whether there is work being carried out in that area or if there is sufficient daylight to allow the lights to be turned off.Of course,this assumes that people or automatic lighting control systems will turn off those lights on such occasions.

The first step in minimising energy use is to avoid overspecifying.

A competent designer should aim to provide as closely as possible the maintained illuminances for each task as specified in the schedule of task illuminances contained in the Code for Lighting also published by the Society of Light and Lighting.

The next step in avoiding unnecessary energy use is to specify the most efficient luminaires that will provide the illumination levelsfor luminaires with high utilisation factors and low energy consumption.

Interaction between daylight and electric light Most office spaces are lit to a varying degree by both daylight and electric lighting. The amount and distribution of daylight varies during the day and from day to day,however the electric lighting tends to be more fixed and is controllable in steps or by area.It is always sensible to explore the feasibility of using the daylight as a source of useful illumination for the office,reducing the amount of electric lighting that needs to be employed.

The guide gives more guidance on the successful use of daylight in an office environment to save energy.Whatever type of lighting controls are installed,manual or automatic,the control zones within the overall space should be localised to groups of workstations and the individual zones should have switches close to the user to allow easy and convenient control of the lights in a local zone.

Window and glazing design

The shape of the windows has a dramatic effect on the availability and distribution of daylight across the space.Tall,narrow windows allow daylight deep into the space but also allow high bright sky to be visible to those working in the space.This can be dazzling and can cause distracting reflections on display screens.Making the widows shorter but wider minimises the area of high,bright sky visible through the window whilst still allowing a good view out and

a reasonable distribution of daylight.

As well as glare,the direct sunlight provides high thermal gain, which can be a cause of considerable discomfort for users near to

windows.Although beyond the scope of this Lighting Guide,consideration should be given to minimising adverse thermal

effects when selecting shading devices.Where the windows provide means of ventilation,then the screening system must be selected to allow users safe and easy access the window’s opening devices.The movement of air through a window can cause annoying flapping or vibration of some screening systems.

Plant life

There is a useful and interesting appendix in the guide on lighting

for indoor plants.For the office spaces themselves,it is normal to select plants which can survive under normal office lighting levels.

For atria and larger entrance halls,and reception areas where special planting may be required,it is reasonable to provide special

lighting to ensure the health and vitality of that planting.

Most light sources encourage plant growth,but it is necessary to consider the composition of the light.For example,‘red’light encourages spindly growth with small leaves,and a small amount of ‘blue’ light is needed to redress the balance.Plants do not respond to light in the same way as the human eye,as can be seen by comparing the sensitivity curve for the eye with that for plant photosynthesis.

The new edition of Lighting Guide LG7:Office lighting costs £25 (plus P&P) for SLL or CIBSE Members and £50 for non-members.It can be

purchased online at www.cibse.org/publications or by calling 020 8772 3618.

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