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Fittings to this standard have been used in virtually all office lighting schemes over the past decade that had to cater for areas where visual display terminals (vdt) are in use. Category 2 fittings have become the standard for such areas, despite never having been mentioned in legislation. But that must now change.

The CIBSE's Society of Light and Lighting, concerned about misinterpretation of LG3 and in particular the Cat 2 specification, has been monitoring the use of lighting technology. As a result, the Guide's three-level category system has been withdrawn.

Mike Simpson, president of the Society of Light and Lighting, explains the rationale: "Some designers are concentrating on the Cat 2 luminaire classification and ignoring other, equally important, aspects of the Society's recommendations for lighting where vdt are in use.

"Cat 2 luminaires are not needed in the majority of modern interiors. The category system was created at a time when few vdt had anti-reflective surface treatment and most displays had bright characters on a dark background. Both of these situations are now rare," he says.

The Society advises that if a vdt has an anti-reflection coating and/or the software used displays only dark characters on a white background there can be two stages of relaxation of the Cat 2 requirements for maximum luminance. In modern offices it is usually possible to relax the luminance limits, says CIBSE, often from 200 to 100 cd/m2. This is stated clearly in LG3 but is often ignored by designers.

The withdrawal of the category system means that the design and installation of office lighting schemes are likely to undergo significant changes.

The Regulations
Cat 2 lighting was introduced almost a decade after CIBSE Technical memoranda 6: Lighting for visual display units, the first attempt at guidance for such areas, was published in 1981. This gave advice on reducing luminances to levels which would not produce reflections on vdt screens. It also covered the wider issues of lighting design for such spaces.

The first edition of CIBSE LG3 replaced TM6 in 1989. This introduced the concept of quality classes for luminaires – from Cat 1, for areas with intensive vdt use, to the minimum standard Cat 3 for areas where vdts may be used occasionally. The categories formed only a small part of LG3, which stresses that "selection of the correct luminaire does not guarantee a successful lighting installation".

The key to success, states the Guide, is the avoidance of sharp contrasts within an area – a luminaire reflected in a screen or seen in the distance should not be overly brighter than the general field of view.

In 1992 the European Display Screen Equipment Directive was implemented in the UK by means of the Health and Safety (Display Screen Equipment) Regulations 1992. This came into force for existing installations in 1996. These Regulations require that all lighting installations in areas where vdts are used be assessed for their suitability but they are very general and set no specific requirements. They are supplemented by an HSE guide which gives practical interpretation but no particular solution: LG3 is included in the references but no mention is made of the category system.

As a result of this, the Lighting Industry Federation (LIF) introduced a certification scheme for luminaires complying with the Cat 2 specification that is relevant to the majority of commercial buildings. This, unfortunately, led to claims that category 2 luminaires were mandatory.

A new edition of LG3, published in 1996, attempted to put the emphasis on the wider lit scene and reduce the emphasis on luminaire design. It also recognised that both hardware and software had developed so that the straight Cat 2 specification was relevant in a decreasing proportion of situations.

The need for change
The problem determined by CIBSE is not the Cat 2 specification in itself, but the fact that it is frequently taken in isolation, with room surface reflectances and luminances being ignored in lighting designs. This often leads to unsatisfactory lighting installations which, the Society believes, often do not meet the overall quality requirements of LG3. If the whole publication was taken into account this would not happen, it states.

"By ignoring large parts of our publication, those designing lighting schemes run the risk of producing gloomy interiors which are unpleasant to work in," states Simpson. "More to the point, they cannot claim to be complying with our recommendations. It is not possible to comply with our recommendations with a lighting scheme using solely Cat 2 luminaires," he says.

It is the tendency for developers to use Category 2 as a standard that has led to its misuse and ultimately the decision to withdraw the specification. The Society intends that the category system be replaced by good design practice where luminance limits are checked against the specific requirements of each installation. "This will result in more thought being given to the choice of luminaire and the overall visual environment it creates," predicts Simpson.

The move has been welcomed by manufacturers. "We have been pushing for change ever since the guidance was published because of the appalling lighting schemes that result from a specification relying only on Cat 2," says Harry Barnitt of Zumtobel Staff Lighting.

Simpson highlights the need for an upward light component to be included in schemes. This is already included in the Interior Lighting Code but will now be emphasised by adding the requirement for at least 10% of the installed flux to be directed to the ceiling.

The Society is actively preparing a Factfile to effect the withdrawal of the category system and emphasise the factors needed for good lighting design. It is due to be issued during September. So if you're working on office services, be ready – the changes in lighting installations are due to begin.