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Technical - CIBSE LG3

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What is LG3?
What is the relevant legislation?
What does this mean in terms of lighting?

How has the latest issue of LG3 changed?

What is LG3?

The Lighting Guide LG3 is a document published by CIBSE (Chartered Institute of Building Services Engineers). Entitled 'The Visual Environment for Display Screen Use'. LG3 details a design process that can be used when illuminating areas containing computers & display screen equipment. Although no specific reference to LG3 exists in any regulation, a guidance document to the Health and Safety Regulations does state in its Schedule of Minimum Requirements that LG3 provides 'specific and detailed guidance' on lighting issues.

Lighting Guide LG3:2001 is the latest development in a design guide that has been in use by the lighting industry since 1989. It has recently been updated to take into account new display screen types, new software and lighting technology and changing national and international standards.

What is the relevant legislation?

A The requirements of EC directive 90/270/EEC (Directive on minimum safety and health requirements for work with display screen equipment) and other European health & safety directives have been written into law through the Health and Safety Regulations. The requirements cover all aspects of operator / workstation interaction including visual environment, seating position and keyboard angle. There are four regulations that contain comments and requirements for lighting. These are :-

The Management of Health and Safety at Work Regulations

The Workplace (Health, Safety and Welfare) Regulations

The Provisions and Use of Work Equipment Regulations

The Health and Safety (Display Screen Equipment) Regulations

A new DSE ergonomics standard was introduced after LG3:1996 went to print. BS EN ISO 29241,'Ergonomic requirements for office work with visual display terminals' is referred to in guidance to the Heath and Safety (Display Screen Equipment) Regulations. Although there is no requirement to comply with this new ISO standard, the Display Screen Regulations do state that 'employers may find standards helpful as workplaces satisfying [BS EN ISO 29241] would meet and in most cases go beyond the minimum requirements in the Schedule of the Regulations'.

What does this mean in terms of lighting?

A Two statements made in The Health and Safety (Display Screen Equipment) Regulations directly affect the specification and implementation of artificial lighting. These statements are 'The screen shall be free from disturbing glare and reflections' and 'There shall be an appropriate contrast between the screen and its background'. Lighting must adequately illuminate the working area while limiting veiling reflections on the screen. It may therefore be necessary to control any light produced at a high enough angle to directly reflect back into the line of sight of the operator. Obviously in small enough areas this is not a problem but for larger open plan or glass partitioned areas, greater attention must be paid to the design and distribution of luminaires which are to be used.

BS EN ISO 29241 states that 'It is...important that the luminance peaks diverge as little as possible from the average values'. A sharp cut-off in the luminous output of a luminaire may cause scalloping at the tops of the walls, resulting in visual fatigue. Harsh cut-offs in the luminous output of a luminaire should also therefore be avoided.

How has the latest issue of LG3 changed?

A LG3:1996 gave guidance allowing the designer use of a standard range of downlights for instances where display types and locations were unknown. It set out measurable limits to three categories of luminaires to match different concentrations and applications of display screens. Each Category had an associated limiting angle, above which the luminance of the luminaire could not exceed a predetermined value. The limiting angles were measured from the downward vertical at all angles of azimuth, Category 1 having the tightest control, down to CAT3 which was far more relaxed.

Since the introduction of the category system there has been a tendency amongst some in the industry to specify category rated luminaires in all environments where display screens are to be used. Very little consideration has been given to the actual working environment into which the luminaires are to be installed, or to whether category rated luminaires are appropriate. This has often resulted in inappropriate poorly designed lighting schemes that could be viewed as not fit for the purpose. For this reason the Category system has been withdrawn from LG3:2001. The specification of luminaires for use in display screen areas should in future be based on data provided to the designer by the client detailing the type, position and orientation of the screens to be used.

BS EN ISO 29241 acknowledges that measurement of peak luminance on complex luminaires has always been difficult and for this reason the 'Patch Test' has also been removed from LG3:2001. However, manufacturers are still required to design luminaire systems that avoid obvious bright patches as far as is practicable.

Sub-section 8.3.2 of LG3:2001, under the heading of 'unknown screen geometries', deals with areas where details are not available. It states that if information on display screen position and orientation is not available, the designer must select luminaires with an appropriate luminance limit based on the degree of anti-reflective screen surface treatment. This luminance limit would be applied at 65° from the downward vertical unless it is known that the display screen tilt angle was likely to require a lower limit angle. This section of the guide also reminds designers that the use of a luminaire with a defined distribution pattern does not guarantee successful results. Moreover, only consideration of all aspects of the guidance will result in design compliance with Lighting Guide 3.

Appendix 2 of LG3:1996 has also been withdrawn and replaced with guidance for selecting appropriate luminaires based on luminaire lighting distribution. It states that where details of the anti-reflective properties of display screens in an area are unavailable, the designer should select luminaires with a luminance limit of 200cd/m² at 65° elevation. In special circumstances, the luminance limit angle may be reduced to 55°. It should be noted at this point that assuming the patch test was ignored, a luminaire with a luminance limit of 200cd/m² at 65° elevation would constitute LG3:1996 Category 2 compliance for display screens with poor surface treatment.

When working with a known client and where full details of the display screens have been provided, the specified luminaires may have luminance limits of 500cd/m², 1000cd/m² or 1500cd/m² with the limit angle defined by the designer.

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