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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