GLARE
–noun, (glehr). 1. A light within the field of vision that is brighter than
the brightness to which the eyes are adapted. Glare is caused by a
significant ratio of luminance between the task and the glare source. DESCRIPTIONS
ILLUMINANCE
Lux levels to calculate light.
LUMINANCE
Brightness.
ELONGATED LUMINAIRES
Luminaires with a base length ratio of 3:1 or more. eg: Luminaires
that utlilise horizontally mounted linear fluorescent tubes.
NON-ELONGATED LUMINAIRES
Luminaires with a base length ratio of 3:1 or less. eg: Luminaires that
utlilise tungsten filament lamps and compact fluorescent lamps.
DARK-SIDED LUMINAIRES
Recessed Luminaires with a vertical depth of the luminous opening
less than 30mm. Also luminaires with opaque side panels.
BRIGHT-SIDED LUMINAIRES
All other luminaires different to the above.
THRESHOLD ZONE
Where the lighting changes over two extremes over an appropriate
distance where the observer can easily adapt to the opposing
transition of light from where he / she is moving from.
CANDELA
Luminous intensity at a specific angle.
ONE
GLARE
A term used where effects are produced due to the luminance of
a visual system and the visual field in the space being significantly
different. The two major types of glare are:
1. DISABILITY GLARE
Visual impairment where eyesight is temporarily or permanently
effected. Veiling luminance is a main effect with disability glare.
Veiling Luminance – when a light source of a high luminance is
viewed directly, then when you look away into a darker space,
obscuring details of after images of the light source are created.
This is light scattering in the eye which produces veiling luminance,
which is then super-imposed over the space. Another term which
supports this is the Transient Adaption Effect.
Transient Adaption Effect – when a long adaption change in your
eye sight occurs due to moving from a brightly lit space to a low dim
space between a short distance. The darker space appears darker
than what it actually is, which is quite dangerous in safety and
security environments.
An example of this is driving into an internal carpark from outside.
The change of daylight to a 40 lux illuminated carpark will cause a
long adaption effect. This is why a Threshold Zone must be applied,
which means between the high and low illuminance, an even
amount of illuminance between the two extremes must be applied
to allow the adaption to evolve quicker and avoid disability glare.
This is why we apply 800 lux in the first 15m of an internal carpark
entry then 150lux the following 4 metres to allow for this adaption.
TWO
2. DISCOMFORT GLARE
Where your peripheral vision captures a source of contrast or glare
in the surrounding environment.
It can be immediate at one time, however, sometimes it can be
evident after an extended period of exposure.
Other factors that provoke discomfort glare are exposure time, the
adaption state of the visual system, fatigue, age, drugs and alcohol.
How to avoid discomfort glare?
ͽͽ Reduce luminance of a glare source.
ͽͽ Reduce size of the glare source.
ͽͽ Increase background reflectance or illuminance.
ͽͽ Re-position glare sources away from line of sight.
GLARE SOURCE, LUMINANCE Lᵥ
PLANE OF
THE EYE
DIRECTION OF VIEW
E EYE
THREE
REDUCING GLARE FROM WINDOWS
Windows can cause discomfort glare. Not only can the sun glare
through the window cause this, particular when the worker is
stationed in line of sight of the sunlight, but a brightly lit overcast sky
and highly reflective buildings, also known as exterior obstruction
luminance, can cause levels of visual discomfort.
Ways to avoid glare from windows:
ͽͽ Reduce the size of the windows.
ͽͽ Arrange work positions so windows are not in line
of sight.
ͽͽ Installing curtains, blinds, or opaque louvers to diffuse
the luminance.
ͽͽ Use of tinted/low transmission windows, also known
as ‘glazing.’
ͽͽ Arrange for other light sources in the room (ceiling luminaire /
windows on the opposite side of the room) to reflect light into
the walls adjacent to the windows to decrease contrast between
a dark wall and bright window.
Ways to avoid glare from luminaires:
ͽͽ Choosing luminaires with low illuminance.
ͽͽ Arranging work positions or re-locating luminaires so that they
are not in line of sight.
ͽͽ Use high reflectances on ceilings and upper walls along with high
illuminance/luminance levels to avoid low contrasts.
FOUR
ͽͽ Choosing luminaires with high visual comfort and excellent lamp
shielding.
CUT-OFF ANGLES
The Cut-off angles for interiors such as offices, school classrooms,
and other industrial interiors is 60 degrees.
Cut off angles are required to block out any discomfort glare.
Partly enclosed luminaires (with open reflectors / louvers) screen the
lamps from being viewed directly from the horizontal plane down to
at least the required cut-off angle. If the luminaire is open, glare is
visible in all directions.
POSSIBLE CUT OFF
ANGLES (SEE NOTE)
POSSIBLE CUT OFF
ANGLES (SEE NOTE)
FIVE
REFLECTORS – MATT OR SPECULAR
MATT: A reflector on the luminaire where you get a shaded diffused
view of the light source which produces minimal visual discomfort.
SPECULAR: Another reflector, however, if the luminaire is
positioned in line of sight, you will get a direct reflection of the light
source, which can cause visual discomfort.
NOTE: If task lighting is also provided in a space, the assumption is taken that the task
lighting will not contribute to any increase of discomfort glare.
CONTROL OF DISCOMFORT GLARE FROM
ELECTRIC LIGHTING
There are two main systems to control this procedure. They are:
1. The CIE Unified Glare Rating (UGR) system (also commonly known
as Glare Index (GI)
2. Luminaire selection / Luminance Limiting system.
NOTE: Some areas require more attention when controlling discomfort glare:
a. The major part of the space is screen-based.
b. The room is large.
c. Visual tasks are difficult to view, for example, low contrast, small
detail, etc.
d. Workers viewing a subject higher than the horizontal plane, for
example, a lecture theatre.
e. Room reflectances are dark.
SIX
1. UGR/GI, which can be calculated in AGi32, is determined by
calculating the glare’s luminous intensity (usually measured in
candela’s per square metre) from various positions / angles of the
space. For example, standing in the corner of a room, the glare
source is aimed at 225 degrees. This is the direction taken
into calculations.
Some angles will show a significantly higher glare reading than
others due to the likelihood of having a direct view of a light source
reflecting off a luminaires reflector. This direction/angle is worst
case scenario of the highest glare reading in the environment being
calculated, which should be the priority angle taken into calculations.
This method provides values of UGR taken from the following scale:
13, 16, 19, 22, 25, 28.
We take the maximum UGR value being calculated, and if that falls
under the required glare reading then the calculation complies.
For the purpose of complying, if the figures don’t meet the exact
value they are to be rounded up or down using intervals of 1.5.
For example, if a maximum glare reading for a meeting room at 225
degrees reads 20.4, this figure being less than 1.5 to 19, then the
figure gets rounded down to 19. If it’s 20.5, then it’s rounded up to
22.
NOTE: Refer to LUX booklets where the Maximum UGR/GI reading required is stated for
all environments.
2. Luminaire Selction / Luminance Limiting system. If the lighting
designer needs to select a lighting system based on minimal
information given to him/her, a luminaire selection process can
be applied providing a uniform array of one type of luminaire is
being used.
SEVEN
The luminaire selection system is based on reducing the risk of
discomfort by controlling the luminance in luminaires in different
directions, depending on the size of the space and the luminaire’s
being used.
Depending on how much information the designer is given, the
luminance limiting method can still exceed compliance. For example,
a luminaire may already be installed next to a window.
However, if there is very minimal information provided, the designer
can then state his/her assumptions based on a set criteria:
ͽͽ The maintained illuminance is not greater than 600 lux.
ͽͽ The maximum luminous opening of the luminaires must not
exceed 1200mm x 600mm.
ͽͽ The intensity distribution is not an extreme bat-wing distribution.
ͽͽ Room reflectances for ceiling/walls/ floor are 70/50/20.
ͽͽ The ‘worst case’ glare positions are on the C0 and C90 planes of
the luminaires orientation.
ͽͽ The visual display is similar to a reading / writing / typing
environment.
LINE OF SIGHT
EIGHT
MAXIMUM LUMINANCES OF LUMINAIRES IN
LIGHTING SYSTEMS:
Firstly, we must take into account:
a. The type of luminaire being used. (elongated / non-elongated /
etc) – refer to page 1 descriptions page.
b. The particular angles where luminance limitation is necessary.
c. The maintained illuminance which is recommended for the space
(refer to LUX booklet)
Through this process we take into account viewing directions along
and across a room. Luminance limits are based on a scale measured
in kilocandelas per square metre:
1, 1.5, 2, 3, 4, 6, 8, 12, 16.
We then continue to determine which angles we apply the
luminance limiting method to.
Based around the CO and C90 planes of a luminaire, the main angles
which we deal with mostly where luminance limits apply are:
ͽͽ 55 degrees
ͽͽ 65 degrees
C90
C0
55
NINE
85
75
65
85
75
65
55
And dealing with maintained illuminances less than 400 lux we need
to know that at:
ͽͽ 55 degrees – 6 kilocandelas per square metre is what the
luminaire will have. (6000 candelas per square metre)
ͽͽ 65 degrees – 3 kilocandelas per square meter.(3000 candelas per
square metre)
HOW DO WE KNOW IF OUR CHOICE OF LUMINAIRE
COMPLIES WITH THIS METHOD?
Refer to the NATA test report of the luminaire where the Average
Luminance (cd/sq.m/klm) table is provided. Look at the values on the
CO and C90 grids where they line up with 55 and 65 degrees. If those
values fall under kilocandles required, then everything complies.
HOW TO WE CALCULATE THIS?
If you’re using a 1x28w lamp wattage, we can then calculate if our
choice of luminaire complies. If the value on the test report lining up
the C0 and 55 degrees reads 528 candelas per meter square, and the
requirement being 6000 candelas at that angle, then your luminance
limiting method complies. The same calculation must be applied for
both C0 and C90 planes to comply.
AVERAGE LUMINANCE (cd / sq.m / klm)
GAMMA
C0
C45
C90
45.0
2314
3459
2248
55.0
528
1409
1325
65.0
117
160
54
75.0
37
28
26
85.0
31
33
33
TEN