Glare can be caused by disability glare or discomfort glare. Disability glare reduces visibility while discomfort glare causes pain. The Threshold Increment (T.I.) measure quantifies disability glare in road lighting but does not account for discomfort glare or factors like source size. T.I. values should be adjusted upwards for older drivers as glare impacts increase with age. For complex areas like junctions, the Glare Rating (GR) measure may provide a better assessment of disability glare than T.I. or G-classes. Further research is needed on how source size, particularly of LEDs, affects glare.

1. Glare
ILP Professional Lighting Summit - June 2019
Nic Winter - Road Lighting Sales Manager
TRT Lighting Ltd

2. Contents
• What determines glare?
• Disability & Discomfort glare
• T.I. & G Classes
• Tunnels
• T.I. deconstructed
• T.I. & age
• Sports/outdoor work environment Lighting
• GR and area lighting – example junction
• L.E.D.s source size – D classes
• Office Lighting – UGR – source size

3. What Determines Glare?
• How “bright” the light source is – how much
light is coming from the source, in the
direction of the viewer?
• How dark the surroundings are?
• Is the light source in the line of sight?
• How big is the light source?

4. • How “bright” the light source is – the photometry of the fitting
determines what % of the output of the fitting is directed
towards the viewer and what power/flux the output is obviously
also affects brightness. Photometry & Power.
• How dark the surroundings are - ambience - the Luminance of
the known surfaces.
• How directly the light source is in the line of sight, affects the
intensity of the illuminance on the eye. Relative position.
• How big the light source is – source size.
• How old the observer is -Glare, like all lighting, is the interaction
of light & a person.
What Determines Glare?

5. Factor G classes T.I.
Power No Yes
Photometry Yes Yes
Luminance No Yes
Position No Yes
Source Size No No
What Determines Glare?

6. Two Types of Glare
• Disability Glare – too much light entering the
eye relative to the luminance of the visual
field causes scattering of light in the eye and a
veiling luminance, reducing the ability to see.
• Discomfort Glare – too high an intensity of
light falling on a section of the retina causes
discomfort or even pain by overloading
receptors.

7. Disability Glare
• Measured by T.I. in street lighting.
• Varies with column height & position, general
luminance of visual scene as well as with
photometry.
• Generally agreed & quantified though still
varying from person to person & the impact
worsens with viewers’ age.

8. Discomfort Glare
• Caused by over stimulated receptors
• There is not an established, commonly accepted measure for quantifying
discomfort glare, it is to an extent subjective.
• The impact of glare as reported by subjects in tests varies very
significantly.
• Extreme discomfort glare also inhibits vision, overloaded receptors fail,
which differs from veiling luminance
• Designers may sometimes use G classes as a “work around” for the
control of discomfort glare. This is beyond the intended scope of G classes.
However, G classes:
• take no account of the power of the lantern.
• Take no account of a lantern’s location relative to an observers line of
sight.
• Take no account of the adaptation of the eye to the luminance of the
visual field.

9. G Class restrictions do not
necessarily limit disability glare
As measured by T.I.

10. Photometric Evaluation

11. Photometric Evaluation

12. Glare control in tunnels is measured by T.I.
relating to disability glare – a key difference to
road lighting being that the luminance of the walls and
ceiling are known as well as the roadway. Discomfort
glare is not separately accounted for. The road surface
can be made brighter (reducing T.I.) by closely spaced
luminaires with a distribution that does not overly
illuminate the eye (which causes veiling luminance).
This could partly explain why there are some reports of
discomfort glare in tunnels lit with L.E.D.s despite T.I.
limits being met.
Tunnel Lighting

13. Threshold Increment
TI = 65 (Lv / L0.8)
Where TI is the Threshold Increment, Lv is the
equivalent veiling luminance (cd/m2) and L is
the average road surface luminance (cd/m2)

14. T.I. = 65
Threshold Increment
As measured by T.I.
En qn
-2n
1
( )L0.8
For the n* light source
E = Illuminance of the observers eye from the n* source
q = The angle between line of sight and the n* source
L = Average road surface luminance
10

15. T.I. is proportionate to:
Eq-2
L0.8
Threshold Increment
Where:
E = Flux & distribution of the lantern
q = Relative position of lantern to observer
L = Background light level

16. T.I. is determined by:
Flux & Distribution of Lantern
Background Light & Relative Position
Threshold Increment

17. Glare & older drivers
As measured by T.I.

18. T.I. = 65
Threshold Increment
AGE
En qn
-2n
1
( )L0.8
10

19. T.I. = 65
Threshold Increment
AGE
En qn
-2n
1
( )L0.8
10
This constant (k) can be adjusted for age

20. K = 9.05
Effects of Age
( )1+
(AGE
66.4
4
)
K = 9.05
( )1+
( 25
66.4
4
)
For a 25yr old person
= 9.05 x 1.02 = 9.23
REF: Virginia Tech Transportation Institute

21. K = 9.05
Effects of Age
( )1+
(AGE
66.4
4
)
K = 9.05
( )1+
(66.4
66.4
4
)
For a 66.4yr old person
= 9.05 x 2 = 18.1
REF: Virginia Tech Transportation Institute

22. • However, T. I. cannot be used to calculate disability
glare at many junctions and conflict areas as the
calculation grid for T.I. starts 60m in a straight line
from the observer and in many situations this view
will not be available, hence G4 is recommended to
limit the risk of glare.
• GR calculations can however assess disability glare
from many viewing angles and distances.
T.I. Calculations / G Class

23. Glare Rating
LVL = Veiling luminance on the eye produced by the luminaires for
one point.
LVE = Veiling luminance on the observer's eye produced by the
environment.
Is a disability glare based measure like T.I. but using multiple
viewing positions and directions, generally used in sports
lighting. Constructed by the CIE for sports and area lighting could
this measure be used for conflict areas? Lower is better.
i.e. snow improves/lowers GR as it increases Lve

24. Lighting of Work Places
EN 12464-2:2007 (E)

25. Glare Rating
i.e. snow improves/lowers GR as it increases Lve

26. Glare Rating
i.e. snow improves/lowers GR as it increases Lve

27. What’s Changed…? - LEDs.
• Do our existing measures of glare show any difference
between LEDs and lamp based lanterns?
• If not, does this mean that LEDs are not actually “more glary”
or that our current measures of glare are insufficient?
• Previous BS (2003) took account of source size

28. Glare & Source Size
EN13201 2003

29. Source size
• Lumens per mm2 of emitter or Candelas per
steradian ?
• What is the emitter – the LED chip, the lens, the
cover ?
• Is far field single point photometry now failing?
• At 20m distance from a lantern on a 10m column
the eye can delineate as finely as an 8mm separation
between 8mm wide lines.

30. Source size UGR
• Used in interior lighting to quantify discomfort glare despite being a
luminance based measure.
• UGR accounts for the contrast in Luminance of a lantern to the
surrounding luminance.
• It takes account of the source size and the relative position of the observer
as part of the calculation of the luminance of the lantern.
• UGR =8log((0.25/background luminance) x Luminance
(cd/m2) of source in direction of observer position/ position
index )
• However: it only refers to a standardised layout of luminaires
on a ceiling which prevents it being of use in exterior lighting
& takes the complete optical area into account to calculate
luminance which may not be appropriate for LED.

31. What to Do?
• Consider the T.I. of illuminance based designs when the road
layout is broadly appropriate but balance this with the need for
vertical illuminance.
• There is not an established measure of discomfort glare in
street lighting.
• G classes can be misleading
• T.I. is an increasing concern with increasing driver age
• For conflict areas G4 as per the B.S. guidance is playing safe but
potentially unnecessarily restrictive in some situations.
• Consider using GR in conflict areas.
• Research on the effect of source size is needed.