LEDs and Road Lighting, Signs
and Signals
Steve Jenkins
Road Lighting
• The promise of LED-based luminaires is
great with advantages in low power
consumption, long life, low
maintenance, emitting white light and
ease of being implemented into an
adaptive road lighting scheme.
Road Lighting
The stance of the Report is that the widespread use of
LED road lighting is to be vigorously pursued.
To help to make this happen, it is imperative to point out
the hurdles that are currently preventing its
widespread use.
By detailing the concerns of users, road lighting
designers and visual ergonomists, these can be
addressed urgently so that the potential of LED road
lighting is realised in a short space of time.
Road Lighting
• Major issues of concern are:
• the lack of long term experience of LED luminaires in the road
environment,
• claims made by LED manufacturers
• the likely failure modes,
• the lighting quality of an LED road lighting scheme and how to
specify it,
• how to evaluate the cost/benefit of an LED scheme,
• the test and measurement requirements for an LED luminaire
and
• what monitoring is needed of the performance of an LED road
lighting installation over time
LED Efficacy, Manufacturers’ Claims
LED Efficacy, Manufacturers’ Claims
Lumen Output
Likely Failure Modes
• Catastrophic failure of driver – estimate
0.1%-0.2% per 1000 hours
• Failure of individual LED – rare and usually
goes short-circuit. Consequence depends on
optics, multiple layer or multiple spot.
• Degradation throughout life, then need to
know when installation no longer complies
with Standard.
Types of Optics
Harmonics
4 luminaires don’t comply with the EN61000-3-2 standard
Conclusion-Deswert and Putteham (Belgium)
No major energy-saving
Not flexible in maintenance
Very expensive
Problems with drivers
Technology not mature yet !
Will become important for the future
Road Lighting Quality
• LED luminaires can be designed with quite
sharp cut-off beams, so that specific areas of
the road corridor can be lit.
• Two studies have looked at how people
judge the quality of a road lighting
installation.
Park Path Lighting
• Surround lighting was
appreciated by 87% of
evaluators as providing
safety and visibility
• Sharp lighting effects
were not appreciated
• Shadows and poor
uniformity were not
appreciated
Residential Street (Cat P) Lighting
• 71% of evaluators
preferred the street and
building façade to be
illuminated
Cat P Lighting Quality
• Lower contrast between road and path is
appreciated.
• Perceived safety is well correlated to brightness of
path and façade.
• The ratio of path illuminance to road illuminance
should be about 40%.
• It is recommended to reduce contrast between the
street and the pavements and the surrounding area
instead of increasing the illuminance level on the
road
Road Lighting-Light Quality
Parameters
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Assessment of glare
Lighting of surrounds
Ensuring sufficient vertical illuminance
Preference for warmer colour luminaires
Maintenance – dirt accumulation
Road Lighting – Mesopic Photometry
• CIE are about to publish the CIE System for
mesopic photometry.
• Mesopic luminous efficiency function over the
range 0.005 to 5.0cd/m2 is defined as:
• M(m)Vmes(l) = m V(l) +(1-m)V’(l)
• 0<m<1 and M(m) is a normalising constant such that the
maximum value of Vmes(l) is 1.
• m depends on photopic luminance (adaptation)
level and lamp spectrum (S/P)
Road Lighting – Mesopic Photometry
• Mesopic Luminance is then:
• Lmes = 683  Le(l) Vmes(l) dl/ Vmes(l0)
• Vmes(λ0) is the value of Vmes(λ) at 555
nm
• Le(l) is the absolute spectral radiance in
W.m-2.sr-1.nm-1
Road Lighting – Mesopic Photometry
Road Lighting – Mesopic Photometry
• Adoption of mesopic photometry will result in energy
savings for sources with higher S/P ratios
• The visual task must be considered. For Cat V roads the
task is essentially foveal and rods are not significantly
involved. So always use photopic luminance values.
• Cat P roads will benefit from higher S/P sources.
• More work needs to be done at the implementation stage as
all Cat P lighting is in terms of illuminance NOT
luminance.
• What is the adaptation luminance level in Cat P roads?
LEDs as Signals
• Difficult to match to existing optics
• LED colour shifts with temperature
• Colour of white phosphor-coated LEDs can vary
with angle
• Light output varies with temperature
• Veiling reflection problems or Sun-phantom
• Need to ensure reasonably uniform appearance at
all viewing distances
LEDs as Flashing Signals
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Advantages:
Robust, long life
More efficient at generating coloured light
Colours are saturated, white can have a high CCT
Fast turn on and off – grater conspicuity
Luminance output easily controlled
Low voltage operation
LEDs as Flashing Signals
• Disadvantages:
• Difficult to achieve sharp cut-off for coloured
sector lights
• Measurement issues with high frequency PWM
• Colour has to be uniform throughout beam extent
• I cd and Dominant l can be different between
flashing and steady state modes.
LEDs for Signs
• Problem of maintaining good contrast in the
presence of veiling reflection
• Measurement issues for complex signs such
as ESLS - luminance measurement of red
annulus
• Speed signs that are not full-matrix may
have “confusion” problems