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

Harmonics4 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

• 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() = m V() +(1-m)V’() • 0<m<1 and M(m) is a normalising constant such that the

maximum value of Vmes() 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() Vmes() dVmes()

• Vmes(λ0) is the value of Vmes(λ) at 555 nm

• Le() 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

• 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 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