How to compare lighting designs for road and street applications

Defining and evaluating lighting design specifications can be a challenging task on its own. But as many professionals are coming to realize, comparing lighting design proposals from different companies can be even more challenging.

Given all the criteria that professionals have to take into account, defining a complete and unambiguous set of specifications can be a big help towards comparing proposals accurately. Lighting solutions, after all, can differ significantly depending on the parameters they’re based on — so comparing specifications on a like-for-like basis makes it possible to reach the right conclusions.

This article will focus on how to establish the correct lighting design criteria for road- and street-lighting projects.

Given the many factors involved in providing excellent lighting at a sporting venue, it might be useful to define the key considerations that sports lighting professionals need to take into account.

Lighting criteria

The first task is to define the lighting class and your main reference standard.

The reference standard (which describes the class in question, the criteria associated with it, the calculation process, etc.) can be a national norm or recommendation, or the European road lighting norm EN13201. There can also be additional criteria —for glare (G* class could be indicated), for roads under wet conditions (the minimum uniformity U_ow), or if facial recognition is a priority (the minimum vertical illuminance E_v,min or minimum semi-spherical illuminance E_sc,min).

As some parameters must be maintained over a certain time, it’s also important to link them to a certain maintenance factor. (See the section on light depreciation, below.) Moreover, to ensure accuracy of calculations, it’s important, when defining targeted lighting criteria, to use the same number of decimal places that the revision of European norm EN13201:2015 uses.

Color quality

As both CRI (Color Rendering Index) and CCT (Correlated Color Temperature) influence the efficacy, final performance and quality of a solution, we need to define them as well.

CCT, measured in Kelvin (K), defines the color appearance of a white light source. A warm light (with a yellowish-white color) is around 2700-3000 K; a neutral white light registers around 4000 K; and a cool white light (with a bluish-white color) posts a CCT of 5000K and above.

CRI measures the ability of an artificial light source to reveal an object’s colors in comparison to natural light, which has a CRI of 100. (CRI is also referred to as Ra.) For road and street lighting, CRI 70 and 80 are the most commonly used.

Geometric parameters

When defining the geometric parameters that may influence a lighting solution, you need to look at both certain road configuration aspects and the actual lighting solution’s geometry.

Road configuration includes parameters such as the dimension of the surface to be lit, the number of traffic lanes on the road, and the road surface’s reflective properties. All these have a direct impact on the lighting calculation.

When it comes to the geometry of the lighting installation, you need to consider aspects such as:

o The height at which the luminaire is mounted

o The spacing between the luminaires

o Light overhang, which refers to the distance between the edge of the surface to be lit and the position(s) of the optical center(s) of the light source module(s).

o The distance of the pole from the edge of the roadway

o The tilt angle of the luminaire

o The actual arrangement of luminaires on the road

Energy consumption

It’s important to emphasize that when calculating and comparing the energy consumptions of different solutions, you need to evaluate the system power of both a specific product and the complete installation of which it’s a part.

System power of the product

The total power consumption of a luminaire is the sum of the electrical power that the product’s components consume. We need to consider all of the individual components that contribute to this system power.

System power of the installation

When evaluating the power consumption of an installation we need to consider two different situations.

● Whether it’s a refurbishment — in which case you need to make your calculations using the system power of the product and the annual energy consumption indicator when the dimming scenario is programmed.

● Or whether it’s a new installation — in which case you can evaluate the system power of the installation in W/m or W/km and using the power density indicator (PDI) and annual energy consumption indicator (AECI).

Power density and annual energy consumption indicators

The European standard on road lighting has defined two indicators with which to assess and compare energy performance: the power density indicator (PDI) and the annual energy consumption indicator (AECI).

Power density indicator (PDI ; D_P)

PDI is defined as the energy needed to fulfill lighting criteria at an area. It refers to the capability of a luminaire’s optics and sources to convert electrical power into light on a surface.

Annual energy consumption indicator (AECI ; D_E)

AECI is defined as the total electrical energy that a lighting installation consumes during a year, in proportion to total lit area. It takes into account adaptive lighting, which is inconstant throughout a given period of time (whether a single night or an entire season).

System flux: Luminous flux and efficacy

Luminous flux and efficacy have to be handled carefully, as they can prove confusing parameters in the absence of sufficient information about where luminous flux and efficacy come from.

Luminous flux and efficacy can be considered at different levels: source level and luminaire level. For a project, the meaningful criteria to be compared are at the luminaire level (system flux and system efficacy), as these represent the performances delivered by the luminaire that contribute to the lighting level.

Performances at the luminaire level are unique, and thus comparable. Performances at source level can be expressed in different ways, however — hence the difficulty of comparing them. When comparing lighting products, the product that delivers the lowest system flux is the one that offers the most efficient light distribution.

Light depreciation

To ensure that all lighting designs are based on the same criteria, the customer should indicate the useful lifetime of the installation in question. If the customer indicates no value, the following installation lifetimes can apply:

Outdoor applications	Defualt annual operating hours (EN13201-5)	Average time to refurbishment	Average installation life time
	t0	Years	Hours
Street	4000	25	100 000
Tunnel (entrance)	4000	25	100 000
Tunnel (interior)	8760	12	100 000
Sport (recreational)	1250	20	25 000
Area	4000	25	100 000

Maintaining lighting installations is essential, as it keeps the performance of the system within the design limits and promotes both safety and energy efficiency. In the lighting design phase, the deployment of the overall maintenance factor (MF), which combines several factors into one, accounts for maintenance.

MF = lamp lumen maintenance (LLMF) x luminaire maintenance (LMF)

LLLMF describes the light depreciation of the light source over the course of its lifetime. LMF describes how often an installation undergoes cleaning, a factor that has much to do with the functional characteristics of a luminaire, given that dirt can seriously affect its performance.

Dimming solution

Municipalities understand that it’s unnecessary to keep a light burning 100 percent of the time on any given night. Defining the right dimming profiles and dimming solutions is therefore crucial. Such profiles and solutions help reduce environmental impact and energy costs.

But the lighting level that a specific moment and place require depends on several parameters. Those parameters may vary throughout a night or season. A dimming level is generally defined as the percentage of light that a lighting installation delivers. At first approximation, the dimmed light percentage equals the dimmed system power percentage — an installation burning at 50 percent of its full light level will be consuming 50 percent of the energy that it usually does. We can determine dimming scenarios using the European technical report (TR EN13201-1), which considers the various parameters and their relative influences on light levels.

Dimming solution

Total Cost of Ownership (TCO) represents the overall cost of a lighting installation over its lifetime. It includes three main cost items:

- Initial investment: the cost of the luminaires and any related materials, as well as the cost of the initial set-up of the lighting installation.

- Energy cost over lifetime: the cost of the electricity that the lighting installation consumes over its lifetime.

- Maintenance-related costs over lifetime: this includes maintenance operation costs during the lifetime of a lighting installation.

Total cost of ownership (TCO) lets us compare how much an alternative lighting solution will cost in comparison with a current lighting installation. A TCO calculation takes into account both an installation’s area and the chosen lighting solutions. It can be used to determine how long it will take a more significant investment to pay off financially as compared with a lesser investment. (In other words, to determine ROI.)

Evaluating lighting design plans using the right criteria can be challenging. It can be especially so when you need to compare proposals from different suppliers. The How to compare lighting design guide can help professionals in the field overcome that challenge. It provides a clear list of the criteria that can help in defining project specifications and in comparing designs for lighting projects, with a focus on road and street applications.

To download the complete guide, click here .

Florian Hupont

Lighting Application Specialist - Product Management Outdoor