Introduction

In road lighting design, the surround ratio (SR) and edge illumination ratio (Rei) are indicators for evaluating whether the lighting system can meet the lighting requirements. Both SR and Rei measure the continuity of lighting in the surrounding area adjacent to the road. High SR and Rei values ​​usually indicate better visibility at the edge of the road, which is beneficial for drivers to judge boundaries and obstacles and thus reduce the risk of accidents. However, there are also certain differences between the two. The former was proposed by EN13201:2004 and was replaced by Rei in the EN13201:2015 street lighting standard in 2015. This article mainly explains what SR and Rei are, how to calculate them, and what Rei values ​​are more appropriate. Usually, bad Rei is caused by unreasonable lighting systems (including luminaire arrangement, poor light distribution). Therefore, optimizing these parameters can obtain a better lighting solution, that is, a smaller wattage + a more reasonable Rei value, which is helpful for ensuring the energy efficiency (What’s luminaire efficacy and how to improve it?) and safety of the road lighting system. Now let’s go into the main text to learn more about it.

What’s SR?

SR is short for surround ratio, it’s mentioned in EN13201-2:2004 street lighting standard. SR refers to the ratio of the horizontal illuminance value in the area adjacent to the roadway to the corresponding illuminance value on the roadway. It essentially describes the lighting of surrounding area compared to the road surface. The old EN13201 standard states that SR must be greater than 0.5 for M1 to M5 lighting class(More about M lighting class and its selection). Ps: there isno requirement for M6 lighting class. A good road lighting system should ensure that SR>0.5 so that the driver can see objects and obstacles in the surrounding area, such as pedestrians or potential hazards. Below is the requirement of surround ratio in EN13201:2004.

Class	Luminance of the carriageway for the dry road surface condition			Disability glare TI	Lighting of surroundings
	Lav  [cd/P2]	Uo	Ul	TI  [%]	SR
ME1	2	0.40	0.70	10	0.50
ME2	1.5	0.40	0.70	10	0.50
ME3a	1	0.40	0.70	15	0.50
ME3b	1	0.40	0.60	15	0.50
ME3c	1	0.40	0.50	15	0.50
ME4a	0.75	0.40	0.60	15	0.50
ME4b	0.75	0.40	0.50	15	0.50
ME5	0.5	0.35	0.40	15	0.50
ME6	0.3	0.35	0.40	15	No requirements

What‘s Rei?

Rei is short for edge illuminance ratio (or Environment illuminance ratio), which is firstly proposed in EN 13201:2015 street lighting standard. Edge illuminance ratio (REI) is defined as the average horizontal illuminance (Horizontal illuminance and vertical illuminance) on two longitudinal strips of equal width each adjacent to the two edges of the carriageway, and lying off the carriageway ( E1 , E4 ), divided by the average horizontal illuminance on two longitudinal strips of equal width each adjacent to the edges of the carriageway, but lying on the carriageway (E2, E3). This concept is a bit confusing. Let’s take a 7-meter two-way road as an example. Rei is related to the illuminance of the motor vehicle lane on one side (E2) and the illuminance on its edge strip (E1), and also to the illuminance of the motor vehicle lane on the other side (E3) and the illuminance on its edge strip (E4). The value of Rei is the smaller of E1/E2 and E4/E3. Edge illuminance ratio is to improve spatial orientation, so that the areas beside the carriageway, if not illuminated themselves, are still recognisable. Below is the requirement of edge illuminance ratio in EN13201:2015.

Class	Luminance of the carriageway for the dry road surface condition			Disability glare TI	Lighting of surroundings
	Lav  [cd/P2]	Uo     (Uow)	Ul	TI   [%]	EIR
M1	2	0.40   (0,15)	0.70	10	0.35
M2	1.5	0.40   (0,15)	0.70	10	0.35
M3	1	0.40   (0,15)	0.60	15	0.30
M4	0.75	0.40   (0,15)	0.60	15	0.30
M5	0.5	0.35   (0,15)	0.40	15	0.30
M6	0.3	0.35   (0,15)	0.40	20	0.30

How is SR calculated?

We directly propose the formula SR=Esurround/Eroad, which is the ratio of ambient illuminance to illuminance on the road. So what is the value of a 7-meter road after the lamps are installed? EN13201-2:2004 gives a more detailed calculation formula, which points out that SR=[Ehav(strip 1)+Ehav(strip 4)]/[ Ehav(strip 2)+Ehav(strip 3)], where strip 2 and strip 3 refer to two 3.5-meter lanes on the road, and strip 1 and strip 4 are virtual calculation surfaces distributed on both sides of the lanes (the width is also 3.5 meters). In order to verify whether this calculation formula is correct, ZGSM performed lighting simulation (More about ZGSM lighting simulation) with Dialux version 4.13, because it still uses the EN13201:2004 street lighting standard. In the simulation, we set up a 7-meter motorway and a 3.5-meter non-motorway on each side. The lamps were arranged at the bottom of the single side, with an installation height of 8 meters and a pole spacing of 30 meters. The lamps were Rifle series 50W (ZGSM Rifle series street lights). The simulation results are as follows. In the simulation, the illumination of the motorway was 12.4 lux, the illumination of sidewalk 1 was 11.8 lux, and the illumination of sidewalk 2 on the other side was 5.19 lux. Through the formula, we get SR=（11.8+5.19）/(12.4+12.4) =0.69, which is highly consistent with the results in the simulation.

How is Rei calculated?

The calculation formula of Rei is also related to the illuminance on different strips, that is, Rei=E surround/Eroad is also applicable. EN13201-2:2015 also gives a detailed calculation formula Rei=Minimum [Ehav(strip 1)/Ehav(strip 2); Ehav(strip 4)/Ehav(strip 3)]. Since Dialux 4.13 does not support the calculation of Rei values, we can only perform lighting simulation in Dialux evo (Dialux EVO for street lighting design) and verify whether the results are reliable. For lighting simulation, we also refer to the layout of road and luminaire arrangement of Dialux 4.13, that is, a 7-meter motor vehicle lane (the non-motor vehicle lanes of 3.5 meters on both sides need to be removed), the lamps are arranged at the bottom on one side, the installation height is 8 meters, and the distance between lamp poles is 30 meters. In the simulation, we obtained that the illumination of the two motorway lanes was 14.26 lux (strip 2) and 10.59 lux (strip 3), the illumination of sidewalk 1 (strips 1) was 11.8 lux, and the illumination of sidewalk 2 (strips 4) on the other side was 5.19 lux. Through the calculation formula, we obtained Rei1=11.8/14.26=0.83, Rei2=5.19/10.59=0.49, and finally we took the smaller one to get Rei=0.49. This result is also highly consistent with the simulation result of Dialux evo.

Is the higher Rei, the better?

When the luminance or illuminance of the main road is the same, the higher the Rei, the better. However, there is another important premise, that is, the project uses lamps with the same wattage. In practical applications, ZGSM believes that Rei only needs to meet the corresponding requirements of EN13201 (0.3-0.35) street lighting standard. From the content of the previous sections of the text, we can know that the higher the Rei, the more light on the road. For example, we also use lamps of different wattages to meet the brightness requirements of the M4 lighting class, but the result of the former uses Falcon 60W street light (ZGSM Falcon street lights), Rei=0.49, and the result of the latter is Falcon 80W street light, Rei=0.75. Although the latter has a higher Rei, the former is more in line with actual needs from the project side. Therefore, in practical applications, we also need to make a lot of adjustments to get a reasonable Rei value.

How to modify to get more reasonable Rei with lower wattage?

Different light distribution

ZGSM analyzed the results of IES of Rifle series 60W but different lenses under the condition of certain street profile and luminaire arrangement (7-meter motorway, luminaires are single-side bottom layout, installation height is 8 meters, and pole spacing is 30 meters). The results show that the Rei value of the former Type II light distribution in the Dialux simulation result is 0.49, but the Rei value of Type III light distribution is 0.66, both of which meet the requirements. However, there is also a significant difference in luminance on the lanes of the two, the former is 0.93cd/m2, and the latter is 0.78cd/m2 (just meeting the luminance requirement of 0.75cd/m2). Obviously, the former has more room for optimization. When we reduce the wattage from 60W to 50W, Lav can still meet the requirements, and the Rei value will not change. The final conclusion we get is that Type II light distribution is more advantageous in the above road conditions, which concentrates more light on the motorway, while Type III light distribution has a wide lateral light distribution (Check to find details about light distribution), which directs more light to the sidewalk on the opposite side. Although the Rei is higher, the luminance on the motorway is also lower.

Road layout	Lav	Uo	Ul	Rei	Eav of sidewalk
Without sidewalk Type II	0.93	0.46	0.86	0.49	/
Without sidewalk Type II	0.78	0.54	0.89	0.66	/
With sidewalk Type II	0.93	0.46	0.86	0.84	7.25
With sidewalk Type III	0.78	0.54	0.89	0.85	8.40
With two sidewalks Type II	1.72	0.36	0.91	0.33（<0.35）	/
With two sidewalks Type III	1.55	0.5	0.91	0.53	/

Different layout of roads (street profile)

When the road distribution is different, the Rei results will also be different. For example, the arrangement of the lamps (More about luminaire arrangement of street light) is the same as before, but there is a 2-meter sidewalk on one side of the road. After importing street lights with different light distribution, we get different results, as shown in the table above. In the results, we can see that when there is a sidewalk on one side, the Type III light distribution is even less meaningful, and its Rei value changes very little, only the Eav of sidewalk increases by 8.4lux, but the Lav also decreases a lot. So under what circumstances is Type III more advantageous? If there are sidewalks on both sides of the road and the light poles are far away from the roadside, type III light distribution is relatively suitable, see line 5 and line 6 in the above table. We can see that the Rei of type II light distribution is only 0.33, which is lower than the requirement of M2 lighting class Rei 0.35 in EN13201:2015 street lighting standard, while the Rei with Type III light distribution is 0.53.

ZGSM LED lights

ZGSM has a variety of street light products, and the application scenarios cover main roads, rural roads, sidewalks, parking lots and parks. Different application scenarios often require street lights with different light distribution types to meet the SR or Rei requirements of road lighting. Similarly, we can achieve SR or Rei requirements by fine-tuning the street luminaire arrangement. If you are interested in these street light products, you can click on the link below to get more information. If you have any questions about how to choose light distribution and adjust the luminaire arrangement to achieve the required SR or Rei requirements, you are also welcome to contact us. In addition, for other requirements in lighting standards, such as uniformity, glare, illuminance/brightness, etc., you can also click on the corresponding link to learn more or contact us.

Summary

In the EN12301 street lighting standard, surround ratio (SR) and edge illumination ratio (EIR) are two key parameters. They are both used to evaluate the difference between the illuminance at the edge of the road lighting area and the target area of ​​the road lighting. Due to the update of the standard, in practical applications, we now need to focus on Rei. This article discusses the calculation methods of SR and Rei, and gives the calculation methods and formulas. These basic knowledge are very helpful for facilitating everyone to optimize the Rei value in lighting design. At the same time, we also briefly introduce how to modify to get more reasonable Rei with lower wattage. They include but are not limited to choosing a suitable light distribution, and the impact of layout of road on Rei results. ZGSM believes that the value of Rei can be appropriately small when the requirements are met, because this is very helpful for optimizing lighting design. When meeting the street lighting standard, the lower the Rei value, the higher the energy efficiency (LPD, and what’s LPD?) of the lighting system, which is very helpful in helping you get the project. If you want to know more about SR and Rei, please contact ZGSM for more information.

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