How to choose the right light distribution types for streetlights？

Introduction

The type of street lamp lighting directly affects the lighting effect, visual comfort, and energy efficiency.

Brightly lit cities are the main places for human activities at night, and outdoor lighting plays a vital role in this. Therefore, outdoor lighting (such as road, parking lot, and stadium lighting) has become a rapidly growing part of the lighting market in recent years. In this development process, we also need to pay attention to the quality of lighting. The basic factors that affect the quality of lighting include brightness level and its uniformity, maintaining appropriate contrast between objects and backgrounds, limiting glare to ensure viewing comfort, etc., which are closely related to the light distribution types of lamps. This article mainly explains how to choose the right light distribution to ensure that the lighting meets the corresponding standards. These standards include the 115-2010 “Roadway Lighting for Automotive and Pedestrian Traffic” standard issued by the International Commission on Illumination (CIE), the European standard EN 13201:2015 “Roadway Lighting”, and the North American standard ANSI/IESNA (American National Standards Institute/Illuminating Engineering Society of North America) RP-8-00 “Roadway Lighting”. By choosing the right light distribution, we can meet the lighting needs with the minimum wattage while ensuring brightness, illumination, uniformity, and controlling glare ( How to control glare? ), which is crucial. So, let’s go into detail step by step.

Three main reasons why light distribution types

Lighting power density ( LPD )

LPD is an important indicator for measuring the energy efficiency of lighting systems. It represents the lighting power per unit area. The lower the LPD value, the more efficient the lighting system. In lighting design, LPD consideration helps to optimize the energy consumption of lighting systems and reduce the energy consumption of road systems, thereby reducing operating costs and reducing environmental impact. To reduce LPD, lighting designers can choose more efficient light sources and lamps with more reasonable light distribution. Generally speaking, the more reasonable the light distribution, the smaller the wattage required to meet the corresponding standards, that is, the smaller the LPD.

Uniformity

Uniformity ( What’s uniformity in lighting? ) is a parameter that describes the uniformity of light distribution in the lighting area. It is usually obtained by calculating the ratio of the minimum illuminance (or brightness) to the average illuminance (or brightness) in the lighting area. Good uniformity can ensure that the light intensity at each position in the lighting area is similar, thereby avoiding areas that are too bright or too dark, and improving visual comfort and safety. In lighting design, the more reasonable the light distribution of the lamp, the smaller the difference in illuminance (brightness) at each point in the lighting area, that is, the higher the uniformity.

Glare

Glare refers to the visual phenomenon that causes visual discomfort and reduces the visibility of objects due to inappropriate brightness distribution or brightness range, or extreme brightness contrast in the field of vision. In lighting design, many standards have put forward requirements for glare, so glare control is very important. For example, to reduce glare in road lighting, we can appropriately increase the installation height of the lamp pole, keep the lamp horizontally installed, or use a sunshade, etc. If these cannot effectively reduce glare, then the glare can only be reduced by replacing the light distribution of the lamp to ensure that it meets the requirements of the corresponding standards.

Classifications of light distribution types

The light distribution pattern mainly refers to the spatial distribution characteristics of light when it leaves the lamp. Each lighting fixture will produce a different light distribution pattern due to its design, LED position, structural characteristics, lens selection, etc. To simplify and standardize, the lighting industry usually classifies these patterns into several classified and widely accepted light distributions types, including Type I, Type II, Type III, Type IV and Type V. For details, please refer to the light distribution definition of different types of lamps such as road lights and area lights by the Illuminating Engineering Society of North America ( Find more about IESNA).

Lateral light distribution

Type I, Type II, Type III, Type IV, and Type V actually refer to the lateral light distribution of the lamp, that is, how the light emitted by the lamp is distributed on several lanes of the street. The lateral light distribution types of the lamp are determined by analyzing the half-maximum candela trajectory. It provides some guidance on how to choose the light distribution according to the road width and installation height, but there is no necessary connection.

In short, Type I is suitable for roads with 1-2 lanes or one lane + sidewalk, Type II is suitable for roads with 2-3 lanes or 2 lanes + crosswalk, Type III is suitable for roads with more than 3 lanes, and Type IV is suitable for parking lots. Of course, this is generally for reference. The lighting simulation results should be used to determine which light distribution is suitable for your project. The results can well reflect the lighting effect after the lamps are installed.

According to experience, Type xS light distribution is suitable for 2.5-3 times the pole spacing, Type xM light distribution is suitable for 3.5-4 times the pole spacing, and Type xL and xVL light distribution is suitable for more than 4 times the pole spacing. These are also for reference only. 

How to choose the right light distribution for your project

Lighting distribution on road lighting

To help you better understand the impact of light distribution types on lighting effects, we will demonstrate the specific role of light distribution through lighting simulation. The following is the road condition we simulated: the road width is 7.0 meters, with 2-meter-wide sidewalks on both sides, the installation height of the lamp is 9 meters, and the distance between the lamp poles is 32 meters. We selected three different lenses for comparison and listed the simulation results in the below table.

	Average Luminance Lav [cd/m2] >1.00	Overall Uniformity Uo [-] >0.40	Longitudinal Uniformity Ul >0.60	Threshold Increment FTI [%] <15	Edge Illuminance ratio REI [-] ≥0.30	Em ≥7.50	Emin ≥1.50	Em ≥5.00	Emin ≥1.00
Option 1	1.00	0.4	0.76	14	0.46	9.71	6.52	5.87	3.45
Option 2	1.00	0.4	0.75	14	0.51	10.23	6.95	4.87	2.75
Option 3	0.75	0.59	0.78	11	0.68	7.55	4.60	7.97	5.06

The following is a diagram of a false colour rendering result. In option 3, the lighting of the sidewalk and the middle motorway is insufficient because the unreasonable vertical light distribution causes the light to fail to cover the sidewalk and the motorway. In this case, we have to install more lamps, which means more installation space for lights. will be shortened. In option 2, the brightness of the motorway meets the requirements, but the lateral light distribution of the crosswalk far away from the lamps is unreasonable, and the illumination does not meet the requirements of P4 sidewalk lighting in EN13201. On the contrary, option 1 provides uniform lighting on both the sidewalk and the motorway, and the illumination meets the requirements of the EN13201 standard. Both the vertical light distribution and the horizontal light distribution perform well, ensuring the safety of pedestrians and drivers at night.

Lighting distribution on parking lot lighting ( Type IV )

To save energy, the customer chose to replace the LED lamps. We recommended the most suitable lens to the customer through lighting simulation. The size of the parking lot is 40*60 meters, with a total of 90 parking spaces. The lighting simulation shows that when the T2M lens is selected, the average illumination is 75lux and the uniformity is 0.288. When we change to the Type IVM lens, the average illumination is 82lux and the uniformity is 0.375. We can conclude that in this case, the Type IVM lens is better than the Type IIM in terms of both illumination and uniformity. The reason is very simple. The Type IIM lens is not suitable for covering the wide area of two parking spaces plus one lane in terms of vertical lighting distribution. In the false color rendering image below, we can intuitively see that the lane cannot be effectively illuminated, which ultimately leads to insufficient uniformity. At the same time, some light is projected to the roadside, resulting in a lower average illumination. In this case, to achieve the same illumination, we have to use higher-wattage lamps, and to improve uniformity, we have to place more lamps or replace lenses, check to find more about Hylele parking lot lighting solutions.

Lighting distribution on pedestrian lighting

The lighting of pedestrian crossings requires street lamps with asymmetric light distribution in both C0-180 and C90-270. These street lamps can provide vertical illumination to directly illuminate pedestrians on the side of or on the pedestrian crossing and remind motor vehicle drivers of the existence of the pedestrian crossing. To have better vertical illumination, these lamps are usually placed on both sides of the pedestrian crossing. For example, a one-way single-lane or a two-way two-lane road lamp only uses a street lamp with a right-polarized light or left-polarized light design (C0-180), which can achieve effective lighting of the pedestrian crossing. If it is a one-way, two-way, three-way or four-lane, it is often necessary to combine two street lamps ( Find more Hylele streetlights )with different light distribution types to obtain higher illumination and better uniformity. As shown below, we compare the use of conventional lenses and asymmetric light distribution (C0-180+C90-270) street lamps. The simulation results show that the former requires 30W to achieve the required 30lux illumination, while the latter only requires 20W, and the uniformity is better than the former.

Summary

This article mainly explains the importance of light distribution, the different street light distribution types, and the application of light distribution in different scenarios, such as ordinary roads, parking lots, sidewalks, etc. Through these, we can conclude that reasonable light distribution can better meet the requirements of contrast and uniformity in lighting standards. Lamps with unreasonable light distribution often require higher wattage to meet the requirements of illumination. In addition, to achieve the ideal uniformity, the project party has to place additional lamps to achieve the good uniformity. In some special applications, we need customized lenses to meet the lighting requirements, such as sidewalks, parking lots, stadiums, etc.