What are the types of IESNA streetlight light distribution patterns and their applications?

The light distribution of LED fixtures refers to the distribution, direction, and intensity of light emitted by the fixtures. It significantly impacts lighting efficiency, energy efficiency, and visual comfort. Proper light distribution can provide appropriate illuminance and uniformity, save energy, reduce glare, enhance safety, decrease light pollution, and create a comfortable lighting environment. LED lighting companies should design appropriate light distributions to ensure the advantages of their fixtures, while lighting engineers and designers should select suitable light distribution patterns based on specific needs to achieve optimal lighting effects from LED fixtures.

When a light source emits light, its propagation direction may not align with the expected direction. In such cases, specific structures (such as lenses and reflectors) need to be designed to alter the light’s propagation direction. This involves adjusting the spatial distribution of light to achieve the desired effect. This method of controlling the propagation direction of light is called the luminaire photometric curve or light distribution.

The light intensity distribution curve, commonly known as the photometric curve or light distribution curve (LDC), illustrates the spatial distribution of luminous intensity from the fixture. The intensity distribution curve is directly obtained through measurements of the fixture using a photometer. Common formats for intensity distribution curves include IES (North America) and LDT (Europe). As buyers, we can also obtain these files by contacting the fixture manufacturers. There are two methods for representing photometric curves: for floodlights, a rectangular coordinate system is generally used, while polar coordinates are adopted for indoor and roadway lighting.

Light Distribution Curve (Polar Coordinates)

In a measurement plane passing through the center of the light source, the luminous intensity values of the fixture at different angles are measured. Starting from a certain direction, the luminous intensity at each angle is marked and represented by vectors. Connecting the endpoints of these vectors forms the polar light distribution curve/photometric curve of the fixture, as shown on the left side of the following image.

Light Distribution Curve (Linear Coordinates)

This distribution curve is typically used for devices such as LED spotlights and floodlights. Since the beams of these fixtures are concentrated in a very small solid angle, it is challenging to represent their spatial luminous intensity distribution using polar coordinates. Thus, some manufacturers use linear coordinate light distribution curves/photometric curves to represent their light distribution. The vertical axis represents luminous intensity (I), while the horizontal axis represents beam angle, as shown on the right side of the following image.

IESNA Lighting Distribution Type Classification

Since its establishment in 1906, the Illuminating Engineering Society of North America (IESNA) has a history of over a century. The classification of lighting distributions introduced by IESNA is still widely used today. The fixture distribution classification system is clearly defined in the ANSI/IESNA RP-8-1983 standard. The types of fixtures defined by the Illuminating Engineering Society of North America are standardized categories used to describe the light distribution patterns of fixtures. These lighting distribution types help lighting professionals and designers understand how fixtures produce light and how light propagates within a given area. IES defines several standard distribution types, each represented by a two-letter code. Common IES fixture distribution types include Type I, Type II, Type III, Type IV, and Type V, followed by Roman numerals (I-V), where S, M, L represent Short, Medium, and Long respectively. The specific classification is determined by the 50% and maximum intensity points of the IES file, which will be detailed in the subsequent sections.

Currently, there are many standardized formats for photometric files, with common ones including EULUMDAT, CIE102, and IESNA LM-63. IESNA LM-63 is used in North America, EULUMDAT is used in Europe, while CIE102 is used in New Zealand. The current standard from 2002 has been approved and recognized by the American National Standards Institute (ANSI). IESNA LM-63-2002 has become the dedicated photometric file format for North America, with the file extension “*.ies”.

IESNA lighting distribution types define the light distribution of fixtures more precisely based on the shape of the illuminated area. For lateral light distribution, this pattern describes how light scatters from the fixture and is characterized by the point where the intensity reaches 50%. This distribution pattern involves the fixture’s ability to project light both forward and backward. In simple terms, if you want to illuminate a single lane, Type I may be suitable; if you want to illuminate two lanes, Type II may be more appropriate. However, this is not a strict rule and is influenced by factors such as mounting height, tilt angle, arm length, and the distance of the fixture from the edge of the road. IESNA has defined five main light distribution patterns: Type I, Type II, Type III, Type IV, and Type V. These classifications are commonly used to determine suitable spectra for roads of varying widths.

In the standards published by IESNA, the road is longitudinally divided into five regions, as shown in the above image. Lateral light distribution is classified based on the area where the 50% maximum intensity point is located. For the light distribution curves of the above fixtures, if the 50% intensity point falls within the Type III area, then its corresponding light distribution type is classified as Type III. From the diagram, we can generally infer that this distribution is suitable for three-lane roads. Different lateral light distributions are suitable for various application scenarios, as detailed below:

• النوع الأول: 1-1 MH, when the 50% light intensity trajectory is located between 1 MH on the fixture side and the street side, we refer to it as Type I narrow symmetrical or asymmetrical light distribution. Suitable for sidewalks, paths, and single-lane roads.
• النوع الثاني: 1-1.75 MH, when the 50% light intensity trajectory is located between 1 MH and 1.75 MH on the fixture’s street side, we refer to it as Type II narrow asymmetrical light distribution. Suitable for 1-2 lane roads, main roads, and highways.
• النوع الثالث: 1.75-2.75 MH, when the 50% light intensity trajectory is located between 1.75 MH and 2.75 MH on the fixture’s street side, we refer to it as Type III wide asymmetrical light distribution. Suitable for main roads, highways, and parking lots.
• النوع الرابع: 2.75-3.75 MH, when the 50% light intensity trajectory is located between 2.75 MH and 3.75 MH on the fixture’s street side, we refer to it as Type IV wide asymmetrical forward light distribution. Suitable for parking lots, plazas, and wall-mounted area lighting.
• النوع الخامس: Symmetrical circular pattern, with circular symmetrical distribution around the fixture, providing even light distribution both in front and back. Suitable for parking and area lighting.

Vertical and Longitudinal Light Distribution

Vertical light distribution refers to the vertical light distribution of a fixture, based on the position of maximum luminous intensity (measured in candelas) in the grid parallel to the TRL. The road along the TRL is divided into different regions according to its distance from the TRL (expressed as multiples of installation height). Longitudinal light distribution involves the fixture’s ability to project light to the left and right sides, defined by the maximum intensity point of the fixture. According to IESNA’s definition, the “S” category applies to pole spacings of less than 2.25 times the mounting height, “M” applies to pole spacings between 2.25 and 3.75 times, and “L” applies to pole spacings of between 3.75 and 6.0 times. However, this is not a strict rule and is influenced by factors such as fixture arrangement and road conditions. Generally, fixtures classified as “S” are suitable for smaller pole spacings, while “L” classified fixtures are suitable for larger spacings.

In the standards published by IESNA, the road is transversely divided into three regions, as shown in the above image. Longitudinal light distribution is classified based on the area where the 100% maximum intensity point is located. For the light distribution curves of the fixtures shown, if the maximum light intensity point of the streetlight falls within the “medium” area, then its corresponding lighting distribution type is classified as “Type II Medium”. From the diagram, we can infer that this lighting distribution pattern has a pole spacing of about 3.0-3.5 times the pole height. Different longitudinal light distributions are suitable for different pole spacing scenarios, as detailed below:

• VS, 0-1.0 MH: When the 100% light intensity trajectory is located between 0 to 1.0 MH in the transverse road lines, it is referred to as VS (Very Short) longitudinal light distribution.
• س, 1.0-2.25 MH: When the 100% light intensity trajectory is located between 1.0 to 2.25 MH in the transverse road lines, it is referred to as S (Short) longitudinal light distribution.
• م, 2.25-3.75 MH: When the 100% light intensity trajectory is located between 2.25 to 3.75 MH in the transverse road lines, it is referred to as M (Medium) longitudinal light distribution.
• ل, 3.75-6.0 MH: When the 100% light intensity trajectory is located between 3.75 to 6.0 MH in the transverse road lines, it is referred to as L (Long) longitudinal light distribution.
• VL, >6.0 MH: When the 100% light intensity trajectory is located beyond 6.0 MH in the transverse road lines, it is referred to as VL (Very Long) longitudinal light distribution.

Application Characteristics of Longitudinal Light Distribution

We used DIALux evo to analyze the applications of Type II S and Type II M streetlights.

The road conditions are as follows: width of 7 meters, three lanes, 0.8-meter overhang, pole height of 8 meters, fixture spacing of 36 meters, in accordance with EN13201 M4 lighting level.

After importing the photometric data of Type II S and Type II M into DIALux evo for analysis, the results are very clear. For Type II S type photometric data, it was found that its longitudinal light distribution is relatively short and fails to effectively distribute light to the sides of the fixtures. Therefore, the spacing between the two fixtures needs to be reduced to 33 meters to ensure that the middle position between the two fixtures receives adequate lighting. This adjustment is necessary to meet the required uniformity. In contrast, the Type II M distribution performs better in this regard with fixture spacing of 36 meters, and all simulation parameters meet M4 standards. The 36-meter spacing is 4.5 times the mounting height, slightly exceeding the recommended 3.75 times by IESNA. Thus, it is advisable to use the lighting simulation results as the basis for the final selection of fixtures.

The above primarily introduces the concept of IESNA light distribution, covering Type I, Type II, and Type III distributions, as well as the concepts of short, medium, and long light distribution types, and briefly outlines their practical applications. Understanding these concepts can quickly determine the appropriate photometric type required for a project. For instance, Type II distribution is suitable for 1-2 lane roads, while the short distribution type is suitable for scenarios where the pole spacing is three times the mounting height. The medium distribution type is suitable for pole spacings of four to five times the mounting height. Of course, these are not absolute, and validation should be conducted using DIALux or other road lighting simulation software.