Introduction

Currently, the light distribution of LED streetlights is typically classified as Type IM, IIM, IIIM, VS, and so on; however, some streetlight bidding projects (More about requirements as per street lighting tender) still specify requirements using beam angles. Why is this the case? The answer lies in the use of two classification systems: the former employs the IES Luminaire Classification System (LCS), while the latter uses the NEMA Classification—which is, of course, also included in the IESNA Handbook. Both describe the spatial distribution of light emitted by luminaires, but each has its own characteristics. This article will focus on the IES Luminaire Classification System (LCS) and NEMA Classification, as well as their applications in street light distribution classification. This includes different light distributions of street lights, as well as the street light beam angle.

What’s IES luminaire classification system (LCS)?

The luminaire classification system proposed by the IES serves as the cornerstone of road lighting design, defining luminaire types based on the lateral and longitudinal distribution of light on the road surface. Lateral light distribution, also known as transverse light distribution, determines the light distribution type based on the ratio of the luminaire’s mounting height to the road width. Lateral light distribution is determined by analyzing the half-maximum candela trace. Type I is suitable for narrow sidewalks, where the light is primarily distributed within a range of 1 times the mounting height on either side of the luminaire (i.e., the half-maximum candela trace falls between 1 MH on the house side and 1 MH on the street side of the luminaire position); Type II and Type III are suitable for roads of medium width, with Type III’s half-maximum intensity point falling between 1.75 and 2.75 times the installation height in front of the luminaire. Type IV is suitable for wide-area lighting, featuring extremely strong lateral projection, with the half-maximum candela trace extending beyond 2.75 times the installation height. It also introduces longitudinal light distribution, which is determined by analyzing the maximum candela trace to classify the luminaire’s distribution type (short, medium, or long). Generally, when the luminaire’s maximum intensity point falls within a range of 1.0 MH to 2.25 MH on either side of the luminaire’s position, it is classified as having a short distribution characteristic. For luminaires with medium light distribution characteristics, the point of maximum luminous intensity lies within the range of 2.25 MH to 3.75 MH. For luminaires with long light distribution characteristics, the point of maximum luminous intensity lies within the range of 3.75 MH to 6.0 MH. These three types of light distribution are suitable for pole spacing that is small, medium, and large, respectively. Light distribution is typically designed for a range of typical conditions, including luminaire installation height, lateral (overhang) position, longitudinal spacing, the width of the road requiring effective illumination, and luminaire arrangement (What’s luminaire arrangement in street lighting?).

Same LCS light distribution means same lighting result?

The classification of LCS light distributions is based on the half-maximum candela trace and the maximum candela trace. Since light distributions falling within the same range are defined as identical, the lighting results for the same LCS light distribution will inevitably differ, and in some cases, these differences can be significant. In road lighting simulations, we also use Type II lateral light distributions to observe whether different lanes are illuminated; the results clearly show that the former provides significantly better lighting performance than the latter. The same applies to the same Type xxM vertical light distribution. ZGSM therefore recommends that clients, if your project has specific information regarding pole height (What factors decide height of street lights?), pole spacing, pole cantilever and setback, and tilt angle, select an appropriate light distribution for lighting simulation based on your lighting requirements, rather than simply choosing a light distribution based on pole height versus road width or pole spacing, as the results may not necessarily meet your project’s requirements.

What’s NEMA classification?

The National Electrical Manufacturers Association (NEMA, which is well know as NEMA socket) has established a luminaire classification system based on the distribution of luminous flux within the beam produced by the luminaire. This system is primarily applied to sports lighting and indoor lighting fixtures. It defines seven distribution types (i.e., Types 1 through 7), with beam widths increasing sequentially from narrow to wide. This classification system and other related classification methods use “beam angle” and “field angle” to define the light distribution characteristics of luminaires. The beam angle is defined as the maximum angle at which the luminous intensity drops to 0.50 times its maximum value, measured from the center of the light distribution. As shown in the NEMA classification diagram below, if the maximum luminous intensity of the luminaire is 100,000 cd, then the beam angle is the angle formed by the two lines connecting the points at 50,000 cd on either side to the origin (0 cd). Taking the ZGSM Zoom series floodlight as an example, by reviewing its photometric test report, we know that the luminaire’s maximum intensity is 5,542 cd, half of the maximum intensity is 2,771 cd, and its beam angle at C0/180 is 25.1°, which matches the claimed beam angle of 25° as stated in the datasheet.

Same beam angle means same lighting result?

Not always. Based on our definition of beam angle, we know that even if the beam angles are similar, the light distribution of the luminaires can still vary significantly. As shown in the figure above (left and right), although the angles of these high mast lights (ZGSM high mast lighting solutions) between the 50% intensity points and the origin are both around 15 degrees, the light from the former is noticeably more concentrated, while a portion of the light from the latter extends beyond the 15-degree angle. In photometric test reports, we can also see that at the same height, there is a significant difference in illuminance at the center point: the former measures 890 lux @30m distance, which is far higher than the latter’s 924 lux @30m distance. In lighting simulations, the results for the two luminaires also differ significantly. As shown below, using a standard soccer field as an example, we compared two field lights with a claimed beam angle of 15°; the average illuminance of the former is noticeably lower than that of the latter. For the street light beam angle, the situation is the same. Analysis reveals that the primary reason is the former’s lack of light concentration, causing a portion of the light to fall outside the field or into the sky, thereby reducing the amount of light reaching the field. Furthermore, light directed toward the periphery not only causes light spill but also creates glare—neither of which is desirable for ideal sports field lighting. Please read our blog to learn more about how to reduce light trespass in sports lighting.

Light distribution of LED street light

The light distribution of LED streetlights is currently classified primarily using the IESNA LCS system, commonly referred to as Type IS, Type IIM, Type IVS, and so on. When a streetlight project specifies requirements for street light beam angle, we must classify the light distribution according to NEMA classification standards. As we know, streetlights are typically installed on one side of the road; even when equipped with cantilevers, they are generally positioned closer to one side rather than directly in the center of the road. Therefore, LED streetlights often employ asymmetric light distributions (Asymmetrical light distribution and its benefits) to meet the requirements of road lighting. So, how is the NEMA classification system applied?

Different types of light distribution by IES outdoor luminaire classifications

As mentioned earlier, there are descriptions of the different types of light distribution for streetlights. ZGSM has written similar articles in the past, so I won’t go into too much detail here. If you’re interested, you can check out the ZGSM blog post titled “IESNA Lighting Distribution Types and Their Applications”

Beam angle of LED street light – street light beam angle

The beam angle of an LED streetlight refers to the angle of light dispersion emitted by the light. Specifically, it is the angle formed as the light extends outward from the brightest point at the center until the light intensity drops to 50% of the maximum intensity at the center (i.e., the half-peak intensity). However, as we know, streetlights use an asymmetric light distribution, meaning that the light emitted by the streetlight differs in directions such as C0/180, C30/210, C60/240, and C90/270. We often see product specifications for streetlights labeled as 70° × 150°, indicating two street light beam angles at both C0/180 and C90/270. These two angles represent the beam angles at the C0-180 and C90-270 characteristic planes. The former describes the light distribution along the width of the road, while the latter describes the light distribution along the length of the road.

Taking the ZGSM Alca streetlight (Alca street lights)as an example, let’s explain how the street light beam angle is determined. For C0-180 plane, the maximum luminous intensity is approximately 4,500 cd, so the half-maximum intensity will be 2,250 cd. As shown in the diagram, there are two red lines. Each line connects the origin to the half-maximum intensity point. This gives us an angle of 69.6°. Therefore, the street light beam angle for the C0-180 plane is 69.6°. For the street light beam angle @C90-270, it differs slightly from that at @C0-180. As we can see, the maximum intensity is around 10,500 cd. Obviously, the angle between the two lines connecting the origin to the half-maximum intensity point will be less than 160°, since the half-maximum intensity point lies below the 80° line. So what’s the issue? Actually, for the beam angle @C90-270, we use the center light intensity of the LED luminaire, which is approximately 3000 cd. This gives us a street light beam angle @C90-270 of about 160°, which is quite close to the result shown in the diagram.

For streetlights, the beam angle can be simply understood as determining whether the light cast on the ground is “concentrated” or “dispersed.” For instance, if the street light beam angle for lateral light distribution (C0-180) is 70°, the light is primarily concentrated within that range; conversely, a 150° street light beam angle for vertical light distribution (C90-270) results in less concentration. Selecting the appropriate beam angle is crucial for streetlights: an excessively wide C0-180 beam angle can cause light to spill onto the roadside, leading to light pollution, while an angle that is too narrow may leave parts of the traffic lanes unilluminated. A narrower C90-270 beam angle is suitable for roads with shorter pole spacing and offers better glare control, whereas a wider angle suits roads with greater pole spacing but tends to result in higher glare (Glare and how to reduce it). Additionally, a combination of a narrow beam angle and low pole height can create dark zones between poles, leading to uneven road surface brightness.

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Summary

In the selection and bidding process for LED streetlights, the IES Luminaire Classification System (LCS) and the NEMA classification system are often used in parallel. The IES Luminaire Classification System categorizes luminaires into Types I–IV and short, medium, and long distribution types based on the horizontal and vertical distribution of light on the road surface, serving as a common foundation for road lighting design. The NEMA classification system , on the other hand, uses beam angle and field angle as classification criteria, dividing light distribution into Types 1–7, and is primarily used for sports and indoor lighting. While the definitions and application scenarios of the two systems differ, some streetlights currently still label their light distribution patterns directly using the “beam angle” terminology from sports and indoor lighting—a topic central to this article.

Using the ZGSM Alca series of streetlights (Click to view more ZGSM street lights) as an example, this article combines the non-symmetrical light distribution characteristics of LED streetlights to provide a detailed explanation of how to calculate street light beam angles within the two characteristic planes (C0-180 and C90-270). It also analyzes the impact of beam angles on the road illumination range, illuminance uniformity (What’s light uniformity and how to improve it?), glare, and the formation of dark zones. The paper emphasizes that the selection of beam angles must be based on lighting simulations conducted using the project’s actual parameters; existing classifications or fixed angle values cannot be directly applied. It also points out that luminaires within the same LCS classification may not necessarily produce consistent lighting results. Since this classification is based solely on the trajectories of the half-maximum and maximum luminous intensity, the simulated results after luminaire installation may vary significantly. Similarly, luminaires with identical beam angle values may exhibit differing lighting performance; variations in the concentration of luminous intensity distribution can lead to significant deviations in central illuminance, average illuminance, and illuminance uniformity. For more information on LED luminaire light distribution and lighting design (ZGSM lighting design solution), please contact ZGSM.

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