Introduction

In the implementation of municipal road lighting and high-power lighting projects for industrial facilities, designers have long relied on single parameters—such as luminaire power, beam angle (What’s beam angle?), and central illuminance—to plan lighting layouts. This approach is highly prone to causing issues such as alternating light and dark patches on road surfaces, excessive illuminance variations on workshop work surfaces, and overall lighting energy consumption exceeding standards. In some cases, this can also result in unreasonable illuminance levels in certain areas, leading to poor uniformity; in severe cases, it may even cause localized glare. As a visual representation that clearly illustrates the illuminance distribution across an entire plane, the isolux diagram allows for the intuitive identification of high-illuminance zones, low-illuminance zones, lighting blind spots, and areas of excessive overlap and brightness. It serves as a core technical reference linking luminaire optical performance, lighting design , on-site installation and commissioning, energy-saving retrofits, and project acceptance. This article focuses on two major categories of high-power lighting products—roadway lighting and industrial lighting—and provides a step-by-step explanation of the definition of isolux diagram, how to generate them, and how to apply them during the project design phase for both types of lighting fixtures. Let’s dive into the main text to learn more.

What’s isolux diagram?

An isolux diagram is an important concept in photometry used to describe the distribution density of luminous flux on an illuminated surface. Illuminance refers to the luminous flux received per unit area, typically measured in lux (lx) or foot-candles (fc). Generally speaking, illuminance is higher on an illuminated surface the closer it is to the light source or the more perpendicular the angle of incidence; conversely, it is lower. Connecting points with the same illuminance value forms isolux lines, and multiple isolux lines together constitute an isolux diagram. The isolux diagram corresponds to the isocandela diagram; the former represents illuminance, while the latter represents luminous intensity. Isolux curves vividly illustrate the distribution pattern of illuminance, helping designers assess the distribution of light and shadow within an illuminated area. To better understand isolux curves, we can draw an analogy between isolux diagrams (isolux curves) and “contour lines” in geography. On a map, contour lines connect points of equal elevation, thereby depicting the undulations of mountain ranges; similarly, in lighting design (ZGSM lighting design solutions), isolux lines connect points of equal illuminance, depicting the “peaks” and “valleys” of illumination. The illuminance (or “height”) in the area between two isophotes (contour lines) lies between the two values. Generally, the center of the closed curve represents the area of strongest illumination (such as directly beneath streetlights or highbay lights), while the outward spread of the curve indicates a gradual decrease in illuminance.

Differences in isolux diagrams of street lights and highbay lights

Due to the differences between street lighting and industrial lighting, the isolux diagrams of streetlights and industrial (highbay) lights also exhibit distinct characteristics. Streetlights on roads use a batwing asymmetric light distribution (What’s asymmetric light distribution?), and their isolux diagrams take the form of narrow, elongated ellipses or rectangles, with light extending along the direction of traffic flow. When the contours of adjacent streetlights overlap, the illuminance in the overlapping area increases significantly; therefore, streetlights can achieve a smooth transition in road surface illuminance even when installed at wider intervals. Highbay lights, on the other hand, are based on symmetrical light distribution. Typically, their isolux diagrams form concentric circles; for high-bay lights with narrow beam angles, the isolux diagram lines are more compact and concentrated, while for those with wide beam angles, the isolux lines are relatively more dispersed. Of course, this is not set in stone; for example, some industrial and mining lights also feature rectangular light distribution patterns, resulting in rectangular isolux diagram that are well-suited for scenarios such as the long, linear layouts of factory assembly lines and the narrow, elongated aisles of warehouses. The differences in isolux diagrams between streetlights and highbay lights are fundamentally influenced by the design of the luminaire’s light distribution structure.

Where to find the isolux diagram?

Isolux diagram in IES reports or photometric test reports

Both the luminaire IES files and photometric test reports include isolux diagrams; both are issued by the luminaire manufacturer and serve as the core reference materials for the luminaire’s optical parameters. IES and LDT files are obtained through actual measurements of the luminaires using a goniophotometer (More about Photometric testing) and serve as the foundational data source for compiling photometric test reports. Photometric test reports are in PDF format and embed isolux diagram (which also called isolux plot), making them the most convenient reference materials to consult during the luminaire selection phase. While photometric test reports are easy to read, IES and LDT files must be imported into lighting simulation software to generate a false-color image (equivalent to isolux plots). Of course, if you have professional software capable of opening IES files, you can also view the isolux diagram / plots directly. IES and LDT files are primarily used for comparative luminaire selection and preliminary lighting scheme calculations during the early stages of new construction projects. After data processing through simulation software, the lighting effects (including illuminance) are displayed more intuitively, and the results are more reliable. IES and photometric test reports may be provided by luminaire manufacturers; however, formal LM79 test reports issued by accredited third-party professional laboratories following comprehensive three-dimensional photometric performance testing of finished streetlights and industrial luminaires carry greater credibility. Including isoilluminance (isolux) plots, LM79 reports fully specify key colorimetric parameters such as the Color Rendering Index (CRI and what’s CRI?), Correlated Color Temperature (CCT), and color tolerance.

Isolux diagrams in professional lighting simulation software

We are all familiar with the industry-standard lighting simulation software such as Dialux, Dialux evo, AGi32 (AGi32 and its application), and Relux. Information such as brightness, illuminance, and uniformity—which we commonly refer to—can be obtained through these lighting simulation programs, which also include isolux diagrams and false-color maps. As core tools in the lighting design phase, we can also use them to generate customized isolux diagrams. Taking road lighting as an example, when we import luminaire IES photometric files into the software, place the luminaires, and set actual on-site parameters—such as installation height, beam elevation angle, ground surface reflectance, and lighting maintenance factor—the software can perform illumination superposition calculations for single luminaires or combinations of luminaires, thereby generating isolux diagrams or false-color maps. Standalone IES files or photometric test reports typically simulate the independent light pattern (here means isolux plots) of a single luminaire. In contrast, lighting simulation software is capable of performing global illuminance simulations for an entire road or factory building after the lighting layout is finalized. Lighting simulation is also the most widely used method during the lighting design and construction phases because it more accurately reflects the overall lighting conditions.

Application of isolux diagram in street and industrial lighting

In indoor lighting applications (such as warehouses and factories), we can use isolux diagrams to accurately predict lighting effects. These maps also provide guidance for luminaire layout, thereby ensuring that the illuminance and uniformity at work surfaces meet requirements. Let’s analyze step by step how we use isolux diagrams to select and arrange luminaires, as well as the lighting effects they achieve. When planning factory or workshop lighting, if 150 lux is defined as the standard minimum illuminance value, the 75-lux isolux on the isolux diagram is crucial for determining luminaire spacing. This is because the light from two adjacent factory luminaires can be combined on the evaluation surface, allowing the floor to achieve a total illuminance of 150 lux. As shown in the figure below, the 75 lux isolux line corresponds to an 8-meter spacing at a 10-meter installation height of ZGSM HB11 highbay lights (More about HB11 series high bay lights). Therefore, we recommend an installation spacing of 16 (8 × 2) meters @ 10-meters height. Of course, if the installation height changes, the isolux lines will also shift, thereby affecting the installation spacing. To verify the above calculations, we conducted a simulation using lighting software. In a 60 × 60-meter space, we configured 16 luminaires, each spaced 8 meters apart and installed at a height of 10 meters. The simulation results showed that illuminance at most calculation points reached 150 lux (averaging 170 lux). However, illuminance levels were lower in the four directions farthest from the luminaires (around 120 lux), which is partly due to the circular shape of the isolux lines. Additionally, the emissivity characteristics of the floor, ceiling, and walls also played a role. In conclusion, ZGSM believes that isolux diagrams can be used to estimate installation spacing; however, lighting simulation software remains the tool of choice for professional lighting designers to confirm lighting layouts. We also recommend conducting on-site lighting tests to achieve the best lighting layout results.

When it comes to road lighting, the calculations are relatively complex. As shown in the figure below, based on the IES file and photometric test report for a single luminaire, we know that the illuminance near the center is approximately 13 lux. In the lighting simulation, when the luminaire is installed at a height of 10 meters, no correction factors are required. The simulation shows that the calculated average illuminance near the center point is 13 lux when a single luminaire is installed. On a road, adjacent luminaires all affect the same area, though those closer to the illuminated area have a greater impact than those farther away. After superimposing the light from two adjacent luminaires, the center illuminance beneath the luminaires reached 15 lux. Similarly, in the area between the two streetlights, the primary illumination comes from the two adjacent luminaires. From the isolux diagram, we can see that at a distance of four times of the installation height, the illuminance is approximately 3.3 lux. After superimposing the lighting effects of the two lights, the illuminance is approximately 7 lux, which is essentially consistent with the 7.3 lux reported in the simulation report. When performing calculations for different points—and in addition to calculating illuminance (for pathway lighting, sidewalk lighting, and parking lot lighting)—it is also necessary to calculate luminance (especially for public lighting), glare (What’s glare?), and SR. This method is clearly impractical; therefore, using lighting simulation is the preferred solution. It allows us to overlay the isolux diagrams of two or even more lights to calculate the illuminance at specific points, thereby deriving parameters such as luminance, uniformity (What’s light unifomity?), and Rei—parameters that are difficult to estimate using isolux diagrams alone. It should be noted that the isolux diagram method can only be used for preliminary estimates prior to simulation; the actual lighting effect still requires continuous adjustment and optimization within lighting simulation software.

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Summary

This article provides a systematic introduction to the basic concepts of isolux diagrams and their application in lighting projects. First, the article clarifies the definition of an isolux diagram—an optical chart that represents the distribution of illuminance on a plane using contour lines—and highlights the typical differences between isolux diagrams for streetlights (ZGSM streetlights) and industrial lights: the former features a narrow, elongated distribution to follow the course of the road, while the latter exhibits a circular or wide-angle symmetrical distribution to meet the need for uniform illumination over a large area. Subsequently, the article identifies two primary methods for obtaining isolux diagrams: first, by directly extracting them from photometric test reports such as those from the IES; and second, by generating them from reports in the Dialux lighting simulation software. The former provides actual light distribution data, while the latter primarily serves to validate and optimize the data from the former. At the application level, the article explores the engineering value of isolux diagrams in industrial lighting versus road lighting—in factories, they are used to determine luminaire installation heights and spacing to avoid dark zones; in road lighting, they are similarly used to evaluate installation spacing (pole distance and how to determine the street light distance?) and post-installation illuminance levels, as well as to optimize light pole layouts. For brightness, uniformity, and glare analysis, the article recommends relying on lighting simulations. We hope this information will help you more efficiently achieve design goals such as meeting illuminance standards, saving energy, and reducing costs. For more information, please contact ZGSM.

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