Introduction

IES-LM-79-2019 is a new standard that replaces the 2008 edition and applies to solid-state lighting (SSL) products that use light-emitting diodes (LEDs) as light sources for illumination. SSL products include inorganic light-emitting diodes (LEDs) and organic light-emitting diodes (OLEDs), both of which are covered by this standard; however, LEDs remain the most common in lighting applications, such as ZGSM’s streetlights, sports field lights, highbay lights, and floodlights. With an LM-79-19 report, we can determine the photometric parameters of the corresponding product, allowing professional users to select products suitable for their specific lighting scenarios based on this information. Furthermore, since these reports are provided by third parties, they are highly credible. In addition to common metrics such as wattage, luminous efficacy, and luminous flux (What’s lumens – luminous flux?), the standard requires testing of spectral distribution, CCT, CRI, luminous intensity diagram, and isolux diagram. These reports serve as valuable references for both residential and professional products. Let’s delve into the process and content of LM-79 testing, with a particular focus on photometric and color parameters.

Why we need LM-79-19 or what it tells to us?

LM-79-19 is primarily used to characterize the initial photometric and electrical performance of complete luminaires as they leave the factory and is a mandatory submission requirement for several energy efficiency certifications in North America. This standard applies only to finished luminaires equipped with drivers and heat sinks, or LED light engines; bare chips, LED chips, and modules without drivers are not within the scope of testing. It collects only “zero-hour” data after the sample has been warmed up and stabilized, covering three major aspects: electrical, photometric, and chromaticity parameters. Electrical characteristic measurements for AC-powered SSL products include root-mean-square (RMS)* AC voltage, RMS AC current, AC active power, power factor, total harmonic current distortion, and voltage frequency. Measurements of the electrical characteristics of DC-powered SSL products include DC voltage, DC current, and power. For this approved method, the determination of these parameters is considered an electrical measurement. The photometric and color data generally required for SSL products are total luminous flux (lumens), luminous efficacy (lumens per watt), luminous intensity in one or more directions (candelas), chromaticity coordinates, correlated color temperature (CCT) and color rendering index (CRI). In addition, radiant intensity, photon intensity, radiant flux, photon flux, radiant efficiency and photon efficiency are required for specific lighting applications of SSL products, but they are not needed for general lighting products such as streetlights (More about ZGSM LED street lights), highbay lights and floodlights.

How the LM-79 test is carried?

The complete LM-79-19 test suite is conducted in a controlled, standardized laboratory, where the ambient temperature must be maintained at 25°C with a tolerance of ±1.2°C. Airflow, humidity, and stray light interference are strictly controlled in accordance with the standard’s requirements. Test specimens are limited to complete, finished solid-state lighting products, such as finished luminaires and integrated light engines; components such as bare LED chips and modules without drivers are not included in the scope of the test. Prior to testing, the test specimens are powered on under rated operating conditions and maintained in the operating installation orientation specified by the manufacturer until all thermal, electrical, and photometric readings have stabilized; the stabilization time is approximately 30 minutes for small luminaires (50 minutes for ZGSM LED lights), while large luminaires may require several hours.

For electrical testing, a power analyzer is used to measure the input voltage, current, active power, power factor, and total harmonic distortion at the rated voltage; the measurement results include the power consumption generated by the drive circuit itself. For photometric measurements, either an integrating sphere or a C-type goniophotometer may be used; the goniophotometer scans luminous intensity across the entire solid angle to obtain total luminous flux, beam angle, and polar luminous intensity distribution curves, and can also output IES files for use in lighting simulation software. Chromaticity testing relies on a spectroradiometer to collect raw SPD (What’s spectral power distribution?) in the 380–780 nm wavelength range; spectral calculations are used to derive correlated color temperature, color coordinates, and color rendering index (CRI), and optional TM-30 color quality parameters can also be output.

All final values in the report must be traceable to the original test tables, spectral curves, and goniometric data sets. The summary result (see above picture) on the home page is intended for quick reference only and cannot be used on its own as the basis for compliance determination. LM-79-19 measures only initial performance at zero hours, and does not evaluate light decay or product lifespan (What’s lifetime, projecting long term lumen maintenance by LM-80 and TM-21?). Also, laboratory test results cannot fully replicate on-site installation conditions and do not represent on-site glare or actual illuminance on the work surface. See the official ANSI/IES LM-79-19 standard document for complete procedural details. For more testing method and procedures refer to the IES-LM-79-2019 standard.

Electrical parameters in LM-79-19 report

The electrical parameters in the LM-79-19 report are used to characterize the power consumption characteristics of the entire LED luminaire, reflecting its power consumption level and power quality. Key parameters include input power, power factor (PF and what’s power factor?), total harmonic distortion (THD), as well as test voltage, frequency, input current, and other metrics. Input power is the active power of the entire luminaire, which includes the driver’s own losses, and serves as the denominator for calculating the luminaire’s luminous efficacy. Power factor represents the ratio of active power to apparent power; the closer the value is to 1, the lower the reactive power loss in the power grid. Many overseas projects set a threshold of PF ≥ 0.9. THD (Total Harmonic Distortion) reflects the purity of the current waveform; the lower the percentage value, the less interference it causes to the power grid. LM-79 testing collects data under specified voltage and frequency conditions; for the same luminaire model, power, PF, and THD will vary slightly under different supply voltages. Therefore, voltage and frequency are also displayed in the report, as are the photometric and colorimetric results discussed later.

Photometric parameters in LM-79-19 report

Luminous flux and luminous efficacy

Luminous flux and system luminous efficacy are the two most critical metrics among the LM-79-19 photometric parameters. Total luminous flux is measured in lumens (lm) and represents the total amount of visible light emitted by the luminaire into the entire space. This is a measured value for the entire unit and differs from the nominal lumens of an LED chip, as it already accounts for light losses caused by the driver, heat dissipation, and optical lenses. System luminous efficacy is measured in lm/W and is calculated by dividing the total luminous flux by the luminaire’s total active input power. It characterizes the amount of visible light produced per watt of electrical energy and serves as a key criterion for determining compliance with DLC energy efficiency thresholds.

It is important to note that luminous efficacy (What’s luminous efficacy and how to improve it?) reflects only the electro-optical conversion efficiency; a high value does not necessarily indicate superior overall luminaire performance. The luminous intensity distribution and isolux diagram discussed later directly determine the actual lighting effect. For typical residential applications, luminous flux and luminous efficacy are the primary considerations; however, for specialized lighting projects such as roads and sports fields, special attention must be paid to how light is distributed across the illuminated surface—that is, the luminous intensity distribution. Additionally, while luminous flux, luminous efficacy, and colorimetric parameters are not directly correlated, colorimetric parameters do affect lighting comfort and will be discussed in greater detail in subsequent chapters.

Luminous intensity and Isolux diagram

LM-79-19 uses a goniophotometer to scan target points, capturing the luminaire’s three-dimensional luminous intensity distribution data, and outputs the results in two formats: polar coordinate light distribution curves (Polar candela distribution) and luminous intensity data tables. Polar candela distribution allows for an intuitive assessment of the direction of light projection and help identify whether a luminaire has a flood, spot, symmetrical, or asymmetrical light distribution (What’s light distribution?). They also provide the beam angle—the angle at which the luminous intensity decays to 50% of its peak value. The luminous intensity data table records candela (cd) values for each C-plane and Gamma angle; it serves as the raw data for generating IES or LDT light distribution files. These files can be directly imported into software such as Dialux or AGi32 (More about AGi32) for lighting simulation and constitute core data for evaluating luminaire light distribution characteristics in professional projects such as roads and sports venues.

An isolux diagram (More about isolux diagram and its application) is used to illustrate the distribution of illuminance received on a work surface or the ground under assumed installation conditions, measured in lux (lx); it should be distinguished from a luminous intensity distribution map. Luminous intensity maps describe the luminous intensity of the luminaire itself in three-dimensional space, which is determined by the luminaire’s inherent characteristics; in contrast, isolux diagrams are calculated based on a preset installation height and the specific scene. The closed curves in the picture are isolux curves, allowing for a visual assessment of the light beam’s coverage area and illuminance uniformity. While these maps can help predict the lighting effects at a site, the actual on-site illuminance still requires further verification using lighting simulation software.

Chromaticity parameters in LM-79-19 report

All chromaticity parameters (color characteristics) in the LM-79-19 report are calculated from the SPD (Spectral Power Distribution) curve and serve as the core basis for evaluating a luminaire’s color rendering. All chromaticity calculations are performed on the raw spectral data for the visible light spectrum (380-780 nm). The first is the Correlated Color Temperature (CCT) measured in kelvin (K). This tells us if the light is going to look warm or cool. The CIE 1931 color coordinates x and y can be used to identify a color point accurately, and the report also applies u’ and v’ to evaluate the angular color uniformity, which measures the color difference in relation to various light emission angles of the luminaire. Additionally, color coordinates can be used to calculate Duv (i.e., color difference SDCM). Based on the McAdam ellipse, this metric characterizes the deviation between the measured color point and the target center point. Generally, a deviation of ≤3 steps is imperceptible to the human eye, while ≤5 steps is the requirement for commercial lighting and some road lighting projects. Furthermore, the report includes the color rendering index (CRI), which is based on Ra and focuses on R9 (red rendering). The latest LM-79-19 standard also requires TM-30 parameters, where Rf represents color fidelity and Rg reflects color saturation; accompanying vector diagrams provide a visual representation of color deviation. For more information on CRI and TM-30, please refer to our blog – CRI vs. TM-30.

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Summary

This article introduces the IES LM-79-19 testing standard for solid-state lighting products. LM-79-19 establishes a unified, reproducible laboratory testing method for complete LED luminaires (rather than individual LED components). Testing is conducted in a temperature-controlled laboratory environment, and three major categories of data are collected once the luminaire has reached thermal steady state. Electrical parameters include metrics such as input power and power factor under specified operating conditions. Photometric parameters—including luminous flux, luminous efficacy, luminous intensity distribution, and isolux diagrams—are used to describe the luminaire’s light output and illumination coverage. Chromaticity parameters (color characteristics) are calculated from the 380–780 nm visible light SPD spectrum and include correlated color temperature, color coordinates, color difference (SDCM), CRI (What’s CRI and its relationship with luminous efficacy?), and TM-30; color coordinates can also be used to calculate Duv (i.e., color difference SDCM), which differs from the angular color uniformity assessed by CIE 1976 u’ and v’ coordinates. In summary, the LM-79-19 report provides reliable baseline performance data for product validation, certification, and the acceptance of lighting projects.

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