Illuminating the Path to Unravel the Secrets of lighting performance evaluation

Lighting plays a crucial role in our lives, illuminating our surroundings and shaping our experiences. As energy efficiency and sustainability have become increasingly important, the focus on optimizing lighting technologies has intensified. In this quest for efficient illumination, energy reporting standards such as LM-79, LM-80, and TM-21 have emerged as key tools for evaluating and understanding the performance of lighting systems. These standards provide valuable insights into the longevity, reliability, and energy consumption of light sources. Moreover, advanced measurement devices such as Photometers and Goniophotometers enable precise and comprehensive analysis of lighting characteristics, including luminous intensity, spectral distribution, and light distribution patterns.

To evaluate the longevity of LEDs, there are some standard industry reports used to evaluate the lifetime of LEDs and establish the performance characteristics of LED lighting products. The 3 most common reports are LM-79, LM-80 and TM-21 that are approved by Illuminating Engineering Society of North America (IESNA).

LM-79: a standard for testing the performance of the LED lighting product.

LM-80: a standard for testing the lumen maintenance of the LED package (LED chips and phosphor) over time.

TM-21: estimates the LED lifetime based on accelerated testing.

Now, we discuss some more information about these 3 reports for LEDs:

LM-79 report: This report is a testing procedure that evaluates the optical and electrical characteristics of LED lighting products, including their total light output, efficacy, color characteristics, and distribution patterns. It provides a standardized method of comparing LED products from different manufacturers to ensure that they meet minimum performance standards. The LM-79 test measures the luminous flux (total light output), electrical power consumption, and other photometric and colorimetric (spectrophotometer) characteristics of the LED product. The results of the LM-79 test are used to calculate the product's luminous efficacy, which is the ratio of the total light output to the electrical power consumption, expressed in lumens per watt (lm/W) (Picture 1).

Picture 1: LM-79 Test Report (shows Colorimetric, Photometric and Electric parameters)

LM-80 report: This report is a standardized test method to evaluate the lumen maintenance of LED packages, arrays, and modules over time. This test measures the change in light output of an LED device over a specific period. LM-80 defines procedures for testing the lumen maintenance of LEDs and provides a way to compare different LED products in terms of their expected performance over time. The data generated by LM-80 testing can be used to estimate the expected lifetime of an LED product and provide information for manufacturers and consumers to make informed decisions about the quality and reliability of LED lighting products (Picture 2).

Picture 2: LM-80 Test Report (shows the total time (hours) that estimates lumen maintenance of LED)

In an LM-80 test report, "In-Situ Temperature Measurement Test" (ISTMT) inputs, commonly referred to as "In-Situ inputs," are crucial for accurately assessing the performance of LED products under realistic operating conditions.

Definition: In-Situ inputs refer to the temperature measurements of an LED system or fixture in its actual operating environment, as opposed to the controlled environment of the LM-80 testing. These measurements are typically taken at the LED's Tc point, which is the temperature measurement point specified by the LED manufacturer. This Tc point is critical for evaluating the thermal performance of the LED in its intended application.

Purpose: The purpose of including In-Situ inputs in an LM-80 report is to ensure that the thermal conditions under which the LEDs operate in the real world are accurately represented. Since the lifetime and performance of LEDs are significantly affected by their operating temperature, knowing the actual temperature conditions helps in predicting the real-life performance and lifespan of the LED product more accurately.

Application: These measurements are used along with the LM-80 test data to perform TM-21 calculations, which project the long-term lumen maintenance of LED light sources. By incorporating In-Situ inputs into these projections, manufacturers, designers, and engineers can obtain a more accurate estimation of LED performance over time in specific applications.

This section shows the percentage of initial lumens. the main 3 different percentages are L70, L80, and L90.

• L70 means the LED maintains 70% of its initial light output, representing a 30% reduction in brightness.
• L80 means the LED maintains 80% of its initial light output, representing a 20% reduction in brightness.
• L90 means the LED maintains 90% of its initial light output, representing a 10% reduction in brightness.

Examples of L70, L80, and L90: The comparison aims to illustrate the impact of 3 distinct LED operating temperatures (60°C, 70°C, and 80°C) on lumen maintenance levels (L70, L80, and L90) and their corresponding lifetimes (Lx0 in hours). This analysis highlights the differences in how long an LED can maintain a specific percentage of its initial light output (70%, 80%, and 90%) across varying temperatures.

Illustration of L70: The results indicate that the L70 hours lifetime is consistent (>72000), across the measurements, with minimal variance observed in the lumen maintenance over time. 

Illustration 1: The result for L70 hours and Lumen maintenance at time

Illustration of L80: The results reveal that as the LED temperature increases, the lifetime, measured in hours for L80, correspondingly decreases (this is attributed to the impact of temperature on the LED's longevity over time, leading to a reduced lifespan). Additionally, there is a slight decrease in lumen maintenance observed.

Illustration 2: The result for L80 hours and Lumen maintenance at time

Illustration of L90: The results mirror those observed for L80, as described previously above.

TM-21 report: The TM-21 report estimates the lifetime of LEDs based on accelerated testing, which involves subjecting the LEDs to extreme conditions to simulate many years of use in a short period of time. According to the latest TM-21 reports, LED lamps can last up to 100,000 hours or more, depending on the specific type of LED and its intended use (Picture 3).

Picture 3: TM-21 Test Report (shows the lifetime of LEDs based on the 3 different LM-80 test conditions)

Here is a table that compares the different parameters for these 3 test reports (Table 1).

Testing and Measurement Facilities for LM-79, LM-80, and TM-21
as mentioned before, ensuring accurate and reliable performance data is crucial for manufacturers, designers, and consumers. To achieve this, standardized testing procedures have been established, such as LM-79, LM-80, and TM-21. To conduct these tests effectively, specialized facilities and devices are utilized which are discussed here.

Integrating Sphere: An integrating sphere is a key component used to measure the total luminous flux output of an LED lighting product. It consists of a hollow sphere with a highly reflective interior surface. The device captures the emitted light from the LED product and ensures accurate measurement of the total light output (Picture 4).

Picture 4: Integrated Sphere (Copyright: Labsphere)

Sphere internal parts
An integrating sphere is a spherical device used in photometric and radiometric measurements to collect and measure light emitted by a source. Let's explore its key components and how it works:

Sphere Structure: The integrating sphere consists of a hollow spherical shell with a highly reflective interior surface. The interior is coated with a white or highly reflective material, such as barium sulfate or Spectralon. This ensures that light incident on the sphere is scattered and evenly distributed.

Light Source Input: The light source input refers to the opening or port on the sphere through which the light from the source under test is introduced. This port can be a small aperture or a larger opening depending on the size of the source. The source is positioned at the center of the sphere's input port.

Baffle: A baffle is a structure within the sphere that helps to control the directionality of the light entering the sphere. It helps to minimize direct reflections and scattered light that may interfere with the accuracy of the measurements. The baffle helps ensure that the light undergoes multiple reflections within the sphere to achieve uniformity.

Detector: The detector, also known as a photodetector or radiometer, is a light-sensitive device used to measure the light intensity or radiant flux inside the integrating sphere. It is typically placed at a specific location within the sphere to capture the scattered light.

Scattered Light Collection: When the light from the source enters the integrating sphere, it undergoes multiple reflections off the highly reflective interior surface. This scattering process ensures that the light is evenly distributed within the sphere. The scattered light is then collected by the detector.

Calibration: To obtain accurate measurements, the integrating sphere needs to be calibrated. This involves calibrating the detector's response to known reference standards. The calibration process accounts for factors such as the sphere's reflectance, detector sensitivity, and any losses or corrections needed.

The integrating sphere allows for the collection of light emitted by the source from all angles, ensuring that the measurement is representative of the total light output. By measuring the light at a specific location within the sphere using the detector, the sphere provides an average value of the light output from the source (Picture 5).

Picture 5: Integrated Sphere Construction (detailed)

Spectroradiometer:
A spectroradiometer measures the Spectral Power Distribution (SPD) of the LED lighting product. It analyzes the intensity of light at different wavelengths, allowing precise characterization of color quality, chromaticity, and other photometric parameters (Picture 6).

Picture 6: Spectrometers (Copyright: Konica Minolta)

Spectroradiometer internal parts
A spectroradiometer works by breaking down the light into its constituent wavelengths and measuring the intensity of each individual wavelength (Picture 7). Here is a brief explanation of how a spectroradiometer works:

Light Input: The spectroradiometer has an input port where the light to be measured is directed. This can be light emitted by a source or light reflected or transmitted through a sample.

Optical System: The spectroradiometer contains an optical system that manipulates the incoming light to separate it into different wavelengths. This is usually achieved using a combination of diffraction grating or prism and lenses.

Wavelength Dispersal: The diffraction grating, or prism disperses the light, causing it to split into its component wavelengths. This dispersion spreads out the light onto a detector.

Detector Array: The spectroradiometer is equipped with a detector array that measures the intensity of light at different wavelengths. It consists of multiple individual detectors, each sensitive to a specific wavelength range.

Signal Processing: The spectroradiometer captures the intensity readings from the detector array and processes the data. It applies corrections, such as dark current subtraction and calibration factors, to ensure accurate measurements.

Spectral Data Output: The processed data is then presented as a spectral power distribution, which shows the intensity of light at different wavelengths. This information can be displayed as a graph or stored in a digital format for further analysis.

Picture 7: Spectrometer connected to the sphere and the software extract spectral data

Goniophotometer:
A goniophotometer is employed to measure the angular distribution of light emitted by an LED lighting product. It provides information about the light distribution pattern, enabling accurate assessment of beam angles, luminous intensity, and light efficiency (Picture 8).

Picture 8: Goniophotometer structure (Copyright: Intertek)

A goniophotometer is an instrument used to measure the light distribution pattern of a light source or luminaire in different directions. It consists of several key components that work together to provide comprehensive angular measurement data (Picture 9). Here are the different parts of a goniophotometer:

Light Source: The light source is mounted on a rotating or moving platform to allow measurements at different angles.

Rotating or Moving Mechanism: This mechanism enables controlled rotation or movement of the light source and ensures precise angular measurements at various positions.

Photodetector: A photodetector is positioned at a fixed location within the goniophotometer setup. It measures the intensity of light emitted by the light source at different angles. The photodetector can be a photodiode, photomultiplier tube (PMT), or other light-sensitive devices.

Angle Measurement System: The goniophotometer includes an angle measurement system to accurately determine the position and orientation of the light source. This system may utilize encoders, rotary stages, or other angular measurement devices.

Goniophotometer enclosure (chamber): The goniophotometer enclosure is designed to provide a controlled dark environment for accurate measurements. The enclosure minimizes ambient light interference and stray reflections that could affect the measurement accuracy.

Data Acquisition and Control System: The goniophotometer is connected to a data acquisition and control system that records and analyzes the measurements. This system captures the intensity readings from the photodetector at different angles and stores the data for further analysis.

Software and Data Analysis: Specialized software is used to process and analyze the data collected by the goniophotometer. The software generates angular distribution data, intensity curves, polar diagrams, and other graphical representations of the light distribution pattern.

Angular Positioning Control: The goniophotometer may have a control system that precisely positions the light source at specific angles. This allows for measurements at predetermined angular positions or for scanning the light source in a continuous manner.

Optical Components: The goniophotometer may incorporate optical components such as mirrors, lenses, or diffusers to ensure uniform illumination of the photodetector and minimize stray light effects.

Calibration Standards: To maintain accuracy, the goniophotometer is calibrated using reference standards with known characteristics. Calibration ensures that the measurement results are traceable and reliable.

Picture 9: Goniophotometer structure (detailed)

Goniophotometers come in various types and configurations, each designed to cater to specific measurement requirements. The specific types of goniophotometers can vary based on their intended applications, size, and measurement capabilities. Here are some common types of goniophotometers:

Type A (Type Aα and Type Aβ): Type A is commonly used for measuring the total luminous flux distribution and intensity distribution of light sources or luminaires. Type Aα goniophotometers measure the forward light distribution, while Type Aβ goniophotometers measure the backward light distribution.

Type B: Type B is typically used for measuring the spatial luminous intensity distribution of a luminaire. They provide detailed information about the luminous intensity at different angles in a specified plane.

Type C: Type C is a specialized instrument used for measuring the luminous intensity distribution in both the horizontal and vertical planes simultaneously. They provide a more comprehensive assessment of the light distribution pattern.

Type D: Type D is designed to measure luminaires' spatial color distribution. They enable the evaluation of color uniformity and color consistency across different angles.

Type α/β: Type α/β combines the capabilities of both Type Aα and Type Aβ instruments. They measure the total luminous flux distribution in both forward and backward directions, providing a complete assessment of the light output.

Type C-γ: Type C-γ is primarily used to measure luminaires' spatial color distribution. They offer the ability to assess color characteristics, including chromaticity, color temperature, and color rendering properties, at different angles.

As mentioned, the selection of a specific type depends on the desired measurements, application requirements, and standards compliance. Goniophotometers play a crucial role in evaluating the photometric and colorimetric properties of light sources and luminaires, providing valuable data for lighting design, quality control, and research purposes.

Below is a table that compares different aspects of the 3 different facilities (Table 2).

 Table 3: The comparison of different aspects of the 3 different facilities

LM-80 testing focuses on evaluating the lumen maintenance of LED packages, arrays, and modules over an extended period. The following facilities and devices are utilized for LM-80 tests:

LED Aging Climate Chamber: An aging climate chamber provides a controlled environment to simulate real-world operating conditions and accelerate the aging process of LED devices. It subjects the LEDs to a constant temperature, humidity, and electrical stresses over the test duration.

LED Driver: An LED driver is a crucial component used in LM-80 testing. It supplies power to the LEDs and maintains a constant current or voltage, ensuring accurate and stable measurements during the test.

Data Logging System: A data logging system records the operating conditions and the lumen output of the LED devices over time. It collects data at regular intervals, allowing the calculation of lumen maintenance and extrapolation of performance beyond the test duration.

TM-21 testing extrapolates the lumen maintenance data obtained from LM-80 tests to predict the long-term performance of LED lighting products. Although TM-21 is a projection test, it requires certain facilities and devices for accurate estimation:

Data Analysis Software: TM-21 requires specialized software capable of analyzing the LM-80 data and projecting the lumen maintenance curve beyond the available test duration. The software utilizes mathematical algorithms to generate reliable predictions based on statistical models.

Temperature Monitoring System: As the temperature has a significant impact on LED performance, a temperature monitoring system is employed to measure and record the temperature of the LED devices during the test. This information is crucial for accurate performance projection.