Integrating spheres for measuring the radiant or luminous flux of light sources

The standard specifies that the size of the integrating sphere is based on the size of the lamp under test. But experts explain that for many product development and testing purposes, smaller spheres can provide acceptable measurement accuracy.

1. What is the function of integrating sphere and what is integrating sphere?
1.1 Integrating sphere instruction:
The integrating sphere is a quick and convenient tool for measuring the radiation or luminous flux of a light source. Such spheres are often used to characterize light sources, such as packaged LED components and finished luminaires of various sizes. Clearly, the product development and test measurement teams wanted to record accurate results from sphere testing. But the measurement practices specified in some standards lead to high costs associated with the need for very large spheres. Let’s consider a series of laboratory tests, trying to determine how much accuracy is affected when a sphere smaller than the specified range is used in routine testing, not for the purpose of reporting accredited laboratory results.

LPCE-2(LMS-9000)High Precision Spectroradiometer Integrating Sphere System

1.2 Integrating sphere spectroradiometer system measurement
Maximizing the accuracy of a measurement system requires consideration of multiple variables during the selection and use of system hardware and software, especially when strictly adhering to CIE standards. Because of the enormous scale and cost requirements that such compliance entails, many companies want to understand how compromises affect measurement accuracy. Compromises can minimize the routing costs associated with testing, as long as the accuracy can be shown to meet the requirements of the task at hand.

LPCE-3 CCD Spectroradiometer Integrating Sphere Compact System

2. How to choose the right size integrating sphere？
2.1 The test principle of integrating sphere
Measurements in compliance with the CIE S 025/2015 LED product measurement standard must meet specific dimensional requirements. There are two common sphere measurement geometries – 2π and 4π. The 4π configuration is the most commonly used configuration and requires the DUT (device under test) to be mounted in the center of the sphere. In tests where the light source doesn’t radiate backwards, you can more conveniently measure the total flux of the DUT mounted outside the sphere, and irradiate the radiation to a port on the side of the sphere – called a 2π geometry.

In 4π measurement geometry, the area of the DUT must be less than 2% of the inner diameter of the sphere. This corresponds to a DUT area of 1/10 the diameter of the sphere. For 2π measurements made externally, the port diameter must be ≤ 1/3 the diameter of the sphere.

2.2. Integrating sphere application area
The CIE S025 standard is a global document aimed at harmonizing LED measurements in countries around the world. The terms of this regulation are now available in the European and US IESNA light measurement standards. The end result is that only small diameter lamps can be measured in most practically sized integrating spheres. Larger light sources and luminaires must be measured on very large integrating spheres or using goniometers. Large integrating spheres, eg more than 3 meters in diameter, are expensive and require a lot of laboratory space. Equally expensive goniometers require constant ambient conditions and distance from light measuring instruments. Both solutions are unacceptable to many companies and institutions for day-to-day engineering and testing tasks.

2.3. Size of the integrating sphere
The integrating sphere works with a Spectroradiometer to do the photometry, colorimetry and radiometry parameters measurement.
• IS-0.3M/IS-0.5M is for LEDs, LED modules, mini LED bulbs & other small lamps. The flux testing range is 0.001 to 1,999 lm
• IS-1.0MA is for CFL or LED bulbs. The flux testing range is 0.1 to 199,990 lm
• IS-1.5MA/IS-1.75MA is for CFL, LED bulb and tube, fluorescent lamp, CCFL. The flux testing range is 0.1 to 1,999,900 lm
• IS-2.0MA is for HID lamps or high power lamps. The flux testing range is 0.1 to 1,999,900lm

3. Break the regular rules
3.1. Golden rules broken situation
So what happens when the “golden rule” of integrating sphere measurements is broken when measuring very large DUTs? In practice, companies using in-house standards for testing have adopted measurements that allow luminaires up to 30% of the diameter of the sphere, compared to those seeking accredited laboratory status. Expected measurement uncertainty under laboratory conditions is 3-4%.

3.2. DUT limits spherical reflections
Due to the relatively small sphere and larger DUT, the error increases as the DUT limits spherical reflections, which translates to lower measurement accuracy. Still, what compromises can be considered while still allowing engineers to achieve meaningful results for internal testing? In this article, we detail the test results, which identified a small increase in uncertainty compared to strict adherence to the CIE standard.

4. What testing method is used and what equipment will use?
4.1. Test method of the equipment
We first designed a measuring table and a set of DUTs that could simulate different luminaire sizes. Basically, each DUT relies on the same LEDs mounted on housings of different sizes and shapes to simulate different DUT test interference situations.

Our lab team maintains stringent measurement conditions throughout testing for each DUT configuration:
• Programmable and stable TDK lambda power supply
• Constant LED integration time and LED on time
• LED cooling for 3 minutes or more between measurements
The test is repeated several times for each DUT configuration. The DUT is a phosphor-converted white LED with a power consumption of 5.6W at a drive current of 0.6A.

Measurement results of various luminaire simulations and DUTS

4.2. Test equipment configuration
Our tests were performed using Lisun’s LPCE-2 (LMS-9000C) high-precision spectrometer integrating sphere system. LPCE-2 Integrating Sphere Spectroradiometer LED Testing System is for single LEDs and LED lighting products light measurement. LED’s quality should be tested by checking its photometric, colorimetric and electrical parameters. According to CIE 177, CIE84,  CIE-13.3, IES LM-79-19, Optical-Engineering-49-3-033602, COMMISSION DELEGATED REGULATION (EU) 2019/2015, IESNA LM-63-2 and ANSI-C78.377, it recommends to using an array spectroradiometer with an integrating sphere to test SSL products. The LPCE-2 system is applied with LMS-9000C High Precision CCD Spectroradiometer or LMS-9500C Scientific Grade CCD Spectroradiometer, and A molding integrating sphere with holder base. This sphere is more round and the test result is more accruacy than the traditional integrating sphere. However, for the requirements of this set of experimental measurements, these measurements use a reference or benchmark test to measure the LEDs on top of the rod or measuring table. The base case is the smallest of the DUTs. Results for other DUT configurations were compared with the baseline case. Figure 2 shows different DUT configurations.

5. What was the test result?
The result was better than expected. Even if the recommendations made in the standard exceed the standard several times, the error is only 2%.
The impact of house size and structure is surprising. Our 15×25, 15×55, 15×67, 15×80, 50×67 cm DUTs are made of black foam, while the round DUTs are made of light colored cardstock that fills most of the ball volume. The latter produces less flux measurement error than the smaller black foam DUT. The measurement results are shown in the table.

Lightsource test report

6. In conclusion
Accredited laboratory testing must of course meet applicable standards. But in internal testing, the company will find that changes in luminaire dimensions correlate with small differences in luminous flux and colorimetric measurements.
As the size of the device under test increases, the luminous flux reading decreases, and the necessary self-absorption compensation is significant. However, after recalculation, the measurements are very repetitive. This means that even a relatively large DUT, a well-designed measurement system with a reflective coating index higher than 97% will produce a dozen reflections in the sphere.

It must be noted that the sphere, the self-absorption coefficient of the spectroradiometer system should be defined for each wavelength, and thus its total amount defines the flux absorbed by the measured object. Depending on the object size and color, the coefficients may vary across the entire measurement range, necessitating the use of an accurate spectroradiometer or spectrometer.

More general conclusions about the measurement principles defined in the standard require additional testing and comparison of different measurement systems. However, as it turns out when the correct systems and practices are used, repeatable and reliable measurements can be obtained for light sources significantly larger than those specified in the standard.

Lisun Instruments Limited was found by LISUN GROUP in 2003. LISUN quality system has been strictly certified by ISO9001:2015. As a CIE Membership, LISUN products are designed based on CIE, IEC and other international or national standards. All products passed CE certificate and authenticated by the third party lab.

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