Integrating Spheres for lumen measurement of Light Sources

Integrating Spheres can conveniently lumen measurement and test radiation power of light sources, thus it can be used for testing various lighting products.

What is an integrating sphere?
An integrating sphere, also known as a photosphere, is a hollow sphere with a highly reflective interior surface that is used as an efficient light-gathering device to collect the scattered or emitted light from samples located within or placed nearby and close to a window. The inner walls of the integrating sphere are coated with a white diffuse material, that is, a material with a diffuse reflectance coefficient close to 1, materials such as magnesium oxide or barium sulfate are usually sprayed on the inner surface after mixing uniformly with a rubber adhesive.

The basic principle of the integrating sphere is that light is collected through the sampling port and scattered uniformly inside the integrating sphere after multiple reflections. When using the integrating sphere to measure the light flux, the measurement results can be more reliable, and the integrating sphere can reduce and eliminate the measurement errors caused by the difference in light shape, divergence angle, and responsivity at different positions on the detector.

Spheres are widely used in photometry, and in most applications these common optical components are used in conjunction with other photonic devices such as photometers and radiometers. In research laboratories, integrating spheres can be used to measure the transmittance or reflectance of materials. Balls are also often used to calibrate and measure light in electronic imaging devices.

Method of measuring reflectance of integrating spheres:
The sample is kept in the 0-degree port, and the incident beam is used to determine the reflectance through the 180-degree port. The sphere spatially integrates the total reflected radiation, and it is measured with a baffle detector. The reflected specular radiation can be cleaned using the sample holder, which uses vertical incidence to reflect any incident specular radiation.

The “specular plus diffuse” reflectance can be measured with the 8-incidence sample holder. By measuring both and taking their ratio, the reflectance of a known standard sample can be calculated. To avoid errors due to the sample’s reflective nature, the sample and the standard should have the same reflectance. A double-beam system can be used to eliminate such a potential source of measurement error. The detector is mounted on a 90-degree port.

Measurement of Total Flux:
The total optical flux produced by a light source (such as a lamp) is measured by placing the light source inside the optical element, where the light is often reflected multiple times by the sphere walls until it is detected by the photodetector. This concept of luminance is widely used in industry for comparing the lumen output of a light source for manufacture quality control. Other basic elements required for this configuration include observation-vision response detectors and diffusers.

Measurement of Transmission Ratio of Integrating Spheres:
The light that is incident on the sample and pass through the sample is known as the transmittance. If the sample has low scattering (such as clean dilute solution), almost all the light that is not absorbed will be transmitted. In addition to reflectance measurement, another important factor to be considered while using Integrating Sphere is the measurement of transmittance.

Spectral Radiance Flux Measurement:
In order to measure the spectral radiance flux generated by the light source, an Integrating Sphere is used with a Spectroradiometer. This configuration is same as that used for measuring total flux, but instead of photodetectors, a spectroradiometer is used. Furthermore, this set-up is applicable for measuring colorimetric indexes, correlated color temperature and chromaticity coordinates.

Testing Imaging Sensor:
The spheric port allows the optical element to be used as a uniform light source diffuser where a uniform irradiance originating from a diffuse source can be used to test imaging systems such as CCD cameras or array detectors. This application set-up is realized by placing lamps inside the sphere. The selection of lamps depends on the required irradiance. For this configuration, no detectors are needed.

Laser Power Measurement:
Spheres are widely used for laser power measurements in laboratory and industries where these optical elements are suitable for testing high powered and low powered industrial lasers. When doing laser power measurements, the correct use of a shield aids in preventing direct view of the laser hot spot. Spatial integration makes an Integrating Sphere a suitable choice for testing laser diodes and other devices exhibiting asymmetric and divergent characteristics. Moreover, the highly reflective coating of sphere helps in protecting the surface material from extreme high temperatures.

Testing Solid State Lighting Product:
Performance characteristics of LED lamps and other solid state lighting products are obtained by testing and evaluating the lumen efficacy and the total flux generated by the light source; using an Integrating Sphere, accurate measurements of colorspace data and total flux can be obtained. Although these optical elements enable precise measurements of the mentioned parameters, they do not measure spatial distributions of the beam. In order to measure performance characteristics of these light sources, Spheres are usually used in combination with gonio-photometer. Gonio-photometer can measure spatial distributions accurately.

The total luminous flux testing of integrating sphere includes at least two parts – one part for a sample, and the other for a detector. The detector is calibrated for radiation flux, but basically it tests the illuminance on the inner surface of the sphere. When using an integrating sphere to measure LED luminous flux, it is necessary to consider the package size and type of LED.

The integrating sphere must meet the following requirements to ensure that all curved surfaces have the same illuminance (the illuminance value in the tested area will represent that of the whole sphere):
the ratio of the diameter of the sphere to the diameter of the detector interface should be the maximum of 1:3;

ignore the self-absorption of the light from the sample interface or the auxiliary light; the inner coating has a high reflection and diffuse reflectivity coefficient (BaSO4);

in accordance with the requirements of CIE 127:2007 and IES LM-79 for other luminous flux testing conditions.

According to the standard, there are three different configurations of integrating sphere testing:
2π configuration: sample is placed on the inner surface of the integrating sphere – used for LEDs without reversed light. Often used for single LEDs and LED arrays.

4π configuration: sample placed in the center of the sphere through an interface – often used for solid-state light sources, auxiliary lamps are necessary to compensate for the absorption of light by the sample and the connector.

Partial luminous flux testing configuration: the sample is placed a certain distance away from the sample interface – a solid angle is defined by an accurate aperture in front of the interface.

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, IES-LM-80 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.

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

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.

Our main products are Goniophotometer, Integrating Sphere, Spectroradiometer, Surge Generator, ESD Simulator Guns, EMI Receiver, EMC Test Equipment, Electrical Safety Tester, Environmental Chamber, Temperature Chamber, Climate Chamber, Thermal Chamber, Salt Spray Test, Dust Test Chamber, Waterproof Test, RoHS Test (EDXRF), Glow Wire Test and Needle Flame Test.

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